The physiological basis of the widely differing growth rates between broiler and layer chickens is still poorly understood. Recurrent findings have included differences in rates of protein breakdown, especially in young birds (1.2- fold to 10- fold higher in layers e.g. Muramatsu et al., 1987; Flannery and Beynon, 1991). Ubiquitin is abundant in many tissues including muscle. It has important functions including the tagging of proteins for degradation by ATP-dependent and lysosomal pathways, and a role in cell surface receptor down-regulation; transcription of its mRNA is increased in many catabolic states in mammals (Attaix et al., 1994; Strous et al., 1996). The aim of this experiment was to test the hypothesis that differing regulation of ubiquitin pathways might be responsible in part for the divergence of growth rates seen in chicken breeds.

Week-old male broiler (Ross No.l) and layer (ISAbrown) chicks were randomly assigned to cages, as pairs of the same breed. Chicks had free access to water and a standard crumb starter diet. Birds were slaughtered at four weeks and rapidly dissected. Tissues were snap-frozen in liquid nitrogen and stored at -70°C until use.