Oocytes in sheep homozygous for a mutation in bone morphogenetic protein receptor 1B express lower mRNA levels of bone morphogenetic protein 15 but not growth differentiation factor 9.

The aim of this study was to test the hypothesis that the high ovulation rate in ewes (BB) homozygous for a mutation in the bone morphogenetic protein receptor type 1B (BMPR1B) gene is linked to lower BMP15 and/or GDF9 mRNA in oocytes compared with those in wild-type (++) ewes. Cumulus cell-oocyte complexes (COC) and granulosa cells (GC) were recovered from ≥1 mm diameter follicles of BB and ++ ewes during a prostaglandin-induced follicular phase. Expression levels of GDF9 and BMP15 were measured by multiplex qPCR from individual COC. The gonadotropin-induced cAMP responses of the GC from each non-atretic follicle were measured following treatment with FSH or human chorionic gonadotropin. In a separate validation experiment, GDF9 and BMP15 expression was present only in oocytes and not in cumulus cells. There was no effect of follicular diameter on oocyte-derived GDF9 or BMP15 mRNA levels. The mean expression levels of BMP15, but not GDF9, were significantly lower in all non-atretic follicles, including the subsets containing either FSH- or LH-responsive GC in BB, compared with ++, ewes. No genotype effects were noted for FSH-induced cAMP production by GC either with respect to dose of, or number of follicles responding to, FSH. However, ovaries from BB ewes contained significantly more follicles responsive to LH, with respect to cAMP production in GC. We propose that these findings are consistent with the hypothesis that the higher ovulation rate in BB sheep is due, at least in part, to lower oocyte-derived BMP15 mRNA levels together with the earlier onset of LH-responsiveness in GC.