The effect of TN-QTLl and other QTLs on lean meat yield and meat quality in sheep and its evaluation using visa

This was an SAC-led project, to which Roslin contributed genetic expertise. The project had two main themes of (i) Evaluation of direct and indirect effects of 3 muscling quantitative trait loci (QTL)/genes (ii) Evaluation of video image analysis (VIA) and its use to measure the effects of the muscling genes/QTL. General activities and outcomes were as follows.
(i) Evaluation of direct and indirect effects of 3 muscling QTL/genes
(i-a) TM-QTL: This QTL on chromosome 18, originally identified in purebred UK Texel sheep, had been reported to increase UMD muscle depth by around 4% to 7%. Muscling in other parts of the body had not been investigated.
• The QTL increased loin muscling by 4-11% in heterozygous animals (crossbreds out of Mule ewes and purebred Texels) when they inherit the mutation from the ram.
• Phenotypic effects on muscling were largely restricted to the loin area, with only minor effects on other parts of the carcass.
• TM-QTL appeared to show polar overdominance for muscling traits, implying that only heterozygous animals inheriting the favourable allele from the ram showed increased muscling.
• Homozygous lambs had increased body weights (from birth to slaughter by ~7 to 8%) and higher carcass weights (by 9% or 1.4kg). Unlike the muscling effects, body growth seemed to show an additive mode of inheritance.
• There was a small but significant increase in shear force (a measure of MQ) at meat ageing A. The mutation is located on chromosome 2 in the region of the myostatin gene. Polymorphisms in this gene in several other mammalian species result in “double-muscling”. However, from the results of studies carried out in New Zealand, the effects of this ovine mutation seem small to moderate. Phenotypic effects of this QTL were investigated in crossbred lambs out of Mule and Welsh Mountain ewes.
• In contrast to TM-QTL and LM-QTL, the phenotypic muscling effects of MM-QTL are not confined to the loin area. In general, lean tissue growth is increased and fat growth is reduced. This mutation seems to act mostly in a partial recessive way, indicating that heterozygous animals are closer to the homozygous wildtype animals and larger effects occur only in homozygous carrier animals.
• Heterozygous carriers have higher lean tissue (by ~3%) and muscle areas (2 to 5%) and total fat weight is decreased (by 6-16%). Increased CT-measured muscle densities in heterozygous lambs have been observed which is indicative of lower intramuscular fat content.
• Carrying two copies of MM was associated with a significant positive effect on 8 week weight, a negative effect on UFD and a substantial decrease in MLC fat class. Two copies of MM-QTL 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, and loin muscle width, depth and area.

(ii) Evaluation of VIA and its use to measure the effects of the muscling genes/QTL
(ii-a) Evaluation of VIA
• Previous work closely related to this project had shown that VIA, under abattoir conditions and at line speed (800 lambs/h), improved the prediction of primal meat yields compared to the current MLC EUROP carcass classification.
• VIA has been calibrated in this study against dissection. It was shown that VIA accurately and precisely reflected the value of sheep carcasses: the R2 values for predicting leg, chump, loin, breast and shoulder primal joints, and total of the primals, were 0.97, 0.94, 0.91, 0.88, 0.96 and 0.99, respectively.
• Dataset of >7000 animals with pedigree information and 630 with repeated VIA measures from this project were used to estimate genetic parameters (heritabilities values and genetic correlations) for VIA-based predictions of primal weights. The repeatability for VIA traits was high (> 0.9) and h2 values were low to moderate (0.08 to 0.26). High repeatability estimates of VIA traits and moderate heritabilities of the most valuable carcass joints suggest that including VIA information in breeding programs would be useful in order to improve carcass quality.
• Using the same dataset it was shown that the h2 values for subjective characters (fat class, conformation) were low (h2 = 0.1). This is likely to be a reflection of the subjective nature of this assessment.
• VIA based dimensional carcass measurements showed moderate to high h2 values (0.20-0.53), suggesting that using this VIA dimensional information in the evaluation of purebred terminal sire breeds is likely to improve conformation on crossbred lambs.

(ii-b) Capability of VIA to measure phenotypic effects of the muscling QTLs/genes
• Although VIA gave accurate assessments of overall carcass yield, it was unable to reliably identify individual animals carrying the QTL investigated. However, when scaled up to the population level this lack of fine-scale precision effect will reduce, as animals with better carcasses will (on average) be given better assessments.

Genetically improving the quality of lamb carcasses, and capturing these benefits so that the lamb producers are appropriately rewarded, is a different technical problem with interesting underlying science. In this SAC-led project we evaluated three genetic loci that contribute to increased carcass quality (i.e. more meat) and investigated their genetic control. As well as precisely quantifying their benefits, we demonstrated that the primary target of the study (a quantitative trait locus on chromosome 181 in Texel sheep known as TM-QTL) shows an unusual form of imprinting: it is only expressed if the lamb inherits the favourable allele from the sire and not from the dam. This is scientifically intriguing and may have wide-ranging implications for our understanding of complex traits. Additionally, we refined and calibrated a video image analysis (VIA) system of lamb carcass classification, which is designed to be implemented directly within abattoirs. This has immense practical benefit for the meat industry, enabling objective assessment of carcasses in the abattoir. Potentially it leads to fairer payment to farmers, providing the incentive for them to produce improved carcasses, and also it will enable faster genetic progress in the sheep industry.

We showed that a quantitative trait locus (QTL), observed in the Texel breed, for increased muscling has a large impact on the quantity of meat produced by the animal, e.g. ca. 10% increase in highly priced part of the carcass. Further, we did not observed any major negative impacts on eating quality, a feature sometimes seen with other similar major gene effects for muscling in sheep. This QTL is termed the TM-QTL.



We demonstrated that the TM-QTL exhibits a form of imprinting known as paternal polar overdominance, in which for the trait improvements to be expressed the favourable allele must be inherited from the sire and not from the dam. This mechanism underlying this unusual mode of inheritance is of fundamental scientific interest, as understanding this would lead to a greater understanding of the genetic control of many complex trait. This pattern of inheritance makes it ideal for exploitation in a terminal sire breed which is to be crossed with a maternal breed lacking this QTL: slaughter progeny of homozygous sires would always express this phenotype.



We also quantified the phenotypic effects of a similar QTL found in the Poll Dorset breed (LM-QTL), which maps to a similar position in the genome. It is not known yet whether this QTL is the same or allelic to the TM-QTL. Additionally, we quantified the benefits of a third muscling QTL, found on chromosome 2 in Texel sheep, for which the causative mutation (a micro-RNA mutation affecting myostatin expression) has been identified. This QTL has been demonstrated to be at high frequency in Texel sheep, moderate frequency in Charollais and apparently absent in Suffolks. Further, it can be immediately exploited by the UK sheep industry, and indeed is currently being exploited.



A second and major class of outcome from this project was a refined video image analysis (VIA) system of lamb carcass classification, which is designed to be implemented directly within abattoirs. This was developed by calibrating VIA with computerised tomography (CT) and carcass dissection data. It now provides objective, accurate and precise predictions of carcass primal weights. It works under industry conditions and at slaughter line speeds and could be the backbone for a payment system that rewards farmers providing good quality carcasses. VIA has inherent electronic data capture capability which, if combined with electronic identification of individual animals, could allow numerous carcass characteristics to be derived, stored, and then matched with other performance and pedigree data to facilitate more robust genetic parameter estimates for exploitation within sheep breed improvement programs. In contrast to subjective carcass scoring of fat class and conformation, the more objective VIA-measured traits are moderate to highly heritable, promising substantial selection response when included in a breeding programme targeting improved carcass conformation.