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Keywords:

  • female choice;
  • fish;
  • genetic introgression;
  • hybrid zone;
  • male competition;
  • mating system;
  • Pecos River;
  • reproductive isolation;
  • sexual selection;
  • speciation

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Abstract Rapid and extensive genetic introgression has occurred between Pecos pupfish (Cyprinodon pecosensis) and sheepshead minnow (Cyprinodon variegatus) in the wild. We studied both female mate choice and male–male competition for mates among C. pecosensis, C. variegatus, and their F1 hybrids to determine what role these behaviours played in the formation of the hybrid swarm. Female C. pecosensis preferred male C. variegatus to conspecific males, C. variegatus females displayed no significant preference when given a choice between purebred males, and neither C. pecosensis nor C. variegatus females discriminated against F1 hybrid males. We found no evidence for female olfactory recognition of mates. Male F1 hybrids and C. variegatus were more aggressive than C. pecosensis males, achieving greater reproductive success under two different experimentally-induced mating systems. Hybrids were superior to C. variegatus when only two males competed (dominance interactions), but the two types were competitively equivalent in a territorial mating system. Our results indicate that active inter- and intra-sexual selection contributed to the accelerated hybridization between these two species. By including the possibility that some aspects of a hybridization and introgression event may be under positive selection, researchers may better understand the dynamics that lead to hybrid zone stability or the spread of introgressed genetic material.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

When allopatric populations are brought into secondary contact, the mechanisms that retard gene flow between them are of interest because they provide insights into the processes of speciation (Hewitt, 1988; Arnold, 1997; Boake, 2000). Divergent sexual selection (e.g. West-Eberhard, 1983; Boake, 2000; Uy & Borgia, 2000), natural selection (e.g. Schluter, 1998), and random genetic divergence (Dobzhansky, 1940; Muller, 1942; Orr, 1995) are each expected to reduce reproductive compatibility between geographically isolated populations (but seeRice & Hostert, 1993). The reproductive isolating mechanisms produced by these processes can be assigned to two broad categories (1) premating isolating mechanisms, which limit the number of matings between heterospecifics, and (2) post-mating isolating mechanisms, which limit the survival and reproduction of hybrid offspring.

In contrast to the two well-studied outcomes of hybridization (random genetic mixing and slower-than-random gene flow between hybridizing populations), the mechanisms that drive faster-than-random fusion of genomes have received little study. Sexual selection could promote gene flow between populations if hybrids benefit from female mate choice preferences or asymmetries in male–male competition for mates. Both processes, acting alone or in combination, would promote heterospecific matings and could result in faster-than-random fusion of genomes. Pre- and post-mating processes that promote greater inter-population cohesion than intra-population cohesion (sensuTempleton, 1989) have been documented. Pre-existing sensory bias (e.g. Kaneshiro, 1976; Ryan & Wagner, 1987; Ryan & Rand, 1993, 1995; Pfennig, 2000), resulting in female preferences for heterospecific males, is one phenomenon that may produce higher than random rates of heterospecific matings. Heterosis (hybrid vigor) exemplifies a post-mating mechanism that could promote gene flow between divergent populations (e.g. Scribner, 1993). Either pre-existing sensory bias or heterosis could lead to rapid genetic introgression (fusion of gene pools; Anderson, 1949) when formerly allopatric populations are brought into secondary contact. Because the role of sexual selection in promoting inter-population cohesion is poorly understood, its operation during the formation of hybrid zones may lead to incorrect interpretation of the forces producing genetic structure in hybrid swarms.

We conducted a series of experiments to determine the role of female mating preferences and male competitive interactions in promoting the extremely rapid and extensive genetic introgression that occurred in the wild between Pecos pupfish (Cyprinodon pecosensis Echelle and Echelle) and sheepshead minnow (Cyprinodon variegatus Lacepede). We studied female visual and olfactory mate preferences because of their importance in mate recognition in other fishes (e.g. swordtails: McLennan & Ryan, 1999; pupfish: Strecker & Kodric Brown, 1999, 2000). In species with territorial mating systems, male competitive interactions are indicative of male vigor and reproductive success (examples in Andersson, 1994).

Study System

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Cyprinodon pecosensis is endemic to the southern Pecos River drainage of New Mexico and Texas (Echelle & Echelle, 1978). Cyprinodon variegatus has a much broader geographic range, stretching from the coast of Massachusetts to the Yucatán Peninsula (Lee et al. 1980); the species is widely introduced into inland water bodies where it has become established. Sometime during the early 1980’s, C. variegatus were introduced to the Pecos River of Texas (Echelle & Connor, 1989). In <5 years, hybrids between C. pecosensis and C. variegatus had completely replaced the endemic (C. pecosensis) throughout more than 500 Km of the Pecos River. From the initial, presumably localized site of introduction, the geographic range of the hybrids has expanded beyond the historical southern limit of C. pecosensis (Echelle & Connor, 1989). Genetic analyses of the hybrid swarm (Echelle & Connor, 1989; Childs et al., 1996) suggest that the introduction of C. variegatus occurred only once, from a naturalized source population (probably in Lake Balmorhea, Reeves County, TX), and that only a small number of individuals were originally introduced. The proportion of introduced genetic material exceeds 50% throughout most of the hybrid population and both sexes of both species have contributed to formation of the hybrid swarm (Echelle & Connor, 1989; Childs et al., 1996). Pure populations of Pecos pupfish remain in two main areas: Bitter Lake National Wildlife Refuge (BLNWR) and Bottomless Lakes State Park (BLSP), both in Chavez County, New Mexico (Echelle et al., 1997). These populations are physically isolated from the Pecos River and the hybrid swarm.

