CLASS-3 HOX GENES IN INSECTS AND THE ORIGIN OF ZEN

We have cloned, from a beetle and a locust, genes that are homologous to the class 3 Hox genes of vertebrates. Outside the homeobox they sha re sequence motifs with the Drosophila zerknullt (zen) and z2 genes, a nd like zen, are expressed only in extraembryonic membranes. We conclu de that the zen genes of Drosophila derive from a Hox class 3 sequence that formed part of the common ancestral Hox cluster, but that in ins ects this (Hox) gene has lost its role in patterning the anterio-poste rior axis of the embryo, and acquired a new function. In the lineage l eading to Drosophila, the zen genes have diverged particularly rapidly .