Euteleost fish genomes are characterized by expansion of gene families

The presence of additional hox clusters in the zebrafish has led to the hyp othesis that there was a whole genome duplication at the origin of modern F ish. To investigate the generality of this assumption, we analyzed all avai lable actinopterygian fish gene families, and sequenced nuclear receptors F rom diverse teleost fish. The origin and timing of duplications was systema tically determined by phylogenetic analysis. More genes are indeed found in zebrafish than in mouse. This abundance is shared by all major groups of e uteleost fish, but not by eels. Phylogenetic analysis shows that it may res ult From frequent independent duplications, rather than from an ancestral g enome duplication. We predict two zebrafish paralogs for most mouse or huma n genes, thus expressing a note of caution in functional comparison of Fish and mammalian genomes. Redundancy appears to be the rule in fish developme ntal genetics. Finally, our results imply that the outcome of genome projec ts cannot be extrapolated easily between fish species.