THE ROLE OF HOMEOTIC GENES IN THE SPECIFICATION OF THE DROSOPHILA GONAD

Background: In Drosophila, the gonads are composed of two cell populat ions: the germ line, derived from the pole cells, and a somatic compon ent, derived from the mesoderm of abdominal segments 5-8. Formation of the gonad requires the function of a specific homeotic gene, abdomina l-d (abd-A). Other genes of the bithorax complex, Ultrabithorax (Ubx) or Abdominal-B (Abd-B), cannot substitute for this requirement when ab d-A is removed. Results: We show here that, in embryos lacking the ent ire bithorax complex, experimentally induced expression of either ABD- A or UBX protein in the mesoderm will rescue the expression of a gonad -specific marker, 412 RNA. Ubiquitous expression of these homeotic pro teins within the mesoderm results in the formation of ectopic gonad ti ssue anterior to the normal location of the gonads. In the absence of any endogenous bithorax-complex gene expression, however, mesoderm exp ressing gonad markers still condenses preferentially in the posterior segments of the abdomen, even in the absence of pole cells. Conclusion s: The specific requirement for abd-A and not Ubx in gonad development does not reflect differences in the properties of the proteins that t hese genes encode, but presumably reflects differences in their regula tion. In normal development, the restriction of gonad formation to the posterior abdomen does not depend on the overlap of abd-A and Abd-B e xpression, but must depend on the regulation of abd-A and Ubx in the s ub-population of the mesoderm that forms the gonad. Factors other than homeotic gene expression provide some cues that direct gonadal mesode rm to condense in the correct location.