The Hox genes are implicated in conferring regional identity to the anteroposterior axis of the developing embryo. We have characterized the organization and expression of hox genes in the teleost zebrafish (Danio rerio), and compared our findings with those made for the tetrapod vertebrates. We have isolated 32 zebrafish hox genes, primarily via 3'RACE-PCR, and analyzed their linkage relationships using somatic cell hybrids. We find that in comparison to the tetrapods, zebrafish has several additional hox genes, both within and beyond the expected 4 hox clusters (A-D). For example, we have isolated a member of hox paralogue group 8 lying on the hoxa cluster, and a member of hox paralogue group 10 lying on the b cluster, no equivalent genes have been reported for mouse or human. Beyond the 4 clusters (A-D) we have isolated a further 3 hox genes (the hoxx and y genes), which according to their sequence homologies lie in paralogue groups 4, 6, and 9. The hoxx4 and hoxx9 genes occur on the same set of hybrid chromosomes, hinting at the possibility of an additional hox cluster for the zebrafish. Similar to their tetrapod counterparts, zebrafish hox genes (including those with no direct tetrapod equivalent) demonstrate colinear expression along the anteroposterior (AP) axis of the embryo. However, in comparison to the tetrapods, anterior hox expression limits are compacted over a short AP region; some members of adjacent paralogue groups have equivalent limits. It has been proposed that during vertebrate evolution, the anterior limits of Hox gene expression have become dispersed along the AP axis allowing the genes to take on novel patterning roles and thus leading to increased axial complexity. In the teleost zebrafish, axial organization is relatively simple in comparison to that of the tetrapod vertebrates; this may be reflected by the less dispersed expression domains of the zebrafish hox genes.
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