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Keywords:

  • Butomus umbellatus;
  • clonal reproduction;
  • cost of reproduction;
  • flowering rush;
  • plants;
  • reproductive allocation;
  • trade-offs

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

That trade-offs result from the allocation of limited resources is a central concept of life history evolution. We quantified trade-offs between sexual and clonal reproduction in the aquatic plant, Butomus umbellatus, by experimentally manipulating sexual investment in two distinct nutrient environments. Increasing seed production caused a significant but nonlinear trade-off. Pollinating half of all flowers strongly reduced clonal bulbil production, but pollinating the remaining flowers did not cause any further trade-off. Trade-offs were not stronger under low nutrient conditions that clearly limited plant growth. Experimentally induced trade-offs were not reflected in negative phenotypic correlations between sexual and clonal allocation among plants within eight populations grown in a uniform greenhouse environment. Diminishing effects of increased sexual allocation plus a lack of accord between experimental manipulations and phenotypic correlations suggest that trade-offs between sexual and clonal reproduction are unlikely to constrain the evolution of reproductive strategy in this species.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Trade-offs among life functions that contribute to fitness is a central concept of life history theory (Roff, 1992). For instance, investment of resources in sexual reproduction is expected to reduce somatic growth and energy reserves. This physiological trade-off should lead to a demographic trade-off: increased reproduction reduces survival and future reproductive success (reviewed in Bell & Koufopanou, 1986). The physiological and demographic costs of reproduction have been quantified to varying extents in a wide variety of plants. Several studies have quantified phenotypic correlations between sexual reproduction and vegetative growth (Jurik, 1985; Elle, 1996), survival and future reproduction (e.g. Law, 1979; Geber, 1990; Cain & Damman, 1997) or, more rarely, both physiological and demographic parameters (Karlsson et al., 1990). Investment in sexual reproduction has also been experimentally manipulated (e.g. flower removal, controlled induction of flowering, hand-pollination) to directly measure the effect on resource allocation (e.g. Reekie & Bazzaz, 1987, 1992; Muir, 1995; Saulnier & Reekie, 1995) or demography (e.g. Horvitz & Schemske, 1988; Primack & Hall, 1990; Calvo, 1993; Ågren & Willson, 1994; Lehtilä & Syrjänen, 1995; Primack & Stacy, 1998). However, the extent to which trade-offs revealed by experimental manipulations are reflected in phenotypic or genetic correlations is unclear. The very few studies that have combined experimental and correlative approaches have found little correspondence between them (Fox & Stevens, 1991; Jackson & Dewald, 1994).

For most perennial plants, understanding the cost of reproduction is complicated because they reproduce both sexually by seed and asexually through vegetative clonal propagation, and it is thought that sexual and clonal reproduction compete for resources (Abrahamson, 1980). The resulting trade-off is expected to constrain the evolution of the reproductive strategy by influencing the evolutionary response of populations to environmental unpredictability, including temporal and spatial variation in habitat quality and pollination (Schmid, 1990; Gardner & Mangel, 1999). Yet, the relation between reproductive modes remains poorly understood (Bazzaz, 1997). Some studies detected negative correlations between sexual and clonal reproduction (Sohn & Policansky, 1977; Law et al., 1983; Geber et al., 1992; Cheplick, 1995; Worley & Harder, 1996; Prati & Schmid, 2000; Ronsheim & Bever, 2000; van Kleunen et al., 2003) while others have not (Douglas, 1981; Pitelka et al., 1985; Reekie, 1991; Cain & Damman, 1997). Interpreting these phenotypic or genetic correlations is difficult because individuals may vary widely in their ability to acquire resources as well as how they allocate them. Genetic and/or environmental variation in resource acquisition among individuals can cause positive covariation between competing functions that may obscure trade-offs (reviewed in Reznick et al., 2000). This is especially true of sexual and clonal reproduction because the net effect of one on the other will depend on how they compete directly for resources as well as how they independently affect and are affected by vegetative growth (Worley & Harder, 1996). This necessitates experimental manipulation and quantitative estimation of all allocation components to test for reproductive trade-offs. The few studies that have manipulated sexual reproduction and examined the consequences for clonal reproduction have produced conflicting results (Snow & Whigham, 1989; Westley, 1993; Saikkonen et al., 1998; Méndez, 1999). Moreover, very few studies have examined the demographic trade-off between sexual and clonal reproduction (Douglas, 1981).

Here, we manipulate sexual reproduction and also quantify phenotypic correlations in a broad sample of populations to investigate physiological and demographic trade-offs between sexual and clonal reproduction in an aquatic plant, Butomus umbellatus L. (Butomaceae, flowering rush). In doing so, we address several difficulties that have impeded clear demonstration of reproductive trade-offs. First, reproductive organs may act partially as resource sources rather than wholly as resource sinks. For instance, some organs involved in sexual reproduction may photosynthesize and contribute toward the cost of their construction and maintenance (Bazzaz, 1997). Likewise, clonal organs may enhance resource acquisition and storage (Grace, 1993), thereby enhancing investment in sexual and vegetative structures. In contrast, sexual reproduction, clonal reproduction and vegetative growth in B. umbellatus involve distinct, easily measured structures. Clonal reproduction occurs via pea-sized, vegetative bulbils produced on rhizomes, from which they detach almost as soon as they are formed. These clonal propagules function only in terms of genotypic multiplication and dispersal and could be produced at the expense of vegetative and sexual structures. Bulbils are also formed, although to a lesser extent, on inflorescences and may, therefore, compete with sexual reproduction for inflorescence meristems in addition to resources (Watson, 1984). The unambiguous nature of clonal bulbils also allows sexual and clonal reproduction to be expressed in a common currency, propagules, to evaluate the demographic fitness trade-off between reproductive modes.

