poirot, a new regulatory gene of Drosophila oskar acts at the level of the short Oskar protein isoform.

Embryonic germ cell formation and abdomen development in Drosophila requires localisation and site specific translation of oskar mRNA in the posterior part of the oocyte. Targeting of oskar function to the posterior pole of the oocyte needs a large set of proteins and RNAs, encoded by posterior group genes. Consequently, mutations in the posterior group genes can result in embryos without abdomens and/or germ cells. During a systematic hobo-mediated mutant isolation screen, we identified poirot, a novel posterior group gene, owing to its germ cell-less phenotype. We show that the lack of poirot activity dramatically decreases OSK protein levels, without affecting the oskar mRNA distribution. In poirot mutant oocytes, delocalised OSK protein is observed, indicating that wild-type poirot has a role in the anchoring process of the OSK protein at the posterior pole. Furthermore, we demonstrate that poirot acts in an isoform-specific manner, only the short OSK isoform is affected, while the long OSK isoform remains at wild-type levels in poirot mutants.