Study organisms

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Cyprinodon pecosensis and C. variegatus are closely related, but C. pecosensis has probably been isolated from other Cyprinodon species for approximately 10 000 years (Echelle & Echelle, 1978, 1992). Cyprinodon pecosensis differs from C. variegatus in scalation, morphology, emphasis of male mating colours, and shape of ventrolateral markings; the two species also display fixed differences in allozyme markers and mtDNA sequences (Echelle & Echelle, 1978; Echelle & Connor, 1989; Childs et al., 1996). The two species are not sister taxa (Echelle & Echelle, 1992). Both species have a promiscuous breeding system in which males compete to establish breeding territories where they court females (Itzkowitz, 1977, 1981; Kodric-Brown, 1977, 1983). Females evaluate males and their territories and indicate a willingness to mate by swimming close to the substrate where spawning takes place. Females lay eggs on the substrate and may mate with several males each day (Kodric-Brown, 1977, 1983; Itzkowitz, 1978). There is no parental care of eggs other than the incidental product of males guarding territories. Male courtship displays are similar between the two species.

We collected C. pecosensis from Figure 8 Lake in BLSP (1997–1999) and from BLNWR (1999). Cyprinodon variegatus were purchased from a commercial breeder (1997–1999; Aquatic Biosystems, Ft. Collins, CO, USA); collected from Sea Rim State Park, Jefferson County, TX (1997) and from Lake Balmorhea, TX (1999); and donated by Dr P. Klerks, Louisiana State University-LaFayette, LA (2000). All of these populations had genetic ancestry from populations in the northern coast of the Gulf of Mexico. These fish and their offspring, including C. variegatus × C. pecosensis (Cv × Cp) and C. pecosensis × C. variegatus (Cp × Cv) F1 hybrids, were used in experiments. All F1 hybrids were bred and reared on the UNM campus.

All fish were allowed to acclimate to laboratory conditions and our animal care protocol for at least 3 weeks before use in a trial. Fish were housed in 37.85 and 75.70 L aquaria at similar densities in single-sex groups and fed ad libitum on flake food mixed with freeze-dried brine shrimp. Aquaria were filled with dechlorinated tap water mixed with artificial sea salt to 10 ppt; healthy wild populations of adult C. pecosensis are found in salinities ranging from 3.5 to 50 ppt (New Mexico Department of Game and Fish, 1999) and C. variegatus also have extremely broad salinity tolerances (Haney, 1999).

Visual preference experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Three 37.85 L aquaria (50 × 25.5 × 29 cm) were placed parallel to each other, 1 cm apart. The two lateral aquaria were divided into five equal sections with 0.57 mm transparent plexiglas dividers. The centre aquarium was undivided. Aquaria were illuminated from above using three full-spectrum, fluorescent bulbs (model: GE, F20T12-Pl/AQ, General Electric, Fairfield, CT, USA, http://catalog.gelighting.com). Fish in the aquaria were observed from above through holes in a horizontal screen. Cardboard dividers were placed between the three aquaria during the 15-min acclimation period, and removed before each trial.

A single mature female was placed in the centre aquarium. Each female was used in only one trial. A single male was placed in each of the five compartments of the two divided aquaria. We measured the total length of the female and each male. Males of one type (C. pecosensis, C. variegatus, Cp × Cv hybrid, or Cv × Cp hybrid) were placed in one of the divided aquaria and males of another type were placed in the other divided aquaria. To avoid side-bias, males of a particular type were placed on different sides of the female aquarium in different trials. The transparent dividers in lateral aquaria allowed males to see and display to conspecific males but prevented physical contact between males. Males were selected at random with respect to coloration, but we tried to match opposing males (those in the opposite sections of the two lateral aquaria) so that they were of roughly equal mass. The relationship between mass and length differs among the parental species and hybrids, so mass was estimated using species-specific length-mass relationships (J.A. Rosenfield, unpublished data). Males were stored separately from females and thus had no intercourse for at least 1 week prior to the trial.

Male coloration intensity and behaviour are affected by interactions with neighbouring males and breeding status (Kodric-Brown, 1983, 1996; Kodric-Brown & Mazzolini, 1992). We rearranged and replaced males between trials to expose each female to a different combination of male phenotypes. Between trials, one pair of competing males was removed from the lateral aquaria and replaced with a different pair of competing males. At least two males in each aquarium were moved to different sections of their aquarium between trials. This protocol could not completely eliminate the potential effect of pseudoreplication as there was some overlap in the males that certain females were exposed to. Completely replacing all 10 males for each trial would have required far more males than we were able to support. Compared with an experimental design where all females are exposed to the same males, our methodology substantially reduced any impact of pseudoreplication. In addition, by exposing each female to five conspecific (10 total) males simultaneously, we reduced the possibility that species-level preferences were obscured by the particular attributes of individual males. For each male, coloration intensity was scored for the dorsal fin, caudal fin, and dorsum on a scale from 0 to 6 (0 = coloration absent, 6 = peak coloration) before and after each trial. The summed pre- and post-trial scores were averaged for each male.

