Application of myostatin in sheep breeding programs: A review

Myostatin or growth and differentiation factor 8 (GDF8), has been known as the factor causing double muscling phenotypes in which a series of mutations make the myostatin protein inactive, hence disabling it to regulate the deposition of muscle fibre. This phenotype happens with high frequency in a breed of sheep known as the Texel. Quantitative trait loci (QTL) studies show that a portion of the OAR2 that encompasses GDF8 has a major effect on muscular growth of Belgian Texel, on the muscling and fat depth in New Zealand Texel sires, and UK Texel and Charollais sheep. The functional polymorphism resides inside the GDF8 non-coding region. To date, there have been studies showing biallelic SNPs with significantly different allelic frequencies between hyper-muscled Texel and control animals including one in the 3'UTR  (g.+6223G>A) and one in 2.5 kb upstream from the GDF8 transcription start site (g.-2449G>C). The GDF8 allele of the Texel sheep is characterized by one G to A transition in the 3'UTR, creating a target site for mir1 and mir206 which are highly expressed in skeletal muscles. This prevents myostatin gene translation, thus contributing to the double muscling of Texel sheep. Therefore, the GDF8 g.+6223A allele seems to be a causative variable increasing muscularity in the Texel rams and could be identified as a quantitative trait nucleotide.