Sex-specific gene expression in preimplantation mouse embryos

The daily deluge of marketing advertisements might have us believe that our entire lives are occupied with sex. Recent work from Kobayashi and colleagues [1] suggests that this may not be too far from the truth. It has long been observed that gene-expression differences correlated with genetic sex exist at the preimplantation stages. This was first observed 30 years ago, when it was noticed that 50% of eight-cell-stage mouse embryos expressed the histocompatibility Y (Hya) antigen. At the time these were correctly presumed to be the male embryos [2]. Later work [3,4] confirmed their prediction. Since then, several studies have confirmed the existence of a handful of genes that appear to be expressed in a sex-specific fashion in the preimplantation embryo (reviewed in [5]). Kobayashi et al. [1] recently increased that handful of genes by two orders of magnitude when they observed 591 transcripts that appeared to be differentially expressed in either male or female blastocysts.To study sex-specific expression, they analyzed RNA derived from more than 1,000 sexed mouse blastocysts on a DNA microarray representing 20,000 transcripts [1]. Although gene arrays have been used before with staged preimplantation embryos and have identified a complicated milieu of signaling pathways [6], they have not been used to look at differences between the sexes. To overcome the technical problem of correctly sexing so many embryos, Kobayashi et al. [1] used a strain of transgenic mice harboring a constitutively expressed X-linked green fluorescent protein (GFP) transgene (XGFP) [7]. By breeding XGFPY transgenic males to non-transgenic (XX) females, female progeny could be distinguished at preimplantation stages from male littermates by their green fluorescence, which results from the zygotic expression of the paternally inherited GFP transgene. Male blastocysts, which have only a maternally inherited X chromosome, were identifiable by the lack of fluorescence.While the identification o