Secondly, it is widely predicted that trade-offs will only be manifested in environments where critical resources are limiting (reviewed by Bell & Koufopanou, 1986; Roff, 1992; Reznick et al., 2000). A few studies have shown that the effect of sexual reproduction on vegetative growth varies with nutrient or light availability (Jurik, 1985; Reekie & Bazzaz, 1987), and stronger costs of reproduction have sometimes been observed after defoliation of experimental plants (Lubbers & Lechowicz, 1989; Primack & Hall, 1990; but see Lehtilä & Syrjänen, 1995). However, the few studies investigating the effect of environmental quality on trade-offs between sexual and clonal reproduction suggest that variation in light and nutrients has no effect on phenotypic correlations between reproductive modes (Reekie, 1991; Cheplick, 1995; Ronsheim & Bever, 2000; but see Wijesinghe & Whigham, 1997) or experimental manipulations of sexual reproduction (Saikkonen et al., 1998). We manipulated sexual reproduction along with the availability of key nutrients, nitrogen in particular, which may strongly influence the cost of reproduction (Saulnier & Reekie, 1995). Moreover, the growth of B. umbellatus in natural habitats appears strongly influenced by nutrients (Hroudová & Zákravsky, 1993a,b).

Thirdly, it is generally expected that trade-offs will be strongest between functions that compete for resources at the same time (Bell & Koufopanou, 1986). Although temporal separation between vegetative growth, flowering and the production of clonal propagules varies widely among plants and is expected to strongly reduce trade-offs (Ågren & Willson, 1994; Bazzaz, 1997; Gardner & Mangel, 1999), it has rarely been formally quantified in observational or experimental studies (Cheplick, 1995; Méndez, 1999). We quantified investment in vegetative growth and clonal reproduction before and after manipulation of investment in sexual reproduction to directly relate the degree of temporal overlap between components of allocation to the effect of increased sexual reproduction.

Fourthly, almost all previous experimental studies of trade-offs involving sexual reproduction have used simple two-level manipulations of investment in sex. We used three treatments to detect nonlinear trade-offs (Schmid, 1990) and better interpret the ecological relevance of our experimental manipulations.

Finally, we investigate the relatively untested assumption that trade-offs between reproductive modes revealed by experimental manipulations translate into negative phenotypic correlations between sexual and clonal reproduction that might constrain the evolution of reproductive strategies (Jackson & Dewald, 1994). To compare with the results of our experimental manipulations, we quantified among-plant phenotypic correlations between sexual and clonal allocation for eight widely distributed populations of B. umbellatus while statistically controlling for confounding genetic and/or environmental variation in resource acquisition.

Study species

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Butomus umbellatus is an emergent, aquatic monocot from a monotypic family, that was introduced from Eurasia to North America approximately 100 years ago, and has spread across the northern United States and southern Canada (White et al., 1993). The reproductive and vegetative morphology of B. umbellatus is ideal for studying reproductive trade-offs. Individual ramets consist of a monopodial prostrate rhizome that produces thin upright leaves from rhizome meristems. Hence, vegetative growth involves extension and branching of the rhizome and leaf production. Axillary rhizome meristems also form small clonal bulbils, hundreds per ramet per season, which readily detach and quickly develop on moist soil or the water surface. Rhizome bulbils function solely in genet multiplication and dispersal and are unambiguously organs of clonal reproduction. Individual rhizomes may break into several fragments over winter, but their contribution to multiplication and dispersal is negligible (K. Lui, F.L. Thompson & C.G. Eckert, unpublished data). Rhizome meristems also form inflorescences, often several per ramet per season, each of which bears an umbel of 20–50 pink flowers on a long, thin stalk. Each flower has six carpels, including approximately 200 ovules that yield approximately 35 seeds (Eckert et al., 2000). Flowers are fully self-compatible but require insect visitation for pollination and seed set. They do not spontaneously self-pollinate (Eckert et al., 2000). Although levels of self-fertilization and outcrossing have not been quantified, it is likely that plants are highly outcrossing (Bhardwaj & Eckert, 2001). Individual flowers exhibit strict dichogamy, thereby preventing self-pollination within flowers (autogamy). Moreover, the sex expression of flowers within inflorescences is synchronized such that umbels tend to be wholly male or wholly female at any given time, thereby severely limiting opportunities for self-pollination between flowers (geitonogamy). Population surveys of floral sex expression indicate that strong dichogamy within flowers and inflorescences is a consistent feature of populations across North America (Bhardwaj & Eckert, 2001; F.L. Thompson & C.G. Eckert, unpublished data). In addition to producing clonal bulbils on rhizomes, B. umbellatus also produces bulbils at the base of inflorescences (Eckert et al., 2000). These form near the end of flowering, and remain attached to the inflorescence until it decomposes. Trade-offs between sexual and clonal reproduction may, therefore, occur within inflorescences and at the whole ramet level.

Experimental manipulations

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

In July 1999, we randomly sampled 20 widely spaced (>10 m) plants from throughout a large, sexually fertile, diploid population of B. umbellatus population (ONGL) in eastern Ontario, Canada (Appendix S1). Although we attempted to maximize the number of genotypes included in the experiment, we recognize that, because of extensive clonal reproduction, our 20 experimental plants may not be distinct genotypes. Hence, we refer to variation among the sampled plants as ‘plant’ rather than ‘genotype’ effects.

