Abstract
BackgroundResearch into energy balance and growth has infrequently considered genetic sex, yet there is sexual dimorphism for growth across the animal kingdom. We test the hypothesis that in the chicken, there is a sex difference in arcuate nucleus neuropeptide gene expression, since previous research indicates hypothalamic AGRP expression is correlated with growth potential and that males grow faster than females. Because growth has been heavily selected in some chicken lines, food restriction is necessary to improve reproductive performance and welfare, but this increases hunger. Dietary dilution has been proposed to ameliorate this undesirable effect. We aimed to distinguish the effects of gut fullness from nutritional feedback on hypothalamic gene expression and its interaction with sex.MethodsTwelve-week-old male and female fast-growing chickens were either released from restriction and fed ad libitum or a restricted diet plus 15% w/w ispaghula husk, a non-nutritive bulking agent, for 2 days. A control group remained on quantitative restriction. Hypothalamic arcuate nucleus neuropeptides were measured using real-time PCR. To confirm observed sex differences, the experiment was repeated using only ad libitum and restricted fed fast-growing chickens and in a genetically distinct breed of ad libitum fed male and female chickens. Linear mixed models (Genstat 18) were used for statistical analysis with transformation where appropriate.ResultsThere were pronounced sex differences: expression of the orexigenic genes AGRP (P < 0.001) and NPY (P < 0.002) was higher in males of the fast-growing strain. In genetically distinct chickens, males had higher AGRP mRNA (P = 0.002) expression than females, suggesting sex difference was not restricted to a fast-growing strain. AGRP (P < 0.001) expression was significantly decreased in ad libitum fed birds but was high and indistinguishable between birds on a quantitative versus qualitative restricted diet. Inversely, gene expression of the anorectic genes POMC and CART was significantly higher in ad libitum fed birds but no consistent sex differences were observed.ConclusionExpression of orexigenic peptides in the avian hypothalamus are significantly different between sexes. This could be useful starting point of investigating further if AGRP is an indicator of growth potential. Results also demonstrate that gut fill alone does not reduce orexigenic gene expression.
Highlights
Research into energy balance and growth has infrequently considered genetic sex, yet there is sexual dimorphism for growth across the animal kingdom
We have previously shown that the expression of agoutirelated protein (AGRP) mRNA in the arcuate nucleus is increased many-fold in broiler breeder chickens under feed restriction compared to those fed ad libitum, whereas the anorectic peptide POMC mRNA was relatively unchanged [10]
Effect of sex and diet on basal hypothalamic neuropeptide gene expression plus physiological parameters Food intake and body and organ weight Restricted birds were fed 46 g/day which equates to approximately 25% of the food intake of the average ad libitum intake (181 g/day) at 12 weeks of age, as previously observed [10]
Summary
Research into energy balance and growth has infrequently considered genetic sex, yet there is sexual dimorphism for growth across the animal kingdom. As in most mammals, the male grows larger and faster than the female; the domestic chicken displays one of the clearest sexual dimorphisms in body weight, with males around 20% heavier and with a clear difference in growth rate long before sexual or somatic maturity [11, 12]. This dimorphism for body weight holds true across the spectrum of chicken lines: in fast-growing meat-type chickens, in crosses between OShamo game bird and white leghorns and in egg-laying strains [11, 13, 14]. From a practical point of view, selecting either sex for improved growth rate would be effective
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