Abstract
Within the plant kingdom, many genera contain sister lineages with contrasting outcrossing and inbreeding mating systems that are known to hybridize. The evolutionary fate of these sister lineages is likely to be influenced by the extent to which they exchange genes. We measured gene flow between outcrossing Geum rivale and selfing Geum urbanum, sister species that hybridize in contemporary populations. We generated and used a draft genome of G. urbanum to develop dd‐RAD data scorable in both species. Coalescent analysis of RAD data from allopatric populations indicated that the species diverged 2–3 Mya, and that historical gene flow between them was extremely low (1 migrant every 25 generations). Comparison of genetic divergence between species in sympatry and allopatry, together with an analysis of allele frequencies in potential parental and hybrid populations, provided no evidence of contemporary introgression in sympatric populations. Cluster‐ and species‐specific marker analyses revealed that, apart from four early‐generation hybrids, individuals in sympatric populations fell into two genetically distinct groups that corresponded exactly to their morphological species classification with maximum individual admixture estimates of only 1–3%. However, we did observe joint segregation of four putatively introgressed SNPs across two scaffolds in the G. urbanum population that was associated with significant morphological variation, interpreted as tentative evidence for rare, recent interspecific gene flow. Overall, our results indicate that despite the presence of hybrids in contemporary populations, genetic exchange between G. rivale and G. urbanum has been extremely limited throughout their evolutionary history.
Highlights
A key factor influencing the evolutionary trajectory of lineages is their level of genetic exchange with related taxa (Abbott, Barton, &Good, 2016; Coyne & Orr, 2004); for example, genetic exchange can introduce adaptive mutations (e.g., Paoletti, Buck, & Brasier, 2006; Whitney, Randell, & Rieseberg, 2010) or restrict the lineages’ independent evolution
Contemporary populations display widespread evidence for hybridization between outcrossing and inbreeding sister species (e.g., Mimulus (Brandvain, Kenney, Flagel, Coop, & Sweigart, 2014; Kenny & Sweigart, 2016; Vickery, 1978), Rhinanthus (Ducarme, Vrancken, & Wesselingh, 2010), Centaurium (Brys, Vanden Broeck, Mergeay, & Jacquemyn, 2014), but we have little understanding of whether this leads to significant introgression between them
Introgression may be of most importance to inbreeders, whose evolutionary potential is otherwise compromised by low genetic diversity and effective recombination rates (Arunkumar, Ness, Wright, & Barrett, 2015; Charlesworth, Morgan, & Charlesworth, 1993, 2003; Glemin & Ronfort, 2013)
Summary
A key factor influencing the evolutionary trajectory of lineages is their level of genetic exchange with related taxa The study species (Figure 1) comprise Geum rivale (outcrossing rate t = 0.8), which possesses a pendulous flower adapted to bee pollination and typically occupies open, moist habitats, and inbreeding Geum urbanum (t = 0.15), which bears erect flowers adapted to fly pollination and grows in shaded, well-drained sites (Ruhsam, Hollingsworth, Squirrell, & Ennos, 2010; Taylor, 1997a, 1997b). Both species are perennial and ancient allohexaploids (2n = 42; Smedmark et al, 2003). (v), we relate estimates of an individuals’ hybridity to its morphology to test whether a plant’s morphology reflects its genotypic hybrid status and to test whether morphology varies with focal RAD loci
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