Genetic mapping of quantitative trait loci for meat quality and muscle metabolic traits in cattle.

A whole-genome scan was carried out in New Zealand and Australia to detect quantitative trait loci (QTL) for live animal and carcass composition traits and meat quality attributes in cattle. Backcross calves (385 heifers and 398 steers) were generated, with Jersey and Limousin backgrounds. The New Zealand cattle were reared and finished on pasture, whilst Australian cattle were reared on grass and finished on grain for at least 180 days. This paper reports on meat quality traits (tenderness measured as shear force at 4-5 ages on two muscles as well as associated traits of meat colour, pH and cooking loss) and a number of metabolic traits. For meat quality traits, 18 significant QTL (P < 0.05), located in nine linkage groups, were detected on a genome-wise basis, in combined-sire (seven QTL) or within-sire analyses (11 QTL). For metabolic traits, 11 significant QTL (P < 0.05), located in eight linkage groups, were detected on a genome-wise basis, in combined-sire (five QTL) or within-sire analyses (six QTL). BTA2 and BTA3 had QTL for both metabolic traits and meat quality traits. Six significant QTL for meat quality and metabolic traits were found at the proximal end of chromosome 2. BTA2 and BTA29 were the most common chromosomes harbouring QTL for meat quality traits; QTL for improved tenderness were associated with Limousin-derived and Jersey-derived alleles on these two chromosomes, respectively.

Limousin myostatin F94L variant affects semitendinosus tenderness.

Texture parameters (peak force and compression), muscle myofibre diameter, and hydroxyproline were measured in semitendinosus samples from a cattle gene-mapping herd. The data were analysed to determine the relationships between these traits. The traits were also mapped by genetic linkage analysis to identify quantitative trait loci, and hence, candidate genes for these traits. Neither texture parameters were affected by the muscle structural traits of myofibre diameter or collagen content (as measured by hydroxyproline), despite significant variation in these traits between animals. QTL for the texture parameters of peak force and compression, as well as collagen content, were found on cattle chromosome 2 (BTA2) and attributed to the myostatin gene. Within the cattle population used for the QTL mapping, a gene variant of myostatin, F94L, has been previously shown to increase muscle mass, predominantly in the semitendinosus. It was determined herein that the F94L myostatin homozygous animals had more tender meat as measured by both peak force and compression. The variant was also responsible for a reduction in the collagen/elastin content of muscle. The myostatin F94L variant had no effect on muscle myofibre diameter of the semitendinosus, even though the variant causes substantial increases in muscle mass. Consequently, the increase in muscle mass of the variant must be due to myofibre hyperplasia and not hypertrophy. In addition, myostatin effects on tenderness are caused by changes in the extracellular matrix rather than muscle myofibre diameter.

Vitamin A and marbling attributes: Intramuscular fat hyperplasia effects in cattle.

Twenty Angus steers were fed a diet low in ß-carotene and vitamin A for 10months. Ten steers were supplemented with vitamin A weekly, while the other ten steers did not receive any additional vitamin A. The results demonstrated that the restriction of vitamin A intake increased intramuscular fat (IMF) by 46%. This was a function of the total number of marbling flecks increasing by 22% and the average marbling fleck size increasing by 14%. Vitamin A restriction resulted in marbling flecks that were less branched (22%) and slightly more round (4%) with an increased minor axis length (7%). However, restricting vitamin A did not affect the size of the intramuscular or subcutaneous adipocyte cells or the subcutaneous fat depth. The results suggest that vitamin A affects the amount of marbling and other attributes of the marbling flecks due to hyperplasia rather than hypertrophy. This may explain why vitamin A restriction specifically affects IMF rather than subcutaneous fat deposition.

Effect of low vitamin A status on fat deposition and fatty acid desaturation in beef cattle.

A group of Angus beef cattle was removed from temperate pastures and fed a very low beta-carotene cereal-based ration in a feedlot for over 300 d. Half the group was supplemented weekly with retinyl palmitate (at the rate of 60,000 IU vitamin A/100 live weight (LW)/day), sufficient to offset clinical vitamin A deficiency; the other half received no supplement. Blood was sampled from all animals at biweekly intervals to assess beta-carotene and vitamin A status. Adipose tissue was sampled by biopsy on three occasions throughout the experimental period and at slaughter to assess FA composition. Muscle was sampled at slaughter to determine the intramuscular fat content. The mean plasma concentration of beta-carotene of all animals fell from an initial value of 20.1 to 5.2 microg/mL at 14 d, to 1.4 microg/mL at 35 d, and to zero at 105 d. Mean vitamin A in plasma was not significantly different between the treatment groups initially. The values then rose to almost twice their initial values by 35 d, but subsequently fell to below initial values by day 119. Thereafter, plasma vitamin A of the supplemented group was significantly greater than that of the unsupplemented group (P < 0.05). Muscle samples at slaughter from supplemented animals contained significantly (P < 0.01) more intramuscular lipid (13.0 vs. 9.6%). Major changes occurred over time in FA composition in both groups. Saturated FA decreased as monounsaturated FA increased over the first 60 d. An index of desaturation of FA was significantly lower (P < 0.001) in the vitamin A-supplemented group than in the nonsupplemented group. M.P. of the adipose tissue of nonsupplemented animals was 32.3 degrees C, significantly less (P< 0.05) than that of supplemented animals (34.1 degrees C). Feeding vitamin A was associated with less intramuscular fat but with a less desirable (less unsaturated, more solid) FA profile.

