A comparison of Hox3 and Zen protein coding sequences in taxa that span the Hox3/zen divergence.

The class 3 Hox genes of insects have diverged--in expression domain and functional role during embryogenesis--compared to those of other bilaterians. Whereas the canonical ortholog (Hox3) is involved in axial patterning of the embryonic body, the insect ortholog (zen) is involved in extraembryonic development. In this paper, we present sequence data from the centipede Strigamia maritima, the collembolan Folsomia candida, and the insect Thermobia domestica. With these data, complete coding sequences are now known for orthologs in all four arthropod classes and all three great bilaterian clades. We make use of this large Hox3/Zen ortholog data set to define differences in the protein sequences encoded by insect zen genes compared to all other Hox3 orthologs. Folsomia and Thermobia are particularly relevant to determining when zen diverged from Hox3 over evolutionary time. Intriguingly, the orthologs of these two species have some protein sequence features typical of Hox3 and some typical of Zen, and they differ from one another for these features.