Female C. pecosensis were exposed to male C. pecosensis competing against either male C. variegatus (n = 20) or male F1 hybrids (n = 18). Female C. variegatus were exposed to male C. variegatus competing against either C. pecosensis (n = 20) or F1 hybrids (n = 20). A female was judged to be inspecting a particular male when she oriented towards that male while swimming with her snout <2 cm from the glass (a ‘contact’). In the wild, such active investigation of males by females would usually result in a mating (Kodric-Brown, 1977, 1983). Studies of other fish species have found that female association behaviour of this type is a good predictor of female mating preferences (e.g. Ryan & Wagner, 1987; Kodric-Brown, 1993). Trials continued for 15 min after the first contact. The species identity and aquarium section of contacted males and duration of each contact were recorded. Trials where females did not contact any male within 25 min or those where females contacted males for a total of <90 s during the 15-min trial period, were discarded. All C. variegatus females met these minimum standards of activity, but 10 female C. pecosensis did not. We controlled female side-bias by switching competing male types between sides of the aquaria, between trials and by observing females for 5 min prior to the trial (i.e. with screens in place). Wilcoxon Matched Pairs tests were used to test an overall effect of female side-bias. Individual females that displayed a side-bias during the pretrial observation period were not used in subsequent trials.

Analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

For each combination of male and female types, a Wilcoxon Matched Pairs test was used to determine whether females preferred conspecifics or heterospecifics. Because the two species are known to hybridize in the wild, we employed a two-tailed test.

The effect of interspecific differences in female behaviour on total contact time was evaluated using a Mann–Whitney U-test. Only those trials in which females had a choice between purebred males (C. pecosensis vs. C. variegatus) were used in this analysis. The effect of male species on female activity level was evaluated within female species. We compared female activity levels in trials where they chose between purebred males only to those where females were given a choice between conspecifics and hybrids. This allowed us to compare the activity levels of females exposed to hybrid heterospecifics with those exposed to purebred heterospecifics. Finally, we used a Mann–Whitney U-test to compare differences in female response to the two reciprocal crosses of hybrid males (individual females were exposed to one hybrid cross or the other, not a mixture of the two).

Expression of female preferences for male coloration and length were evaluated in two ways. Females might prefer males that displayed an extreme of either of these two characteristics. To detect such a preference, we studied the individual males that females contacted the most (‘preferred’ males) within each trial and ranked them for length and colour score as either high (among the five largest or most colourful males) or low (among the five smallest or least colourful males). A chi-square goodness of fit test was used to determine if preferred males were more colourful or larger than males they competed against. Our second analysis of female preference for male colour and length was designed to determine if females displayed a graded response to increasing coloration intensity or length. Because we observed species-specific differences in length (caused by controlling for mass) and coloration, this analysis could confound length or colour intensity preferences with preferences for other species-specific attributes. Thus, in this analysis, we ranked males within species for length, colour, and female preference (contact time). We generated a probability distribution for the relationship between female preference ranks and colour or length rank by producing 1000 randomized replicates of actual female preference ranks and colour or length ranks. We then compared the Spearman Rank Correlation between female preference and male colour or length in the actual data set to the randomized distribution to determine the probability that our observations could occur by chance alone. In this analysis, a significant correlation between contact time and colour or length would reveal a female preference for colour or length independent of male species.

Olfactory preference experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Female olfactory preference was evaluated using methods modified from McLennan & Ryan (1999) and Strecker & Kodric Brown (1999). A single female was placed in a 303 L aquarium (91 × 41 × 38 cm) half-filled with dechlorinated, salt water (10 ppt). Two lines drawn on the outside of the aquarium divided its length into three equal sections. The aquarium was surrounded by opaque, white plexiglass to eliminate external visual signals. Females were given 15 min to acclimate to this environment. Approximately 1 L of water (treated water) was taken from one of two 76 L aquaria containing 15 to 25 mature conspecific males. To reduce the potentially attractive scent of food, treated water was taken from aquaria where males had not fed for at least 40 h. Treated water was placed into a clean intravenous (IV) bag and dripped into one end of the female's aquarium. The drip rate was set at two drops per second using an IV drip-regulator. Into the other side of the female's aquarium, dechlorinated salt water (untreated water) was supplied from an IV at the same rate as treated water. At this drip rate, coloured waters dripped into opposing ends of the female tank were observed to mix after about 15 min. Female behaviour was videotaped for 15 min and data were recorded from the tape. Female preference for a given type of water was scored as the amount of time she spent in the third of the tank closest to each of the two water sources. Females were observed for 15 min prior to the trial in order to detect side-bias; no side-bias was detected. The side of the aquarium to which treated water was supplied was randomly assigned for each trial. The female's aquarium was cleaned between trials as per McLennan & Ryan (1999).

