Donor-dependent developmental competence of oocytes from lambs subjected to repeated hormonal stimulation

An unpredictability of ovarian response still remains the major problem concerning ovine reproductive programs. The influence of several environmental, genetic, and ovarian cycle effects on oocyte/embryo yield from donor females has been previously reported. The present research has been designed to exclude aforementioned causes of variability, thus to verify embryogenic competence in homogenous groups of animals. For this purpose we used prepubertal ewes kept under identical conditions. Initially, we stimulated three groups of prepubertal ewes at various ages and used a number of gonadotropin treatments to assess differences in oocyte competence between individuals. The results revealed the repeatability of response within individual donor lambs throughout the study. Moreover, once the variability in both oocyte and embryo yield between homogenous groups of donors was revealed alongside the influence of age and type of gonadotropin treatment (P < 0.001), we investigated whether the individual donor effect persisted among genetically similar animals. Therefore, we compared oocyte and subsequent embryo output of sibling lambs derived from the most efficient donor. Here the genetic homogeneity of sisters kept under identical conditions substantially improved the uniformity of either follicular response or embryo production, suggesting that the genotype plays a primary role in establishing follicular recruitment and developmental capability of oocytes. This observation consents to predict the ovarian performance from a single ewe already in early prepuberty (i.e., to qualify the female to breeding programs).