Phylogeny, morphological evolution, and taxonomic diversity of the mycoheterotrophic genus Epirixanthes (Polygalaceae)

Abstract The Asian genus Epirixanthes (Polygalaceae) is one of the several mycoheterotrophic lineages of eudicots, being also the sole non-photosynthetic group in the order Fabales. Epirixanthes is subdivided into seven species, two of which were described as recently as in the 2010s. Despite the low species diversity and broad geographical distribution across tropical Asia of some of its species, the evolution and biology of Epirixanthes have been insufficiently understood, and this has, in turn, hindered its taxonomic understanding and specimen identification. In this study, we provide the first comprehensive phylogenetic reconstructions of Epirixanthes that cover all its seven currently accepted species. Three of these species have never been included before in any phylogenetic analysis. Using high-throughput sequencing, we obtained original data on the DNA structure of 16 specimens belonging to the seven species of Epirixanthes (including the type gatherings of E. compressa and E. confusa) as well as of three outgroup species (Polygala karensium, Salomonia ciliata, and S. cantoniensis). The sampling was supplemented by 17 previously studied specimens. We employed the ITS and matK markers used in the earlier reconstructions of Epirixanthes, and expanded the set of markers by utilizing the nuclear ETS region and 18S gene and 10 protein-coding plastid genes. We also explored the morphological evolution of Epirixanthes through ancestral state reconstructions of 13 characters of inflorescences, flowers, and fruits. We uncovered the species-level relationships within the genus, and demonstrated that E. confusa cannot be treated as a species distinct from E. elongata; this idea is supported by the morphological evidence, and therefore we formally synonymized E. confusa with E. elongata. We reconstructed a partial portrait (comprising seven characters) of the last common ancestor of Epirixanthes. Evolution of most characters was found to be homoplastic, which is partly explained by intraspecific variation, and partly also by the considerable morphological similarity between the distantly related E. compressa and E. elongata. Our results provide a basis for a taxonomic revision of Epirixanthes at a worldwide scale, as well as a framework for the DNA barcoding of its species.