Genetic differentiation and admixture between sibling allopolyploids in the Dactylorhiza majalis complexf

Allopolyploidization often happens recurrently, but the evolutionary signicance of its iterative nature is not yet fully understood. Of particular interest are the gene ow dynamics and the mechanisms that allow young sibling polyploids to remain distinct while sharing the same ploidy, heritage and overlapping distribution areas. By using eight highly variable nuclear microsatellites, newly reported here, we investigate the patterns of divergence and gene ow between 386 polyploid and 42 diploid individuals, representing the sibling allopolyploids Dactylorhiza majalis s.s. and D. traunsteineri s.l. and their parentsat localities across Europe. We make use in our inference of the distinct distribution ranges of the polyploids, including areasin which they are sympatric (that is, the Alps) or allopatric (for example, Pyrenees with D. majalis only and Britain withD. traunsteineri only). Our results show a phylogeographic signal, but no clear genetic differentiation between the allopolyploids, despite the visible phenotypic divergence between them. The results indicate that gene ow between sibling Dactylorhiza allopolyploids is frequent in sympatry, with potential implications for the genetic patterns across their entire distribution range. Limited interploidal introgression is also evidenced, in particular between D. incarnata and D. traunsteineri. Altogether the allopolyploid genomes appear to be porous for introgression from related diploids and polyploids. We conclude that the observed phenotypic divergence between D. majalis and D. traunsteineri is maintained by strong divergent selection on specic genomic areas with strong penetrance, but which are short enough to remain undetected by genotyping dispersed neutral markers.