Male competition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

We evaluated male competitive ability by tallying male–male aggressive interactions and mating behaviours and, where appropriate, estimating the size of male territories. All male competition trials were conducted outdoors on the UNM campus, Albuquerque, NM between 17 May and 2 August of 1998–2001. Aggressive behaviours included (1) males chasing after, (2) lunging at, and (3) fighting with their opponents. Mating behaviours included male pursuit of females and spawning (a jerking motion of the male and female when their bodies are in contact). Individual males were used in only one trial.

In order to provide a comprehensive picture of male–male competitive dynamics, we employed two different experimental protocols: (1) dominance trials, where two males competed for access to females but males did not establish or defend territories, and (2) territoriality trials, where four males competed for spawning territories and access to females. These two different scenarios allowed us to record different types of valuable information. In the Dominance Experiment, we were able to observe the behaviours of individual males. In the Territoriality Experiment (where four males competed), we were not able to track the behaviour of individuals between observations; however, competition between multiple neighbouring males is normal in the wild, so the Territoriality Experiments represented a more typical context than the Dominance Experiment. The protocols reflected an effort to produce experimental conditions that allow for complete enumeration of all relevant behaviours (simplicity) and those that represent somewhat natural conditions (complexity). The Cyprinodon mating system is affected by local population density and habitat size (Soltz, 1974; Kodric-Brown, 1981, 1988a,b). By staging male–male competition under different population density conditions, we were also able to observe whether the outcome of male–male competition for mates was influenced by changes in the local mating system. The protocols are described below in detail.

Dominance experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

A single male from each of two types (C. pecosensis, C. variegatus, or F1 hybrids) competed for mating access to three or six females in a 1.8-m diameter stock tank that was divided with opaque plexiglass into two semicircular arenas. Independent trials were conducted simultaneously in each arena (area = 1.27 m2 each, population density = 3.94 or 6.30 m−2). Females in each arena represented a range of sizes and were a mixture of the three genotypes (i.e. C. variegatus, C. pecosensis, and F1 hybrid). A cinder block, with travertine rock glued over its holes, was placed in the centre of each arena to provide a focal territory. Competing males were placed in the arena on the morning of a trial (females remained in the arenas between trials) and, after a 15-min acclimation period, were observed from behind a blind in two to four 15-min observation periods conducted between 1000 and 1700 hours. We recorded the frequencies of male aggressive behaviour (chases, lunges, and fights) and mating behaviour (pursuits of females and spawnings). Observation periods were discarded if no aggressive or courtship activity was observed; trials were discarded if aggressive or courtship behaviours were not observed in at least two observation periods. We conducted trials with the following combinations of male types: C. pecosensis vs. C. variegatus (n = 21), C. pecosensis vs. F1 hybrids (n = 21), C. variegatus vs. F1 hybrids (n = 24). Trials were completed within 24 h because dominance was quickly established by one of the competing males.

Analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

For each male, an aggressiveness score was calculated as the number of lunges at and chases of the opposing male; fights (aggressive events in which there was no clear winner) were analysed separately. The number of male pursuits of females and spawnings were analysed separately. Data were standardized by dividing each variable by the total number of minutes in a trial. These standardized counts were square-root transformed to produce distributions that met the assumptions of parametric statistics. We used the transformed data and two-tailed, paired t-tests to test whether male types differed in the frequency of agonistic behaviours or in courtship and mating success. In trials where C. variegatus males competed with F1 hybrid males, the data for male pursuits of females were non-normal; for this comparison only, a Wilcoxon Matched Pairs test was used. We looked for significant correlations between the estimated difference in mass of competing males and differences in (1) aggressive, (2) courtship, and (3) mating behaviour of competing males using Kendall's Tau. For these correlations, a Bonferroni adjustment of alpha (adjusted α = 0.017) was performed because we searched for correlations between the mass differences of competing males and each of three independent variables. To determine if certain combinations of male types engaged in fights more often than others, anova was performed with the combination of competing males as the independent variable and the number of fights (square-root transformed) as the dependent variable.

Territoriality experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

A pair of males from each of two types (C. pecosensis, C. variegatus, or F1 hybrids) competed for territories and mating access to six females of one type (either C. pecosensis or C. variegatus) in 1.2-m diameter stock tanks (area = 1.13 m2, population density = 8.85 m−2). Four trays of gravel and travertine chips were placed equidistant from each other to serve as focal territories for males and spawning substrates for females. We conducted 28 trials with C. variegatus males and C. pecosensis males competing (n = 17 with C. pecosensis females, n = 11 with C. variegatus females). Twelve trials were conducted with C. pecosensis males and F1 hybrid males competing; female C. pecosensis were used in each of these trials. Twelve trials were conducted with C. variegatus and F1 hybrids competing; C. variegatus females were used in each of these trials. As it was difficult to distinguish C. variegatus males from F1 hybrid males when they competed with each other, males in this treatment were first marked according to type with white or yellow latex paint injected subcutaneously into the caudal peduncle and the area behind the dorsal fin (males were anaesthetized with aqueous MS-222 prior to injections). Males were marked 2 days prior to use in trials allowing sufficient time for them to fully recover from the injection. These identifying marks were clearly visible for over a week and had no adverse effect on the males’ behaviour (see Results). Treatments involving C. pecosensis males did not require marking males because they are easily distinguished from C. variegatus and F1 hybrid males.

