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The Amaranthus genus comprises several important weeds grouped into two subgenera. Homonym subgenus Amaranthus includes long-known weeds of cosmopolitan distribution (Mosyakin & Robertson, 2003). The most important among these belong to the Amaranthus hybridus aggregate (Holm et al., 1991), encompassed by monoecious species native of Central and North America, these are predominantly self-pollinated. Subgenus Acnida includes dioecious species restricted to North America (Mosyakin & Robertson, 2003). Sex in these species is controlled by a chromosomal XY-type system (Murray, 1940). Except for Amaranthus palmeri (Sauer, 1957), species of Acnida have gained notoriety as weeds only recently, with Amaranthus tuberculatus/Amaranthus rudis representing one of the most dramatic cases of weed invasion in the midwestern USA.
Amaranthus tuberculatus was previously subdivided into two species, A. tuberculatus and A. rudis (Pratt & Clark, 2001). Amaranthus rudis (initially named Acnida tamariscina) was first described in Oklahoma in 1830 and has since shown continuous northward and eastward accretion into midwestern States, overlapping with A. tuberculatus, of static range, in sandy and muddy stream banks, lakeshores and pond margins, along the Missouri, Mississippi and Ohio river systems (Sauer, 1957, 1972). Although neither A. rudis nor A. tuberculatus were considered significant weeds of agriculture, herbarium samples from crop fields often had intermediates (Sauer, 1957), perhaps suggesting that hybrid forms were favored as weeds. A combination of overlapping ranges and hybridization precludes distinguishing A. tuberculatus and A. rudis and, therefore, a single taxon is presently recognized with A. tuberculatus taking priority in the name (Pratt & Clark, 2001). (Some authors propose A. tuberculatus var. rudis for the more invasive variant; Costea et al., 2005.)
Over the last 20 yr, A. tuberculatus has gone from virtual anonymity to becoming the most significant problem weed in one of the world's premier agricultural regions, the midwestern USA (Steckel, 2007). Its success as a weed is attributed to, among other things, its remarkable ability to evolve resistance to herbicides. Recently, resistance to the herbicide glyphosate, a fundamental herbicide in the production of commodity crops, has been reported in A. tuberculatus (Legleiter & Bradley, 2008). Populations of A. tuberculatus exist in which a single plant can withstand normally lethal doses of at least nine different herbicides, severely restricting chemical weed management (Patzoldt et al., 2005). What is exceptional is that cases of multiple resistances in A. tuberculatus result from the accumulation of independently evolved mutations (i.e. not broad resistances cause by single gene effect), some of which have been reported only in this species (Gressel & Levy, 2006; Patzoldt et al., 2006).
A major epidemiological concern is found in predicting whether rare herbicide resistance alleles found in A. tuberculatus can naturally introgress into widely distributed taxa of the A. hybridus aggregate. Such introgression could exacerbate the occurrence of herbicide-resistant weed populations. Prior research assessing genetic exchange between A. tuberculatus and A. hybridus shows that the two species can hybridize at high frequencies under field conditions (Trucco et al., 2005a,b) and that genetic exchange may occur within a homoploid background (Trucco et al., 2005c). Moreover, the low fecundity of first-generation hybrids may be quickly overcome in the first backcross generation (Trucco et al., 2005c). However, although transfer of a herbicide resistance allele from A. hybridus to A. tuberculatus has been shown experimentally (Tranel et al., 2002), there is no evidence of the reciprocal transfer. In fact, an experimental attempt to transmit an A. tuberculatus herbicide resistance allele at the ALS locus to a monoecious A. hybridus background was unsuccessful. This lack of transfer was hypothesized to be caused by linkage of the ALS locus to some other locus associated with, for example, hybrid sterility or sex identity (Trucco et al., 2005c).
To test this linkage hypothesis, we expanded on our previous study by examining homoploid BC1 individuals for inter-specific introgression at 199 loci. Our goal was to profile the patterns of allele exchange between the two species to: access genetic exchange throughout the genome; determine if fecundity penalties are associated with genetic exchange; and compare the overall pattern of exchange with that of two important herbicide resistance genes, namely the acetolactate synthase (ALS) and the protoporphyrinogen oxidase (PPO) genes. Our findings suggest there is a generalized phenomenon operating after hybrid formation to bring about asymmetric genetic exchange between A. hybridus and A. tuberculatus.
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As stated previously, A. tuberculatus is known for evolving resistance to herbicides including triazines, ALS-inhibitors, PPO-inhibitors and glyphosate (Heap, 2009). However, some of these resistances have not been reported in widely distributed weeds of the A. hybridus complex, despite high rates of interspecies hybridization with A. tuberculatus (Trucco et al., 2005a,b) and some reproductive viability in the F1 (Trucco et al., 2006). Moreover, sympatric populations of A. tuberculatus and A. hybridus show independent evolution for ALS-inhibitor resistance alleles (P. J. Tranel, unpublished), even though one would predict resistance to evolve via gene flow before selection of de novo mutations (Jasieniuk et al., 1996). This raises the question of whether gene introgression can proceed after the F1 generation.
In a previous study, we used DNA content analysis to examine the patterns of gene introgression after the F1 generation and observed that most progeny produced upon backcrossing hybrids with A. tuberculatus or A. hybridus were homoploid and recombinant (Trucco et al., 2005c). However, just as observed herein, A. tuberculatus ALS alleles did not show introgression into the monoecious A. hybridus background. We suggested then that lack of introgression resulted from linkage of ALS to a hybrid sterility locus associated with sex determination, a taxonomically discriminating character for these species. It is evident now, after failing to transfer a second herbicide resistance allele – in this instance at the PPO locus – that genetic exchange likely is limited by a phenomenon beyond circumstantial linkage.
