Mitochondrial rDNA phylogeny of the asteroidea suggests the primitiveness of the paxillosida.

Asteroids display four distinct modes of developmental patterns: the indirect mode, the nonbrachiolarian mode, the direct mode, and the mode with a barrel-shaped larva. Among them the former two are planktotrophic, whereas the latter two are lecithotrophic. The direct mode and the mode with a barrel-shaped larva are thought to have evolved from the more primitive planktotrophic mode, the nonbrachiolarian and the indirect mode, respectively. However, whether the nonbrachiolarian mode or the indirect mode is the more primitive in asteroids is unresolved, despite discussion since early this century. A key aspect of this problem is the phylogenetic status of paxillosidans, since the nonbrachiolarian mode and the mode with a barrel-shaped larva are seen only in paxillosidans. To resolve this problem, we performed a molecular phylogenetic study of asteroids, based on the nucleotide sequences of mitochondrial rDNAs. Phylogenetic trees support a close relationship between the Asterinidae and the Solasteridae. We suggest that the paxillosidans are not a monophyletic group; rather, the Luidiidae (one family of Paxillosida) is a sister group to the rest of the asteroids. Although some aspects of our results contradict a recent study by Lafay et al. (1995, Syst. Biol. 44: 190-208) based on 28S rRNA sequences, both studies agree on a paraphyletic nature for the paxillosidans. We conclude that characters shared by paxillosidans are primitive; hence the primitive mode of development in asteroids is the nonbrachiolarian mode.