The Anterior Determinant Bicoid of Drosophila Is a Derived Hox Class 3 Gene

The Drosophila gene bicoid functions as the anterior body pattern organizer of Drosophila. Embryos lacking maternally expressed bicoid fail to develop anterior segments including head and thorax. In wild-type eggs, bicoid mRNA is localized in the anterior pole region and the bicoid protein forms an anterior-to-posterior concentration gradient. bicoid activity is required for transcriptional activation of zygotic segmentation genes and the translational suppression of uniformly distributed maternal caudal mRNA in the anterior region of the embryo. caudal genes as well as other homeobox genes or members of the Drosophila segmentation gene cascade have been found to be conserved in animal evolution. In contrast, bicoid homologs have been identified only in close relatives of the schizophoran fly Drosophila. This poses the question of how the bicoid gene evolved and adopted its unique function in organizing anterior-posterior polarity. We have cloned bicoid from a basal cyclorrhaphan fly, Megaselia abdita (Phoridae, Aschiza), and show that the gene originated from a recent duplication of the direct homolog of the vertebrate gene Hox3, termed zerknüllt, which specifies extraembryonic tissues in insects.