Egg Cytoplasmic Factors that Regulate Developmental Pattern

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A recent molecular-phylogenetic study based on the comparison of 18S rDNA sequences has suggested that formation of tadpole larvae is a key developmental event to understand the origin and evolution of chordates. During embryogenesis of ascidians, about forty unicellular and striated muscle cells as well as forty notochord cells are formed in the tail of the larva. Therefore, it is important to study molecular mechanisms involved in differentiation of these cells to understand not only ontogeny by also phylogeny of animals.We have isolated an ascidian homolog of the mouse T (Brachyury) gene. The ascidian T gene is expressed exclusively in notochord cells and the expression begins with the time of restriction of their developmental fates. Therefore, the T gene is cue to understand the chordate evolution. We are now studying the T gene expression in ascidians which develop tailless larvae.Muscle cell differentiation in ascidian embryos has been studied at the levels of muscle actin genes, myogenic regulatory gene, and cytoplasmic factors (or muscle determinants). We have also studied muscle actin genes in ascidians with tailless larvae, and it was found that the ascidian with tailless larvae contains several pseudogenes of muscle actin, which were not found in ascidians with tailed larvae under usual research conditions we adopted.