A novel follicle-cell-dependent dominant female sterile allele, Star(Kojak), alters receptor tyrosine kinase signaling in Drosophila

Abstract

We describe a new dominant allele, Star(Kojak), that alters receptor tyrosine kinase signaling in the follicle cells and in the eyes in Drosophila. We isolated Star(Kojak) in a screen for follicle-cell-dependent dominant female sterile mutations. We show that Star(Kojak) and revertants of Star(Kojak) do not complement Star loss-of-function mutations. We propose that Star(Kojak) is a novel type of allele of Star that has both dominant gain-of-function phenotypes early in development and dominant loss-of- function phenotypes later in development. Star encodes a putative transmembrane protein that has previously been shown to be a critical component of the epidermal growth factor receptor tyrosine kinase signaling pathway. Early in oogenesis, Star mRNA expression is higher in Star(Kojak) egg chambers than in wild-type egg chambers, consistent with its gain-of- function phenotype. Later in oogenesis, Star mRNA expression is lower in Star(Kojak) follicle cells than in wild-type follicle cells, consistent with its loss-of-function phenotype. By genetically analyzing Star(Kojak) and its revertants, we present evidence that Star is involved in anterior-posterior axis formation both in the female germline cells and in the somatic follicle cells. We also demonstrate that at least part of the dominant female sterile phenotype of Star(Kojak) is restricted to the posterior-pole follicle cells. We propose that Star functions by processing pro-Gurken to mature Gurken, which is thereby released in the region between the oocyte and the follicle cells and binds to the epidermal growth factor receptor in the follicle cells.