The effects of a mutation in the myostatin gene on meat and carcass quality
Introduction
The myostatin gene (also known as GDF-8) regulates development of muscle and inactivation of this gene or the gene products results in extended muscular development. The effects of the gene were first described in mice, where loss of myostatin expression in knock-out mice was associated with both an increase in the number of muscle fibres (hyperplasia) and an increase in fiber size (hypertrophy) (McPherron, Lawler, & Lee, 1997). Later, an extreme form of muscularity (“double muscling”) seen in the Belgian Blue and Piedmontese cattle breeds was shown to result from mutations in the coding region of the myostatin gene (Grobet et al., 1997, Kambadur et al., 1997, McPherron and Lee, 1997). In the case of the Belgian Blue, the mutation is an 11-bp deletion in the third exon, while the Piedmontese carries a point mutation in the same exon. Other heavily muscled cattle breeds have also been shown to carry polymorphisms in the myostatin gene (Dunner et al., 2003, Grobet et al., 1998), lending support to this being the causative gene for the “double muscled” phenotype. Myostatin is a negative growth factor that inhibits both the terminal differentiation of myoblasts (Langley et al., 2002, Rios et al., 2002) and the proliferation of myogenic cells (Thomas et al., 2000). The myostatin gene has been shown to influence muscle metabolism and gene expression (Cassar-Malek et al., 2007, Hocquette et al., 2007, Hocquette et al., 1998, Potts et al., 2003). In cattle, the effects of myostatin are seen during foetal development, where the loss of myostatin function affects muscle fibre development and results in hyperplasia (Deveaux, Picard, Bouley, & Cassar-Malek, 2003). Double-muscled cattle have been reported to have substantially greater numbers of muscle fibres than non-double-muscled animals at birth (Gerrard & Judge, 1993) and at slaughter, a greater proportion of muscle and lower proportions of bone and fat (Ansay and Hanset, 1979, Arthur, 1995, Clinquart et al., 1998, Ménissier, 1982).
An earlier report showed that the South Devon cattle breed carries, at moderately high frequency, the 11-bp deletion first reported in the Belgian Blue (Smith, Lewis, Wiener, & Williams, 2000). It was subsequently shown that this allele is associated with increased muscling, calving difficulty, and decreased fat depth in this breed (Wiener, Smith, Lewis, Woolliams, & Williams, 2002). These observations were in agreement with those of others on the effects of myostatin mutations in different breeds. Alleles associated with increased muscling have also been associated with variation in other traits, including collagen content, meat colour, energy metabolism and hormone levels (Bellinge et al., 2005, Hocquette et al., 1998). These characteristics may affect the quality of meat produced from animals carrying different myostatin alleles (Aldai et al., 2006, Oliván et al., 2004), although there have been conflicting reports on the effects of double muscling on meat tenderness (Arthur, 1995).
Consumers define meat quality initially in terms of appearance, particularly colour and visible fat, to guide selection at purchase, and then, at the time of consumption, primarily by texture and flavour (Colmenero, 1996, Neely et al., 1998). However, consumers are now also considering health implication of meat composition, e.g. poly-unsaturated versus saturated fat, in purchase choices. In addition to effects of nutrition, animal management and carcass treatment on carcass and meat quality traits, genetic factors also play a significant role (Wheeler, Cundiff, Shackelford, & Koohmaraie, 2004). Among the genetic factors, the full impact of myostatin genotype on meat-related traits requires further clarification.
The quality of meat from double-muscled breeds has been compared with meat produced from normal cattle of other breeds, however, in these cases, the observed variation may be an effect of differences in genetic background rather than of myostatin genotype. Few studies have compared phenotypes of animals from the same breed with different myostatin genotypes. The current study examined variation in growth rate, carcass characteristics, and meat quality from British South Devon cattle with different myostatin genotypes. This comparison allowed us to directly investigate the effect of the myostatin gene on traits of economic importance within the same genetic background.
Section snippets
Study population
Most of the animals in the study population came from 12 farms where blood samples of calves and parents as well as detailed calving records were collected. Additional blood samples were collected at abattoirs at the time of slaughter. Animals were managed, recorded and slaughtered according to commercial farm practices.
Calving and growth data
Data on calving included sex, weight at birth, calving score (1–5, increasing with the difficulty of the calving), whether the calf was a twin, and if the animals died at birth.
Results
A summary of the effects of myostatin gentoype for traits other than growth is shown in Table 1. Additive, dominance and substitution effects are given in Table 2.
Discussion
The results presented here suggest that the myostatin allele with the 11-bp deletion (MH) segregating in the South Devon breed affects several traits related to beef production. Some of the effects could be considered as advantageous from the point of view of the producer, while others are disadvantageous. In the present study the MH allele was associated with heavier calves at birth but slower growth, leading to lighter adult animals. However, although adult MH/MH animals had a lower
Acknowledgements
We are very grateful for the enthusiastic participation of the South Devon Herd Book Society without whom this project could not have been completed. Particular thanks go to Caroline Poultney and the other staff at the Society. We also acknowledge assistance from Tim King (Roslin), Victoria Bingham (Roslin), Kevin Gibson (Bristol), Duncan Marriott (Bristol), Ann Baker (Bristol), Kim Matthews (EBLEX Ltd.) and the following participating abattoirs: Jaspers Treburley Ltd., ABP York, St. Merryn
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