To reduce nongenetic variation because of carryover effects of the clonal maternal environment (Schwaegerle et al., 2000), plants were trimmed to a standard size, and grown for 16 weeks in a randomized greenhouse environment. Plants were moved to cold storage in November, just after they began to senescent, and stored in the dark at 5 °C until May 2000. Clonal maternal effects were further reduced by generating 16 replicate ramets per original plant from the rhizome bulbils produced in a common environment. We expect that clonal maternal effects did not contribute to phenotypic variation among plants in this experiment for two reasons: (1) Clonal bulbils were very small (mean ± SD dry mass = 0.021 ± 0.005 g) compared with the mature plants they produced (final plant mass = 12.6 ± 5.5 g). (2) It is expected that carry-over effects are primarily caused by variation in the size of the propagules used to generate experimental plants (Schwaegerle et al., 2000). However, the size of individual rhizome bulbils was not affected by our experimental nutrient treatment, which caused a twofold difference in overall plant mass [see below; mean ± SE bulbil dry mass: low nutrients = 20.6 ± 5.3 mg; high nutrients = 20.9 ± 4.3 mg; P = not significant (n.s.)].

Individual bulbils were planted in a randomized array in plug trays, and developing plants were transplanted 1 month later to randomly-positioned 10-cm pots. Half the clonal replicates from each plant were randomly assigned to each of two nutrient treatments: either 0.11 or 0.037 g plant−1 week−1 of 20-20-20 (N-P-K) fertilizer. For each nutrient treatment, two clonal replicates per plant were randomly assigned to each of three pollination treatments to vary investment in sexual reproduction: (1) All flowers were left unpollinated and did not produce seed (see above). (2) Every second flower was cross-pollinated with pollen from another plant. Hand pollination always results in fruit production. (3) All flowers were cross-pollinated.

Just before pollination, we harvested two ramets for each plant in each nutrient environment to determine investment in vegetative, clonal and sexual structures before sexual reproduction was manipulated. For the remaining ramets, we counted and collected the flowers and fruits on each inflorescence. In November 2000, just as the aboveground structures began to senesce, all ramets were harvested, and sexual, clonal and vegetative structures were separated, counted, dried at 70 °C until constant mass, and weighed to 0.001 g. Sexual structures included inflorescence stalks, peduncles and fruits. Vegetative structures included leaves and the rhizome. Clonal structures included bulbils produced on rhizomes and inflorescences.

We used three-way mixed model anova to test for the effects of plant (random effect), pollination treatment and nutrient treatment (both fixed effects), on each component of biomass (Neter et al., 1990, p. 850). F-tests for the effect of plant used a synthetic denominator mean square (MS) (MSplant × pollination + MSplant × nutrient − MSplant × pollination × nutrient); pollination treatment used MSplant × pollination; nutrient treatment used MSplant × nutrient; plant × pollination, plant × nutrient, and pollination × nutrient used MSplant × pollination × nutrient; and plant × pollination × nutrient used MSresidual · Log10-transformation of the data normalized residuals and eliminated associations between residuals and predicted values. In some cases (15%) plants were only represented by a single replicate within a given combination of pollination and nutrient treatment, hence tests of significance used the Satterthwaite method (Sokal & Rohlf, 1995). Only 48% of ramets produced inflorescence bulbils, so we considered this a two-state variable (0 vs. at least 1 bulbil) and used two-way categorical anova to test the effects of pollination and nutrient treatments.

Phenotypic correlations: controlling resource acquisition

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

We evaluated the phenotypic correlation between sexual and clonal allocation among plants while statistically controlling for variation in resource acquisition (see Introduction). This is usually done by expressing sexual mass and clonal mass as proportions of total mass (e.g. Cheplick, 1995; Saikkonen et al., 1998; Prati & Schmid, 2000). However, this use of ratio variables has been seriously criticized (Sokal & Rohlf, 1995; Jasienski & Bazzaz, 1999), so we independently corrected both sexual and clonal mass for covariation with vegetative mass (i.e. leaves and rhizome). This is an appropriate method for statistically controlling resource acquisition because the leaves and rhizome acquire and store resources that may limit reproductive allocations, vegetative mass does not include any component of sexual or clonal mass, both reproductive components covaried positively with vegetative mass (see below), and most vegetative mass was accumulated before investment in clonal or sexual structures (Fig. 1).

image

Figure 1. Differential timing of biomass investment in sexual and clonal vs. vegetative structures in Butomus umbellatus. Bars are mean ± 1 SE dry mass before the imposition of the pollination treatments (prepollination) vs. at final harvest (averaged across pollination treatments). Statistics are calculated from plant means (n = 20).

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Phenotypic correlations: replicated plants from ONGL

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

First, we tested for variation among plants in relative investment (allocation) to sexual reproduction, clonal reproduction and vegetative growth using multivariate profile analysis. Profile analysis tests for differences among plants in the slopes connecting vegetative, clonal and sexual biomass (Ronsheim & Bever, 2000). A profile × plant interaction indicates that plants vary in their pattern of allocation to the three functions. A profile × plant × pollination interaction indicates that the pattern of variation in allocation among plants differs between pollination treatments. A profile × plant × nutrient interaction indicates that the pattern of variation in allocation differs between nutrient treatments.

We then used the residuals of log10 (sexual or clonal mass) for each plant from an ancova model including the combination of nutrient and pollination treatment as main effect (d.f. = 5) and log10 (vegetative mass) as covariate. The effect of vegetative mass was strong, significant (mean standardized β = +0.41, both P < 0.001), and did not vary among the six pollination × nutrient environments for either sexual or clonal mass (both P = n.s.). We then used ancova to test the overall phenotypic correlation between sexual and clonal allocation and whether it varied among environments.

