Donor-Dependent Developmental Competence of Oocytes from Lambs Subjected to Repeated Hormonal Stimulation

An unpredictability of ovarian response still remains the major problemconcerning 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 excludeaforementioned causes of variability, thus to verify embryogenic competence inhomogenous groups of animals. For this purpose we used prepubertal ewes keptunder identical conditions. Initially, we stimulated three groups of prepubertal ewes at various ages and used a number of gonadotropin treatments to assessdifferences in oocyte competence between individuals. The results revealed therepeatability of response within individual donor lambs throughout the study.Moreover, once the variability in both oocyte and embryo yield between homogenousgroups of donors was revealed alongside the influence of age and type ofgonadotropin treatment (P < 0.001), we investigated whether the individual donor effect persisted among genetically similar animals. Therefore, we compared oocyteand subsequent embryo output of sibling lambs derived from the most efficientdonor. Here the genetic homogeneity of sisters kept under identical conditionssubstantially improved the uniformity of either follicular response or embryoproduction, suggesting that the genotype plays a primary role in establishingfollicular recruitment and developmental capability of oocytes. This observation consents to predict the ovarian performance from a single ewe already in earlyprepuberty (i.e., to qualify the female to breeding programs).[...]