Comparative studies on mammalian Hoxc8 early enhancer sequence reveal a baleen whale-specific deletion of a cis-acting element

Variations in regulatory regions of developmental control genes have been i mplicated in the divergence of axial morphologies. To find potentially sign ificant changes in cis-regulatory regions, we compared nucleotide sequences and activities of mammalian Hoxc8 early enhancers. The nucleotide sequence of the early enhancer region is extremely conserved among mammalian clades , with five previously described cis-acting elements, A-E, being invariant. However, a 4-bp deletion within element C of the Hoxc8 early enhancer sequ ence is observed in baleen whales. When assayed in transgenic mouse embryos , a baleen whale enhancer (unlike other mammalian enhancers) directs expres sion of the reporter gene to more posterior regions of the neural tube but fails to direct expression to posterior mesoderm, We suggest that regulatio n of Hoxc8 in baleen whales differs from other mammalian species and may be associated with variation in axial morphology.