Systematics and the evolution of flower symmetry of Posoqueria (Rubiaceae)

Sammanfattning: The genus Posoqueria is a Neotropical group of shrubs and trees in the coffee family Rubiaceae. Posoqueria is phylogenetically poorly understood and presents several internal taxonomical difficulties. Previous studies using limited molecular data and taxon sampling suggest the species in Posoqueria could be split into two monophyletic groups based on the presence or absence of a unique, specialized pollination system known as the Pollen Catapult Mechanism (PCM). This split implies the need for the resurrection of the genus Stannia, which groups the species lacking the PCM. Here, 15 species were sampled with target sequence capture and a phylogeny of Posoqueria was generated based on 177 genes and the multispecies coalescent model. The study confirms the monophyly of most Posoqueria taxa and provides evidence of the phylogenomic distinctiveness of species such as P. mutisii, P. longiflora, P. maxima, P. grandiflora, and P. williamsii, but questions the current circumscriptions of P. chocoana and P. costaricensis. Particularly, the phylogeny supports the re-circumscription of P. latifolia, suggesting that populations from Central America and Colombia belong to a yet undescribed species in the genus. Furthermore, this phylogenomic study indicates that the unique catapult mechanism linked to zygomorphic flowers in Posoqueria is the product of several evolutionary transitions and is, therefore, a non-homologous trait. The results challenge the hypothesis that this trait played a significant role in the diversification of the genus and reject the recognition of Stannia as a valid taxon. This novel view of the phylogenetic relationships in Posoqueria expands the horizons for future research in the group, especially from the alpha-taxonomy, developmental genetics, pollination biology, ecology, and biogeography points of view.