Six class homeobox genes in Drosophila belong to three distinct families and are involved in head development

The vertebrate Six genes are homologues of the Drosophila homeobox gene sin e oculis (so), which is essential for development of the entire visual syst em. Here we describe two new Six genes in Drosophila, D-Six3 and D-Six4, wh ich encode proteins with strongest similarity to vertebrate Six3 and Six4, respectively. In addition, we report the partial sequences of IZ Sir gene h omologues from several lower vertebrates and show that the class of Six pro teins can be subdivided into three major families, each including one Droso phila member. Similar to so, both D-Six3 and D-Six4 are initially expressed at the blastoderm stage in narrow regions of the prospective head and duri ng later stages in specific groups of head midline neurectodermal cells. D- Six3 may also be essential for development of the clypeolabrum and several head sensory organs. Thus, the major function of the ancestral Sir gene pro bably involved specification of neural structures in the cephalic region. ( C) 1999 Elsevier Science Ireland Ltd. All rights reserved.