Effect of selection for commercially productive traits on the plasticity of cardiovascular regulation in chicken breeds during embryonic development

Domesticated animal breeds have experienced profound anatomical and physiological changes as a result of human-driven genetic selection. In poultry, this selection process has resulted in many distinct phenotypes from the ancestral bird, the Red Jungle-fowl. Growth rate and egg-laying capacity are 2 traits that have been commercially prioritized, and this has resulted in a fast-growth breed, the broiler, and a prolific egg layer, the White Leghorn. In this study, we investigated basic cardiovascular physiology in these 3 breeds at 90% of incubation. We aimed to identify breed-specific features of arterial blood pressure and heart rate as well as the physiological mechanisms regulating them. Specifically, we investigated mechanisms mediated by the autonomic nervous system by means of cholinergic and adrenergic receptors. Our overriding hypothesis was that selection for rapid growth would require an acceleration of heart rate and arterial pressure development in broilers compared with White Leghorns and the ancestral breed. The embryonic broiler is characterized by resting relative hypertensive bradycardia, whereas the White Leghorn is hypotensive. All 3 breeds maintained resting arterial pressure and heart rate via a similar beta- arid alpha-adrenergic receptor tone; however, cholinergic tone on heart rate was absent in the embryonic White Leghorn. Each breed responded differently to incubation in chronic hypoxic conditions (14% O-2). White Leghorn relied on augmenting cholinergic heart rate tone, and broilers relied on reducing beta-adrenergic tone on pressure. We concluded that selection for rapid growth shifts cardiovascular regulatory plasticity to emphasize mechanisms that modulate pressure, and that selection for egg-laying capacity emphasizes mechanisms that modulate heart rate.