After a 24-h period, during which they established territories, competing males were observed from behind a blind in 30-min observation periods conducted between 1000 and 1700 hours. Trials were continued for approximately 1 week in order to evaluate the stability of male territories. We recorded spawning events for each male; observation periods were discarded if no males spawned and trials were discarded if spawning was not observed in at least two observation periods. We tallied agonistic encounters (fights and chases) between conspecifics separately from those between heterospecific males. We also recorded territory size using gravel trays and other marks in the stock tanks to define territory boundaries.

Analysis

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Species-specific differences in spawning success were detected by using paired t-tests to compare the number of spawning attempts (square-root transformed) attributable to each type of male in a trial. For this analysis, results for conspecific males were summed across the two observation periods within each trial because the identity of conspecific individuals in the same trial could not be tracked between observation periods. Using anova, the sum of fight frequency and chase frequency (square-root transformed) was compared among treatments to determine whether aggression was greater in one combination of male types than others.

Female preference – visual experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Female C. pecosensis spent more time in contact with male C. variegatus than conspecific males (Wilcoxon Matched Pairs: z = 2.016, n = 20, P < 0.05; Fig. 1). C. variegatus females also spent more time in contact with C. variegatus males, but the preference was not statistically significant (z = 1.792, n = 20, P = 0.072). Neither C. pecosensis females nor C. variegatus females discriminated between conspecific males and F1 hybrid males (z = 0.9799, n = 18, ns and z = 0.7467, n = 20, ns, respectively). Side-bias was not detected among C. pecosensis females (Wilcoxon matched pairs test: z = 0.7396, n = 38, ns) or C. variegatus females (z = 1.665, n = 40, P = 0.096).

image

Figure 1. Mean duration (+SE) of females’ contact with competing male types in visual choice experiments. Significance values are indicated by asterisks (*, P < 0.05). Cyprinodon pecosensis are represented by clear bars, Cyprinodon variegatus are represented by black bars, and F1 hybrids are represented by half-tone bars. Twenty trials were conducted with each male-female species combination, except for C. pecosensis males vs. F1 hybrid males (n = 18).

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Female activity did not vary significantly between the two species or with regard to the species of competing males. When females were given a choice between purebred males, the activity levels of C. variegatus females and C. pecosensis females were not significantly different (mean total contact time: 500.1 and 412.5 s, respectively; Mann–Whitney U-test: U20,20 = 155, ns). Activity levels of C. pecosensis females exposed to purebred males were not significantly different from those where females chose between C. pecosensis and F1 hybrid males (311.3 s; Mann–Whitney U-test: U20,18 = 139, ns). Likewise, C. variegatus females exposed to competing purebred males displayed total activity levels similar to those of females exposed to male conspecifics competing against F1 hybrids (407.4 s; U20,20 = 164, ns). Neither C. variegatus nor C. pecosensis females displayed a significant preference for one cross of hybrid males as compared with hybrid males of the reciprocal cross (variegatus females: U13,7 = 29, ns; pecosensis females: U11,7 = 20, P = 0.09).

Female preference was not related to male coloration intensity or male length. For both species of females, preferred males were not significantly more colourful than the other competing males (Chi-square for pecosensis females: inline image = 0.947, ns; variegatus females: inline image = 1.256, ns; Fig. 2). Similarly, there was no significant correlation between male colour rank (ranked within male species, for this analysis) and female preference rank when our data were compared with a distribution of 1000 randomized replicates (Spearman's correlation: C. pecosensis females, r = −0.031, ns; C. variegatus females, r = −0.024, ns). The coloration of male C. variegatus was generally more intense than that of male C. pecosensis in the visual choice trials (mean scores = 3.6 and 2.5, respectively). Also, hybrid males had higher colour intensity scores than C. pecosensis males (mean scores = 2.9 and 2.2, respectively) but lower scores than C. variegatus males (mean scores = 3.3 and 3.9, respectively). The distribution of length ranks among preferred males did not deviate significantly from random for female C. pecosensis (Chi-square: inline image = 0.105, ns; Fig. 2) or C. variegatus (inline image = 0.9, ns). No significant correlation between male length rank and female preference rank was detected when our data were compared to a distribution of 1000 randomized replicates (Spearman's correlation: C. pecosensis females, r = 0.018, ns; C. variegatus females, r = 0.07, ns).

image

Figure 2. Distribution of the colour and length ranks of preferred males (the single male that an individual female contacted for the longest time). No significant preference for either male length or male coloration intensity was detected among female Cyprinodon pecosensis or Cyprinodon variegatus.

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Female preference – olfactory experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Neither female C. pecosensis nor female C. variegatus distinguished between water taken from an aquarium with conspecific males and plain, untreated water. In 14 trials, seven C. pecosensis females ‘preferred’ male-treated water (Wilcoxon Matched Pairs test: z = 0.878, ns). In 10 trials, five C. variegatus females ‘preferred’ male-treated water (z = 0.801, ns).

