Isolation of germ line-dependent female-sterile mutation that affects yolk specific sequestration and chorion formation in Drosophila

Two loci on the X chromosome have been implicated in choriogenesis by in situ hybridization of poly A-containing RNA from choriogenic eggchambers to Drosophila polytene chromosomes (A. C. Spradling and A. P. Mahowald (1979): 7E and 12E. At least two genes coding for major eggshell proteins map to region 7E (A. C. Spradling, M. E. Digan, A. P. Mahowald, M. Scott, and E. A. Craig (1980). In an effort to elucidate the functional role of the 12E gene product, 3600 EMS-treated X chromosomes were screened for recessive female-sterile mutations that mapped within the region 11F10-12F1. Four independent female-sterile mutations were recovered, three of which fell into one complementation group (fs29, fs117, and fs445). Mapping by analysis of recombinant progeny as well as of trans heterozygotes utilizing other deficiency chromosomes showed that the three noncomplementing mutations all mapped to region 12E1-12F1. Studies comparing chorion morphology and protein synthesis indicate localized perturbations in the extracellular assembly of eggshell components in mutant eggchambers. The germ line dependence of the mutations was established using germ line mosaics constructed by pole cell transplantation. Analysis of eggchamber protein accumulation patterns showed reduced amounts of yolk polypeptides (YPs) in the mutants. The elevated concentrations of YPs found in mutant hemolymph coupled with the normal YP biosynthetic patterns and active uptake of trypan blue by mutant oocytes suggest that 12E sequences play a role in yolk-specific sequestration.