No abstract

The vasa gene is essential for germline formation in Drosophila. Vasa-related genes have been isolated from several organisms including nematode, frog and mammals. In order to gain insight into the early events in vertebrate germline development, zebrafish was chosen as a model. Two zebrafish vasa-related genes were isolated, pl10a and vlg. The pl10a gene was shown to be widely expressed during embryogenesis. The vlg gene and vasa belong to the same subfamily of RNA helicase encoding genes. Putative maternal vlg transcripts were detected shortly after fertilization and from the blastula stage onwards, expression was restricted to migratory cells most likely to be primordial germ cells.

The engrailed locus of Drosophila melanogaster has the characteristics of both a homoeotic gene and a segmentation gene: like a homoeotic gene, it specifies the development of specific compartments of the Drosophila embryo (the posterior compartments of each segment), and, like mutations of segmentation genes, lethal alleles of engrailed affect also the pattern of segmentation of the embryo. Here we report that like many of the homoeotic genes of the bithorax and Antennapedia complexes, engrailed has a 'homoeo box' sequence: also, like the segmentation gene fushi tarazu, the engrailed gene displays a periodic pattern of expression in Drosophila embryos.

Murine and human sequences homologous to the paired box of the Drosophila segmentation gene paired have been reported previously. Here we describe a zebrafish (Brachydanio rerio) paired box‐containing clone, pax[zf‐a], which is clearly distinct from reported vertebrate Pax genes. The putative protein encoded by pax[zf‐a] contains a paired box and a paired‐type homeobox separated by a glycine‐rich, acidic linker and a carboxy‐terminal end which is remarkably rich in serine, threonine and proline residues. By in situ hybridization to embryonic tissue sections and whole mount embryos, pax[zf‐a] transcripts were found within restricted regions of the central nervous system and the eye. In contrast to the murine Pax genes recently characterized, pax[zf‐a] is not expressed in the segmented mesoderm. At the 17 h stage, pax[zf‐a] expression is detected in a defined area of the diencephalon which circumscribes the presumptive thalamus. This suggests an involvement of pax[zf‐a] in pattern formation in the rostral brain. The pax[zf‐a] gene is also expressed throughout the hindbrain and spinal cord. This hybridization signal is restricted to a longitudinal column which includes the basal plate. Later in development, at 36 h post‐fertilization, pax[zf‐a] transcripts are no longer restricted to a specific region of the diencephalon, but are distributed over the entire developing brain.

This study describes the isolation and characterization of zebrafish homologues of the mammalian Pax3 and Pax7 genes. The proteins encoded by both zebrafish genes are highly conserved (>83%) relative to the known mammalian sequences. Also the neural expression patterns during embryogenesis are very similar to the murine homologues. However, observed differences in neural crest and mesodermal expression relative to mammals could reflect some functional divergence in the development of these tissues. For the zebrafish Pax7 protein we report the first full-length amino acid sequences in vertebrates and show the existence of three additional isoforms which have truncations in the homeodomain and/or the C-terminal region. These novel variants provide evidence for additional isoform diversity of vertebrate Pax proteins.