The Phylogenetic Position of Vincetoxicum pannonicum (Borhidi) Holub Supports the Species' Allopolyploid Hybrid Origin

Abstract

The Pannonian endemic species <em>Vincetoxicum pannonicum </em>was described from specimens collected in Hungary and occurs at only few locations. It is considered “vulnerable” according to the International Red List. The chromosome set was reported to be tetraploid, and the species was hypothesized to be an allotetraploid hybrid of the Balkan species <em>V. fuscatum </em>and the Adriatic species <em>hirundinaria </em>subsp. <em>adriaticum. </em>We investigated the origin of <em>V. pannonicum </em>using molecular phylogenetic methods by separately analyzing the multicopy nuclear ribosomal internal transcribed spacer (nrITS) and the plastid-encoded <em>trn</em>H-<em>psb</em>A DNA regions and by evaluating discrepancies between the produced gene trees. Paralogs in the nrITS region clustered in two main groups, one of which was closest to <em>V. fuscatum</em>, and the other included <em>V. hirundinaria </em>subsp. <em>adriaticum</em>. According to <em>trn</em>H-<em>psb</em>A sequences, <em>V. pannonicum </em>and <em>V. hirundinaria </em>subsp. <em>adriaticum </em>formed a single group. Our results show that <em>V. pannonicum </em>diversified because of hybrid speciation, in which <em>V. fuscatum </em>was the pollen donor. We discovered a similar placement of <em>V. maeoticum</em>, which suggests a further hybridization event between <em>V. fuscatum </em>and a species of the <em>V. hirundinaria </em>group. Our genome-size estimate indicates almost sixfold larger genome size in <em>V. pannonicum </em>compared to the maternal diploid parent, suggesting hexaploidy; however, <em>V. pannonicum </em>is tetraploid. This may suggest cytological diploidization in the allopolyploid <em>V. pannonicum</em>. We observed substantial genetic distance between <em>V. hirundinaria </em>subsp. <em>adriaticum </em>and all other subspecies of <em>V. hirundinaria</em>, and we therefore propose that <em>V. adriaticum </em>should be regarded as a separate species.