MULTIPLE ORIGINS OF ANURAL DEVELOPMENT IN ASCIDIANS INFERRED FROM RDNA SEQUENCES

Ascidians exhibit two different modes of development. A tadpole larva is formed during urodele development, whereas the larval phase is modi fied or absent during anural development. Anural development is restri cted to a small number of species in one or possibly two ascidian fami lies and is probably derived from ancestors with urodele development. Anural and urodele ascidians constitute a model system in which to stu dy the evolution of development, but the phylogeny of anural developme nt has not been resolved. Classification based on larval characters su ggests that anural species are monophyletic, whereas classification ac cording to adult morphology suggests they are polyphyletic. In the pre sent study, we have inferred the origin of anural development using rD NA sequences. The central region of 18S rDNA and the hypervariable D2 loop of 28S rDNA were amplified from the genomic DNA of anural and uro dele ascidian species by the polymerase chain reaction and sequenced. Phylogenetic trees inferred from 18S rDNA sequences of 21 species plac ed anural developers into two discrete groups corresponding to the Sty elidae and Molgulidae, suggesting that anural development evolved inde pendently in these families. Furthermore, the 18S rDNA trees inferred at least four independent origins of anural development in the family Molgulidae. Phylogenetic trees inferred from the D2 loop sequences of 13 molgulid species confirmed the 18S rDNA phylogeny. Anural developme nt appears to have evolved rapidly because some anural species are pla ced as closely related sister groups to urodele species. The phylogeny inferred from rDNA sequences is consistent with molgulid systematics according to adult morphology and supports the polyphyletic origin of anural development in ascidians.