Changes in fitness traits associated with selection for divergence in yolk cholesterol concentration

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Abstract
1. Correlated changes in egg production, hatchability, egg weight, age at sexual maturation and body weight of lines selected for divergence in yolk cholesterol were studied in two different populations. 2. Divergence in yolk cholesterol resulted in a highly significant difference in percentage egg production with a negative correlation between yolk cholesterol and egg numbers. However, the calculated total daily mass of cholesterol excreted into the egg was similar in both lines. 3. These results indicate that the physiological mechanism by which these populations responded to selection pressure on the concentration of cholesterol in the yolk was by changing egg numbers rather than total amount of cholesterol excreted. 4. Body weights of the high and low yolk cholesterol lines of the meat type, but not of the layer‐type populations, were significantly different. After four generations the line selected for low yolk cholesterol from the meat‐type population weighed 80% of the line selected for high yolk cholesterol levels. 5. Sexual maturity and egg weight were also significantly different between the high and low yolk cholesterol lines, but these differences could not be related to selection response. No changes in hatchability between the cholesterol lines were observed.