Investigation of polymorphic variation in the growth differentiation factor-9 gene (GDF9) in New Zealand sheep breeds

Litter size is an important factor that can improve sheep production. There are many genes that have been investigated for their association with litter size, including the Booroola gene and Inverdale gene. GDF9 is a protein that is produced in the ovaries and regulates folliculogenesis, but has yet to be widely investigated for use in sheep breeding programmes in New Zealand. This study will therefore look for polymorphic variation in GDF9 in sheep breeds found in New Zealand. Blood samples were taken from 16 sheep breeds from a range of sheep farms and collected on FTA cards. The breeds included Romney, Suffolk and Merino, which are found commonly throughout New Zealand. PCR-SSCP analysis was use to genotype a portion of the ovine GDF9 gene. Each of the variants found were also DNA sequenced. Two variants of GDF9 were found using PCR-SSCP analysis. From the sequencing results it was found that one variant contained the variations previously described as G5 and G6 and the other did not. G5 is an alteration from the sequence in GenBank (accession number AF078545.2) involving the substitution A978G and G6 describes the substitution G994A. G6 results in the change of valine to isoleucine in the protein sequence. In all samples investigated in this study G5 and G6 always occurred together if they were present at all. There was variation in frequency of the two variants of GDF9 in the Corriedale and South Down breeds compared with the other breeds investigated. These two breeds were also significantly different from each other. Based on findings from other research papers to date, neither of the polymorphisms have a known effect on litter size. Further studies need to be carried out on the rest of the GDF9 sequence in New Zealand sheep breeds to determine if they are carrying any of the other known, or possibly unknown, polymorphisms in GDF9, which may influence fecundity and litter size