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Divergent Selection for Body Density in Japanese Quail (Coturnix coturnix japonica)1
BREEDING AND GENETICS Divergent Selection for Body Density in Japanese Quail (Coturnix coturnix japonica)1 V. A. GARWOOD U.S. Department of Agriculture, Agricultural Research Service, Poultry Research Laboratory, Georgetown, Delaware 19947 K. C. DIEHL, JR. and C. G. HAUGH Agricultural Engineering Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received for publication October 19, 1988)
1989 Poultry Science 68:1033-1039 INTRODUCTION
Body density is a trait that has received limited attention by poultry researchers probably because of the lack of an accurate, easy, nondestructive method of measuring body volume of the bird. As a consequence, little is known regarding its relationships to body or carcass components, viz. ash, fat, moisture, and protein. But knowledge of these relationships may be important in effecting changes in body composition as these components differ in density. Thus, any change effected in body density should be automatically reflected in the relative proportions of the components. Ergo, increase in body density should favor increases in the more dense components or combinations thereof, whilst the converse should favor the less dense components and combinations. Garwood and Diehl (1987) and Diehl et al. (1988) have reported an adaptation of the
Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable.
technique by Day (1964) for the nondestructive measurement of body density in poultry and indicated it to be moderately heritable and associated with lipid content in a randombred population of Japanese quail. The purpose of this report was to present the direct results of divergent selection for body density and the correlated responses in body weight and volume and shank length in Japanese quail. MATERIALS AND METHODS
The base population for the experiment was the Eastern Shore Randombred (ESR) quail population (Coturnix coturnix japonica), developed and maintained as described by Garwood and Diehl (1987). The experiment was conducted in two phases: a selection phase, extending over three generations, and a testing phase, comprised of the fourth generation. In the selection phase, each of three replicates was initially formed from ESR by random selection of 8 males and 24 females at 8 wk of age to form mating groups of 3 females per male. When rate of lay was satisfactory for each replicate, a 9-day egg collection was made. Resulting progeny were identified at hatch and placed in randomly
1033
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ABSTRACT Japanese quail were selected divergently for body density over three generations for study of direct responses and correlated responses in body weight, volume, and shank length. Testing of differences occurred in the fourth generation by comparisons with a control population. Absolute standardized selection differentials were similar throughout generation-replicate subclasses. The line selected for high body density (H) and that selected for low body density (L) differed significantly (P-c.01) for body density and weight and shank length but not for body volume. Total gains in H and L, respectively, were: .015 and -.016 g-cm for body density; -3.6 and -1.2 g for body weight; and .3 and -.4 mm for shank length. Responses to selection were symmetric with respect to the control for body density and shank length but not for body weight and volume. (Key words: selection, body density, body volume, body weight, quail)
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GARWOOD EI AL.
33.58; volume variance, 54.86; and weightvolume covariance, 35.25. The estimates of genetic variances and covariance used were: weight variance, 16.49; volume variance, 16.00; and weight-volume covariance, 13.38. Statistical Analyses. Differences among replicates and between directions of selection in the selection phase were tested by analyses of variances according to the following model with Type I sums of squares: Yhijk = H + b + D; + bDi + rj + dry + Sk + DSjk + rsjk + drsyk + ehijk where \i is the overall mean of the nth observation in the ith direction (D), jth replicate (r), and kth sex (S), b is the linear regression on generation, and bD is the interaction of the direction with regression of Y on generations. All dependent variables were analyzed in regular measure and in transformed measure (common logarithms) except shank lengdi, in which only regular measure was analyzed. Differences were tested at the 1% probability level. As results were similar for the regular and transformed variables, analyses of regular measures are presented. Absolute standardized selection differentials were analyzed according to the following model: Yhijk = u + Di + rj + rdjj + Gk + rg ik + DGjk + ehijk where |J. is the overall mean of the hth differential in the ith direction of selection (D), jth replicate (r), and the kth generation (G). The model for analysis of the data of the test phase was: Yhij = u + Di + rj + rdik + ehij where \L is the overall mean of the hth observation in the ith direction of selection (D) and the jth replicate (r). Dependent variables were transformed as in the selection phase for analyses. Results of analyses were similar for the two types of variables, so analyses of regular measure are presented. Type III sums of squares were derived and differences were tested at the 1% probability level. Realized heritability was estimated for each replicate. The genetic gain, calculated as