Indeed, we were unable to transfer most of 133 AFLP markers from A. tuberculatus to A. hybridus, with the exception of introgression in a smaller group of nonmonoecious BC1s characterized by anomalous phenotypes and F1-like fecundities, or worse. This observation is striking as we uncovered a very different scenario in the reciprocal exchange. We were able to transfer from A. hybridus to A. tuberculatus not only ALS and PPO alleles but also most of our A. hybridus-specific AFLP markers. Although introgression at some loci appeared to be disfavored (i.e. showed negative segregation distortion), ALS and PPO alleles as well as many of the AFLP-markers showed Mendelian segregation in backcross progeny with A. tuberculatus. This was not observed in progeny from reciprocal backcrosses, where almost all markers (strictly species-specific or more permissive markers) showed strong negative distortion and increased introgression was associated with reduced reproductive viability.
Taking monoecism and dioecism as the taxonomic distinguishing characters for A. hybridus and A. tuberculatus, respectively, we may say that contrary to previous speculations (Trucco et al., 2005c), genetic exchange between these species is unidirectional. Even if we considered nonmonoecious recombinant BC1s obtained with A. hybridus, the evolutionary contribution of these individuals seems restricted by the dramatic fitness disadvantage compared with their reciprocals. By contrast, there is no significant association between the level of introgression measured in A. tuberculatus BC1s and their seed output. We should point out however, that a small but significant penalty was observed when we measured micropollen occurrence, an indicator of male sterility (Srivastava et al., 1977), and level of introgression in males. This difference could result from the differential sensitivity of the parameters measured, with micropollen data more closely reflecting meiotic dysfunctionalities than seed yield. Also, there may more hybrid incompatibilities that affect male fertility than those that affect female fertility. Alternatively, or in addition, the difference observed between male and female reproductive fitness may be a reflection of Haldane's rule (Haldane, 1922), as males are the heterogametic sex (Murray, 1940). If this is the case, ovules will be favored over pollen for continued introgression. However, this is a matter for further investigation.
The study of natural populations from hybrid zones has provided evidence on the occurrence of asymmetric introgression among different hybridizing plant (Sweigart & Willis, 2003; Burgess et al., 2005) and animal species (Johannesen et al., 2006; Kraus et al., 2007). Turelli & Moyle (2007) indicate that Darwin was the first to call attention to this phenomenon, which the authors proposed to name ‘Darwin's Corollary’. Asymmetric hybridization is commonly observed in the generation of interspecific hybrids (Lewis & Crowe, 1958; Rick, 1963), especially as influenced by prezygotic reproductive barriers, such as ecological factors and differences in flowering times and characters (Levin, 1978). The occurrence of differential viability in reciprocal hybrids (i.e. postzygotic asymmetries) is also well documented (Tiffin et al., 2001), though it is seldom analysed beyond the first generation. Our study is novel in that postzygotic asymmetry is observed not in F1 viability (i.e. F1 pollen or seed production with either parental species) but in the array of genotypes and fitness values generated upon backcrossing.
Nonsymmetrical postzygotic isolation is commonly attributed to nuclear–cytoplasmic interactions (Levin, 1978; Tiffin et al., 2001), though this type of asymmetry is usually observed in the F1 generation. Interestingly, the F1 hybrids used in our study were produced with A. hybridus as the seed parent. Therefore, contrary to the actual observation, we would expect greater compatibility in the A. hybridus direction, as deleterious A. tuberculatus-A. hybridus nuclear–cytoplasmic interactions should be minimized in this cross.
Chromosomal rearrangement models of postzygotic reproductive isolation predict a symmetrical outcome, as both parents are considered to contribute equally to the chromosomal composition of hybrids (Tiffin et al., 2001). A symmetrical outcome may also be predicted based on classical Dobzhansky–Muller nuclear interactions (Levin, 1978). However, this model may predict asymmetry when sex chromosomes are involved, as is the case with A. tuberculatus (Murray, 1940). For example, consider the existence of a deleterious interaction between an X-linked gene from A. tuberculatus and a recessive gene from A. hybridus. This interaction will not manifest in the F1, as first-generation hybrids will be heterozygous at the recessive gene. Upon backcrossing with A. hybridus, the recessive condition could be obtained and BC1 viability will be observed only in individuals without the X-linked gene (presumably monoecious) or in individuals heterozygous at the recessive gene. Since A. hybridus’ homozygous recessive condition cannot be restituted upon backcrossing with A. tuberculatus, the lethal interaction cannot be obtained in this direction.
From a practical standpoint, results of this research indicate that it may be difficult for A. hybridus to acquire herbicide resistance from A. tuberculatus, despite the species’ cohabitation and propensity to hybridize with each other. This is significant in that numerous populations of A. tuberculatus have evolved resistance to PPO-inhibiting herbicides and/or to the even more widely used herbicide, glyphosate, whereas A. hybridus has not yet evolved these resistances (Heap, 2009). By contrast, our data indicate that A. tuberculatus can readily acquire genetic material from A. hybridus. Did such genetic acquisition foster expansion of A. tuberculatus from its primarily riparian habitat to agricultural fields throughout the midwestern USA? Although our results do not directly address this question, they add to a growing body of evidence that interspecific hybridization has the potential to play a significant role in the evolution of at least some Amaranthus species as weeds. Research is now needed to find a causative relationship between gene introgression and enhanced weediness in A. tuberculatus, and to determine the genomic basis for the herein discovered asymmetric genetic exchange. Such research provides an excellent opportunity for collaboration between evolutionary ecologists and crop scientists.