Localization and expression pattern of type I postplasmic mRNAs in embryos of the ascidian Halocynthia roretzi

The posterior-vegetal cytoplasm (PVC) of fertilized ascidian eggs plays important roles in embryo development. It has been reported that some maternal RNAs are localized to the PVC. We identified four novel type I postplasmic mRNAs that are localized to the PVC through the use of data from a cDNA project of maternal mRNAs in the eggs of Halocynthia roretzi (MAGEST database). The mRNAs are HrGLUT, HrPEN-1, and HrPEM-3, which show similarity to a glucose transporter, a g1-related protein, and Ciona pem-3, respectively; and HrPEN-2, with no similarity. Maternal mRNAs of all four genes were identically localized to the PVC after ooplasmic segregation. During cleavage, they were concentrated in the centrosome-attracting body (CAB) and were then segregated into the small blastomeres located at the posterior pole. This localization pattern is common to all known type I postplasmic mRNAs found so far. HrGLUT, HrPEN-1, and HrPEM-3 were expressed zygotically in various tissues later in embryogenesis: HrGLUT and HrPEM-3 in the mesenchyme and nervous system, and HrPEN-1 in the ectodermal cells.