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assigned pens of batteries (Petersime Incubator Co., Gettysburg, OH) adapted for quail. The chicks were fed Purina Game Bird Startena (30% protein) (Purina Mills, Inc., St. Louis, MO) their first 7 wk and Purina Game Breeder Layena (20% protein) thereafter. Initial brooding temperature was 40.7 C. Each succeeding week the temperature was reduced 2.8 C until 21.1 C was reached. The lighting regimen was: 0 to 21 days, 24 h light; 22 to 42 days, 6 h light; thereafter, light was increased in daily increments of one-half h until 16 h was reached. At 42 days chicks were weighed to the nearest .1 g and measured in an aircomparison pycnometer (Garwood and Diehl, 1987) for body volume to the nearest .1 cm3. Shank length was measured to the nearest .1 mm. Density was calculated as the weight: volume ratio and estimated to the nearest .001 g-cm-3. Two lines were formed by selection within each replicate (approximately 60 males and 60 females) of the initial or zero generation. The H (high density-selected) line was initiated by selecting and intermating the 8 males and 24 females having the highest densities, whereas the L (low density-selected) line was initiated by the 8 males and 24 females having the lowest densities. Thereafter, selection was within each line-replicate in the designated direction for three generations. Husbandry and management were as described for the zero generation. The testing phase, consisting of the fourth generation, was produced in a manner similar for the preceeding generations with the exception that the parents (8 males, 24 females) were selected randomly. From the progeny within each line-replicate subclass, 20 males and 20 females were randomly selected at 42 days, weighed, and measured as above. Males and females of ESR were reared concurrently, randomly selected in equal numbers, and treated similarly to H and L progeny to serve as controls for the test phase. A simulation study was conducted to determine results obtained by the use of a linear index (Gunsett, 1984) based on parameters associated with body density. The program used to simulate selection was oudined by Gunsett (1984) with parameter estimates from Garwood and Diehl (1987) based on the component traits of body density. The phenotypic estimates were: mean weight, 90.0 g; mean volume, 99.6 cm3; weight variance,
1035
BODY DENSITY SELECTION
deviation of the selected line from the control line in Generation 4, and divided by the cumulative selection differentials in the selection phase, was used as the estimate of realized heritability. Values were averaged over replicates. RESULTS AND DISCUSSION
98-
b
96 1 94"
'92-
V.
X^//
y
x
\ %
90
8&
• •
HIGH DENSITY SELECTED LOW DENSITY SELECTED
\. ^^H
FIGURE 1. Performance plotted over generations of selection for high body density and low body density: (a) body density, in grams per cubic centimeter (g/cm) x 1,000; (b) body weight (wt), in grams; (c) body volume, in cubic centimeters (cc); and, (d) shank length, in millimeters.
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Analyses of data from the selection phase of the experiment are given in Table 1 with the least squares means for the directions of selection. Although replicates and sexes were in general significant sources of variation, they were irrelevant to the objectives of the experiment. The differences between sexes are well documented for a number of growthassociated traits. Pertinent differences involve generations, because they concern accumulated genetic gain, and directions of selection, because they relate to the selection differentials exerted. Both these sources of variation were statistically significant and performances of the direction-generation subclasses are depicted in
Figure 1. The least square means (Table 1) indicate that the overall density of H was .924 g-cm-3 and that of L was .914 g-crn--*. Quail of Line H averaged 2.5 g heavier than those in Line L, with 1.6 cm^ greater body volume and shanks .2 mm longer than birds in Line L. The regressions of performance on generations were significant in all cases except shank length, and the coefficients were heterogeneous for the directions of selection. These results are as to be expected in a divergent selection experiment if selection is effective and, at the same time, if genetic correlations of the selection criterion and the secondary traits exist. Regression coefficients for both Lines H and L were negative for density and weight (Table 1). A negative environmental effect as a possible explanation can only be speculated, as no control line was carried in the selection phase because of facility limitations. Because any unequal performance responses could result from unequal selection intensities, the absolute standardized selection differentials for the generation-replicate-direc-
1036
GARWOOD ET AL. TABLE 1. Analysis of body parameters for lines divergently selected for body density
Statistical analysis
df
Body density 3
(g-cm ) 1 1 1 2 2 1 2 1 2 2,504
.065** .057** .052** .110** .013 .038** .044** .022 .009 .004
Body volume
Shank length
(g)
(cm3)
(mm)
