Outcrossing Relatives Research Articles

Mating systems and recombination landscape strongly shape genetic diversity and selection in wheat relatives

Abstract How and why genetic diversity varies among species is a long-standing question in evolutionary biology. Life history traits have been shown to explain a large part of observed diversity. Among them, mating systems have one of the strongest impacts on genetic diversity, with selfing species usually exhibiting much lower diversity than outcrossing relatives. Theory predicts that a high rate of selfing amplifies selection at linked sites, reducing genetic diversity genome-wide, but frequent bottlenecks and rapid population turn-over could also explain low genetic diversity in selfers. However, how linked selection varies with mating systems and whether it is sufficient to explain the observed difference between selfers and outcrossers has never been tested. Here, we used the Aegilops/Triticum grass species, a group characterized by contrasted mating systems (from obligate outcrossing to high selfing) and marked recombination rate variation across the genome, to quantify the effects of mating system and linked selection on patterns of neutral and selected polymorphism. By analyzing phenotypic and transcriptomic data of 13 species, we show that selfing strongly affects genetic diversity and the efficacy of selection by amplifying the intensity of linked selection genome-wide. In particular, signatures of adaptation were only found in the highly recombining regions in outcrossing species. These results bear implications for the evolution of mating systems and, more generally, for our understanding of the fundamental drivers of genetic diversity.

Divergent ecological selection maintains species boundaries despite gene flow in a rare endemic tree, Quercus acerifolia (maple-leaf oak).

Strong gene flow from outcrossing relatives tends to blur species boundaries, while divergent ecological selection can counteract gene flow. To better understand how these two forces affect the maintenance of species boundaries, we focused on a species complex including a rare species, maple-leaf oak (Quercus acerifolia), which is found in only four disjunct ridges in Arkansas. Its limited range and geographic proximity to co-occurring close relatives create the possibility for genetic swamping. In this study, we gathered genome-wide single nucleotide polymorphisms (SNPs) using restriction-site-associated DNA sequencing (RADseq) from 190 samples of Q. acerifolia and three of its close relatives, Q. shumardii, Q. buckleyi, and Q. rubra. We found that Q. shumardii and Q. acerifolia are reciprocally monophyletic with low support, suggesting incomplete lineage sorting, introgression between Q. shumardii and Q. acerifolia, or both. Analyses that model allele distributions demonstrate that admixture contributes strongly to this pattern. Populations of Q. acerifolia experience gene flow from Q. shumardii and Q. rubra, but we found evidence that divergent selection is likely maintaining species boundaries: 1) ex situ collections of Q. acerifolia have a higher proportion of hybrids compared to the mature trees of the wild populations, suggesting ecological selection against hybrids at the seed/seedling stage; 2) ecological traits co-vary with genomic composition; and 3) Q. acerifolia shows genetic differentiation at loci hypothesized to influence tolerance of radiation, drought, and high temperature. Our findings strongly suggest that in maple-leaf oak, selection results in higher divergence at regions of the genome despite gene flow from close relatives.

Digest: Mating systems, intragenomic conflict, and speciation.

Conflict over the degree of maternal investment in an offspring can exist between an offspring's maternally inherited and paternally inherited alleles. Such conflict is not expected under self-fertilization. A new study led by Rifkin and Ostevik suggests that divergence in the degree of conflict between closely related outcrossing and selfing species can lead to aberrant early development of hybrids in morning glories. This dynamic represents a potentially powerful driver of reproductive incompatibility and thus speciation.

Genetic exchange with an outcrossing sister species causes severe genome-wide dysregulation in a selfing Caenorhabditis nematode.

Different modes of reproduction evolve rapidly, with important consequences for genome composition. Selfing species often occupy a similar niche as their outcrossing sister species with which they are able to mate and produce viable hybrid progeny, raising the question of how they maintain genomic identity. Here, we investigate this issue by using the nematode Caenorhabditis briggsae, which reproduces as a hermaphrodite, and its outcrossing sister species Caenorhabditis nigoni We hypothesize that selfing species might develop some barriers to prevent gene intrusions through gene regulation. We therefore examined gene regulation in the hybrid F2 embryos resulting from reciprocal backcrosses between F1 hybrid progeny and C. nigoni or C. briggsae F2 hybrid embryos with ∼75% of their genome derived from C. briggsae (termed as bB2) were inviable, whereas those with ∼75% of their genome derived from C. nigoni (termed as nB2) were viable. Misregulation of transposable elements, coding genes, and small regulatory RNAs was more widespread in the bB2 compared with the nB2 hybrids, which is a plausible explanation for the differential phenotypes between the two hybrids. Our results show that regulation of the C. briggsae genome is strongly affected by genetic exchanges with its outcrossing sister species, C. nigoni, whereas regulation of the C. nigoni genome is more robust on genetic exchange with C. briggsae The results provide new insights into how selfing species might maintain their identity despite genetic exchanges with closely related outcrossing species.

