Localization and function of Bam protein require the benign genial cell neoplasm gene product

Division of a female Drosophila stem cell produces a daughter stem cell and a cystoblast. The cystoblast produces a syncytial cluster of 16 cells by p recisely four mitotic divisions and incomplete cytokinesis. Mutations in ge nes required for cystoblast differentiation, such as bag-of-marbles, block syncytial cluster formation and produce a distinctive "tumorous" or hyperpl astic germ cell phenotype. In this paper, we compare the oogenic phenotype of benign genial cell neoplasm mutations to that of mutations in bam. The d ata indicate that, like barn, bgcn is required for cystoblast development a nd that germ cells lacking bgcn become trapped in a stem cell-like state,On e indication that germ cells lacking bgcn cannot form cystoblasts is that b gcn stem cells resist genetic ablation be Bam misexpression. Misexpression of Bam eliminates wild-type stem cells, apparently by inducing them to divi de as cystoblasts. bgcn stem cells remain active when Bam is misexpressed, probably because they cannot adopt the cystoblast fate. Bgcn activity is no t required for Bam protein expression but is essential for the localization of Bam protein to the fusome. Together, the results suggest that Bam and B gcn cooperatively regulate cystoblast differentiation by controlling locali zation of Bam protein to the fusome. (C) 1999 Academic Press.