6,092.7** 3,516.4** 2,256.0** 736.9** 4.6 16,425.4** 222.9** 445.1** 8.0 44.6
3,135.9** 1,398.9** 694.5** 92.9 172.6 12,885.7** 796.8** 37.6 141.1 70.4
1.1 39.7** 23.8** 24.9** 1.7 104.9** .1 .1 .6 .8
.924 ± .002 .914 ± .002
93.9 ± .19 91.4 ± .19
101.9 ± .24 100.3 ± .23
34.6 ± .02 34.4 ± .02
-.010 ± .006 -.019 ± .006
-1.1 ± .69 -2.8 ± .69
.4 ± .86 -.6 ± .86
.17 ± .09 -.01 ± .09
**P<.01.
tion subclasses (Table 2) were subjected to analysis of variance. No significant difference existed for direction of selection, nor between levels for any other source of variation. Analyses of variances for traits as measured in the test generation are given in Table 3. Significant differences existed between directions of selection (lines) for all traits measured. Orthogonal comparisons of the lines indicated that the direction-difference in body density and in shank length was accounted for entirely by the difference between H and L. The difference between the means of the selected lines and the control line was not
TABLE 2. Absolute standardized selection differentials by generation and replicate for lines selected for high body density (H) and low body density (L) Generation
Replicate
H
L
0
1 2 3 1 2 3 1 2 3
1.303 1.212 1.284 1.089 1.246 1.299 1.019 1.227 .837 1.168
1.239 1.184 1.185 1.109 1.254 1.350 1.095 1.035 1.178 1.181
1
2
X
significant, thereby indicating symmetry of responses. Thus, the magnitude of the gains from selection in H was the same as that from selection in L. For body weight and body volume, the difference between the means of the selected lines and the control was significant, thereby indicating asymmetric responses. The difference between H and L for body weight was significant but not so for body volume. Accumulated genetic gains are given in Table 3. The total gains, derived as differences between least squares means of the selected lines and the control, in Lines H and L for body density were .015 and -.015 g-cmr-^, respectively. In body weight and volume, wherein asymmetric responses were noted, the total gains were .3 vs. -3.6 g and -1.2 vs. -2.5 cm^ for Lines H and L, respectively. These results suggest that, although significant, symmetric, direct responses occurred in body density for Lines H and L, the responses were manifestations of differential asymmetric responses in the basic traits of the density ratio, i.e., body weight and body volume. Correlated gains for shank length were .3 and -.4 mm for Lines H and L, respectively. Conceivably, if the longer shank of H were indicative of a greater general bone structure, the trait either singly or in conjunction with changes in other body compositional traits
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Mean squares of Type I ANOVA Linear regression on generations (B) Directions of selection (D) B x D Replicates (R) D xR Sexes (S) R x S D x S D x R x S Error Least squares means, x ± SE High selected Low selected Regression coefficients on generation High selected Low selected
Body weight
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BODY DENSITY SELECTION
TABLE 3. Analysis of body parameters resulting from test generation of divergent selection for body density
Statistical analysis
2 1 1 2 4 1 2 2 4 359
Body density
Body weight
Body volume
Shank length
(gem 3 )
(g)
(cm3)
(mm)
.027** .054** .000 .018** .004 .000 .008 .025** .021** .002
630.1** 971.6** 288.4** 541.2** 81.4 501.3** 433.3** 85.4 107.5 37.1
198.6** 98.7 298.4** 1,047.0** 85.9 607.8** 632.2** 83.2 159.1** 45.0
13.7** 27.2** .0 14.9** 1.6 22.0** .4 .1 .3 .8
.961 ± .004 .946 ± .004 .931 ± .004
93.0 ± .60 92.7 ± .60 89.1 ± .60
97.2 ± .61 98.4 ± .61 95.9 ± .61
34.9 ± .08 34.6 ± .08 34.2 ± .08
**P<.01.
(e.g., fat, moisture, and protein) could be wholly, or in part, responsible for the line difference in body density. Definitive relationships of changes in ash, fat, moisture, and protein resulting from the divergent selection would depend upon the relative magnitudes of their heritabilities and genetic correlations with body weight and body volume. Heritability and density differences in these compositional traits may be responsible for the differential responses noted above for body weight and volume. A discrepancy between an experimental parameter in the selected and control lines in the test phase must be noted. Effective population sizes were different: that for Lines H and L was 24 throughout the experiment but ESR was maintained with an effective size of approximately 120. Hence, the progeny in the subclasses of the test phase were dissimilar in their inbreeding. An estimate of inbreeding accumulated in each of Lines H and L would be 6% and less than 1% for the ESR control during the three selected generations. Any differences arising from inbreeding depression would be expected to be quite small. Such an effect, if present, generally manifests itself most readily in reproductive traits (Sittmann et al., 1966; Kulenkamp et al., 1973). Examination of rate of lay, fertility, and hatchability in the test phase revealed no significant line differences. Therefore, it appears plausible to
assume that inbreeding depression was not responsible for line differences in the selected and correlated traits. Realized heritability estimates, derived from the accumulated selection differentials and the genetic gain estimated in the test generation, averaged 8 ± 5% and 7 ± 4% over replicates for high and low density, respectively. These values are considerably less than the value of 38 reported by Garwood and Diehl (1987). Part of the discrepancy may reside in the fact that the selection differentials and genetic gains were estimated in different generations. As a consequence, any differences in environmental effects between the generations are inherent in the estimates. In the same report (Garwood and Diehl, 1987), the component traits of density were indicated to be highly heritable (82%, body weight; 66%, body volume). However, a similar situation has been reported in swine for the feed conversion ratio, wherein Bernard and Fahmy (1970) and Jungst et al. (1981) reported realized heritabilities substantially less than estimates from the covariance among relatives (Dickerson and Grimes, 1947; Park, 1965; Robison and Berruecos, 1973; McPhee et al., 1979). Ratios as selection criteria and their erratic responses have been perplexing problems to animal breeders, and attempts to linearize the ratios via indices and thereby produce more conventional responses have been made. Use
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Mean squares of Type III ANOVA Direction of selection (D) High v*. low Control vs. selected Replicates (R) D x R Sexes (S) D x S R x S D x R x S Error Least squares means, x ± SE High selected Control Low selected
df
1038
GARWOOD ET AL.
