Exploring innovative ways, based on skin thickness and brown adipose tissue metabolism, for genetic improvement of new-born lamb survival : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Palmerston North, New Zealand
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2023
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Massey University
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Abstract
A major cause of new-born lamb mortality under pastoral conditions is starvation/exposure. Heritability for lamb survival is low and indirect selection for a trait that is both correlated with lamb survival and is heritable, may provide a more efficacious way to improve lamb survival. In addition to selecting for a lamb that can withstand cold environments, the ability to produce heat via non-shivering thermogenesis through brown adipose tissue (BAT) is essential. Consequently, the main aims of this thesis were to investigate the potential selection for skin thickness and temperature to indirectly improve lamb survival, to examine their interactions with other traits of importance, and to characterise the transcriptome and lipidome of BAT in new-born lambs exposed to a cold environment.
Heritability estimates were moderate for skin thickness and low for skin temperature when considering all sheep data from the FocusPrime, Texel, Romney and Highlander breeds. Positive and favourable correlations between skin thickness and other known insulation traits such as fat depth were found, as well as negative and favourable correlations between skin thickness and skin temperature. Therefore, selecting for thicker skinned lambs could enhance the lamb’s insulation characteristics and survival.
A short-term cold challenge in new-born lambs was carried out to perform a ribonucleic acid sequencing (RNAseq) analysis to characterize the transcriptome of BAT and thyroid tissue. There was no evidence of thermogenic activity from any of the key thermogenic genes, such as UCP1 (uncoupling protein 1) or any of the thyroid receptors. This may suggest that the heat production peak under cold exposure occurs swiftly and thus results in being undetectable in BAT by day three of life. These changes in expression might give way to the whitening process of the adipose tissue, concluding the non-shivering thermogenesis period.
Within the same experimental short-term cold challenge, BAT and plasma samples were collected to identify lipidomic profiles. Profound changes were found after cold exposure, where significant increases in the lipid composition of glycerolipids, glycerophospholipids, sphingolipids and sterol lipids were observed. In some cases, an exponential increase of certain metabolites was recorded through the time of exposure, implying that their increase may be generated as a consequence of BAT activity. The overall differences in lipid types found could be associated with the improvement of lipid metabolism via BAT thermogenic activation, and adipocyte survival during cold adaptation.
In summary, this thesis suggests an alternative pathway to improve lamb survival and provides useful insights into molecular mechanisms and lipidomic composition of thermogenic tissues in new-born lambs.
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Figure 2.1 is reproduced with permission.
Keywords
Lambs, Genetics, Mortality, Effect of cold on, New Zealand