Male competition – dominance experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

No significant difference in competitive balance between types was detected for dominance trials involving six females and those where males competed for access to three females (t-test for independent samples: t22 = 0.093, ns) and the results of the two types of trials were combined for further analyses. F1 hybrid males were significantly more aggressive than either male C. pecosensis (two-tailed t-test for paired samples: t20 = 3.385, P < 0.005) or male C. variegatus (t23 = 2.321, P < 0.05, Fig. 3). Hybrid males also pursued females more frequently than C. pecosensis males or C. variegatus males (t20 = 4.771, P < 0.001, and Wilcoxon Matched Pairs test: z = 2.411, n = 24, P < 0.05, respectively) and spawned more often as a result (t20 = 3.022, P < 0.01 and t23 = 2.421, P < 0.05, respectively; Fig. 3). Cyprinodon variegatus males were more aggressive (t20 = 3.702, P < 0.005) and pursued females more often (t20 = 2.254, P < 0.05) than competing male C. pecosensis; however, the two species had similar spawning success in this experiment (t20 = 2.254, P = 0.086; Fig. 3). Fewer fights occurred in dominance trials involving C. pecosensis than in those where C. variegatus competed against F1 hybrids (anova planned contrast: F1,64 = 12.328, P < 0.001, Fig. 4). No significant correlations were detected between the mass difference of competing males and the outcome of their competitive interaction.

image

Figure 3. Behavioural rates of competing males in the Dominance Experiment. All values are means (+SE) for a 15-min observation period. Male aggression included males chasing or lunging at the opposing male. In all categories, male F1 hybrids were significantly more active than their competitors. Significance values are indicated by asterisks (*P < 0.05; **P < 0.01; ***P < 0.005). Values for each species are represented by the bar patterns described in Fig. 1.

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image

Figure 4. Frequency of fights between different combinations of male types in the Dominance Experiment. Values are means (+SE) for a 15-min observation period. Trials involving Cyprinodon pecosensis males (a) had significantly fewer fights (P < 0.001) than those where Cyprinodon variegatus males competed against F1 hybrid males (b).

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Male competition – territoriality experiment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Cyprinodon pecosensis males were inferior to both C. variegatus and F1 hybrid males at obtaining territories and they spawned less frequently as well. In trials where C. variegatus males competed against C. pecosensis males, C. variegatus established significantly larger territories than C. pecosensis (t-test for dependant samples: t27 = 2.509, P < 0.05; Fig. 5). In this experiment, C. variegatus obtained significantly greater spawning access to females than their C. pecosensis competitors (t27 = 2.6734, P < 0.05). The proportion of spawnings (arc-sin transformed) obtained by C. pecosensis males did not vary significantly between trials involving female C. pecosensis and those where C. variegatus females were used (t-test for independent samples: t25 = 1.397, ns). C. variegatus and F1 hybrids were equally successful in establishing territories (t11 = 0.544, ns) and obtaining spawnings (t11 = 0.434, ns; Fig. 5). F1 hybrid males were more successful at establishing and defending territories than C. pecosensis males (t11 = 2.521, P < 0.05) and obtained more spawnings as well (t11 = 2.951, P < 0.05; Fig. 5). Different treatments (combinations of male types) did not produce significantly different frequencies of fights in this experiment (anova: F1,48 = 0.926, ns).

image

Figure 5. Average spawning frequency and territory size (cm2; +SE) of competing male types in the Territoriality Experiment. In each trial, four males (a pair from each species) were observed in two, 30min observation periods. Values reported here reflect average performance of each species [i.e., (total spawnings for the species)/(two males)/(two observation periods)]. Statistically significant differences are indicated by an asterisk (*P < 0.05). Values for each species are represented by the bar patterns described in Fig. 1.

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Female choice

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

The results provide insight into the role of inter-sexual selection in the initial hybridization between C. vaiegatus and C. pecosensis following the introduction of C. variegatus to the Pecos River. Female C. pecosensis displayed a visual preference for C. variegatus. Cyprinodon variegatus females did not discriminate between purebred types although there is some suggestion of a preference for conspecifics (Fig. 1). In a mixed population, these visual preferences probably created a reproductive advantage for C. variegatus males. The attraction of C. pecosensis females for C. variegatus males represents more than a simple failure of allopatric populations to develop premating isolating mechanisms as such a failure would, by definition, not produce a preference for heterospecifics. Female mating preferences probably played an important role in the extremely rapid genetic fusion between Pecos pupfish and sheepshead minnow in the Pecos River.

We were not able to identify particular characters that allow female discrimination between male C. variegatus and C. pecosensis. No evidence of preference for males on the basis of length or colouration intensity was detected. Kodric-Brown (1983) found a preference among C. pecosensis females for colour intensity in male conspecifics. In our visual choice trials, males did not attain the intensity of coloration seen among breeding males in the wild or those seen in the male competition experiments (A. Kodric-Brown & J.A. Rosenfield, unpublished data). The most intense male colourations appear after males spawn for the first time on a given day (Kodric-Brown, 1996, 1998). Males in our trials had not reproduced for several days prior to exposure to females. With respect to male colouration then, our visual choice experiment forced females to evaluate male attractiveness before males mated. This experiment could not eliminate the possibility that females respond to visual signals that were not measured (for example, evaluation of male courtship behaviour or territory defense).

Females of neither parental species discriminated visually between conspecifics and F1 hybrids. Failure of both parental species to discriminate against hybrids would facilitate introgression in the wild. The lack of a clear female preference for hybrid or conspecific males suggests that the rate and extent of genetic introgression likely depended on the outcome of male–male competition for mates.