TABLE 4. Genetic change in body density by direct selection on the body density ratio and by a linear index of the ratio as simulated in increments (I) of five percent selection intensity (SI) on themale side only
SI
(%)
1 2 3 4 5 6 7 8 9 10
5 10 15 20 25 30 35 40 45 50
.014 .012 .011 .010 .009 .008 .007 .007 .006 .006
Linear index selection .016 .014 .012 .011 .010 .009 .008 .008 .007 .006
of a linear index as the basis of selection for a ratio-defined trait initially was studied by Turner (1959). Lin (1980) approximated the ACKNOWLEDGMENTS linear weights for the component traits that would maximize the genetic change in the The authors are appreciative of the efforts ratio. Gunsett (1984), using simulation, deter- of D. Mitchell for assistance in breeding and mined the efficiency of direct selection for a caring for the quail and in obtaining the ratio as compared with a linear index wherein measurement data. The authors are pleased to the correlation between the genotype of the acknowledge the efforts of F. Gunsett in ratio and the phenotype of the index was conducting the simulation aspects of the study. maximized. Interesting results were obtained for various combinations of parameters for the numerator and denominator traits of the ratio. REFERENCES Table 4 presents the genetic change in the C , and M. H. Fahmy, 1970. Effect of selection on (body density) ratio resulting from direct Barnard, feed utilization and carcass score in swine. Can. J. selection and from selection on a linear index Anim. Sci. 50:575-584. in increments of selection intensity of 5% on Day, C. L., 1964. Device for measuring voids in porous materials. Agric. Eng. 45:36-37. the male side only based on simulation. The derived coefficients for the index were 3.1 and Dickerson, G. E., and J. C. Grimes, 1947. Effectiveness of selection for efficiency of gain in Duroc swine. J. -2.126 for body weight and body volume, Anim. Sci. 6:265-287. respectively. Diehl, K. C , V. A. Garwood, and C. G. Haugh, 1988. Volume measurement using the air-comparison pycThe present absolute genetic gains (.015 nometer. Trans, of ASAE 31:284-287. g-cm~3 for both high and low density selecV. A., and K. C. Diehl, Jr., 1987. Body volume tion) are in agreement with those obtained by Garwood, and density of live Coturnix quail and associated simulated selection. However, all else being genetic relationships. Poultry Sci. 66:1264-1271. equal, the actual gains would be expected to be Gunsett, F. C , 1984. Linear index selection to improve traits defined as ratios. J. Anim. Sci. 59:1185-1193. slightly less than the simulated gains in that the selection intensities differed. In the actual Jungst, S. B., L. L. Christian, and D. L. Kuhlers, 1981. Response to selection for feed efficiency in individual experiment, approximately 15% of the males fed Yorkshire boars, J. Anim. Sci. 53:323-331. and about 50% of the females were selected in Kulenkamp, A. W., C. M. Kulenkamp, and T. H. Coleman, each generation-replicate subclass. From in1973. The effects of inbreeding (brother x sister) on various traits in Japanese quail. Poultry Sci. 52: spection of the simulated estimated gains 1240-1246. (Table 4), it can be seen that index selection Lin, C. Y., 1980. Relative efficiency of selection methods would be little more efficient than direct for improved feed efficiency. J. Dairy Sci. 63: selection with high selection intensity. As 491-494. selection intensity decreases, efficiency of the McPhee, C. P., P. J. Brennan, and F. Duncalfe, 1979. Genetic
Downloaded from http://ps.oxfordjournals.org/ at University of Manitoba on June 10, 2015
I
Direct selection
index decreases rapidly. This situation is undoubtedly a function of the characteristics of the ratio. With traits of the numerator and denominator being highly correlated genetically and phenotypically, there is little variation in the ratio upon which selection can act. Furthermore, change in either direction for body density requires changes in body weight and volume of opposite directions, a circumstance in conflict with the high positive correlation between the two. Thus, in summary, from both the biological and simulated work performed, it can be concluded that genetic gain in the ratio body density can be effected by selection. However, this change, particularly wherein a high correlation exists between the numerator and denominator, is expected to be small. The use of index selection to increase efficiency of selection was not shown to be adequate.
BODY DENSITY SELECTION and phenotypic parameters of Australian Large White and Landrace boars performance tested when offered food ad libitum. Anim. Prod. 28:79-85. Park, Y. I., 1965. Age-constant feed efficiency of pigs. J. Anim. Sci. 24:819-822. Robison, O. W., and J. M. Bemiecos, 1973. Feed efficiency in swine. I. A comparison of measurements and methods of expressing feed efficiency. J. Anim. Sci. 37:
1039
643-649. Sittmann, K. H., H. Abplanalp, and R. A. Fraser, 1966. Inbreeding depression in Japanese quail. Genetics 54: 371-379. Turner, H. N., 1959. Ratios as criteria for selection in animal or plant breeding with particular reference to efficiency of food conversion in sheep. Aust. J. Agric. Res. 10:565-580.
