Mitochondrial metabolism: a driver of energy utilisation and product quality?

High feed efficiency is a very desirable production trait as it positively influences resource utilisation, profitability and environmental considerations, albeit at the possible expense of product quality. The modern broiler is arguably the most illustrative model species as it has been transformed over the past half century into an elite feed converter. Some producers are currently reporting that 42-day-old birds gain 1 kg of wet weight for every 1.35 kg of dry weight consumed. Its large breast muscle is exclusively composed of large, low mitochondrial-content Type IIB fibres, which may contribute to low maintenance costs and high efficiency. In an effort to gain a better understanding of individual variation in chicken feed efficiency, our group has been exploring the biology of the mitochondrion at multiple levels of organisation. The mitochondrion is the organelle where much biochemical energy transformation occurs in the cell. Using Cobb-Vantress industrial birds as our primary experimental resource, we have explored the tissue content, structure and function of the mitochondrion and its relationship to growth, development, efficiency and genetic background. While much remains to be understood, recent highlights include (1) variation in muscle mitochondrial content that is associated with performance phenotypes, (2) altered muscle mitochondrial gene and protein expression in birds differing in feed efficiency, (3) variation in isolated mitochondrial function in birds differing in feed efficiency and (4) evidence for an unexpected role for the mitochondrially localised progesterone receptor in altering bird muscle metabolism. Mitochondrial function is largely conserved across the vertebrates, so the same metabolic principles appear to apply to the major production species, whether monogastric or ruminant. A speculative role for the mitochondria in aspects of meat quality and in influencing postmortem anaerobic metabolism will conclude the manuscript.