Differences in delta9 desaturase activity between Jersey- and Limousin-sired cattle.

An experiment examined delta9 desaturase activity and FA composition in subcutaneous adipose tissue in two differing breeds of cattle. Jersey-sired cattle had significantly higher rates of desaturase activity than Limousin-sired cattle (1.55 vs. 0.75 nmol/mg protein/min). This difference was also demonstrated by a lower concentration of individual (e.g., 18:0) and total saturated FA (38.3 vs. 45.1 wt%), and a higher concentration of individual (e.g., 16:1) and total monounsaturated FA (58.2 vs. 52.7 wt%) in the Jersey animals. Other indices of desaturation calculated from the FA composition showed this same difference. The slip point of adipose tissue of Jersey cattle (36.8 degrees C) was significantly lower than that of Limousin cattle (39.2 degrees C), but Jersey adipose tissue had a greater content of beta-carotene. The positive relationship between adipose tissue beta-carotene and desaturation opposes the negative relationship between dietary beta-carotene and desaturation determined elsewhere. These results, however, lead to the hypothesis that some cattle have a reduced capacity to metabolize beta-carotene to various forms of vitamin A, a compound that can reduce delta9 desaturase enzyme activity. In addition, the higher level of intramuscular fat in Jersey cattle (6.97 vs. 3.82%) is possibly related to a lack of inhibition of the adipocyte differentiation genes by vitamin A.

beta-carotene and oxidative desaturation of fatty acids: a plausible explanation of the conflicting responses of coronary heart disease to beta-carotene?

Studies of the desaturation of saturated fatty acids in animals may help explain conflicting reports of the response of coronary heart disease (CHD) to beta-carotene in humans. A negative relationship exists between desaturation and adipose beta-carotene in cattle when they consume different quantities of beta-carotene. Opposing this finding, however, is a positive relationship between desaturation and adipose beta-carotene when cattle are fed the same quantity of beta-carotene. The reason for this apparent contradiction appears to be due to differences in consumption, or variability in the metabolism of beta-carotene. Animals that efficiently metabolize beta-carotene to vitamin A have low desaturation but high antioxidant potential. These results in animals show some similarity between the consumption of the antioxidant beta-carotene and the risk of coronary heart disease where the oxidation of low-density lipoproteins (LDL) is believed to play a role in the development of atherosclerotic plaque. Genetic differences in carotenoid metabolism in humans, similar to those in animals, would assist in explaining differences in lipoprotein oxidation in humans and variation in the risk of coronary heart disease.

Genotypic effects of calpain 1 and calpastatin on the tenderness of cooked M. longissimus dorsi steaks from Jersey x Limousin, Angus and Hereford-cross cattle.

Single nucleotide polymorphisms (SNPs) in the calpain 1 (CAPN1) and calpastatin (CAST) genes were studied to determine their effects on meat tenderness in Bos taurus cattle. Strip loins (M. longissimus dorsi) were removed from cattle in four resource populations after slaughter (n = 1042), aged under controlled conditions until fixed times after rigor mortis, cooked and measured using a tenderometer. Animals were genotyped for the CAPN1 SNP c.947C>G (p.Ala316Gly; AF252504) and for the CAST SNP c.2959A>G (AF159246). Frequencies of CAPN1 C alleles ranged from 23% to 68%, and CAST A alleles from 84% to 99.5%. From all data combined, the CAPN1 CC genotype (compared with the GG genotype) was associated with a 20.1 +/- 1.7% reduced average shear force at intermediate stages of ageing (P < 0.001) and with a 9.5 +/- 1.3% reduction near ultimate tenderness (P < 0.001). The heterozygote was intermediate. For CAST, corresponding values for AA compared with AG genotypes were reductions of 8.6 +/- 2.0% and 5.1 +/- 1.6% respectively (both P < 0.001), but there were too few GG genotypes for comparison. There were small interactions between the CAPN1 and CAST genotypes. For the CAPN1 and CAST genotypes combined, the maximal genotype effect in average shear force was 25.7 +/- 5.5% (P < 0.001) at intermediate stages and 15.2 +/- 4.8% near ultimate tenderness (P < 0.01).