her1, a zebrafish pair-rule like gene, acts downstream of notch signallingto control somite development

During vertebrate embryonic development, the paraxial mesoderm becomes subd ivided into metameric units known as somites. In the zebrafish embryo, gene s encoding homologues of the proteins of the Drosophila NOTCH signalling pa thway are expressed in the presomitic mesoderm and expression is maintained in a segmental pattern during somitogenesis, This expression pattern sugge sts a role for these genes during somite development. We misexpressed vario us zebrafish genes of this group by injecting mRNA into early embryos. RNA encoding a constitutively active form of NOTCH1a (notch1a-intra) and a trun cated variant of deltaD [deltaD((Pst))], as well as transcripts of deltaC a nd deltaD, the hairy-E(spl) homologues her1 and her4, and groucho2 were tes ted for their effects on somite formation, myogenesis and on the pattern of transcription of putative downstream genes. In embryos injected with any o f these RNAs, with the exception of groucho2 RNA, the paraxial mesoderm dif ferentiated normally into semitic tissue, but failed to segment correctly. Activation of NOTCH results in ectopic activation of her1 and her4. This mi sregulation of the expression of her genes might be causally related to the observed mesodermal defects, as her1 and her4 mRNA injections led to effec ts similar to those seen with notch1a-intra, deltaC and deltaD seem to func tion after subdivision of the presomitic mesoderm, since the her gene trans cription pattern in the presomitic mesoderm remains essentially normal afte r misexpression of delta genes. Whereas NOTCH Signalling alone apparently d oes not affect myogenesis, zebrafish groucho2 is involved in differentiatio n of mesodermal derivatives.