Females of neither species discriminated between male-treated water and untreated water. This negative result may reflect some deficiency in our experimental design; however, other Cyprinodon species displayed olfactory recognition of conspecifics and discrimination between conspecifics and heterospecifics in identical experiments (Strecker & Kodric Brown, 1999, 2000). Also, allopatric fish species in other genera discriminate between conspecific and heterospecific mates when exposed to a similar protocol (e.g. McLennan & Ryan, 1999). Therefore, it is unlikely that either C. variegatus or C. pecosensis females use olfactory cues for mate recognition.

Male competition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Our behavioural studies reveal a significant role for intra-sexual selection in promoting hybridization and genetic introgression between the two parental species in the Pecos River. Male C. variegatus and F1 hybrids were superior to male C. pecosensis in competition for mates under each of the protocols we used. This competitive asymmetry almost certainly promoted the rapid replacement of C. pecosensis by hybrids in the Pecos River because territorial male pupfish achieve much higher mating success than nonterritorial males (Itzkowitz, 1977; Kodric-Brown, 1977, 1988a) and females are attracted to vigorous male courtship (a factor not measured in our female preference experiments; Itzkowitz, 1978; Kodric-Brown, 1983, 1996).

Unlike competitive trials involving C. pecosensis, the outcome of male competition between C. variegatus and F1 hybrids was affected by the different experimental protocols we used. Our results indicate that hybrids have an advantage over both parental types in dominance situations where individual males control large areas of spawning territory and receive the vast majority of spawning attempts (a ‘dominance hierarchy’; Fig. 3). These conditions occur in the wild when habitat size is not limiting and population density is low (Soltz, 1974; Kodric-Brown, 1981, 1988a). Under more typical conditions, where breeding habitat is limited and the density of competing males is high, hybrids are competitively equivalent to C. variegatus, although they still outcompete C. pecosensis for territories and mates. This interaction between ecology and behaviour may help to explain why pure C. variegatus were eliminated from the Pecos River so soon after their introduction. Future studies that focus on behavioural mechanisms operating in hybrid zones should help us to understand how ecological conditions (Hubbs, 1955, 1961; Mayr, 1963) interact with mate choice systems and hybrid genotypes to promote hybridization and introgression.

Male–male competitiveness is only one measure of the viability of hybrid offspring. The rapid introgression observed in the wild would only have occurred if hybrid offspring were also ecologically viable. Under laboratory conditions, growth rates among F1 hybrid juveniles are intermediate to those of C. variegatus and C. pecosensis (J.A. Rosenfield, unpublished data). Also, mature F1 hybrids and backcross hybrids display greater swimming endurance than mature C. pecosensis (J.A. Rosenfield, unpublished data). The assumption that hybrids are competitively inferior to purebred parentals does not hold in this case; F1 hybrid males are competitively superior to C. pecosensis males in several experimental contexts and backcross hybrids have replaced C. pecosensis in all habitats to which they had access.

Reproductive compatibility and the ‘Species Problem’

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Our results would lead some biologists to conclude that C. pecosensis and C. variegatus are not different species (sensuMayr, 1963). Female C. pecosensis’ choice of mating partners and the success of F1 hybrid males actively promoted hybridization and introgression between the two parental species; thus, our results reveal cohesive forces operating beyond ‘species’ boundaries, something that cannot (or should not) occur under most species concepts (see Arnold, 1997). However, C. pecosensis and C. variegatus, have diverged in morphometric and meristic measures, coloration patterns, and display fixed differences in allozymes and mtDNA (Echelle & Echelle, 1978; Echelle & Connor, 1989; Childs et al., 1996). Additionally, the behavioural and ecological differences between parental species (current study; J.A. Rosenfield, unpublished data) demonstrate that these two taxa are not ecologically or demographically interchangeable (Van Valen, 1988; Templeton, 1989; Smith et al., 1995). Finally, the behaviour and ecology of hybrids (Echelle & Connor, 1989; current study; J.A. Rosenfield, unpublished data) reveals a synergy between the parental genomes; this positive interaction is evidence of genetic divergence for the same reasons that partial or complete reproductive isolation (negative interactions between genomes) would be interpreted as evidence of divergence. The lack of pre- or post-mating reproductive isolation between such divergent forms reveals that the mechanisms that produce lineage cohesion may remain intact despite substantial evolution and differentiation in other areas (Smith et al., 1995).

Mating behaviour and the study of genetic introgression

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

Both premating mechanisms (female mate choice and male competition for mates) and a post-mating mechanism (hybrid vigour) promoted rapid genetic fusion between these two allopatric pupfish species upon secondary contact. These findings contradict the assumption that individuals will always mate preferentially with conspecifics when a choice is available (e.g. Tinbergen, 1953; Paterson, 1985). Examples of females in geographically isolated lineages expressing an attraction for heterospecific males are becoming more common (Kaneshiro, 1976; Ryan & Wagner, 1987; Pierotti & Annett, 1993; Ryan & Rand, 1993, 1995; Grant & Grant, 1997; Pfennig, 2000; current study). Our results also stand in contrast to the common finding that hybrid offspring are inferior to parental species in obtaining mating opportunities. This inferiority may occur because hybrids are less viable ecologically (Dobzhansky, 1940; Barton & Hewitt, 1985; Hatfield & Schluter, 1999) or because they are discriminated against by potential mates (e.g. Vamosi & Schluter, 1999). However, hybrid inferiority across habitats is far from universal (e.g. Scribner, 1993; Arnold & Hodges, 1995; Arnold, 1997) and some studies show that hybrid organisms may actually have an advantage in intra-sexual competition for mates (e.g. Pierotti & Annett, 1993; Good et al., 2000; current study). If hybrid offspring are fertile and outcompete one or both parental species for access to mates, rapid and extensive introgression is likely.

