Variations in regulatory regions of developmental control genes have been implicated in the divergence of axial morphologies. To find potentially significant 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 sequence is observed in baleen whales. When assayed in transgenic mouse embryos, a baleen whale enhancer (unlike other mammalian enhancers) directs expression of the reporter gene to more posterior regions of the neural tube but fails to direct expression to posterior mesoderm. We suggest that regulation of Hoxc8 in baleen whales differs from other mammalian species and may be associated with variation in axial morphology.

Different hypotheses have been proposed on the phylogenetic relationships of branchiobdellidans and aphanoneurans among the Annelida based on the anatomical and embryological characters . The 18S ribosomal RNA gene sequences have been analyzed from representatives of the three major taxa of the Annelida plus the branchiobdellidans and aphanoneurans to assess their phylogenetic relationships to each other . In this preliminary study, all of the phylogenetic analyses show the branchiobdellidans as a sister group to the leeches, rather than the oligochaetes . The position of the aphanoneurans is stable as an independent taxon that evolved after the polychaetes branched from the evolutionary stem, but before the ancestral oligochaetes emerged .