The evolution of recombination in self-fertilizing organisms.

Cytological data from flowering plants suggest that the evolution of recombination rates is affected by the mating system of organisms, as higher chiasma frequencies are often observed in self-fertilizing species compared with their outcrossing relatives. Understanding the evolutionary cause of this effect is of particular interest, as it may shed light on the selective forces favoring recombination in natural populations. While previous models showed that inbreeding may have important effects on selection for recombination, existing analytical treatments are restricted to the case of loosely linked loci and weak selfing rates, and ignore the stochastic effect of genetic interference (Hill-Robertson effect), known to be an important component of selection for recombination in randomly mating populations. In this article, we derive general expressions quantifying the stochastic and deterministic components of selection acting on a mutation affecting the genetic map length of a whole chromosome along which deleterious mutations occur, valid for arbitrary selfing rates. The results show that selfing generally increases selection for recombination caused by interference among mutations as long as selection against deleterious alleles is sufficiently weak. While interference is often the main driver of selection for recombination under tight linkage or high selfing rates, deterministic effects can play a stronger role under intermediate selfing rates and high recombination, selecting against recombination in the absence of epistasis, but favoring recombination when epistasis is negative. Individual-based simulation results indicate that our analytical model often provides accurate predictions for the strength of selection on recombination under partial selfing.

Chromosome-Level Reference Genomes for Two Strains of Caenorhabditis briggsae: An Improved Platform for Comparative Genomics.

The publication of the Caenorhabditis briggsae reference genome in 2003 enabled the first comparative genomics studies between C. elegans and C. briggsae, shedding light on the evolution of genome content and structure in the Caenorhabditis genus. However, despite being widely used, the currently available C. briggsae reference genome is substantially less complete and structurally accurate than the C. elegans reference genome. Here, we used high-coverage Oxford Nanopore long-read and chromosome-conformation capture data to generate chromosome-level reference genomes for two C. briggsae strains: QX1410, a new reference strain closely related to the laboratory AF16 strain, and VX34, a highly divergent strain isolated in China. We also sequenced 99 recombinant inbred lines generated from reciprocal crosses between QX1410 and VX34 to create a recombination map and identify chromosomal domains. Additionally, we used both short- and long-read RNA sequencing data to generate high-quality gene annotations. By comparing these new reference genomes to the current reference, we reveal that hyper-divergent haplotypes cover large portions of the C. briggsae genome, similar to recent reports in C. elegans and C. tropicalis. We also show that the genomes of selfing Caenorhabditis species have undergone more rearrangement than their outcrossing relatives, which has biased previous estimates of rearrangement rate in Caenorhabditis. These new genomes provide a substantially improved platform for comparative genomics in Caenorhabditis and narrow the gap between the quality of genomic resources available for C. elegans and C. briggsae.

Biased Gene Conversion Constrains Adaptation in Arabidopsis thaliana.

Reduction of fitness due to deleterious mutations imposes a limit to adaptive evolution. By characterizing features that influence this genetic load we may better understand constraints on responses to both natural and human-mediated selection. Here, using whole-genome, transcriptome, and methylome data from >600 Arabidopsis thaliana individuals, we set out to identify important features influencing selective constraint. Our analyses reveal that multiple factors underlie the accumulation of maladaptive mutations, including gene expression level, gene network connectivity, and gene-body methylation. We then focus on a feature with major effect, nucleotide composition. The ancestral vs. derived status of segregating alleles suggests that GC-biased gene conversion, a recombination-associated process that increases the frequency of G and C nucleotides regardless of their fitness effects, shapes sequence patterns in A. thaliana Through estimation of mutational effects, we present evidence that biased gene conversion hinders the purging of deleterious mutations and contributes to a genome-wide signal of decreased efficacy of selection. By comparing these results to two outcrossing relatives, Arabidopsis lyrata and Capsella grandiflora, we find that protein evolution in A. thaliana is as strongly affected by biased gene conversion as in the outcrossing species. Last, we perform simulations to show that natural levels of outcrossing in A. thaliana are sufficient to facilitate biased gene conversion despite increased homozygosity due to selfing. Together, our results show that even predominantly selfing taxa are susceptible to biased gene conversion, suggesting that it may constitute an important constraint to adaptation among plant species.

