The animal body plan, the prototypic body segment, and eye evolution

Developmental genetics of Drosophila continue to have a great impact on our understanding of evolution. The specification of the body plan involves four conceptual steps: 1) Localization of maternal mRNAs in the egg cytoplasm. 2) Translation of these RNAs and formation of morphogen gradients. 3) Subdivision of the antero-posterior gradient into a repetitive pattern of body segments. 4) Assignment of a specific identity to each segment by the Hox genes. The discovery of the Hox genes has uncovered a universal principle shared by all bilaterians; they serve as master control genes specifying organization along the antero-posterior axis. The ancestral arthropods presumably consisted of a series of more or less identical segments, which may be represented by recently discovered precambrian Lobopodia which have a pair of legs and a pair of eyes in each segment. The progressive divergence of Hox genes has led to progressive cephalization and caudalization. From the amino acid sequences of the clustered homeodomains we can deduce that the mesothoracic segment represents the prototype from the more anterior and the more posterior segments evolved. Pax6 has been identified as a master control gene for eye development in all bilaterians. Since Pax6 is involved in eye development in all bilaterian phyla, this argues strongly for a monophyletic origin of the metazoan eye. With the same tool box of transcription factors all the different eye-types can be constructed.