Downloaded from http://ps.oxfordjournals.org/ at University of Manitoba on June 10, 2015
1989 Poultry Science 68:1033-1039 INTRODUCTION
Body density is a trait that has received limited attention by poultry researchers probably because of the lack of an accurate, easy, nondestructive method of measuring body volume of the bird. As a consequence, little is known regarding its relationships to body or carcass components, viz. ash, fat, moisture, and protein. But knowledge of these relationships may be important in effecting changes in body composition as these components differ in density. Thus, any change effected in body density should be automatically reflected in the relative proportions of the components. Ergo, increase in body density should favor increases in the more dense components or combinations thereof, whilst the converse should favor the less dense components and combinations. Garwood and Diehl (1987) and Diehl et al. (1988) have reported an adaptation of the
Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable.
technique by Day (1964) for the nondestructive measurement of body density in poultry and indicated it to be moderately heritable and associated with lipid content in a randombred population of Japanese quail. The purpose of this report was to present the direct results of divergent selection for body density and the correlated responses in body weight and volume and shank length in Japanese quail. MATERIALS AND METHODS
The base population for the experiment was the Eastern Shore Randombred (ESR) quail population (Coturnix coturnix japonica), developed and maintained as described by Garwood and Diehl (1987). The experiment was conducted in two phases: a selection phase, extending over three generations, and a testing phase, comprised of the fourth generation. In the selection phase, each of three replicates was initially formed from ESR by random selection of 8 males and 24 females at 8 wk of age to form mating groups of 3 females per male. When rate of lay was satisfactory for each replicate, a 9-day egg collection was made. Resulting progeny were identified at hatch and placed in randomly
1033
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ABSTRACT Japanese quail were selected divergently for body density over three generations for study of direct responses and correlated responses in body weight, volume, and shank length. Testing of differences occurred in the fourth generation by comparisons with a control population. Absolute standardized selection differentials were similar throughout generation-replicate subclasses. The line selected for high body density (H) and that selected for low body density (L) differed significantly (P-c.01) for body density and weight and shank length but not for body volume. Total gains in H and L, respectively, were: .015 and -.016 g-cm for body density; -3.6 and -1.2 g for body weight; and .3 and -.4 mm for shank length. Responses to selection were symmetric with respect to the control for body density and shank length but not for body weight and volume. (Key words: selection, body density, body volume, body weight, quail)
1034
GARWOOD EI AL.
33.58; volume variance, 54.86; and weightvolume covariance, 35.25. The estimates of genetic variances and covariance used were: weight variance, 16.49; volume variance, 16.00; and weight-volume covariance, 13.38. Statistical Analyses. Differences among replicates and between directions of selection in the selection phase were tested by analyses of variances according to the following model with Type I sums of squares: Yhijk = H + b + D; + bDi + rj + dry + Sk + DSjk + rsjk + drsyk + ehijk where \i is the overall mean of the nth observation in the ith direction (D), jth replicate (r), and kth sex (S), b is the linear regression on generation, and bD is the interaction of the direction with regression of Y on generations. All dependent variables were analyzed in regular measure and in transformed measure (common logarithms) except shank lengdi, in which only regular measure was analyzed. Differences were tested at the 1% probability level. As results were similar for the regular and transformed variables, analyses of regular measures are presented. Absolute standardized selection differentials were analyzed according to the following model: Yhijk = u + Di + rj + rdjj + Gk + rg ik + DGjk + ehijk where |J. is the overall mean of the hth differential in the ith direction of selection (D), jth replicate (r), and the kth generation (G). The model for analysis of the data of the test phase was: Yhij = u + Di + rj + rdik + ehij where \L is the overall mean of the hth observation in the ith direction of selection (D) and the jth replicate (r). Dependent variables were transformed as in the selection phase for analyses. Results of analyses were similar for the two types of variables, so analyses of regular measure are presented. Type III sums of squares were derived and differences were tested at the 1% probability level. Realized heritability was estimated for each replicate. The genetic gain, calculated as
Downloaded from http://ps.oxfordjournals.org/ at University of Manitoba on June 10, 2015
