Additional hox clusters in the zebrafish: divergent expression patterns belie equivalent activities of duplicate hoxB5 genes

The evolution of metazoan body plans has involved changes to the Hox genes, which are involved in patterning the body axis and display striking evolut ionary conservation of structure and expression. Invertebrates contain a si ngle Hox cluster whereas tetrapods possess four clusters. The zebrafish has seven unlinked hox clusters, a finding that is difficult to reconcile with the notion that genomic complexity, reflected by Hox cluster number, and m orphological complexity are causally linked, as the body plan of the zebraf ish is not obviously more complex than that of the mouse or human. Why have the additional hox genes in zebrafish been conserved? To address the role of these additional zebrafish hox genes, we have examin ed the duplicate hoxB5 genes, hoxB5a, and hoxB5b. Conservation of gene dupl icates can occur when one gene acquires a new function (neofunctionalizatio n), or when the ancestral function is divided between the two duplicates (s ubfunctionalization). hoxB5a and hoxB5b are expressed in distinct domains, and their combined expression domain is strikingly similar to that of singl e Hoxb5 genes in other species. The biochemical functions encoded by the tw o genes were studied by overexpression, which resulted in identical develop mental defects in the anterior hindbrain and cranial neural crest, suggesti ng strongly that hoxB5a and hoxB5b have equivalent biochemical properties w ith respect to early development. From these studies, we conclude that cons ervation of hoxB5a and hoxB5b is likely the result of division of the ances tral Hoxb5 function between the two genes, without significant changes in b iochemical activity. These results suggest a resolution to the conundrum of the extra hox genes and clusters in the zebrafish, since if any of the add itional hox genes in the zebrafish are similarly subfunctionalized, they ar e unlikely to supply novel genetic functions. Thus, the morphological compl exity potentially conferred by the majority of additional zebrafish hox clu sters may not be substantially greater than that conferred by the four tetr apod clusters.