Effect of follicle size and of the FecB Booroola gene on oocyte function in sheep.

Booroola ewes have a major gene that affects ovulation rate. Gene expression has consequences on ovarian somatic cells but it is unknown whether it also affects germ cells in the adult ovary. Hence, the present study examined (1) whether oocyte growth was similar in FecBFecB and Fec+ Fec+ oocytes during preantral and antral follicular growth, (2) whether the patterns of proteins neosynthesized by oocytes of these two genotypes were identical, (3) whether the ability of the oocytes to resume meiosis was unaffected by genotype and (4) whether, after IVF, oocytes from both genotypes could develop to the blastocyst stage at similar rates. Histological examination of the respective sizes of the oocyte and of the follicle demonstrated that oocytes were larger in FecBFecB versus Fec+ Fec+ preantral follicles. Resolution of the proteins neosynthesized by FecBFecB and Fec+ Fec+ oocytes by one-dimensional PAGE and image analysis demonstrated that quantitative (but not qualitative) differences could be observed between genotypes for bands at 74, 59, 35 and 25 kDa. In addition, a genotype by oocyte size interaction was detected for two additional bands at 45 and 43 kDa. After 24 h of culture in vitro in TCM-199 plus 100 ng ml-1 FSH plus 10% sheep follicular fluid, oocytes from FecBFec+ follicles gained the ability to resume meiosis at a smaller size and a higher proportion of them reached metaphase II irrespective of the size class studied compared with Fec+ Fec+ follicles. In addition, the developmental rate of eggs after IVF was also affected by follicle size and genotype, since FecBFec+ oocytes originating from 1.0-3.5 mm follicles had a greater ability (P < 0.05) to develop to the blastocyst stage than Fec+ Fec+ oocytes. It is concluded that the FecB gene, in addition to its effects on granulosa cell maturation, also affects oocyte development and function. Whether these alterations are related requires further investigation.