Polyploid hybrid speciation in the Calcarata species complex of Viola section Melanium (Violaceae): relating hybrid species to parent species distribution and ecology

Two species of the Calcarata species complex of Viola section Melanium have been hypothesized to be of polyploid hybrid origin. To test this hypothesis, we used a modified restriction site associated (RAD) DNA-sequencing approach along with ITS and plastid DNA sequence data for reconstructing and dating the phylogeny of the group. For the detection of polyploid hybrid speciation and the parental lineages involved we used the four-taxon D-statistic metric (ABBA-BABA). Viola corsica (Elba, Corsica, Sardinia; calcifuge) and V. tineorum (Sicily; calcicole) were confirmed as polyploid hybrid species. They share V. cassinensis subsp. cassinensis (central Apennine; calcicole) as a parental lineage with V. bertolonii (Maritime Alps; serpentine) as the second parent of V. corsica and with V. nebrodensis (Sicily; calcicole) as the second parent of V. tineorum. The geographical distribution of the two hybrid species reflects the geographical distribution of the differential parent lineages. Substrate requirements of V. corsica are different, and those of V. tineorum are identical to those of their respective parents. We conclude that the differential distribution and ecology of V. corsica and V. tineorum can be traced back to properties of their respective parents.