Expression patterns of the rogue Hox genes Hox3/zen and fushi tarazu in the apterygote insect Thermobia domestica

Many embryonic patterning genes are remarkably conserved between vertebrates and invertebrates, and the Hox genes are paradigmatic examples of this conservation. Yet even Hox genes can change dramatically in evolution. Two genes in particular-Hox3 and fushi tarazu-lost their ancestral roles as homeotic genes and play very different developmental roles in the fruit fly Drosophila melanogaster. The Drosophila Hox3 homologs zerknullt and bicoid act in extraembryonic tissues and in establishment of the anteroposterior axis, respectively, whereas fushi tarazu acts in segmentation and neurogenesis. It would be valuable to know what mechanisms allowed Hox3 and ftz to abandon their ancestral roles as homeotic genes and take on new roles. To explore the evolutionary transition of these genes, we analyzed their expression in a primitive insect, the firebrat Thermobia domestica. The expression patterns seem to represent a stage of evolution intermediate between the ancestral state seen in basal arthropods and the derived expression patterns in Drosophila. These expression data help us to narrow the period in which the gene transitions took place. Hox3 appears to have evolved directly into zen within the insects, whereas ftz seems to have adopted the expression patterns of a segmentation and neurogenesis gene earlier in the mandibulate arthropods.