A combined morphological and molecular approach to the phylogeny of asteroids (Asteroidea: Echinodermata)
Résumé
Two recent analyses of asteroid phylogeny have come to surprisingly different conclusions concerning both the tree topology and rooting. We reanalyzed the problem for nine asteroids, with representatives from five of the major higher taxa: Paxillosida, Forcipulatida, Valvatida, Spinulosida, and Velatida. We combined the two previously published morphological data sets to generate a well-supported, though unrooted, phylogenetic tree. We also derived a molecular phylogeny from the analysis of the first 400 bases of the 28S ribosomal RNA sequences for the same nine taxa. This tree provides less resolution but does not contradict the morphological tree. The combined morphological and molecular data were used to derive the most well-supported unrooted topology. To avoid assumptions as to what represents derived versus primitive morphological character states among asteroids, we used only molecular data from other echinoderm classes to identify the root position. Although this approach does not resolve the rooting point unambiguously, evidence favors the Astropectenidae as the sister group to the rest of the asteroids. We then polarized the morphological characters of crown group asteroids and estimated relative rates of molecular evolution, which show a five-fold difference. Molecular data give no support to the recent suggestion that Forcipulatida diverged significantly earlier than did other asteroids.