BrainiacEncodes a Novel, Putative Secreted Protein That Cooperates with Grk TGFα in the Genesis of the Follicular Epithelium

Abstract

brainiac (brn)is involved in a number of developmental events. In addition to being required zygotically for segregation of neuroblasts from epidermoblasts, it is essential for a series of critical steps during oogenesis which also depend upongurken (grk),a TGFα homolog. Animals harboring strong mutations of eithergrkor EGF receptor tyrosine kinase (Egfr) or doubly mutant forbrnand weakgrkorEgfrmutations produce ovarian follicles with multiple sets of nurse cell–oocyte complexes. These follicles frequently have discontinuities in the follicular epithelium that uncover nurse cells but not the oocyte. Gaps first appear in the germarium, suggesting that some nurse cells lack affinity for invading prefollicular cells. This is the first evidence thatgrk,in addition to its involvement in the genesis of anterior–posterior and dorsal–ventral polarity, is also required for Egfr-dependent development of the follicular epithelium that surrounds each nurse cell/oocyte cluster to form an egg chamber. We have used restriction fragment length polymorphisms to localizebrnto a 10-kb region within a 300-kb stretch of DNA on the X-chromosome, and we have identified thebrngene by means of RNA rescue.brncodes for a putative secreted protein.brnis expressed in germ cells at the time follicle cells first surround the nurse cell–oocyte complex. Our genetic data suggest thatbrnacts in a parallel, but partially overlapping pathway to the Grk–Egfr signaling pathway. Thebrnpathway may help to provide specificity to TGFα–Egfr function during oogenesis.