Evaluating the effects of the c.*1232G > A mutation and TM-QTL in Texel × Welsh Mountain lambs using ultrasound and video image analyses

Abstract

The c.*1232G > A (MM) mutation in the myostatin gene and TM-QTL are two muscle-enhancing polymorphisms found on Chr 2 and Chr 18, respectively, in Texel sheep. Previous studies on purebred lambs and crossbred lambs out of Mule ewes showed that MM was associated with increased muscle mass and decreased fat content whereas heterozygous TM-QTL lambs had increased loin muscle (M. longissimus lumborum, MLL) depth as measured by ultrasound and increased MLL weight. Welsh Mountain ewes, like Mule ewes, contribute significantly to UK slaughter lamb production from the UK sheep industry's stratified structure. Therefore, this study aimed to evaluate the direct effects of MM and TM-QTL in the genetic background of Texel×Welsh Mountain lambs.Four Texel sires, all MM carriers and two TM-QTL carriers, were used to inseminate Welsh Mountain ewes. Of the 175 lambs born, 130 could be classified for TM-QTL genotype [107 non-carriers (+/+), 23 heterozygous carriers (TM/+)] and 172 for MM genotype [15 non-carriers (+/+), 129 heterozygous carriers (MM/+), 28 homozygous carriers (MM/MM)]. All lambs were weighed at 8 and ∼23weeks of age and were ultrasound scanned before being slaughtered at ∼24weeks of age. All carcasses were graded for conformation and fat class, based on Meat and Livestock Commission (MLC) classification scores, and then scanned using a video image analysis (VIA) system to predict weight of carcass primal cuts and trimmed primal cuts, muscularity traits and carcass shape traits.Carrying two copies of MM was associated with a significant positive effect on 8 weeks weight, a negative effect on ultrasound fat depth and a substantial decrease in MLC-fat score. Two copies of MM also had a strong positive impact on VIA-estimated weight of the hind leg, chump and loin primal cuts, as well as the muscularity of the hind leg and loin regions with greater loin muscle width, depth and area. Homozygous MM carrier lambs had significantly wider carcasses across the shoulders, breast and hind legs and greater areas of the back view of the carcass when measured by VIA. Leg and carcass compactness were also significantly associated with the presence of two copies of MM. However, lambs carrying a single copy of MM were not significantly different to non-carriers in any of the traits measured. TM-QTL significantly increased ultrasound muscle depth and TM-QTL carriers had significantly greater loin muscle width and area measurements. Carcass weight did not differ significantly between genotypic classes for MM or TM-QTL. These results agree in general with other reports of the effects of MM and TM-QTL, in both direction and magnitude, on carcass traits.