Phenotypic correlations: population survey

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

We sampled approximately 30 plants from each of eight diploid populations (including ONGL) from across North America in 1999 (Appendix S1). We attempted to reduce nongenetic variation by collecting only plants consisting of one unbranched rhizome, and trimming each so that all but two of the most apical leaves were removed. Plants were then grown for the summer and fall of 1999 in a randomized greenhouse environment, over-wintered at 5 °C until May 2000, and regenerated (one replicate per plant) from a 3-cm section of rhizome branch of known mass. Plants were randomly positioned on a greenhouse bench and subjected to the high nutrient conditions as above, and left unpollinated (none produced seed). All biomass components were quantified as above. Initial branch mass did not covary with total plant mass or any individual biomass components. We quantified phenotypic correlations while controlling for resource acquisition as above. All analyses used JMP (version 5, SAS Institute Inc, 2002), and means are presented ±1 SE unless otherwise indicated.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Comparison of biomass components before manipulation of sexual reproduction and at final harvest indicated that pollination treatments were imposed before most resources were invested (Fig. 1). Mean total plant mass increased by 91% between harvests under low nutrients (prepollination = 4.3 ± 0.3 g, final = 8.1 ± 0.2 g) and 155% under high nutrients (6.6 ± 0.4 g, 16.9 ± 0.4 g). Proportionally, the increase was much greater for sexual structures (low nutrient = 102%, high = 206%) and clonal structures (582 and 742%) than vegetative structures (28 and 58%). Expressed in terms of the allocation of biomass accumulated during and after pollination, vegetative structures accounted for 23% (low nutrients) and 28% (high), clonal structures for 58 and 51%, and sexual structures for 19 and 21%. Expressed in terms of the allocation of nonsexual biomass, vegetative structure accounted for 28 and 35%, while clonal structures accounted for 72 and 65%. These results suggest that the opportunity for a trade-off with sexual investment is much greater for clonal than vegetative structures.

Overall ramet size and all components of investment about doubled between low and high nutrients (Figs 1 and 2, Table 1). Pollinating half of or all the flowers increased sexual mass by 21 and 42%, respectively under low nutrients, and 24 and 61% under high nutrients (Fig. 2; Table 1). This involved increased investment in fruits but not inflorescence stalks (F2,41 = 1.0, P = n.s.) or peduncles (F2,41 = 1.7, P = n.s.). Nutrients also strongly affected future inflorescence production, as only 23% of ramets produced >1 inflorescence under low nutrients, whereas 67% produced at least two inflorescences under high nutrients (χ2 = 50.4, d.f. = 1, P < 0.001). Pollination did not affect the probability of producing future inflorescences (no flowers pollinated = 47%, half = 49%, all = 43%, χ2 = 1.1, d.f. = 2, P = n.s.). For individuals that produced future inflorescences, pollination increased rather than decreased the number of future flowers under high nutrients (none = 24.8 ± 2.2, n = 26; half = 24.9 ± 1.9, n = 29; all = 37.4 ± 3.1, n = 30; F2,82 = 8.4, P < 0.001) but not low nutrients (grand mean = 15.4 ± 2.0, n = 26; F2,23 = 0.02; P = n.s.).

image

Figure 2. Effect of experimentally increased sexual reproduction on clonal reproduction and vegetative growth in Butomus umbellatus. Means for sexual, vegetative and clonal dry mass are contrasted among three pollination treatments (none = no flowers pollinated, half = half of all flowers pollinated and all = all flowers pollinated) and two nutrient treatments. Each bar is based on data from 40 ramets, and error bars are ±1 SE for the whole biomass component. Statistical analyses are in Table 1.

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Table 1.  Analysis of variance in sexual, vegetative, and rhizome bulbil mass among plants of Butomus umbellatus in response to three pollination and two nutrient treatments. For all variables, the whole anova model is significant (all P < 0.001). All variables were log10-transformed for analysis. Means are in Fig. 2.
VariableSource of variationd.f.SSFP
Sexual mass r2 = 0.78Plant (P)192.7556.40.022
Pollination treatment (T)21.20121.8<0.0001
P × T381.0310.80.78
Nutrient treatment (N)16.858223.6<0.0001
P × N190.5790.90.61
T × N20.0190.30.76
P × T × N381.3261.00.44
Residual1173.981  
Vegetative mass r2 = 0.91P191.85316.10.0002
T20.00800.50.64
P × T380.33781.20.31
N14.3758307.0<0.0001
P × N190.27821.90.041
T × N20.00020.020.98
P × T × N380.28831.20.24
Residual1188.3232  
Rhizome bulbil mass r2 = 0.88P191.3901.80.097
T20.2255.20.0096
P × T380.8381.40.15
N17.193217.7<0.0001
P × N190.6472.20.020
T × N20.0471.50.23
P × T × N380.5931.20.26
Residual1201.608  

Experimentally induced within-plant trade-offs

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Increasing investment in sexual reproduction did not affect vegetative mass (Fig. 2; Table 1) or either of its subcomponents (leaf mass: F2,41 = 0.16; P = n.s.; rhizome mass: F2,41 = 1.4; P = n.s.). In contrast, pollination substantially reduced investment in rhizome bulbils. This involved reduced bulbil number (F2,41 = 3.8; P < 0.05) but not individual bulbil mass (F2,41 = 0.5; P = n.s.). However, the trade-off with sexual investment was nonlinear. On average, pollinating half the flowers increased sexual mass by 0.339 g and reduced the mass of rhizome bulbils by 0.666 g, whereas pollinating all flowers increased sexual mass by 0.822 g and reduced the mass invested in bulbils by 0.450 g. Thus, increasing fruit production from zero to half of full fruit production caused 2 g of bulbils to be lost for every 1 g of investment in fruit, whereas increasing investment from half to full fruit production did not cause any further loss of rhizome bulbils, as there was no difference in total rhizome bulbil mass between half and all flowers pollinated (F1,20 = 1.0, P = n.s.). Contrary to expectations, the trade-off between sexual reproduction and rhizome bulbils did not vary between nutrient treatments (Fig. 2; Table 1).