The mechanisms that contribute to lineage cohesion between formerly allopatric populations require additional study. For example, the visual preference of C. pecosensis females for heterospecific males suggests the possibility that these females retain a preference for ancestral male sexual signals (e.g. Kaneshiro, 1976; McLennan & Ryan, 1999) displayed by C. variegatus. All but one species in the Gulf Coast assemblage of Cyprinodon species, including C. pecosensis, are believed to derive from a C. variegatus-like ancestor (Echelle & Echelle, 1992). Female preference for ancestral male characteristics raises the question: if sexual selection favours ancestral characters, why were those characters lost or diminished during the evolution of C. pecosensis? The ecological success of hybrids argues against correlated evolution of sexual characters with other characters under strong natural selection (Paterson, 1985). The secondary loss of characteristics under positive sexual selection may be a common phenomenon among isolated species (e.g. Kaneshiro, 1976; Ryan & Rand, 1995) and Cyprinodon species in particular (Loiselle, 1982).

Similarly, the role of aggressive behaviour, and hybrid behaviour in general, in the population dynamics of hybrid swarms should be investigated. Our results indicate that, because of their extreme aggressiveness, hybrid males are competitively superior to males of both parental species under certain conditions. As a result, this example of hybrid vigour is a cohesive force in both the premating (hybrids will get more matings than purebreds) and post-mating (hybrid offspring are more vigorous than some parentals) realms. Female selection for male aggressiveness may drive hybridization and introgression in other hybridizing species pairs (Brodsky et al., 1988; Pierotti & Annett, 1993; Good et al., 2000; McDonald et al., 2001) and elevated aggressiveness among hybrid offspring has been documented in other hybrid zones, particularly those involving avian species pairs (Rohwer & Wood, 1998; Good et al., 2000; McDonald et al., 2001). Additional research is necessary to determine whether elevated aggressiveness among hybrids is a general phenomenon.

Studies of the dynamics of natural hybridization and genetic introgression have largely neglected mating behaviours and have instead focused on the ecological forces that limit expansion of the hybrid swarm (Moore, 1977; Barton & Hewitt, 1985; Harrison, 1986; Howard, 1986). Many recent studies of hybrid zones have attempted to infer the behavioural processes that produce patterns in the population genetic structure of the hybrid swarm (e.g. Rosenfield et al., 2000 and references cited therein). However, inferences drawn purely from descriptions of gene frequencies may provide an incomplete view of the evolutionary dynamics within hybrid zones if mating system structure and sexual selection behaviours are not reflected in population genetic models (e.g. Childs et al., 1996). In the context of speciation and hybridization studies, mating behaviours have historically been viewed only in terms of their potential for decreasing reproductive compatibility. The alternative possibility, that behaviours increase the likelihood of interspecific mating and thereby promote hybridization and introgression, has rarely been considered (but see Good et al., 2000; McDonald et al., 2001). Ecological conditions contribute to hybridization in many animal systems (Hubbs, 1955), but, their effects are likely mediated through the mating system and the behavioural responses of individuals (Grant & Grant, 1997). Researchers investigating other cases of hybridization and introgression should explore the role of mating behaviours and the response of these behaviours to changes in ecological conditions.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References

The authors wish to thank K. Howard, J.H. Brown, D. Schluter, O. Seehausen and an anonymous referee for providing valuable comments on this manuscript. S. Cain conducted the olfactory preference experiments. C.M. Sandoval, S.Cain, S. Lindauer, and T. Angón assisted in conducting the male competition trials. A. Allen helped to conduct the resampling analyses of male colour and length. P. Nicoletto and P. Klerks provided invaluable assistance in procuring C. variegatus for this study. Funding was provided by the National Science Foundation (#IBN-9806483), New Mexico Department of Game and Fish, and Graduate and Biology Research Allocation Committees of the University of New Mexico. J.A.R. was supported by a STAR fellowship from the United States Environmental Protection Agency. This research was conducted under animal care protocols approved by the University of New Mexico Animal Care Committee (Permit No. 97001).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study System
  5. Materials and methods
  6. Study organisms
  7. Female choice
  8. Visual preference experiment
  9. Analysis
  10. Olfactory preference experiment
  11. Analysis
  12. Male competition
  13. Dominance experiment
  14. Analysis
  15. Territoriality experiment
  16. Analysis
  17. Results
  18. Female preference – visual experiment
  19. Female preference – olfactory experiment
  20. Male competition – dominance experiment
  21. Male competition – territoriality experiment
  22. Discussion
  23. Female choice
  24. Male competition
  25. Reproductive compatibility and the ‘Species Problem’
  26. Mating behaviour and the study of genetic introgression
  27. Acknowledgments
  28. References
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