Spatial restriction of germ plasm assembly in Drosophila

Localization of the germ plasm to the posterior of the Drosophila embryo is critical to the anteroposterior patterning of the animal as well as the formation and function of the primordial germ cells. Germ plasm localization occurs during oogenesis and follows a hierarchical pathway that relies on the polarity of the microtubule cytoskeleton. osk mRNA is the first component of the germ plasm to become localized and translation of osk is a nucleating step in germ plasm assembly. While much is known about the process by which the germ plasm becomes localized, the mechanism by which germ plasm assembly is restricted to the posterior pole of the oocyte is poorly understood.;Here we describe the characterization of two maternal effect mutants, X-82 and X-83, that display localization of the germ plasm to ectopic patches on the embryonic cortex. Both X-82 and X-83 lead to ectopically localized germ plasm components in mutant oocytes and affect the polarity of the oocyte microtubule cytoskeleton. We show that X-82 is an allele of baz and likely affects repolarization of the microtubule cytoskeleton at mid-oogenesis by mislocalization of the encoded protein. Our results suggest that X-83 is an allele of Ost48 or His3.3b and leads to a subtle effect on microtubule polarity via increased osk levels or defects in vesicle trafficking within the oocyte. Finally, through analysis of other mutants affecting microtubule polarity in the oocyte, we propose that a focus of microtubule plus ends is sufficient to direct ectopic assembly of the germ plasm.