Characterization and embryonic expression of two zebrafish Achaete-Scute homolog genes

Genes of the achaete-scute family are involved in the differentiation of neural cell types in Drosophila and mammals. Two cDNA clones containing sequences highly homologous to the Drosophila and mammalian achaete-scute genes were isolated from a zebrafish embryonic gene library and were named zebrafish achaete-scute homologs-1a and -1b (zASH-1a and zASH-1b). The expression of these genes during development was analyzed using whole mount in situ hybridization. Transcripts of both genes accumulate in distinct populations of cells in the neural anlage beginning at the end of gastrulation and represent the earliest neural specific markers found in the zebrafish thus far. The distribution of the zASH RNAs during the first 18 hours of development is regionally restricted, and marks future nervous system boundaries and structures. Later in development, the presence of zASH transcripts correlates with regions within the neural tube where neural differentiation is occurring. In most regions there appears to be no zASH gene expression in differentiated cells. However, simultaneous expression of zASH RNA and a neural differentiation marker is found in at least one set of cells: the epiphysis. The expression patterns of the zASH genes during two crucial developmental periods, early nervous system pattern formation and neural differentiation, suggests that these genes could have a dual function in zebrafish embryogenesis.