Abstract:
The Myostatin (MSTN) gene has been linked to the double muscling phenomenon, in which a
succession of mutations rendersthe gene inactive and unable to regulate muscle fibre deposition effectively.
This study aimed to assess impact of the MSTN variants and their combinations, in the South African (SA)
Bonsmara and Drakensberger breeds, on reproduction (age at first calving (AFC), inter-calving period
(ICP), scrotal circumference (SC), longevity), growth traits (direct birth weight (BWDIR), direct weaning
weight (WWDIR), average daily gain (ADG), feed conversion ratio (FCR) and carcass traits based on realtime ultrasound (RTU) measurements (fat, marbling, eye muscle area (EMA)). Genomically enhanced
estimated breeding value (GEBVs) and MTSN genotypes for SA Bonsmara and Drakensberger animals
were available. Thirteen MSTN variants were genotyped using the IDBv3 SNP array. This study was
divided into a phase 1 and phase 2. In Phase 1, three MSTN variants (Nt821, F94L and Q204X) were
observed in 355 animals genotyped across the Bonsmara, Beefmaster, Brangus, Drakensberger and
Limousin breeds and genotypic and allelic frequencies were estimated. In the Limousin population, the
F94L variant wasfixated. The Nt821 and Q204X variants were observed at frequenciesranging from 0.00%
to 46.00% in the remaining breeds. In the association study (phase 2), only Bonsmara and Drakensberger
genotypes were included. Genotypic frequencies of MSTN variants ranged from 1.18% for Q204X to
35.02% for Nt748 in homozygous affected animals in Bonsmara cattle and 2.32% for Nt821 to 12.01% for
Nt748 in Drakensberger cattle. Association analysis indicated no significant association (p > 0.05) between
the different variants and reproduction traits (AFC, ICP and longevity), FCR or fat in the Drakensberger
population. All MSTN variants in the Bonsmara population were significantly associated with longevity (p
< 0.05). In addition, the Q204X variant was significantly associated with all reproduction and growth traits
in the Bonsmara population (p < 0.05). The results of the combined genotypes indicated that there was an
additive effect when more than one MSTN variant was present. This is the first study to report the impact
of MSTN variants on reproduction, growth, and RTU measurements in SA Bonsmara and Drakensberger
breeds.
