BIOLOGY & PRODUCTION PATHWAYS FOR DESIRED PHENOTYPES

The Project aims to improve the market value and productivity of lamb through mechanistic understanding of how iron, omega-3 fatty acid and connective tissue content develop in different muscles of high productivity animals and how variation in these phenotypes influences consumer perceptions of eating quality. We aim to identify variation in narrow genomic regions in close association with genes of large effect on the mechanisms underlying these phenotypes and use this genetic variation to improve these lamb phenotypes. 

Achieving levels of iron, zinc and omega-3 fatty acids that reach official dietetic claims (i.e. a good source) has been flagged by Meat & Livestock Australia’s (MLA) human health and marketing division as a key marketing tool into the future. The current information in data bases held by MLA suggests Zn levels are adequate but that Fe and omega 3 could be improved – the improvement required is some 10-20%, in other words we believe quiet achievable. The work on connective tissue content is associated with lifting the eating quality of the topside – a large muscle in the back leg of lambs. Current Meat Standards Australia recommendations are that the topside cannot be sold as a separate cut since it fails as a grill or roast because it is tough. However as lamb carcase weight increases to around 23kg and above, boning rooms wish to not only extract the round and rump (leaving a traditional easy carve leg) but also the topside so as the remaining ‘new easy carve leg’ is of a better portion size for retail purchase. Literature would suggest strongly that high levels of connective tissue will be responsible for the toughness of the topside.

 

We propose that for mineral, omega 3 and connective tissue content we can discover genes of significant causative impact and that these genes will show genetic variation (SNP). Both the discovery and understanding of these genes will strongly help underpin the new molecular technologies for gene marker assisted selection in these difficult to measure traits. The current 50-70K SNP chips available for livestock are currently unproven and at the request of the molecular geneticists it is desirable to discover mechanistically based SNP where a clear biological pathway suggests this is feasible.

 

The new work proposed for the 2008/9 financial year will be to initiate consumer testing of loin and topside muscles (using grill protocols) from 200 lambs per year from the Information Nucleus. This data will contribute to new work on omega 3 fatty acids plus also underpin the objective meat science measures being undertaken in 3.1.

 

The second area of new work will be to develop an experiment(s) (mainly based around the Information Nucleus) to test a new series of hypotheses around regulation of omega 3 fatty acid levels in lamb. Lipid metabolism and biosynthetic pathways are common among mammals as they share the desaturases and elongases involved in fatty acid synthesis. Nutritional studies in humans plus those in monogastric and ruminant livestock have shown that there is poor conversion of precursor short-chain n-3 polyunsaturated fatty acids, such as linolenic acid (n-3 C18:3, rich in green forage), into the health claimable long-chain n-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). This poor substrate-product relationship suggests that simple nutritional manipulation of precursor fatty acids driven by grass versus grain diets later in life will be largely ineffective in determining variations in ‘claimable’ omega-3 content of muscle. Indeed we hypothesise that the majority of EPA and DHA are sequestered in phospholipid within muscle earlier in life (say up to 35kg live weight) and that the maternal/offspring diet during lactation in combination with genetic potential for both muscle and long chain omega 3 synthesis are the primary determinants of final omega 3 expression. In sheep, preliminary values for heritability of EPA and DHA content of muscle are 0.18 and 0.19, respectively. These heritability values are moderate and the early work shows clear evidence for significant sire-related variation (2 fold) in individual and total omega-3 fatty acids. Human research strongly indicates that children who carry a favourable genotype for the delta-6 desaturase gene (FADS2) showed better conversion of precursor omega 3 fats into the more biologically active EPA/DPA. Given this background we propose to investigate various aspects of the above proposal (maternal diet, muscling potential, grass versus grain finishing) as well as exploring genomic regions relating to fatty acid desaturase and elongation to underpin genetic and management manipulation of EPA/DPA in muscle.