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 mRN A is localized in the anterior pole region and the bicoid protein forms an anterior-to-posterior concentration gradient, bicoid activity is required f or transcriptional activation of zygotic segmentation genes and the transla tional suppression of uniformly distributed maternal caudal mRNA in the ant erior 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 ident ified only in close relatives of the schizophoran fly Drosophila, This pose s the question of how the bicoid gene evolved and adopted its unique functi on in organizing anterior-posterior polarity, We have cloned bicoid from a basal cyclorrhaphan fly, Megaselia abdita (Phoridae, Aschiza), and show tha t the gene originated from a recent duplication of the direct homolog of th e vertebrate gene Hox3, termed zerknullt, which specifies extraembryonic ti ssues in insects.