OBJECTIVE OF THE EXPERIMENT The zebrafish, Danio rerio, is a successful addition to the collection of vertebrate model systems that offers several attractive features: mating pairs are readily available and spawn large numbers of eggs on a regular basis. Development is rapid and occurs entirely outside the mother, and embryos and larvae are completely transparent, allowing the observation of the circulatory system and internal organs in the living organism. Up to a few thousand mutants have now been isolated through mutagenesis screens that allow a systematic dissection of vertebrate developmental genetics, mirroring the success of screens in Drosophila and C. elegans.

Segmentation of the body along the anteroposterior axis is a feature found in arthropods, annelids and chordates. In the zebrafish, as in other vertebrates, segmentation is apparent at embryonic stages in the subdivision of the trunk paraxial mesoderm into somites. Although less obvious, the anterior neural tube is also segmentally arranged, as can be seen in the partition of the prospective hindbrain, the rhombencephalon, into seven rhombomeres.

Here the mechanisms that establish anteroposterior identity among rhombomeres will be analysed at the gene expression level. In particular, the effects of exogenous manipulation of retinoic acid (RA) signalling on the development of the central nervous system will be studied in the zebrafish embryo.

DEGREE OF DIFFICULTY Experiments require breeding pairs of zebrafish and some competence in handling fish and setting up pair matings. Both experiments require basic molecular techniques and care in minimising RNAse contamination.