Heavily-selected livestock production traits rarely come without compromise; altered
physiology arising from intensive selection often gives rise to concern of a welfare
trade-off. A particularly clear example of welfare challenge caused by genetic
selection in chickens is the ‘broiler-breeder paradox’, wherein breeding populations of
broiler-type birds selected for fast growth are feed-restricted in order to reduce growth
and maintain reproductive viability at sexual maturity. In order to better-inform
management and breeding strategies for alleviating reproductive problems resulting
from genetic selection for growth, it is essential to develop a better understanding of
the physiological processes underpinning growth. Whereas the molecular
mechanisms governing energy balance in mammals have been relatively welldescribed,
analogous avian systems have not received as much research attention
and remain somewhat poorly understood. The broad aim of this doctoral project was
to contribute to understanding of avian energy balance, particularly in the context of
selection for high growth.
Using an advanced broiler-layer intercross chicken line (AIL), high- and low-growth
haplotypes at the locus encoding the cholecystokinin A receptor (CCKAR), underlying
the most significant QTL for growth in chickens, were characterised. Of over 300
variations detected, a select panel spaced across the CCKAR locus were tested for
prediction of bodyweight in a diverse cohort of chicken populations. One intronic SNP
was found to be significant (p<0.05) and proximal to transcription factor binding sites.
The effect of this locus on gross bodyweight remained significant into the 20th AIL
generation (~20% at 10wk, p<0.05). In this otherwise effectively genetically
homogeneous population, several specific physiological traits were predicted by
CCKAR haplotype alone, yielding some clues as to the significance of perturbed
cholecystokinin (CCK) signalling in broiler strains. While birds with high-growth
CCKAR haplotype (HG) did not appear to consume more, feed conversion efficiency
(FCE) was improved, at least for males, compared to low-growth (LG) (p<0.05).
Visceral organ anatomies were morphologically disparate, with HG individuals
exhibiting ~1/3 less gallbladder mass (p<0.01), and ~10% shorter GI tract (p<0.01)
and metatarsal bone (p<0.05).
Further gaps in knowledge of the expression of peripheral satiety hormones in chicken
are addressed in this thesis. Tissue distributions for expression of CCK, gastrin,
pancreatic polypeptide (PPY) and peptide YY (PYY), were mapped and their
respective dynamic responses to nutritive state examined. CCK was found to be most
highly expressed in the brain, whereas PYY, PPY and gastrin were far more abundant
in distinct regions of the periphery. Interestingly, peripheral CCK was not responsive
to short-term (<10h) satiety in experimental populations where PYY and gastrin were.
PYY expression was found to be greatest in the pancreas and consistently
upregulated within hours after feeding (p<0.01), whereas gastrin expression was
confined to the gastric antrum and paradoxically highest in fasting birds (p<0.01).
PPY expression is strictly limited to the pancreas and appears dependent on longerterm
energy state. These results highlight similarities and differences to mammalian
systems; notably, the avian pancreas seems to fulfil an exceptional role as a site of
signal integration, perhaps unsurprising considering its disproportionate size
compared to mammals. Indeed, pancreatic PYY appears to act as a primary
peripheral short-term satiety hormone in birds.
This body of work contributes to the understanding of avian energy balance and
growth. An invaluable foundation for future research is formed by the identification of
the major locations of production, and basic nutrient-responsive trends, for several
peripheral avian hormones. Information on the growth role of CCKAR is consolidated
and expanded upon, demonstrating a clear genetic contribution to maintenance organ
morphology and overall growth. Such knowledge can be used to reliably assess and
advise on selection and management of chickens to stem welfare concerns without
compromising production. Comparisons between avian and other vertebrate
endocrine systems make for interesting insight into the adaptive role of energy
homeostatic mechanisms in divergent evolution of mammals and non-mammalian
vertebrates. In some aspects, birds might better represent the ancestral phenotype
from which each vertebrate clade arose