Overall, pollination did not significantly reduce the probability of inflorescence bulbil production (likelihood ratio χ2 = 3.0, P = n.s.). However, the effect of nutrients was strongly significant (χ2 = 76.2, P < 0.001) and interacted with the effect of pollination (χ2 = 7.6, P < 0.05). One-way contingency analysis for each nutrient treatment revealed that, although pollination did not affect bulbil production under low nutrients (χ2 = 4.0, P = n.s.) where only 20% of ramets produced bulbils, it significantly reduced bulbil production under high nutrients where 74% of ramets produced bulbils (none = 86%, half = 75%, all = 60%; χ2 = 7.4, P < 0.05).

Phenotypic correlations: replicated plants from ONGL

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

There was significant variation among plants in vegetative and sexual mass, and among-plant variation for rhizome bulbil mass was almost significant (Table 1). Profile analysis revealed that allocation to vegetative, clonal and sexual structures also varied among plants (Fig. 3; profile × plant: F38,298 = 8.0, P < 0.001). Significant profile × plant effects were also detected when pairs of biomass components were analysed separately (results not shown). The pattern of variation in allocation among plants did not differ between nutrient (profile × plant × nutrient: F38,298 = 1.4, P = 0.085) or pollination treatments (profile × plant × pollination: F76,298 = 1.1, P = n.s.).

image

Figure 3. Profile analysis of resource allocation to sexual, clonal and vegetative structures in Butomus umbellatus. Lines join the mean mass component for each plant. Different shading patterns are used on the lines to emphasize among-plant variation in allocation patterns. For instance, 10 plants (thin, gray lines) invested highly in vegetative structures and moderately in clonal and sexual structures, four (thin black lines) invested moderately in vegetative and clonal structures and highly in sexual structures, and four others (thick black lines) invested little in vegetative, moderately in clonal and very little in sexual structures. These examples indicate the range of continuous variation, not distinct allocation strategies.

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We found only weak evidence for a negative phenotypic correlation between sexual and clonal allocation among experimental plants from ONGL (Table 2). The overall regression between sexual and clonal allocation (corrected for variation in vegetative mass) was weakly negative (standardized β = −0.11) and not quite significant (F1,108 = 3.57, P = 0.061). Within pollination × nutrient environments, standardized β's varied from −0.46 to +0.08 but did not differ among environments (F5,108 = 1.2, P = n.s.). The only significant correlation occurred under low nutrients when all flowers were pollinated (β = −0.46, P < 0.05, all other P > 0.28), but this was largely because of a single plant, and was far from significant when that plant was excluded (β = −0.09, P = n.s.).

Table 2.  Among-plant phenotypic correlations between allocation to sexual vs. clonal reproduction in Butomus umbellatus. Coefficients of variation (CV) indicate substantial variation among 20 clonally replicated plants from population ONGL for both sexual mass and clonal mass within each combination of nutrient and pollination treatments (n = 2 replicates per environment). Phenotypic correlations are indicated by standardized β for regressions of clonal mass on sexual mass after both variables have been adjusted for vegetative size (resource acquisition) using residual analysis. Statistical significance of regression coefficients (0.01 < P < 0.05) is indicated by an asterisk. However, ancova did not detect heterogeneity in correlations among environments (P = n.s.).
Nutrients/pollinationCV sexual (%)CV clonal (%)Phenotypic correlation β
Low/none32.835.3−0.35
Low/half47.734.9+0.08
Low/all33.929.0−0.46*
High/none26.324.2−0.20
High/half30.524.7+0.06
High/all38.723.4−0.08

It could be argued that variation in vegetative mass at final harvest is a consequence of variation in allocation among plants rather than an indicator of variation in resource acquisition ability. Resource acquisition capacity may be better reflected by vegetative mass before allocation to reproductive structures. To address this, we performed the same analysis but corrected for variation in vegetative mass before most allocation to sexual or clonal reproduction using plant means from the prepollination harvest. Within nutrient and pollination environments, prepollination vegetative mass correlated positively with vegetative mass (ancovaβ = +0.32, P < 0.001), sexual mass (β = +0.15, P < 0.05) and clonal mass (β = +0.25, P < 0.001) at final harvest. However, the overall regression between corrected sexual and clonal allocation was even more weakly negative (β = −0.06), far from significant (F1,107 = 0.37, P = n.s.), and did not vary among environments (F5,102 = 0.72, P = n.s.). Using total prepollination plant mass as an indicator of resource acquisition also failed to reveal any negative relation between allocations to sexual vs. clonal reproduction (β = −0.05, F1,102 = 0.71, P = n.s.).

