Gpbox (Psx2), a homeobox gene preferentially expressed in female germ cells at the onset of sexual dimorphism in mice

XX gonads differentiate into ovaries, a morphologic event evident by embryo nic day 13.5 (E13.5) in mice. To identify early markers of oogenesis, sex-s pecific urogenital ridge cDNA libraries were constructed from E12-13 embryo s. After mass excision and isolation of plasmid DNA, approximately 4800 exp ressed sequence tags were determined and compared to existing databases. Fe w cDNAs were specifically expressed in the urogenital ridge, but one, desig nated GPBOX, encodes a 227-amino-acid homeobox protein that is first expres sed at E10.5 in the embryo as well as in the extraembryonic tissues. The Gp box gene is single copy in the mouse genome and is located on the X chromos ome in close proximity to two other homeobox genes, Pem and Psx1. Within th e embryo, its expression is limited to the gonad, and transcripts are not d etected in adult tissues. Although comparable levels are initially present in both sexes, GPBOX transcripts accumulate faster in female germ cells and peak at E12.5 when they are present in fivefold greater abundance than in males. The persistence of GPBOX transcripts in female germ cells until E15. 5 and their virtual disappearance in males by E13.5 suggest that Gpbox may play a role in mammalian oogenesis. (C) 2000 Academic Press.