Mutation in bone morphogenetic protein receptor-IB is associated with increased ovulation rate in Booroola Merino ewes

Ewes from the Booroola strain of Australian Merino sheep are characterized by high ovulation rate and litter size. This phenotype is due to the action of the FecB(B) allele of a major gene named FecB, as determined by statist ical analysis of phenotypic data. By genetic analysis of 31 informative hal f-sib families from heterozygous sires, we showed that the FecB locus is si tuated in the region of ovine chromosome 6 corresponding to the human chrom osome 4q22-23 that contains the bone morphogenetic protein receptor in (BMP R-IB) gene encoding a member of the transforming growth factor-beta (TGF-be ta) receptor family. A nonconservative substitution (Q249R) in the BMPR-IB coding sequence was found to be associated fully with the hyperprolificacy phenotype of Booroola ewes, In vitro, ovarian granulosa cells from FecB(B)/ FecB(B) ewes were less responsive than granulosa cells from FecB(+)/FecB(+) ewes to the inhibitory effect on steroidogenesis of GDF-5 and BMP-C natura l ligands of BMPR-IB, It is suggested that in FecB(B) / FecB(B) ewes, BMPR- IB would be inactivated partially, leading to an advanced differentiation o f granulosa cells and an advanced maturation of ovulatory follicles.