Molecular phylogeny of Urvillea (Paullinieae, Sapindaceae) and its implications in stem vascular diversity

Background and Aims: The tribe Paullinieae have the highest diversity of vascular variants among the seed plants. The developmental diversity is better understood in the speciose genera Paullinia and Serjania, however, the phylogeny and diversity of vascular variants in the smaller genera of Paullinieae remain understudied. Here we investigate the evolution of development of stem vasculatures in the small genus Urvillea. Methods: We generate the first molecular phylogeny of Urvillea derived from 11 markers using a maximum likelihood and Bayesian approach. In combination with phylogenetic reconstruction, stochastic character mapping is used to assess evolutionary changes in stem ontogenies, determined from developmental anatomy of stems collected in the field or from herbarium and wood collections. Key Results: Urvillea is supported as a monophyletic group and sister to Serjania. There are five stem ontogenies in Urvillea, including a regular anatomy and four vascular variants. Most stem ontogenies initiate with lobed stems. Lobed adult stems are conserved in Urvillea, but this ontogeny was lost multiple times. A reversal to regular anatomy occurred in non-climbing species. Phloem wedges, fissured stems and ectopic cambia evolved independently once. Phloem wedges is an intermediate developmental stage in the formation of fissured stems, which is characterized by a continuous fragmentation of vascular tissues. Lobed stems may generate constriction zones and lobes may split or not. Conclusions: Urvillea stands out as the third most diverse genus in number of vascular variants within Paullinieae, but only one ontogeny (fissured stems) is exclusive for the genus. Differential cambial activity and ectopic cambia are the main ontogenetic processes generating stem diversity. The evolutionary history of vascular variants demonstrates the large developmental lability of the cambium in such a small genus and corroborates a scenario of repeated evolution of complex anatomies within Paullinieae lianas.