Phenotypic correlations: population survey

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Overall, plants sampled from eight populations exhibited growth and biomass allocation similar to the replicated experimental plants from ONGL. They attained greater total mass (grand mean ± SE = 20.5 ± 0.3 g plant−1) than plants from the equivalent high nutrient/no pollination environment (16.7 ± 0.5 g), probably because they were grown from rhizome branches, which developed faster than the clonal bulbils used to generate experimental plants from ONGL. However, mass was partitioned in a roughly similar manner among sexual (23% of total mass), clonal (39%) and vegetative structures (38%) compared with the experimental plants (Figs 1 and 2). There was significant variation among populations for total mass and all components (total: range of population means = 16.3–23.3 g, r2 from 1-way anova on log10-transformed data = 0.22; sexual: 3.8–5.7 g, r2 = 0.13; clonal: 5.6–9.3 g, r2 = 0.17; vegetative: 5.7–9.5 g, r2 = 0.31; P < 0.001 for all anovas). Within populations, there was substantial variation among plants in mass invested in both sexual and clonal structures (Table 3), on par with that observed among experimental plants (compare CVs in Tables 2 and 3). However, there was no evidence of a negative phenotypic correlation between reproductive modes after controlling for variation in vegetative mass. ancova detected a weakly positive correlation that was not quite significant (standardized β = +0.15, F1,215 = 3.7, P = 0.055) and variation in β among populations (Table 3) that also was not quite significant (F7,215 = 2.0, P = 0.055). Among plants from ONGL, the phenotypic correlation was, again, weakly negative and nonsignificant (β = −0.23). However, the correlations for six of the remaining seven populations were positive and three were significant. Although our sample of eight populations was insufficient for a powerful analysis, covariation between sexual and clonal allocation among populations also failed to suggest any trade-off (β = −0.05, P = n.s.).

Table 3.  Among-plant phenotypic correlations between allocation to sexual vs. clonal reproduction in eight North American populations of Butomus umbellatus. Coefficients of variation (CV) indicate substantial variation among plants for both sexual mass and clonal mass within each population. Phenotypic correlations are indicated by standardized β for regressions of clonal mass on sexual mass after both variables have been adjusted for vegetative size using residual analysis. Statistical significance of regression coefficients is indicated by asterisks (*P < 0.05, **P < 0.01). However, ancova revealed that heterogeneity in these correlations among populations was not quite significant (P = 0.055).
PopulationNumber of plantsCV sexual (%)CV clonal (%)Phenotypic correlation β
MBNM2825.628.7+0.14
MNFL2432.527.4+0.31
NYCR3733.522.2+0.36*
ONBB3038.425.1+0.26
ONBE2827.330.3+0.50**
ONGL2928.132.7−0.23
ONMA2731.333.3+0.44*
ONRC2816.626.4−0.12

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Our experimental manipulation of investment in sexual structures caused a significant but nonlinear trade-off in biomass between sexual and clonal reproduction in B. umbellatus. Pollinating flowers to increase investment in seed reduced biomass allocated to both rhizome bulbils and inflorescence bulbils. Before interpreting our results further, we note three important caveats. Like most studies, we measured trade-offs in biomass, but recognize that this may not accurately reflect the allocation of critical nutrients (e.g. nitrogen, phosphorus), which may vary among different plant tissues (Bazzaz, 1997). We address this issue below by expressing the trade-off in a common currency more directly related to fitness. Secondly, we examined trade-offs within a reproductive bout. However, increased sexual reproduction may also reduce future clonal reproduction, vegetative growth and survival in long-lived plants (Primack & Stacy, 1998). Finally, we evaluated the trade-off between sexual and clonal reproduction by manipulating investment in sexual structures. A complete analysis of this trade-off requires complementary manipulation of clonal structures, which was not possible with B. umbellatus and has rarely been attempted (Muir, 1995).

Previous investigations of trade-offs between reproductive modes in clonal plants have produced mixed results. Some have detected negative phenotypic correlations between sexual and clonal reproduction, while others have not (see introduction). Experimental evidence is also varied. Increased sexual investment reduced rhizome branching in Tipularia discolor (Snow & Whigham, 1989) and tuber production in Helianthus tuberosus (Westley, 1993) but did not alter clonal allocation in Potentilla anserina (Saikkonen et al., 1998) or Arum italicum (Méndez, 1999). Some of this discord among studies may stem from species-specific differences in whether sexual structures are photosynthetic or stimulate photosynthesis and/or resource uptake in vegetative organs (Bazzaz, 1997). We used hand-pollination to increase investment in fruits, which do not seem photosynthetic and likely contribute little to their construction and maintenance costs. In addition, the increased investment in fruit mostly supported the formation of seeds, which disperse from the plant and thus are a nonrecoverable investment (Ashman, 1994). If we could have manipulated investment in whole inflorescences, the gram-per-gram trade-off with clonal structures might have been weaker because inflorescence stalks and pedicels seem photosynthetic, and remain attached to the rhizome so that resources might be recovered. Increased sexual reproduction did not obviously stimulate photosynthetic capacity or resource uptake in B. umbellatus, as neither leaf number, leaf mass, nor rhizome mass increased with pollination (Table 1).

Variation in the trade-off among species may also result from differences in the timing of investment to competing functions (Gardner & Mangel, 1999), which is rarely quantified in experimental studies (e.g. Cheplick, 1995; Méndez, 1999). Our results support the expectation that trade-offs are strongest when different functions compete for resources simultaneously. When ramets of B. umbellatus first flower, most investment in vegetative structures has been completed, whereas investment in clonal structures has just begun (Fig. 1). In terms of nonsexual dry mass accumulated during and after pollination: more than twice as much biomass is allocated to clonal (69%) than vegetative structures (31%). Thus, pollination had a substantial affect on clonal mass but no detectable effect on vegetative mass (Fig. 2; Table 1).

