Maintaining their genetic distance: little evidence for introgression between widely hybridising species of Geum with contrasting mating systems

Within the plant kingdom many genera contain sister lineages with contrasting outcrossing and inbreeding mating systems that are known to hybridise. 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 G. urbanum, sister species that hybridise 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 revea...