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Divergent Selection for Body Weight and Yolk Precursor in Coturnix coturnix japonica.
Divergent Selection for Body Weight and Yolk Precursor in Coturnix coturnixjaponica. 7. Influence of Genetic Changes in Body Weight and Yolk Precursor on Egg Production 1 K. E. NESTOR, W. L. BACON, N. B. ANTHONY,2 and G. B. HAVENSTEIN Department of Poultry Science, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691 (Received for publication August 15, 1986)
1987 Poultry Science 66:390-396 INTRODUCTION
Growth-selected strains of chickens (Jaap and Clancy, 1968), turkeys (Nestor et al., 1970), and Japanese quail (Bacon et al., 1973) accumulate more yolk material in their ovaries than do their egg production-selected counterparts. A large percentage of ovarian follicles are lost in growth strains than in normal strains due to ovulation into the body cavity, follicular atresia, production of defective eggs, or to a combination of these factors. Lipids synthesized in the liver of laying females are transported by the plasma in the form of lipoprotein complexes to the ovary where they are deposited intact into the follicles (Mclndoe, 1959; Christie and Moore, 1972; Hillyard etal., 1972). Bacon etal. (1982) found
Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Journal Article Number 102-86. 2 Present address: Department of Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
that the phenotypic correlation coefficient between total plasma phosphorus (P) and total plasma lipid from the yolk low density fractions of laying female quail was .96, indicating that P is a good measure of yolk precursor in the blood. Strong et al. (1978) also reported that the genetic correlation between plasma P and the plasma concentration of a lipophosphoprotein complex was large and positive. Nestor et al. (1982) developed lines of Japanese quail from a randombred control population (Rl) by divergent selection for 4-week body weight and total plasma P. Based on 7 generations of selection, realized heritabilities (h2) of 4-week body weight were .37 and .44 in the high (HW) and low (LW) weight lines, respectively. Respective realized h2 of high (HP) and low (LP) yolk precursor lines were .32 and .34. No significant changes occurred in plasma yolk precursor of weight lines or in body weight of yolk precursor lines. Egg production declined in HW and HP but not in LW and LP lines (Nestor and Bacon, 1982). The number of ovarian follicles in rapid development in the ovary increased in HW relative to the control Rl, but no consistent changes occurred in the other lines
390
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ABSTRACT Japanese quail lines were started from a randombred control by selecting for high (HW) or low (LW) 4-week body weight and high (HP) or low (LP) total plasma phosphorus (P), a measure of yolk precursor. In the 9th generation, 2 sublines were initiated from Line HW based on high (HW-HP) or low (HW-LP) level of P in females while males were continually selected for HW. A subline (LP-HW) was started from Line LP in the 9th generation by selecting the males for HW while continuing to select the females for LP. All lines were maintained with 36 parental pairs. Realized heritability (h ) values for 4-week body weight were .38 ± .02 in Line HW and .32 ,02 in Line LW for 15 generations. Respective h were .32 ± .0! and .34 ± .01 for P in Lines HP and LP. Although P increased in Line LW, there was no change in Line HW. Body weight of Lines HP and LP declined slightly, but significantly. Egg production decreased significantly in Lines HW, LW, HP, HW-HP, and LP-HW, but did not change in Lines LP and HW-LP. Decrease in egg production of Line HW-HP was greater than that of Lines HW or HP. These results suggest that if both body weight and P are selected for simultaneously, then egg production will decrease more than if selection pressure is applied on either trait alone, even though the two traits do not appear to be genetically closely correlated. (Key words: body weight, yolk precursor, egg production, quail, plasma)
SELECTION FOR BODY WEIGHT AND YOLK PRECURSOR
MATERIALS AND METHODS
Details on the origin and maintenance of Rl, HW, LW, HP, and LP lines were given in Nestor et al. (1982). Individual selection of both sexes was used in HW and LW lines and for the females of HP and LP lines. Males of HP and LP lines were selected on the basis of their sisters' performance. In the 9th generation of selection of Line HW, sublines were initiated by mass selecting the females for high (HW-HP) and low (HW-LP) plasma yolk precursor as measured by P, whereas males of both lines were selected for HW. Another subline (LP-HW) of Line LP was started in the 9th generation by selecting males for HW and females for LP. All selected lines and the randombred control were maintained with 36 pairs of parents (Nestor, 1977). Increase in inbreeding was measured by one-half the reciprocal of the effective population size (Falconer, 1964). Realized h2 of the selected traits was estimated by regressing accumulated gains (deviation from Rl) on the accumulated selection differential. The standard error of this regression coefficient provided an approximate standard error of the heritability estimates. Prior to the selection of breeders in each line, one female, if available, was randomly selected from each family each generation for measurement of correlated response in egg production and P. A few extra females were randomly selected to replace those families with no females. Egg production was recorded for 120 days measured from the date of first egg for each female except in the first generation of Lines HW, LW, HP, and LP, wherein recording was started when approximately 50% of the females had laid their first egg.
