GONADAL MESODERM AND FAT-BODY INITIALLY FOLLOW A COMMON DEVELOPMENTALPATH IN DROSOPHILA

During gastrulation, the Drosophila mesoderm invaginates and forms a s ingle cell layer in close juxtaposition to the overlying ectoderm. Sub sequently, particular cell types within the mesoderm are specified alo ng anteroposterior and dorsoventral axes. The developmental pathways t hat guide the specification of different cell types within the mesoder m are not well understood. We have analyzed the developmental relation ship between two mesodermal tissues in the Drosophila embryo, the gona dal mesoderm and the fat body. Both tissues arise from lateral mesoder m within the eve domain. Whereas in the eve domain of parasegments 10- 12 gonadal mesoderm develops from dorsolateral mesoderm and fat body f rom ventrolateral mesoderm, in parasegments 4-9 only fat body is speci fied. Our results demonstrate that the cell fate decision between gona dal mesoderm and fat body identity within dorsolateral mesoderm along the anteroposterior axis is determined by the combined actions of gene s including abdA, AbdB and srp; while srp promotes fat body developmen t, abdA allows gonadal mesoderm to develop by repressing sip function. Furthermore, we present evidence from genetic analysis suggesting tha t, before stage 10 of embryogenesis, gonadal mesoderm and the fat body have not yet been specified as different cell types, but exist as a c ommon pool of precursor cells requiring the functions of the tin, zfh- 1 and cli genes for their development.