The Drosophila melanogaster sex determination gene sisA is required in yolk nuclei for midgut formation

During sex determination, the sisterlessA (sisA) gene functions as one of f our X:A numerator elements that set the alternative male or female regulato ry states of the Switch gene Sex-lethal. in somatic cells, sisA functions s pecifically in sex determination, but its expression pattern also hints at a role in the yolk cell, a syncytial structure believed to provide energy a nd nutrients to the developing embryo. Previous studies of sisA have been l imited by the lack of a null allele, leaving open the possibility that sisA has additional functions. Here we report the isolation and molecular chara cterization of four new sisA alleles including two null mutations. Our find ings highlight key aspects of sisA structure-function and reveal important qualitative differences between the effects of sisA and the other strong X: A numerator element, sisterlessB on Sex-lethal expression. We use genetic, expression, clonal, and phenotypic analyses to demonstrate that sisA has an essential function in the yolk nuclei of both sexes. In the absence of sis A, endoderm migration and midgut formation are blocked, suggesting that the yolk cell may have a direct role in larval gut development. To our knowled ge, this is the first report of a requirement for the yolk nuclei in Drosop hila development.