Investigations into a putative role for the novel BRASSIKIN pseudokinases in compatible pollen-stigma interactions in Arabidopsis thaliana

BackgroundIn the Brassicaceae, the early stages of compatible pollen-stigma interactions are tightly controlled with early checkpoints regulating pollen adhesion, hydration and germination, and pollen tube entry into the stigmatic surface. However, the early signalling events in the stigma which trigger these compatible interactions remain unknown.ResultsA set of stigma-expressed pseudokinase genes, termed BRASSIKINs (BKNs), were identified and found to be present in only core Brassicaceae genomes. In Arabidopsis thaliana Col-0, BKN1 displayed stigma-specific expression while the BKN2 gene was expressed in other tissues as well. CRISPR deletion mutations were generated for the two tandemly linked BKNs, and very mild hydration defects were observed for wild-type Col-0 pollen when placed on the bkn1/2 mutant stigmas. In further analyses, the predominant transcript for the stigma-specific BKN1 was found to have a premature stop codon in the Col-0 ecotype, but a survey of the 1001 Arabidopsis genomes uncovered three ecotypes that encoded a full-length BKN1 protein. Furthermore, phylogenetic analyses identified intact BKN1 orthologues in the closely related outcrossing Arabidopsis species, A. lyrata and A. halleri. Finally, the BKN pseudokinases were found to be plasma-membrane localized through the dual lipid modification of myristoylation and palmitoylation, and this localization would be consistent with a role in signaling complexes.ConclusionIn this study, we have characterized the novel Brassicaceae-specific family of BKN pseudokinase genes, and examined the function of BKN1 and BKN2 in the context of pollen-stigma interactions in A. thaliana Col-0. Additionally, premature stop codons were identified in the predicted stigma specific BKN1 gene in a number of the 1001 A. thaliana ecotype genomes, and this was in contrast to the out-crossing Arabidopsis species which carried intact copies of BKN1. Thus, understanding the function of BKN1 in other Brassicaceae species will be a key direction for future studies.

Mating system does not predict niche breath

AbstractAimSelf‐pollinating plants (‘selfers’) have larger geographical ranges and inhabit higher latitudes than their outcrossing relatives. This finding has led to the hypothesis that selfers also have broader climatic niches (‘niches’) because the increased likelihood of successful colonization into new areas and the initial purging of deleterious mutations could offset the inability of selfers to adapt to new environments owing to low heterozygosity. Here, we examine the niches of hundreds of closely related selfing and outcrossing species to determine whether selfers do indeed have larger niche breadths.LocationGlobal.Time periodCurrent.Major taxa studiedTwenty clades of flowering plants comprising 424 species.MethodsWe estimated and compared the climatic niches of 194 pairs of sister species across 15 families, which differed in mating system. We incorporated these results into models predicting niche breadth and its change over time to estimate the effects of mating system on niche breadth. Furthermore, we compared the degree of niche divergence between sister‐species pairs of various mating system combinations.ResultsSelfers did not have wider niche breadths than their outcrossing sister taxa. Sister pairs of selfers also exhibited greater niche overlap than outcrossing sisters, implying that niche expansion becomes limited after the transition to selfing. Furthermore, the niche breadth of selfers was predicted to decrease significantly faster than that of closely related outcrossers.Main conclusionsOur results demonstrate a decoupling in the range size and niche breadth of selfers. The larger geographical range and comparable niche breadths of many selfers is most likely to be a temporary phenomenon caused by a transiently expanded realized niche, and both will become constricted over time in comparison to outcrossers.

Examining the consequences of selfing

Comparative Genomics The Caenorhabditis genus of nematodes includes a mix of closely related outcrossing and self-fertilizing (selfing) species. Genome size differs widely among these different species. Yin et al. generated a genome assembly for the outcrossing nematode C. nigoni and compared it

Rapid genome shrinkage in a self-fertile nematode reveals sperm competition proteins.

