The Drosophila orb gene is predicted to encode sex-specific germline RNA-binding proteins and has localized transcripts in ovaries and early embryos

We report the identification of a new gene, orb, which appears to be expressed only in the germline and encodes ovarian- and testis-specific transcripts. The predicted proteins contain two regions with similarity to the RRM family of RNA-binding proteins but differ at their amino termini. In testes, orb RNA accumulates in the primary spermatocytes and at the caudal ends of the spermatid bundles. In ovaries, orb transcripts display an unusual spatial pattern of accumulation in the oocyte. Preferential accumulation in the oocyte of orb RNA is first detected in region 2 of the germarium and is dependent on Bicaudal-D and egalitarian. While in stage 7 egg chambers orb RNA is localized posteriorly in the oocyte, during stages 8-10 it is localized at the anterior of the oocyte, asymmetrically along the dorsal-ventral axis. In embryos the transcripts accumulate at the posterior end and are included in the pole cells. This pattern of localization and the similarity to RNA-binding proteins suggest that the orb gene product may mediate the localization of maternal RNAs during oogenesis and early embryogenesis.