Evolution of Anural Development in Ascidians: Role of Muscle-specific Differentiation Genes

Abstract

Most ascidians show indirect development with a urodele larva, while a few aseidian species, primarily in the family Molgulidae, have evolved an anural larva. The anural larva lacks differentiated notochord and tail muscle cells. Anural development has evolved several times in different clades of the Molgulidae. Molgula oeculta and Molgula bleizi are closely-related anural developers. Molecular phylogenetic analyses suggest that anural development evolved independently in M. bleizi and M. oeculta. Larval muscle actin genes of both species have mutations in its coding region that would result in nonfunctional actin proteins. However, M. oeculta retains functional transcription factors Controlling muscle-specific gene expression in its vestigial muscle cells. Therefore, the regression of muscle cell differentiation during the evolution of anural larvae may be mediated by changes in muscle-specific differentiation genes, such as actin genes, rather than in transcription factors required for expression of these genes. Fewer larval muscle actin genes in Molgulid ascidians may be in part responsible for polyphyly of anural development in the Molgulidae.