To reveal impacts of sexual mode on genome content, we compared chromosome-scale assemblies of the outcrossing nematode Caenorhabditis nigoni to its self-fertile sibling species, C. briggsaeC. nigoni's genome resembles that of outcrossing relatives but encodes 31% more protein-coding genes than C. briggsaeC. nigoni genes lacking C. briggsae orthologs were disproportionately small and male-biased in expression. These include the male secreted short (mss) gene family, which encodes sperm surface glycoproteins conserved only in outcrossing species. Sperm from mss-null males of outcrossing C. remanei failed to compete with wild-type sperm, despite normal fertility in noncompetitive mating. Restoring mss to C. briggsae males was sufficient to enhance sperm competitiveness. Thus, sex has a pervasive influence on genome content that can be used to identify sperm competition factors.

What causes mating system shifts in plants? Arabidopsis lyrata as a case study

The genetic breakdown of self-incompatibility (SI) and subsequent mating system shifts to inbreeding has intrigued evolutionary geneticists for decades. Most of our knowledge is derived from interspecific comparisons between inbreeding species and their outcrossing relatives, where inferences may be confounded by secondary mutations that arose after the initial loss of SI. Here, we study an intraspecific breakdown of SI and its consequences in North American Arabidopsis lyrata to test whether: (1) particular S-locus haplotypes are associated with the loss of SI and/or the shift to inbreeding; (2) a population bottleneck may have played a role in driving the transition to inbreeding; and (3) the mutation(s) underlying the loss of SI are likely to have occurred at the S-locus. Combining multiple approaches for genotyping, we found that outcrossing populations on average harbour 5 to 9 S-locus receptor kinase (SRK) alleles, but only two, S1 and S19, are shared by most inbreeding populations. Self-compatibility (SC) behaved genetically as a recessive trait, as expected from a loss-of-function mutation. Bulked segregant analysis in SC × SI F2 individuals using deep sequencing confirmed that all SC plants were S1 homozygotes but not all S1 homozygotes were SC. This was also revealed in population surveys, where only a few S1 homozygotes were SC. Together with crossing data, this suggests that there is a recessive factor that causes SC that is physically unlinked to the S-locus. Overall, our results emphasise the value of combining classical genetics with advanced sequencing approaches to resolve long outstanding questions in evolutionary biology.

Background Selection in Partially Selfing Populations.

Self-fertilizing species often present lower levels of neutral polymorphism than their outcrossing relatives. Indeed, selfing automatically increases the rate of coalescence per generation, but also enhances the effects of background selection and genetic hitchhiking by reducing the efficiency of recombination. Approximations for the effect of background selection in partially selfing populations have been derived previously, assuming tight linkage between deleterious alleles and neutral loci. However, loosely linked deleterious mutations may have important effects on neutral diversity in highly selfing populations. In this article, I use a general method based on multilocus population genetics theory to express the effect of a deleterious allele on diversity at a linked neutral locus in terms of moments of genetic associations between loci. Expressions for these genetic moments at equilibrium are then computed for arbitrary rates of selfing and recombination. An extrapolation of the results to the case where deleterious alleles segregate at multiple loci is checked using individual-based simulations. At high selfing rates, the tight linkage approximation underestimates the effect of background selection in genomes with moderate to high map length; however, another simple approximation can be obtained for this situation and provides accurate predictions as long as the deleterious mutation rate is not too high.

The importance of autonomous selfing in preventing hybridization in three closely related plant species

