MCCLINTOCK, J.M.*; JOZEFOWICZ, C.; CARLSON, R.; MANN, D.J.; PRINCE, V.E.: Hox gene patterning and fish evolution

We are using the zebrafish to investigate the evolution of Hox gene patterning functions. Whereas tetrapods possess 4 clusters of Hox genes, zebrafish have 7 distinct clusters. This condition is most likely due to a whole genome duplication, which occurred either early in or prior to the radiation of teleost fish. Zebrafish have at least 47 Hox genes, compared with 39 in mouse and human. Why have these “extra” Hox genes been maintained in the zebrafish? To address this question, we have undertaken an analysis of the zebrafish homologues of Drosophila labial,the members of paralogue group 1 (PG1). Zebrafish have four PG1 genes, whereas mouse and human have three. We find that two of the zebrafish PG1 genes share an expression domain in the ventral midbrain, well anterior to the region that vertebrate Hox genes are thought to pattern based on both expression and functional data. Our data suggest that this patterning role may be an ancestral feature within the vertebrates which has been secondarily lost in mammals. We have used mis-expression to directly examine the function of the four zebrafish PG1 genes. We find that ectopic expression of 3 of the 4 PG1 proteins leads to a homeotic transformation in the hindbrain such that rhombomere 2 (r2) takes on characteristics of r4, as assayed by anatomical and molecular markers. Thus, we find evidence for evolution of PG1 genes at both cis-regulatory and protein-coding levels. We are presently exploring the nature of the changes in both regulation and protein function.