Isolation and characterization of cDNA clones for beta-tubulin genes as a molecular marker for neural cell differentiation in the ascidian embryo.

The central nervous system (CNS) of an ascidian tadpole larva is composed of about 340 cells, the lineages of which are well documented. To elucidate the mechanisms underlying the neural induction of ascidians, appropriate molecular markers are required. In this study, to obtain an early differentiation marker of the neural cells, we isolated and characterized cDNA clones for two beta-tubulin genes (HrTBB1 and HrTBB2) of the ascidian Halocynthia roretzi. We found that the HrTBB1 and HrTBB2 amino acid sequences are highly conserved, with 91-98% identities to other invertebrate and vertebrate beta-tubulins. The expression of HrTBB1 was found to be maternal, while HrTBB2 is expressed both maternally and zygotically. We observed that the zygotic expression of HrTBB2 commences at the neural plate stage and is specific to cells of the differentiating CNS. In the larvae, HrTBB2 expression was restricted to cells of the CNS, some cells of the papilla and cells of the peripheral nervous system. These results indicate that HrTBB2 will be a useful early molecular marker for neural cell differentiation in the ascidian embryo.