Genome-Wide Association Analyses Highlight the Potential for Different Genetic Mechanisms for Litter Size Among Sheep Breeds

Genetic variations in ovulation rate which occur in different breeds of sheep provide useful models to explore the mechanisms regulating the development of antral follicles. The Booroola gene, an autosomal mutation that affects ovulation rate, has been known for over two decades and despite intensive research it has not yet been identified. Using resources from human genome mapping and known data about gene linkage and chromosome location in the sheep, we selected the gene encoding the Bone Morphogenetic Protein receptor (BMPR) type 1 B (ALK-6) as a candidate site for the mutation. The BMPR1B gene in the human is located at the region linked with the Booroola mutation, syntenic to chromosome 6 in the sheep. A fragment of the sheep BMPR1B gene was cloned from an ovarian cDNA and the deduced aminoacid (AA) sequence is over 98% homologous to the known mammalian sequences. cDNA and genomic DNA from 20 Booroola genotypes were screened and two point mutation were found in the kinase domain of the receptor, one at base 746 of the coding region (A in the ++ to a G in FF animals) which results in a change from a glutamine in the wild type to a arginine in the Booroola animals. Another point mutation was identified at position 1113, (C to A) but this mutation does not change the coding aminoacid. The first mutation was confirmed in genomic DNA from 10 ewes from an independent Brazilian flock which segregates the Booroola phenotype. In all instances homozygous FecB gene carrier (n=11) had only the 746 A to G mutation, non gene carriers (n=14) had only the wild type sequence and heterozygote gene carriers (n=5) had both sequences. This mutation in the subdomain 3 of the kinase domain could result in an alteration in the expression and/or phosphorylation of SMADs, resulting in the phenotype characteristic of the Booroola animals which is the 'precocious' development of a large number of small antral follicles resulting in increased ovulation rate.

To analyze the consequences of the absence of GH receptor (GHR) and GH-binding protein (GHBP) on female reproductive function, we used a mouse model in which the GHR/GHBP gene has been disrupted by homologous recombination. The major effect on reproductive function seen in GHR/GHBP knockout (KO) compared with wild-type animals is a dramatic decrease in litter size; this defect is due to a reduction of the ovulation rate. The ovulatory response to exogenous gonadotropin treatment is also 3-fold reduced in GHR/GHBP KO compared with the wild-type ovaries. These results establish that the reduced rate of ovulation is essentially due to an ovarian defect rather than a deficiency in pituitary gonadotropins. The number of follicles per ovary is markedly reduced, although all categories of follicles are represented. Interestingly, the number of healthy follicles from antral and preovulatory stages is dramatically decreased in GHR/GHBP KO in comparison with wild-type follicles. The capacity of follicles to bind LH, FSH, and IGF-I was not diminished. IGF-I treatment using micropumps is not able to rescue either fertility or ovarian responsiveness to exogenous gonadotropins, suggesting that the effect of GH is independent of IGF-I. In conclusion, these results indicate that the reduction of litter size in GHR/GHBP KO mice is the consequence of an alteration of the growth of follicles and suggest that the effects of GH effects on follicular growth are independent of IGF-I.