Summary In plants that are able to self‐fertilize, autonomous selfing has been hypothesized to function as an effective mechanism that prevents heterospecific mating. However, there have only been few studies that have assessed the relative and absolute contribution of different modes of selfing to total reproductive isolation acting between species. Because selfing can be expected to increase fitness costs when offspring are sired from more outcrossing sister taxa, it can be hypothesized that the contribution of post‐zygotic barriers to total reproductive isolation becomes stronger in selfing than in more outcrossing relatives. To assess the efficiency of different modes of selfing against heterospecific mating, we conducted mixed‐pollination experiments in three Centaurium species showing strong differences in mating system (i.e. delayed selfing in Centaurium erythraea, competing selfing in C. littorale and prior selfing in C. pulchellum). The contribution of selfing to total reproductive isolation was further examined by quantifying the strength of three pre‐zygotic (flowering phenology, pollinator fidelity and pollen production) and four intrinsic post‐zygotic (seed set and germination, survival and flowering capacity) barriers for each of the six species pair combinations. Although autonomous selfing as a reproductive barrier was unable to completely impede gene flow, its efficiency as a mechanism to prevents heterospecific pollination strongly depended on the mode of selfing, with prior selfing being by far the most efficient mechanism and delayed selfing only offering a limited protection against hybrid fertilization at the end of a flower's lifetime. Apart from the prevention effect of selfing, differences in pollinator fidelity and pollen production also contributed substantially to total reproductive isolation. Post‐zygotic hybrid costs were high and most prevalent for seed production and germination success. Reductions in hybrid vigour were significantly associated with the mating system of the maternal species, with the highest reductions in the most selfing C. pulchellum and the lowest reductions in predominant outcrossing C. erythraea. Synthesis. Our findings indicate that selfing may prevent species from interspecific gene flow and hybridization. The association between the capacity to self‐autonomously and hybrid vigour further suggests that selfing may foster post‐zygotic costs following hybridization.

Reproductive Mode and the Evolution of Genome Size and Structure in Caenorhabditis Nematodes

The self-fertile nematode worms Caenorhabditis elegans, C. briggsae, and C. tropicalis evolved independently from outcrossing male-female ancestors and have genomes 20-40% smaller than closely related outcrossing relatives. This pattern of smaller genomes for selfing species and larger genomes for closely related outcrossing species is also seen in plants. We use comparative genomics, including the first high quality genome assembly for an outcrossing member of the genus (C. remanei) to test several hypotheses for the evolution of genome reduction under a change in mating system. Unlike plants, it does not appear that reductions in the number of repetitive elements, such as transposable elements, are an important contributor to the change in genome size. Instead, all functional genomic categories are lost in approximately equal proportions. Theory predicts that self-fertilization should equalize the effective population size, as well as the resulting effects of genetic drift, between the X chromosome and autosomes. Contrary to this, we find that the self-fertile C. briggsae and C. elegans have larger intergenic spaces and larger protein-coding genes on the X chromosome when compared to autosomes, while C. remanei actually has smaller introns on the X chromosome than either self-reproducing species. Rather than being driven by mutational biases and/or genetic drift caused by a reduction in effective population size under self reproduction, changes in genome size in this group of nematodes appear to be caused by genome-wide patterns of gene loss, most likely generated by genomic adaptation to self reproduction per se.

Self-pollination and expanding range

Plant Biogeography![Figure][1] Species ranges are larger in plants that are able to self-pollinate than in those that must receive pollen from another individual, such as these dwarf monkey flowers in Oregon PHOTO: © DENNIS FRATES/ALAMY The mating system of a plant species may play an important role in determining its geographic range size. Grossenbacher et al. compared the range size of self-pollinating plant species with the ranges of closely related outcrossing species. They found that selfing species tended to have the wider geographic ranges, and that this tendency increased with the time since a selfing species evolutionarily diverged from its outcrossing relative. Traits that improve mating success may be at least as important as other traits, such as dispersal ability, in helping plants to increase their range size and colonize new areas. Ecol. Lett. 10.1111/ele.12449 (2015). [1]: pending:yes

Simplification and Desexualization of Gene Expression in Self-Fertile Nematodes

Evolutionary transitions between sexual modes could be potent forces in genome evolution. Several Caenorhabditis nematode species have evolved self-fertile hermaphrodites from the obligately outcrossing females of their ancestors. We explored the relationship between sexual mode and global gene expression by comparing two selfing species, C. elegans and C. briggsae, with three phylogenetically informative outcrossing relatives, C. remanei, C. brenneri, and C. japonica. Adult transcriptome assemblies from the selfing species are consistently and strikingly smaller than those of the outcrossing species. Against this background of overall simplification, genes conserved in multiple outcrossing species with strong sex-biased expression are even more likely to be missing from the genomes of the selfing species. In addition, the sexual regulation of remaining transcripts has diverged markedly from the ancestral pattern in both selfing lineages, though in distinct ways. Thus, both the complexity and the sexual specialization of transciptomes are rapidly altered in response to the evolution of self-fertility. These changes may result from the combination of relaxed sexual selection and a recently reported genetic mechanism favoring genome shrinkage in partial selfers.

