Phenotypic responses of chickens to long-term, bidirectional selection for juvenile body weight—Historical perspective

Abstract

A long-term selection experiment for high (HWS) and low (LWS) BW at 8 wk of age (BW8) was conducted in White Plymouth Rock chickens. Over 54 generations of selection, responses to bidirectional selection were profound. Increase in BW8 in line HWS was linear, and there was a significant quadratic response in line LWS for BW at both 4 and 8 wk of age. Although there is no indication that line HWS has come close to approaching a selection limit in more than 50 generations, selection limits occurred in line LWS chickens at generation 48 for females and generation 50 for males. Evidence also exists that one or more beneficial mutations have occurred in line HWS, aiding in progressive increases in BW8 over generations. Analyses of ratios of BW at 4 wk of age with those at 8 wk of age (ratio 4/8) revealed that LWS females grew proportionately faster through 4 wk of age than LWS males or HWS chickens. Comparisons of the selected lines with contemporary lines in which selection had been relaxed (discontinued) indicated that, in line HWS, the relaxed lines generally regressed toward original (preselection) values, suggesting that the linear response to single-trait selection was at least partially due to continued genetic variance. In LWS chickens, a series of plateaus in selection response occurred, but relaxed contemporary lines still regressed toward preselection values for BW8. In spite of the length of this selection experiment (54 generations), genetic variance and beneficial mutations have allowed continued, linear response to selection for increased BW8. Response to selection for decreased BW8 has been tempered by physiological barriers that have decreased survival of young chicks or the ability of females to reproduce. These findings are discussed in a historical perspective.

Chickens from lines artificially selected for juvenile low and high body weight differ in glucose homeostasis and pancreas physiology

“…In the 55th generation of selection, HWS chickens were more than 10-fold heavier than LWS chickens at selection age, with substantial fat accumulation (Dunnington et al, 2013). The HWS are hyperphagic and develop characteristics of metabolic syndrome as juveniles, while the LWS are hypophagic with some anorexic individuals.…”

“…Detailed descriptions of the breeding and maintenance of the lines has been previously published (Dunnington and Siegel, 1996;Marquez et al, 2010;Dunnington et al, 2013). Chickens used in the present experiments were obtained from the Paul S. Siegel Poultry Research Center at Virginia Tech.…”

Is Continued Genetic Improvement of Livestock Sustainable?

“…Estimates of the heritability of 35-day body weight for three current nucleus broiler populations averaged 36% (Kapell et al 2012), similar to average estimates (38% from half-sibs, 31% from offspring-parent) for 56-day weight (the then market weight) in a 1969 summary of published data. During 50 generations of selection for 8-week body weight in a population derived from inbred line crosses, response upward was almost linear, although that in the low lines has slowed, associated with fitness problems (Dunnington et al 2013). Heritability of lactation milk yield in dairy cattle has risen from 25% in the 1950s to 35% currently, likely partly due to improved management.…”

Standing genetic variation as a major contributor to adaptation in the Virginia chicken lines selection experiment

“…The genetic variation entering the population thus represents a sample of the polymorphisms present when these partially inbred lines were founded. Since the founder population was established, the high (HWS) and low (LWS) bodyweight lines have been bred with one new generation per year by single-trait, bi-directional selection for 56-day body weight [10][11][12]. The response to selection has progressed steadily throughout the experiment, resulting in an eight-fold difference in 56-day body weight after 40 generations of selection and currently, in the 57th generation, there is a 16-fold difference between the lines (Fig.…”