Early lethality of embryos derived from transgenic Xenopus females is associated with reduced ovarian grem1 expression

The grem1 gene codes a protein that inhibits the action of multiple members of a growth factor family known as bone morphogenetic proteins (BMPs). Certain members of this BMP family can regulate both fecundity and fertility in mammals via their action on oocyte (egg) development, and grem1 has been identified as a marker of oocyte quality in humans. The model amphibian Xenopus laevis is far more fecund than mammals, producing thousands of eggs in a clutch. However, female transgenic frogs carrying grem1 under the control of a stress inducible hsp70 promoter (G frogs) produce very few viable offspring. Here, we show that this is not due to reduced fecundity or fertilization rate, but results from a significant reduction in subsequent survival over the first day of development. Embryos that successfully survive for the first day were found to go on to develop normally when compared to their peers. Both the morphology and stage distribution of oocytes from G females appears normal, and oocytes develop at expected rates, although stage VI oocytes were found to have a lower response to in vitro progesterone treatment. Unexpectedly, levels of grem1 mRNA were found to be consistently lower in the female ovaries from four independent G transgenic lines than in wild type ovaries. Both transgenic and wild type offspring were equally affected, confirming a maternal effect. Our study shows that transgenic females with the lowest levels of grem1 transcripts in the ovary have the lowest rates of survival past the first day of amphibian embryogenesis, equivalent to pre-implantation staged mammalian embryos. The reduced expression of grem1 in the oocytes of transgenic females suggests transgene supression of an endogenous locus may occur in the Xenopus female germline, an unexpected finding.