POLYGENIC VARIATION IN THE UTILIZATION OF ARGININE AND LYSINE BY THE CHICK

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Abstract
In continued tests of two strains of White Leghorns that had been bred for high requirement (HA) or low requirement (LA) of arginine, after 4 weeks on a basal diet containing only 0.9% of that amino acid, HA chicks of the fifth selected generation attained only 27.6% of the average weight of controls given ample arginine, but for chicks of the LA strain the corresponding figure was 83.5%.Reciprocal crosses between these two strains yielded F1generations in which utilization of arginine, (as measured by rates of growth on the test diet) was intermediate between those of the parent lines. Similarly, chicks from reciprocal backcrosses of F1birds to both parental strains were intermediate in performance between the F1and the parental line concerned.These results show that the difference between the HA and LA strains in utilization of arginine is polygenic.Activity of the enzyme arginase in the kidneys of chicks was inversely proportional to the rate of growth, and was about five times as high in the HA strain as in the LA strain. Chicks of the former line also showed higher levels of free lysine, which are believed to be responsible for the excessive activity of arginase in HA chicks. It is concluded that the selection practised differentiated two strains which differ in ability to thrive on a high-lysine, low-arginine diet.