Evolution of stamen number in Ptychospermatinae (Arecaceae): insights from a new molecular phylogeny of the subtribe.

The palm subtribe Ptychospermatinae (Arecaceae: Arecoideae) is naturally distributed in the South West Pacific area and contains 12 genera and around 60 species, including numerous popular ornamentals. Like many palms, Ptychospermatinae flowers are small, trimerous, unisexual and always grouped into inflorescences of various sizes. However they exhibit a wide diversity in stamen number (a few to several dozen or even hundreds) that is poorly understood from an evolutionary point of view. Although advances have been made in elucidating phylogenetic relationships within Ptychospermatinae, some relationships among and within genera still remain to be clarified. Here we used a combination of five nuclear markers (nrITS2, the conserved nuclear intron BRSC10 and three low copy genes, PRK, RPB2 and AGAMOUS) and three chloroplast markers (matK, ndhA and rps15-ycf1) to propose a new phylogenetic hypothesis for the subtribe. The combination of all these markers improved the resolution and robustness of phylogenetic relationships within the subtribe, allowing us to identify four major clades. This phylogenetic framework was used to examine the evolution of stamen number in the clade. The optimization of stamen number on the phylogeny highlighted the high level of interspecific variability, showing that the character is highly labile and raising questions about the evolutionary and functional significance of this lability.