DEVELOPMENTAL GENETICS OF ASCIDIANS

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Ascidians provide an appealingly simple experimental system to investigate the function of developmentally relevant genes during chordate development. The ascidian tadpole is composed of only 〜2600 cells, which constitute a small set of larval organs including the epidermis, central nervous system, notochord and tail muscle. This configuration of the tadpole represents the basic chordate body plan. Embryogenesis of the best studied ascidians Ciona intestinalis and Ciona savignyi is rapid (about l8 hr), they are hermaphroditic, self-fertilization is feasible, and the entire life cycle takes less than 3 months, facilitating mutagenesis and genetic screens. The aim of the present study was to introduce forward genetic methods to isolate mutants in these ascidians.We first attempted mutagenesis with chemical mutagens. By treating sperm with the chemical mutagen ENU, several mutant lines were established. For example, chongmague affects notochord formation and zebulon affects head formation … More in Ciona intestinalis embryos.The identification of the affected genes is not simple when mutations are caused by single nucleotide substitutions. A strategy for identifying mutated genes involves insertional mutagenesis with the aid of transposable elements. However, previous attempts at insertional mutagenesis in marine invertebrates were not successful. We found that a Tc1/mariner family transposon Minos, originally isolated from Drosophila hydei, can function as a transposable element in Ciona. The Ciona intestinalis genome is AT-rich and Minos targets TA dinucleotides. This may explain why Minos is active in Ciona embryos. By careful examination of excision, deletion and transposable activity, we have established a number of enhancer trap lines in Ciona intestinalis. This method might represent a significant breakthrough for elucidating gene function in Ciona. At present, we are isolating mutants with deficiencies of adult organ formation.Thus, the present study is the first report of possible insertional mutagenesis in marine animals. Less