Long Term Selection for Small Body Weight in Japanese Quail I : Direct Selection Response from 60 to 65th Generations

Abstract

It is important to know the limit of artificial selection for efficient animal production. The aim of this research was to examine whether the selection for small body weight in Japanese quail has reached to the limit after the long term selection over 65 generations. In this research, two lines, SS and RR were compared. RR line was produced by random mating in a closed population as the control, and SS line was selected for small body weight at 6 weeks of age until 65 generations. Additionally, the data of LL line selected for large body weight was used to evaluate a selection limit in SS line though the details were not shown. Each line was composed of about 30 pair-matings, and body weight measurement was performed every week until 6 weeks of age, but the body weights of candidate birds selected for production of next generation were measured until 15 weeks of age. In the first step of selection, 4 males and 4 females were selected as a general rule among the offspring from each pair-mating at about 4 weeks of age. In the second step of selection, one male and one female were chosen by body weight at 6 weeks of age from 8 individuals selected in the first step, and 30 pari-matings were arranged with the individuals having no consanguineous relation in the preceding 4 generations. Mean body weight of SS line during 60-65 generations was 59.5g and that of RR line was 106.5 g, and the difference was significant (P<0.01). Selection pressure (SP) in the two lines was constant throughout the generations. Selection intensity (SI) in SS line was in the negative direction, and that in RR line was near zero. Estimated heritability in the two lines was variable from generation to generation. Realized heritabilities in SS line increased in both males and females during two terms of 54 to 59 and 60 to 65 generations. The selection response increased in the negative direction in response to cumulative selection differential. The observed ratio of mean body weight at 6 weeks of age in SS line to that in LL line (ORM) and observed ratio of difference between mean body weight at the same age in SS line and that in LL line to the standard deviation of body weight in RR line (OR) suggested that selection limit may not have been reached in SS line after the long term selection for 65 generations and it may be possible to expect further genetic improvement. Generally, artificial selection during a very long period would reach to a limit on account of decrease in genetic diversity. However, selection in the direction for small body weight was found to show the different tendency from the selection for large body weight. Miniaturization of body size is an interesting biological phenomenon from the evolutionary point of view. The SS line established in the present study would be useful for the research from molecular biologic and metabolic aspects to have an insight into evolutionary mechanism of miniaturization.

Keywords

• selection limit
• body weight
• small size
• heritability
• japanese quail