Finally, the strength of the trade-off between sexual and clonal reproduction may be contingent on the experimental manipulation used. Using three levels of pollination allowed us to detect marked nonlinearity in the trade-off. The reduction of mass in clonal rhizome bulbils per unit increase of mass in fruits was much higher when only half the flowers set fruit (0.666 g/0.359 g = 1.85) than when all flowers set fruit (0.450 g/0.822 g = 0.55). Increasing sexual investment from half to full fruit production did not lead to any further reduction in rhizome bulbils. Our results do not suggest any explanation for this nonlinearity. Increased sink strength associated with full fruit set may stimulate increased photosynthesis or resource uptake (Reekie & Bazzaz, 1987; Lehtilä & Syrjänen, 1995), but we did not detect any positive effect of fruit production on vegetative structures (Fig. 2; Table 1). Physiological measurements are needed to further evaluate this hypothesis. Below, we discuss how this nonlinearity affects the evolutionary interpretation of the reproductive trade-off.

Effect of environment on trade-offs

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Our results do not support the prediction that trade-offs are most pronounced when resources are scarce. Butomus umbellatus responded dramatically to fertilizer manipulation, with ramets under high nutrients attaining twice the size and producing twice as many inflorescences as those under low nutrients. Although this strongly suggests that nutrients were limiting investment, trade-offs were not stronger under low nutrients. In fact, we only observed a trade-off between sexual investment and inflorescence bulbils under high nutrients, probably because bulbil formation was strongly inhibited by low nutrients regardless of whether plants were pollinated.

Environmental effects on trade-offs between sexual vs. clonal reproduction have been previously investigated by manipulating light and nitrogen availability (Reekie, 1991), soil nutrients (Cheplick, 1995; Ronsheim & Bever, 2000) and soil heavy metals (Saikkonen et al., 1998). None of these studies have found stronger trade-offs under resource-limiting conditions. Moreover, a stronger trade-off under higher nutrients was also found by Ronsheim & Bever (2000) (see also Saikkonen et al., 1998). This contrasts with studies demonstrating that sexual reproduction causes a stronger reduction in growth when the resource acquisition capacity of plants is reduced by defoliation (see introduction). Because the number of studies is small, and effect of these various manipulations on plant resource status cannot be directly compared, it is difficult to draw any conclusions regarding the effect of variation in environmental quality on reproductive trade-offs.

Fitness trade-off between sexual & clonal reproduction?

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

The evolutionary significance of trade-offs in resource allocation depends on whether they affect individual fitness or population demography (Horvitz & Schemske, 1988; Bazzaz, 1997). The trade-off in biomass between sexual and clonal reproduction can be interpreted in terms of fitness if the effects of experimental manipulations are expressed in a common currency: propagules (Douglas, 1981). In B. umbellatus, this is facilitated because clonal reproduction involves discrete bulbils that are only involved in multiplication and dispersal. Moreover, our manipulation of both sexual reproduction and resource availability affected bulbil number but not size. In Table 4, we calculate the trade-off in bulbil number caused by manipulating investment in seed. Although the biomass trade-off was substantial, seeds are much smaller than clonal propagules (as is usually the case), so that the loss of bulbils was compensated for by relatively large increases in seed production. The ratio of seeds gained per bulbil lost varied between 86 and 733, although these values would likely be lower if we could factor in the loss of bulbils associated with net investment in sexual support structures. These calculations further emphasize that the trade-off between sexual and clonal reproduction was not linear.

Table 4.  Trade-off in propagule number between sexual vs. clonal reproduction in Butomus umbellatus. Values of fruits and bulbils per plant in each nutrient and pollination environment are based on back-transformed least-squares means from the analyses in Table 1. Calculation of seeds gained used an average of 250 seeds/fruit based on data from 25 North American populations (Eckert et al., 2000; K. Lui, F.L. Thompson & C.G. Eckert, unpublished data). The use of one mean seeds/fruit for both nutrient environments is justified because nutrient treatment had no effect on the mass of individual fruits when half the flowers were pollinated (P = n.s.) or when all flowers were pollinated (P = n.s.).
Nutrients/pollinationFruits per plantBulbils per plantGain in seedsLoss of bulbilsSeeds gained per bulbil lost
Low/none0133
Low/half8.9118222515148
Low/all17.612744006733
High/none0322
High/half19.026747505586
High/all41.026810,25054190

The evolutionary consequences of this seemingly weak trade-off in propagule number further depend on the relative recruitment success of seeds vs. bulbils in natural populations. Demographic studies on a variety of clonal plants indicate that clonal propagules are usually much more successful, often 10- or 100-times more successful, than seeds (Cook, 1985). However, the relative success of seeds is expected to vary with population age. Seeds, owing to their small size and adaptations for dispersal, should be especially important for founding new populations (Eriksson, 1997). Because B. umbellatus is rapidly spreading across North America (White et al., 1993), high seed production might be favoured in colonizing populations despite any modest loss of bulbil production. It has been suggested that the resource trade-off between sexual and clonal reproduction might even be an adaptive mechanism of reproductive assurance (Westley, 1993). Individual plants would increase investment in clonal propagules when sex is unsuccessful during colonization owing to low pollinator visitation or a scarcity of mates (Paige & Whitham, 1987). This seems unlikely in B. umbellatus owing to the nonlinearity of the reproductive trade-off; pollen limitation would have to reduce fruit production below one-half before any significant increase in clonal bulbil production would be realized.

Genetic analyses of B. umbellatus populations in North America (including the ones studied here) suggest that seed recruitment rarely occurs, although plants produce abundant viable seed (Eckert et al., 2003; A. Kliber & C.G. Eckert, unpublished data). Hence, bulbils rather than seed are contributing to population growth and geographical spread. Seeds seem of little functional importance, and even a modest loss of bulbil production caused by investment in sexual reproduction may have negative demographic consequences. This may lead to selection for reduced sexual investment in invasive populations. However, a comparison of growth and reproduction by plants from native European vs. introduced North American populations does not support this prediction (J.S. Brown & C.G. Eckert, unpublished manuscript).

