Identity of the Calcarata species complex in Viola sect. Melanium (Violaceae)

The Calcarata species complex in Viola sect. Melanium (Violaceae) is a group of species from Italy and neighbouring islands. The complex is of considerable evolutionary interest because several hypotheses about hybrid speciation within the group have been previously proposed. Because the Calcarata complex is not well characterized morphologically, we used 142 samples representing 92 (of c. 120) species of V. sect. Melanium plus three outgroup species. Nuclear ITS and ETS and plastid trnS–trnG intergenic spacer sequences were analysed to test the monophyly of the Calcarata complex and to infer relationships among the constituent species. Both nuclear and plastid sequences resulted in very limited phylogenetic resolution. Based on the nuclear dataset, most species of the Calcarata complex were recovered in four clades that also contained species not previously associated with the complex. Results from the plastid dataset recovered most species of the complex in a large polytomy. However, one larger clade containing only Calcarata complex species could be recovered, and species of all four nuclear clades were part of this larger plastid clade. The Calcarata complex clearly could not be resolved as monophyletic. We hypothesize that the lack of phylogenetic resolution may result mainly from frequent hybridization and hybrid speciation, processes that are well documented for Viola and V. sect. Melanium.