Hybridization between insular endemic and widespread species of Viola in non-disturbed environments assessed by nuclear ribosomal and cpDNA sequences

Viola jaubertiana Marès & Vigin. is a narrow endemic violet of the Balearic Islands, restricted to small, fragmented, and scattered populations living in inaccessible rocky places and calcareous overhangs. V. jaubertiana is entirely glabrous and morphologically very uniform. However, several authors have reported hairy individuals collected at the type locality, suggesting that these rupicolous, pubescent plants are putative hybrids with V. alba Bess. subsp. dehnhardtii (Ten.) W. Becker, a woodland violet growing in the area. Ribosomal ITS sequences of the putative hybrids analysed showed additive species-specific sites of V. alba subsp. dehnhardtii and V. jaubertiana, strongly supporting its hybrid origin from these progenitors. CpDNA sequences of all putative hybrids were uniform, and identical to those present in V. jaubertiana accessions. This suggests that the gene flow between V. alba subsp. dehnhardtii and V. jaubertiana is unidirectional and identifies the endemic V. jaubertiana as the unique ovule donor. The additivity of the ITS sequences, together with the pollen and ovule sterility, suggests that the sampled individuals are primary F₁ hybrids, whereas no trace of introgressive hybridization or hybrid zone has been evidenced by the nuclear and plastid markers used. Judging from herbarium sheets, hybridization between V. alba subsp. dehnhardtii and V. jaubertiana is recurrent and dates back from the XIXth century. Hybrids between these species are not linked to disturbed environments. In fact, they have been always reported in rupicolous habitats, where the maternal species is restricted.