Among-plant correlations

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Trade-offs in resource allocation to sexual vs. clonal reproduction within plants, as demonstrated here by experimental manipulation, should translate into negative phenotypic correlations between reproductive modes among plants. Among-plant correlations may reflect plastic resource allocation. They may also arise from genetic trade-offs because alleles that increase total reproductive output should fix quickly, so the only polymorphisms remaining in populations involve alleles that increase one reproductive mode at the expense of the other (Williams, 1966). We detected substantial variation among 20 clonally replicated plants from ONGL for relative allocation to sexual reproduction, clonal reproduction and vegetative growth (Fig. 3). This concurs with most experiments involving clonal replicates (e.g. Geber et al., 1992; Elle, 1996; Prati & Schmid, 2000; van Kleunen et al., 2003), although few studies have demonstrated variation among plants from the same population (Westley, 1993; Cheplick, 1995; Ronsheim & Bever, 2000). The extent to which the variation among our experimental plants reflects phenotypic plasticity vs. genetic variation in reproductive strategy is unclear. Low levels of genotypic variation at RAPD loci within populations of B. umbellatus (discussed above) suggest that the genetic component of among-plant variation may be limited, although neutral molecular markers generally underestimate within-population genetic variation for life-history traits (Reed & Frankham, 2001).

Regardless of the exact genetic vs. environmental contribution to variation in reproductive allocation, the results from our experimental manipulations predict a negative phenotypic correlation between sexual and clonal reproduction. However, we found no evidence of this, either within ONGL or an additional seven widely distributed North American populations. This was not because of limited variation in sexual and/or clonal investment among plants. The coefficient of variation (CV) among plants from ONGL for sexual mass or clonal mass (mean CV: sexual = 35%, clonal = 29%) was just as high as the CV among pollination treatments within plants (sexual = 31%, clonal = 22%). Similarly, cloned plants from each of the eight populations surveyed varied greatly in sexual (mean CV = 29%) and clonal mass (28%). Flowers were not pollinated on plants involved in the population survey, so the resource cost of sexual reproduction may have been lower than in the experimental analysis. However, this does not explain the absence of the expected negative correlation, because pollination of experimental plants from ONGL did not induce negative covariation between sexual and clonal allocation (Table 2).

The failure to detect negative phenotypic correlations between sexual and clonal reproduction (e.g. Pitelka et al., 1985; Reekie, 1991; Cain & Damman, 1997) is usually attributed to genetic or environmental variation in plant resource acquisition obscuring trade-offs between allocation components (Worley & Harder, 1996). We tried to eliminate variation in resource acquisition among plants by evaluating allocation in a relatively uniform greenhouse environment, generating experimental plants from small propagules produced after an extensive holding period to reduce clonal carry-over effects from the field environment, and statistically controlling for variation in the mass of vegetative structures, which include organs of resource production, uptake and storage. For population ONGL, we also performed alternative analyses in which reproductive allocation was adjusted for variation in vegetative or total mass before most investment in sexual or clonal reproduction. Yet all these analyses failed to detect a significant negative correlation between reproductive modes. It remains possible that we were not entirely successful in eliminating confounding variation in plant resource acquisition, although there is no obvious way of evaluating this possibility.

Very few studies have compared trade-offs detected using experimental manipulations with phenotypic or genetic correlations, and fewer still have broadly surveyed phenotypic correlations in multiple populations (Reekie, 1991). Both Fox & Stevens (1991) and Jackson & Dewald (1994) found that the effects of manipulating sexual reproduction on vegetative growth were not reflected by natural phenotypic correlations. As suggested by Jackson & Dewald, experimental manipulations may not adequately simulate the kind of genetic variation or phenotypic plasticity that causes variation in allocation to sexual reproduction among plants. There may be genetic variation for mechanisms that modify the costs of reproductive investment (Bazzaz, 1997). Nonlinear relations between components of reproductive allocation, as revealed by our experimental manipulations, indicate the possibility of additional complexity (Schmid, 1990). Although the physiological and genetic mechanisms that uncouple within-plant trade-offs from among-plant correlations remain unknown, our results emphasize the difficulty in extrapolating experimental manipulations to higher levels of phenotypic covariation. Predicting the evolution of reproductive strategies in clonal plants will require a better understanding of genetic variation in the costs of reproductive allocation (Bazzaz, 1997) along with experimental analysis of evolutionary trade-offs between reproductive modes (van Kleunen et al., 2003).

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

We thank Michael Bhardwaj, Kelly Bronson, Eva Bruni, Agnes Kliber, Dale Kristensen, Celine Griffin, Sarah Yakimowski and Senarath Yatigammana for help in the greenhouse; Lonnie Aarssen, Jeremy Brown, Markus Fischer, Peter Klinkhamer, Sonia Sultan and Anne Worley for comments on the manuscript; and the Natural Sciences and Engineering Research Council of Canada for a scholarship to FLT and a discovery grant to CGE.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Study species
  6. Experimental manipulations
  7. Phenotypic correlations: controlling resource acquisition
  8. Phenotypic correlations: replicated plants from ONGL
  9. Phenotypic correlations: population survey
  10. Results
  11. Experimentally induced within-plant trade-offs
  12. Phenotypic correlations: replicated plants from ONGL
  13. Phenotypic correlations: population survey
  14. Discussion
  15. Effect of environment on trade-offs
  16. Fitness trade-off between sexual & clonal reproduction?
  17. Among-plant correlations
  18. Acknowledgments
  19. Supplementary material
  20. References
  21. Supporting Information

Appendix A1. Locations of eight North American populations of B. umbellatus used in this study.

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JEB_701_sm_appA1.doc8KSupporting info item

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