assigned pens of batteries (Petersime Incubator Co., Gettysburg, OH) adapted for quail. The chicks were fed Purina Game Bird Startena (30% protein) (Purina Mills, Inc., St. Louis, MO) their first 7 wk and Purina Game Breeder Layena (20% protein) thereafter. Initial brooding temperature was 40.7 C. Each succeeding week the temperature was reduced 2.8 C until 21.1 C was reached. The lighting regimen was: 0 to 21 days, 24 h light; 22 to 42 days, 6 h light; thereafter, light was increased in daily increments of one-half h until 16 h was reached. At 42 days chicks were weighed to the nearest .1 g and measured in an aircomparison pycnometer (Garwood and Diehl, 1987) for body volume to the nearest .1 cm3. Shank length was measured to the nearest .1 mm. Density was calculated as the weight: volume ratio and estimated to the nearest .001 g-cm-3. Two lines were formed by selection within each replicate (approximately 60 males and 60 females) of the initial or zero generation. The H (high density-selected) line was initiated by selecting and intermating the 8 males and 24 females having the highest densities, whereas the L (low density-selected) line was initiated by the 8 males and 24 females having the lowest densities. Thereafter, selection was within each line-replicate in the designated direction for three generations. Husbandry and management were as described for the zero generation. The testing phase, consisting of the fourth generation, was produced in a manner similar for the preceeding generations with the exception that the parents (8 males, 24 females) were selected randomly. From the progeny within each line-replicate subclass, 20 males and 20 females were randomly selected at 42 days, weighed, and measured as above. Males and females of ESR were reared concurrently, randomly selected in equal numbers, and treated similarly to H and L progeny to serve as controls for the test phase. A simulation study was conducted to determine results obtained by the use of a linear index (Gunsett, 1984) based on parameters associated with body density. The program used to simulate selection was oudined by Gunsett (1984) with parameter estimates from Garwood and Diehl (1987) based on the component traits of body density. The phenotypic estimates were: mean weight, 90.0 g; mean volume, 99.6 cm3; weight variance,
1035
BODY DENSITY SELECTION
deviation of the selected line from the control line in Generation 4, and divided by the cumulative selection differentials in the selection phase, was used as the estimate of realized heritability. Values were averaged over replicates. RESULTS AND DISCUSSION
98-
b
96 1 94"
'92-
V.
X^//
y
x
\ %
90
8&
• •
HIGH DENSITY SELECTED LOW DENSITY SELECTED
\. ^^H
FIGURE 1. Performance plotted over generations of selection for high body density and low body density: (a) body density, in grams per cubic centimeter (g/cm) x 1,000; (b) body weight (wt), in grams; (c) body volume, in cubic centimeters (cc); and, (d) shank length, in millimeters.
Downloaded from http://ps.oxfordjournals.org/ at University of Manitoba on June 10, 2015
Analyses of data from the selection phase of the experiment are given in Table 1 with the least squares means for the directions of selection. Although replicates and sexes were in general significant sources of variation, they were irrelevant to the objectives of the experiment. The differences between sexes are well documented for a number of growthassociated traits. Pertinent differences involve generations, because they concern accumulated genetic gain, and directions of selection, because they relate to the selection differentials exerted. Both these sources of variation were statistically significant and performances of the direction-generation subclasses are depicted in
Figure 1. The least square means (Table 1) indicate that the overall density of H was .924 g-cm-3 and that of L was .914 g-crn--*. Quail of Line H averaged 2.5 g heavier than those in Line L, with 1.6 cm^ greater body volume and shanks .2 mm longer than birds in Line L. The regressions of performance on generations were significant in all cases except shank length, and the coefficients were heterogeneous for the directions of selection. These results are as to be expected in a divergent selection experiment if selection is effective and, at the same time, if genetic correlations of the selection criterion and the secondary traits exist. Regression coefficients for both Lines H and L were negative for density and weight (Table 1). A negative environmental effect as a possible explanation can only be speculated, as no control line was carried in the selection phase because of facility limitations. Because any unequal performance responses could result from unequal selection intensities, the absolute standardized selection differentials for the generation-replicate-direc-
1036
GARWOOD ET AL. TABLE 1. Analysis of body parameters for lines divergently selected for body density
Statistical analysis
df
Body density 3
(g-cm ) 1 1 1 2 2 1 2 1 2 2,504
.065** .057** .052** .110** .013 .038** .044** .022 .009 .004
Body volume
Shank length
(g)
(cm3)
(mm)