Global Population Genetic Structure of Caenorhabditis remanei Reveals Incipient Speciation

Mating system transitions dramatically alter the evolutionary trajectories of genomes that can be revealed by contrasts of species with disparate modes of reproduction. For such transitions in Caenorhabditis nematodes, some major causes of genome variation in selfing species have been discerned. And yet, we have only limited understanding of species-wide population genetic processes for their outcrossing relatives, which represent the reproductive state of the progenitors of selfing species. Multilocus-multipopulation sequence polymorphism data provide a powerful means to uncover the historical demography and evolutionary processes that shape genomes. Here we survey nucleotide polymorphism across the X chromosome for three populations of the outcrossing nematode Caenorhabditis remanei and demonstrate its divergence from a fourth population describing a closely related new species from China, C. sp. 23. We find high genetic variation globally and within each local population sample. Despite geographic barriers and moderate genetic differentiation between Europe and North America, considerable gene flow connects C. remanei populations. We discovered C. sp. 23 while investigating C. remanei, observing strong genetic differentiation characteristic of reproductive isolation that was confirmed by substantial F2 hybrid breakdown in interspecific crosses. That C. sp. 23 represents a distinct biological species provides a cautionary example of how standard practice can fail for mating tests of species identity in this group. This species pair permits full application of divergence population genetic methods to obligately outcrossing species of Caenorhabditis and also presents a new focus for interrogation of the genetics and evolution of speciation with the Caenorhabditis model system.

Evolutionary rates of commonly used nuclear and organelle markers of Arabidopsis relatives (Brassicaceae)

Recovering the genetic divergence between species is one of the major interests in the evolutionary biology. It requires accurate estimation of the neutral substitution rates. Arabidopsis thaliana, the first whole-genome sequenced plant, and its out-crossing relatives provide an ideal model for examining the split between sister species. In the study, rates of molecular evolution at markers frequently used for systematics and population genetics, including 14 nuclear genes spanning most chromosomes, three noncoding regions of chloroplast genome, and one intron of mitochondrial genome, between A. thaliana and four relatives were estimated. No deviation from neutrality was detected in the genes examined. Based on the known divergence between A. thaliana and its sisters about 8.0–17.6 MYA, evolutionary rates of the eighteen genes were estimated. Accordingly, the ratio of rates of synonymous substitutions among mitochondrial, chloroplast and nuclear genes was calculated with an average and 95% confidence interval of 1 (0.25–1.75): 15.77 (7.48–114.09): 74.79 (36.27–534.61). Molecular evolutionary rates of nuclear genes varied, with a range of 0.383–0.856×10−8 for synonymous substitutions per site per year and 0.036–0.081×10−9 for nonsynonymous substitutions per site per year. Compared with orthologs in Populus, a long life-span tree, genes in Arabidopsis evolved faster in an order of magnitude at the gene level, agreeing with a generation time hypothesis. The estimated substitution rates of these genes can be used as a reference for molecular dating.

Early evolution in a hybrid swarm between outcrossing and selfing lineages in Geum.

Although often considered as evolutionary dead ends, selfing taxa may make an important contribution to plant evolution through hybridization with related outcrossing lineages. However, there is a shortage of studies examining the evolutionary dynamics of hybridization between outcrossing and selfing taxa. On the basis of differential pollinator attractiveness, production and competitive ability of pollen, as well as levels of inbreeding depression, we predict that the early products of hybridization between outcrossing and selfing lineages will be F1s and first-generation backcrosses sired mainly by the outcrossing lineage, together with selfed F2s containing a limited genetic contribution from the outcrosser. These predictions were tested using amplified fragment length polymorphism and chloroplast markers to analyze the composition of a recent hybrid swarm between predominantly outcrossing Geum rivale and predominantly selfing Geum urbanum. In line with predictions, the hybrid swarm comprised both parental species together with F1s and first-generation backcrosses to G. rivale alone. Chloroplast data suggested that G. rivale was the pollen parent for both observed hybrid classes. However, there was no evidence for F2 individuals, despite the fact that the F1 was fully self-compatible and able to auto-pollinate. The pollen fertility of F1s was only 30% lower than that of the parental taxa, and was fully restored in backcross hybrids. Predicting future evolution in the hybrid swarm will require an understanding of the mating patterns within and among the mix of parental, F1 and backcross genotypes that are currently present. However, these results support the hypothesis that introgression is likely to be asymmetrical from selfing to outcrossing lineages.