SCHMIDT-OTT, U.: Evolution of Hox3 in flies

The members of the evolutionarily conserved Hox-gene-complex, termed Hox genes, are required for specifying segmental identity during embryogenesis in various animal phyla. The Hox3 genes of winged insects have lost this ancestral function and are required for the development of extra-embryonic epithelia, which do not contribute to any larval structure. Higher flies (Cyclorrhapha) such as Drosophila melanogaster contain Hox3 genes of two types, the zerkn�llt-type and the bicoid-type. The zerkn�llt gene is expressed zygotically on the dorsal side of the embryo and required for establishing extra-embryonic tissue. Its sister gene bicoid is expressed maternally and the transcripts are localized at the anterior pole of the mature egg. Bicoid protein, which emerges from this localized source during early development, is required for embryonic patterning. bicoid homologues are confined to Cyclorrhaphan flies and a very recent origin appears likely. Characterization of Hox3 genes of the lower flies Empis livida (Empididae), Haematopota pluvialis (Tabanidae) and Clogmia albipunctata (Psychodidae) suggest that a single Hox3 gene with composite bicoid and zerkn�llt expression characteristics occurs in non-Cyclorrhaphan flies. A model is proposed in which the ancestral Hox3 gene had been duplicated in the stem lineage of Cyclorrhaphan flies. During evolution, one of the gene copies has lost maternal expression and evolved as zerkn�llt; whereas the second copy lost zygotic expression and evolved as bicoid. An important implication of this model is that Hox3 of lower flies carried (some) Bicoid functions. Ongoing studies focus on the evolution of genomic regulatory sequences of the Bicoid target gene hunchback.