6,092.7** 3,516.4** 2,256.0** 736.9** 4.6 16,425.4** 222.9** 445.1** 8.0 44.6
3,135.9** 1,398.9** 694.5** 92.9 172.6 12,885.7** 796.8** 37.6 141.1 70.4
1.1 39.7** 23.8** 24.9** 1.7 104.9** .1 .1 .6 .8
.924 ± .002 .914 ± .002
93.9 ± .19 91.4 ± .19
101.9 ± .24 100.3 ± .23
34.6 ± .02 34.4 ± .02
-.010 ± .006 -.019 ± .006
-1.1 ± .69 -2.8 ± .69
.4 ± .86 -.6 ± .86
.17 ± .09 -.01 ± .09
**P<.01.
tion subclasses (Table 2) were subjected to analysis of variance. No significant difference existed for direction of selection, nor between levels for any other source of variation. Analyses of variances for traits as measured in the test generation are given in Table 3. Significant differences existed between directions of selection (lines) for all traits measured. Orthogonal comparisons of the lines indicated that the direction-difference in body density and in shank length was accounted for entirely by the difference between H and L. The difference between the means of the selected lines and the control line was not
TABLE 2. Absolute standardized selection differentials by generation and replicate for lines selected for high body density (H) and low body density (L) Generation
Replicate
H
L
0
1 2 3 1 2 3 1 2 3
1.303 1.212 1.284 1.089 1.246 1.299 1.019 1.227 .837 1.168
1.239 1.184 1.185 1.109 1.254 1.350 1.095 1.035 1.178 1.181
1
2
X
significant, thereby indicating symmetry of responses. Thus, the magnitude of the gains from selection in H was the same as that from selection in L. For body weight and body volume, the difference between the means of the selected lines and the control was significant, thereby indicating asymmetric responses. The difference between H and L for body weight was significant but not so for body volume. Accumulated genetic gains are given in Table 3. The total gains, derived as differences between least squares means of the selected lines and the control, in Lines H and L for body density were .015 and -.015 g-cmr-^, respectively. In body weight and volume, wherein asymmetric responses were noted, the total gains were .3 vs. -3.6 g and -1.2 vs. -2.5 cm^ for Lines H and L, respectively. These results suggest that, although significant, symmetric, direct responses occurred in body density for Lines H and L, the responses were manifestations of differential asymmetric responses in the basic traits of the density ratio, i.e., body weight and body volume. Correlated gains for shank length were .3 and -.4 mm for Lines H and L, respectively. Conceivably, if the longer shank of H were indicative of a greater general bone structure, the trait either singly or in conjunction with changes in other body compositional traits
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Mean squares of Type I ANOVA Linear regression on generations (B) Directions of selection (D) B x D Replicates (R) D xR Sexes (S) R x S D x S D x R x S Error Least squares means, x ± SE High selected Low selected Regression coefficients on generation High selected Low selected
Body weight
1037
BODY DENSITY SELECTION
TABLE 3. Analysis of body parameters resulting from test generation of divergent selection for body density
Statistical analysis
2 1 1 2 4 1 2 2 4 359
Body density
Body weight
Body volume
Shank length
(gem 3 )
(g)
(cm3)
(mm)
.027** .054** .000 .018** .004 .000 .008 .025** .021** .002
630.1** 971.6** 288.4** 541.2** 81.4 501.3** 433.3** 85.4 107.5 37.1
198.6** 98.7 298.4** 1,047.0** 85.9 607.8** 632.2** 83.2 159.1** 45.0
13.7** 27.2** .0 14.9** 1.6 22.0** .4 .1 .3 .8
.961 ± .004 .946 ± .004 .931 ± .004
93.0 ± .60 92.7 ± .60 89.1 ± .60
97.2 ± .61 98.4 ± .61 95.9 ± .61
34.9 ± .08 34.6 ± .08 34.2 ± .08
**P<.01.
(e.g., fat, moisture, and protein) could be wholly, or in part, responsible for the line difference in body density. Definitive relationships of changes in ash, fat, moisture, and protein resulting from the divergent selection would depend upon the relative magnitudes of their heritabilities and genetic correlations with body weight and body volume. Heritability and density differences in these compositional traits may be responsible for the differential responses noted above for body weight and volume. A discrepancy between an experimental parameter in the selected and control lines in the test phase must be noted. Effective population sizes were different: that for Lines H and L was 24 throughout the experiment but ESR was maintained with an effective size of approximately 120. Hence, the progeny in the subclasses of the test phase were dissimilar in their inbreeding. An estimate of inbreeding accumulated in each of Lines H and L would be 6% and less than 1% for the ESR control during the three selected generations. Any differences arising from inbreeding depression would be expected to be quite small. Such an effect, if present, generally manifests itself most readily in reproductive traits (Sittmann et al., 1966; Kulenkamp et al., 1973). Examination of rate of lay, fertility, and hatchability in the test phase revealed no significant line differences. Therefore, it appears plausible to
assume that inbreeding depression was not responsible for line differences in the selected and correlated traits. Realized heritability estimates, derived from the accumulated selection differentials and the genetic gain estimated in the test generation, averaged 8 ± 5% and 7 ± 4% over replicates for high and low density, respectively. These values are considerably less than the value of 38 reported by Garwood and Diehl (1987). Part of the discrepancy may reside in the fact that the selection differentials and genetic gains were estimated in different generations. As a consequence, any differences in environmental effects between the generations are inherent in the estimates. In the same report (Garwood and Diehl, 1987), the component traits of density were indicated to be highly heritable (82%, body weight; 66%, body volume). However, a similar situation has been reported in swine for the feed conversion ratio, wherein Bernard and Fahmy (1970) and Jungst et al. (1981) reported realized heritabilities substantially less than estimates from the covariance among relatives (Dickerson and Grimes, 1947; Park, 1965; Robison and Berruecos, 1973; McPhee et al., 1979). Ratios as selection criteria and their erratic responses have been perplexing problems to animal breeders, and attempts to linearize the ratios via indices and thereby produce more conventional responses have been made. Use
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Mean squares of Type III ANOVA Direction of selection (D) High v*. low Control vs. selected Replicates (R) D x R Sexes (S) D x S R x S D x R x S Error Least squares means, x ± SE High selected Control Low selected
df
1038
GARWOOD ET AL.
