Direct Response to Selection for Postweaning

The effectiveness d selection for 3-9-week gain was examined in a population of rats with a history of past selection for high 3-9-week gain. Lines were selected for high (U line) and low (D line) 3-9-week gain with two replicates of each line. Two randomly selected lines were also kept, one originating from the same base population as the two selected lines (R line) and the other originating from a population that had been randomly mated for the previous 27 generations (C line). Two replicates of each d these lines were kept. After seven generations of selection, a randomly selected line (relaxed line) was formed from each of the two upwardand each of the two downwardselected lines. Results have been presented for 13 generations of selecticm. The environmental trend for 3-9-week gain, as indicated by the randomly selected R and C lines, was consistently negative in all four lines. Realized heritabilities calculated by deviating the response to selection from the trend in the R or C lines resulted in non-significantly higher values in the D lines than the U lines. Six generations of relaxation of selection indicated no effect of natural selection in the U lines or the D lines. The relative magnitude of the drift, error and common environmental variances were estimated by the methods given by HILL (1971). The estimates of these parameters then led to calculation of the degree of bias in the sampling variances of the realized heritability estimates. As was predicted by HILL (1971), estimates of the variance d realized heritabilities obtained by using standard regression techniques weve less than those obtained using HILL’S formulae. The results are discussed in relation to &er similar studies with rats and mice. SELECTION for weight or weight gain has been studied by a number of workers using the laboratory mouse as the experimental animal. ROBERTS (1965) reviewed this work with particular attention to the contributions of these studies to animal breeding research. These studies have provided valuable experimental information on selection response and selection theory. In contrast to the laboratory mouse, the laboratory rat is a mammal which has been used by only a few workers to investigate selection response for weight o r weight gain (PARK et al. 1966; BAILEY et al. 1970; ROUBICEK and RAY 1971). The objective of this study was to investigate response to selection for postPaper No. 1614 from the Laboratory of Genetics. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison. * Present address: Ruakura Animal Research Station, Hamilton, New Zealand. a Present address: 26Do Pleasant Avenue, Minneapolis, Minnesota 55408. Genetics 80: 171-189 May, 1975 172 R. L. BAKER, A. B. C H A P M A N A N D R. T. WARDELL weaning gain in a population of rats with a history of past selection for the same trait. The extent to which these results are in agreement with results obtained from selection with other laboratory species, particularly the mouse, will provide a broader base for generalization of selection theory to domestic animals. This paper reports the direct response to selection for both high and low postweaning gain, with particular emphasis on methods of estimation of the sampling variance of realized heritability estimates. MATERIALS A N D METHODS The animals used in this selection experiment came from a line of rats previously selected for increased postweaning gain from 3 to 9 weeks of age on a standard diet (about 22% protein fed ad libitum) as part ob a genotype-environment interaction study (PARK et al. 1966). The fmndation stock for the first study was formed by cmsing four highly inbred lines of rats. Details of the origin of these inbred lines and of the crossing procedure are given by PARK et al. (19%). After 16 generations of m a s selectkm the line was maintained by random selection (relaxed selection) for 10 generations (FFR line) before the present study was undertaken. Six lines were folrmd from the FFR line at generation 27. Two lines were selected for increased 3-9-week gain (U lines), two lines fm decreased 3-%week gain (D lines), and two lines for continued random selection (R lines). Throughout the previous 26 generations a contrd line, which had originated from the original four-way cross, had been maintained by random selection. From this population two control lines (C lines) were farmed in generation 27 and were continued as randomly selected lines. For the purposes of this rep? generation 27 has been called generation 0. Eight= single-pair matings were made in each of these lines up to generation 7. Four additional lines were formed in generation 7. From each of the two U lines one relaxed line was formed (UR lines), with selection at random, and one relaxed line was also formed from each of the D lines (DR lines). This increase in number of lines at generation 7 necessitated a reduction in the number of pairs of rats mated per line from 18 ta 15 pairs. Selection was on the basis of individual postweaning gain from 3 to 9 weeks d age. Either 15 or 18 rats (about 45% m 55% of the population), with the highest or lowest gain in each sex, were selected in each generation for single-pair matings in each selection line. The C and R lines were selected at random within sexes and then single-pair mated. In all lines, selection was carried out in the first litters with the restriction that no more than two individuals of each sex be selected from each family. This restriction was an attempt to keep inbreeding to a minimum. Males and females were mated monogamously and at random with the restriction, also to avoid inbreeding, that if passible, no pair should have any grandparents in common. In generation 11 the selection procedure was modified to select 7 males and 14 females in each line. One male was then mated to two females with the same restrictions as above. This mating procedure was an attempt to estimate the sire and dam components d variance from a hierarchal analysis of variance. Due to poor fertility in this mating, the limited data obtained did not yield any meaningful results. The litter size was reduced or increased (by transfer from litters born the same day and if possible from the same line) to six at one day after birth with a sex ratio as clom as possible to 3 males and 3 females. Rats transferred into a litter to make it up to s ix were discarded at weaning. The young were weaned at 3 weeks of age, at which time all lines w m randomly assigned to cages on a within-sex basis, with six rats per cage. The restiction was made that no more than two rats from any one family s h d d be reared in the same cage. The data collected each generation were: litter size at birth, litter weight at 2 weeks of age, individual weights at 3, 6, and 9 weeks of age, and the proportion of females pupping out ob the total number exposed to males. Males were left with the females fw a 3-week mating periad.