Sexual reproduction in animals requires the proper differentiation of the germline into male or female gametes, but when and how is germline sex determined? On , Abbie Casper and Mark Van Doren report that this important event occurs early in embryonic gonad development in Drosophila and is controlled mainly by signals from somatic cells (the soma). The researchers identify genes that are expressed in a sex-specific manner in embryonic germ cells and show that these genes start to be expressed at the time of gonad formation. By altering the sex of the soma relative to that of the germline,they show that germ cells largely take on the sex of these surrounding cells,irrespective of their own sex chromosome constitution. By contrast,inactivating the genes thought to act autonomously in the germline to cause sex determination has little effect on the establishment of germline sexual identity. Thus, signals from the soma are, surprisingly, dominant over germline autonomous cues during the initial stage of germline sex determination in Drosophila.
