No evidence of contemporary interploidy gene flow between the closely related European woodland violets Viola reichenbachiana and V.riviniana (sect. Viola, Violaceae)

Viola reichenbachiana (2n=4x=20) and V.riviniana (2n=8x=40) are closely related species widely distributed in Europe, often sharing the same habitat throughout their overlapping ranges. It has been suggested in numerous studies that their high intraspecific morphological variability and plasticity might have been further increased by interspecific hybridisation in contact zones, given the sympatry of the species and the incomplete sterility of their hybrid. The aims of this study were to: (i) confirm that V.reichenbachiana and V.riviniana have one 4x genome in common, and (ii) determine the impact of hybridisation and introgression on genetic variation of these two species in selected European populations. For our study, we used 31 Viola populations from four European countries, which were analysed using AFLP and sequencing of a variable plastid intergenic spacer, trnH-psbA. Our analysis revealed that V.reichenbachiana exhibited larger haplotype diversity, having three species-specific haplotypes versus one in V.riviniana. The relationships among haplotypes suggest transfer of common haplotypes into V.riviniana from both V.reichenbachiana and hypothetically the other, now extinct, parental species. AFLP analysis showed low overall genetic diversity of both species, with V.riviniana showing higher among-population diversity. None of the morphologically designated hybrid populations had additive AFLP polymorphisms that would have indicated recent hybridisation. Also, kinship coefficients between both species did not indicate gene flow. V.riviniana showed significant population subdivision and significant isolation by distance, in contrast to V.reichenbachiana. The results indicate lack of gene flow between species, high influence of selfing on genetic variability, as well as probably only localised introgression toward V.riviniana.