Anatomical Response to Selection for Four-Week Body Mass in Japanese Quail

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Abstract
Changes were analyzed in masses, proportions and water contents of components of Japanese quail (C. japonica) following long-term selection for 4-wk body mass. Chicks from selected and control lines were sacrificed at 0, 1, 2, 4, 6 and 9 wk of age and dissected into 12 components, including plumage, major muscle masses, visceral organs, and brain. Selection for 40 generations had led to more than a doubling in mature mass and a 26-28% decrease in the time required to achieve that size. The difference in growth rates was expressed only before the age of 2 wk post-hatching, hence before chicks had achieved 50% of mature mass. Proportions of several organs and components during the growth period were not markedly affected, but the mass of the pectoral muscles of the selected chicks increased, and the mass of the brain and the wing area decreased, relative to body size. The absolute growth of the brain did not differ between lines. There were no differences between lines in the relative water contents of tissues of chicks of similar ages. These results suggest that growth rate can be increased without decreases in the apparent levels of functional maturity of tissues or changes in body proportions that lead to a decrease in the growth increment of a constraining tissue.