TABLE 4. Genetic change in body density by direct selection on the body density ratio and by a linear index of the ratio as simulated in increments (I) of five percent selection intensity (SI) on themale side only
SI
(%)
1 2 3 4 5 6 7 8 9 10
5 10 15 20 25 30 35 40 45 50
.014 .012 .011 .010 .009 .008 .007 .007 .006 .006
Linear index selection .016 .014 .012 .011 .010 .009 .008 .008 .007 .006
of a linear index as the basis of selection for a ratio-defined trait initially was studied by Turner (1959). Lin (1980) approximated the ACKNOWLEDGMENTS linear weights for the component traits that would maximize the genetic change in the The authors are appreciative of the efforts ratio. Gunsett (1984), using simulation, deter- of D. Mitchell for assistance in breeding and mined the efficiency of direct selection for a caring for the quail and in obtaining the ratio as compared with a linear index wherein measurement data. The authors are pleased to the correlation between the genotype of the acknowledge the efforts of F. Gunsett in ratio and the phenotype of the index was conducting the simulation aspects of the study. maximized. Interesting results were obtained for various combinations of parameters for the numerator and denominator traits of the ratio. REFERENCES Table 4 presents the genetic change in the C , and M. H. Fahmy, 1970. Effect of selection on (body density) ratio resulting from direct Barnard, feed utilization and carcass score in swine. Can. J. selection and from selection on a linear index Anim. Sci. 50:575-584. in increments of selection intensity of 5% on Day, C. L., 1964. Device for measuring voids in porous materials. Agric. Eng. 45:36-37. the male side only based on simulation. The derived coefficients for the index were 3.1 and Dickerson, G. E., and J. C. Grimes, 1947. Effectiveness of selection for efficiency of gain in Duroc swine. J. -2.126 for body weight and body volume, Anim. Sci. 6:265-287. respectively. Diehl, K. C , V. A. Garwood, and C. G. Haugh, 1988. Volume measurement using the air-comparison pycThe present absolute genetic gains (.015 nometer. Trans, of ASAE 31:284-287. g-cm~3 for both high and low density selecV. A., and K. C. Diehl, Jr., 1987. Body volume tion) are in agreement with those obtained by Garwood, and density of live Coturnix quail and associated simulated selection. However, all else being genetic relationships. Poultry Sci. 66:1264-1271. equal, the actual gains would be expected to be Gunsett, F. C , 1984. Linear index selection to improve traits defined as ratios. J. Anim. Sci. 59:1185-1193. slightly less than the simulated gains in that the selection intensities differed. In the actual Jungst, S. B., L. L. Christian, and D. L. Kuhlers, 1981. Response to selection for feed efficiency in individual experiment, approximately 15% of the males fed Yorkshire boars, J. Anim. Sci. 53:323-331. and about 50% of the females were selected in Kulenkamp, A. W., C. M. Kulenkamp, and T. H. Coleman, each generation-replicate subclass. From in1973. The effects of inbreeding (brother x sister) on various traits in Japanese quail. Poultry Sci. 52: spection of the simulated estimated gains 1240-1246. (Table 4), it can be seen that index selection Lin, C. Y., 1980. Relative efficiency of selection methods would be little more efficient than direct for improved feed efficiency. J. Dairy Sci. 63: selection with high selection intensity. As 491-494. selection intensity decreases, efficiency of the McPhee, C. P., P. J. Brennan, and F. Duncalfe, 1979. Genetic
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I
Direct selection
index decreases rapidly. This situation is undoubtedly a function of the characteristics of the ratio. With traits of the numerator and denominator being highly correlated genetically and phenotypically, there is little variation in the ratio upon which selection can act. Furthermore, change in either direction for body density requires changes in body weight and volume of opposite directions, a circumstance in conflict with the high positive correlation between the two. Thus, in summary, from both the biological and simulated work performed, it can be concluded that genetic gain in the ratio body density can be effected by selection. However, this change, particularly wherein a high correlation exists between the numerator and denominator, is expected to be small. The use of index selection to increase efficiency of selection was not shown to be adequate.
BODY DENSITY SELECTION and phenotypic parameters of Australian Large White and Landrace boars performance tested when offered food ad libitum. Anim. Prod. 28:79-85. Park, Y. I., 1965. Age-constant feed efficiency of pigs. J. Anim. Sci. 24:819-822. Robison, O. W., and J. M. Bemiecos, 1973. Feed efficiency in swine. I. A comparison of measurements and methods of expressing feed efficiency. J. Anim. Sci. 37:
1039
643-649. Sittmann, K. H., H. Abplanalp, and R. A. Fraser, 1966. Inbreeding depression in Japanese quail. Genetics 54: 371-379. Turner, H. N., 1959. Ratios as criteria for selection in animal or plant breeding with particular reference to efficiency of food conversion in sheep. Aust. J. Agric. Res. 10:565-580.
Downloaded from http://ps.oxfordjournals.org/ at University of Manitoba on June 10, 2015