Means were compared within generations using a one-way analysis of variance and Tukey's test (Snedecor, 1959). Linear regression coefficients of means or response on generations were used to estimate trends across generations. Curvilinear response was estimated by fitting the data to quadratic equations. RESULTS AND DISCUSSION
Response to selection for HW was linear in Line HW (Figure 1, Table 1). The realized h2 for 4-week body weight in Line HW was .38 ± .02, which is nearly equal to the estimate (.37 ± .05) reported by Nestor et al. (1982) from the first 7 generations of this study. Response to selection for LW in Line LW appears to be declining in later generations. A significant quadratic trend existed in the response over generations. Realized h2 in Line LW was .32 ± .02 for the 15 generations versus .44 ± .28 (Nestor et al., 1982) from the first 7 generations of selection. Selection differentials, weighted by the number of offspring produced, averaged slightly higher in Line LW than in Line HW (9.5 vs. 8.9 g) over all generations. Selection differentials appear, however, to be declining in Line LW. For Generations 1 to 5, 6 to 10, and 11 to 15, average selection differentials for body weight were -10.4, -9.4, and -8.6 g, respectively. A part of the decline in selection
Body Weight
Generation of Selection
FIG. 1. Influence of selection for high (HW) and low (LW) 4-week body weight and high (HP) and low (LP) plasma phosphorus at the beginning of lay on 4-week body weight expressed as a deviation from a randombred control (Rl).
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(Bacon and Nestor, 1983). These results suggest that the significant decline in egg production of HW and HP lines may have resulted through two different unknown physiological pathways. The purpose of this study was to summarize data collected through 15 generations of selection in HW, LW, HP, and LP lines. In addition, sublines of HW were developed to determine if the effects on egg production of selection for increased growth rate could be increased or decreased by modifying yolk precursor level. A subline of LP was also developed to investigate the possibility that the average body weight of the LP line could be increased without reduction in egg production.
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TABLE 1. Characteristics of the regression of traits on generations of selection Regression coefficient3 1
a2
Line
Body weight, g
HW LW HP LP
Total plasma P, 4 figlmX
HP LP HW LW
41 35 53 76*
Egg production, number/hen
HW LW HP LP
-1.8 -2.4 -2.0 -1.7
1.99 -.37 .98 -.19
Linear
Quadratic
3.34** -5.48*** -.39*** -.37**
NS 5 .16** NS NS
46.1*** -48.0*** 1.26 10.2*
1.59** 1.36*** NS NS
-.99** .65 -.67*** .06
NS -.10* NS NS
1 HW = High body weight line; LW = low body weight line; HP = high phosphorus line; and LP = low phosphorus line. 2 3
Intercept at generation zero. Only the linear regression values are given if the quadratic regression was not significant.
4
Phosphorus.
5
NS = Nonsignificant.
* = P<.05. ** = P < . 0 1 . *** = P<.001.
differentials in Line LW can be attributed to an increased rate of mortality from hatch to four weeks of age (Nestor and Bacon, 1982). The decline in genetic variation for 4-week body weight of Line LW cannot be attributed to inbreeding, because the accumulated inbreeding was slightly greater in Line HW (17.7%) than in Line LW (15.0%). Inbreeding in R1 increased by 7.1% during the 15 generations. There was no significant trend in 4-week body weight with generations in Line Rl. Average 4-week body weight of Line Rl was 88.9 g. Response to selection for either HP or LP, as measured by total plasma P at the beginning of lay, exhibited quadratic trends (Figure 2, Table 1). In Line HP, response to selection for P was greater than that expected due to a significant quadratic trend in later generations of selection. A significant quadratic trend is also evident in Line LP, indicating a plateau was reached in selection response in that line. Realized h2 for total plasma P were .32 ± .01 and .34 ± .01, respectively, in Lines HP and LP for the 15 generations of selection. In the
T o t o l Plasmo
Phosphorus
/ /
S
X4
/A
S ss
-^1-
2
3
4
5
6
7
8
9
10
12
13
14
IS
Generation of Selection
FIG. 2. Effect of selecting for high (HW) and low (LW) 4-week body weight and high (HP) and low (LP) total plasma phosphorus at the beginning of lay on total plasma phosphorus at the beginning of lay as expressed as a deviation from a randombred control (Rl).
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Trait
SELECTION FOR BODY WEIGHT AND YOLK PRECURSOR Ego. Production
Generation of Selection
FIG. 3. Influence of divergent selection for 4-week body weight (HW, LW) and total plasma phosphorus (HP, LP) on 120-day egg production expressed as a deviation from a randombred control (Rl).
significantly with generations. Mean egg production of Line Rl was 113 eggs per hen. Decline in egg production for Line HW was associated with an increase in the production of abnormal eggs (unpublished data) and ovarian follicular production (Bacon and Nestor, 1983). Similar results were observed in chickens (Jaap and Muir, 1968; Reddy and Siegel, 1976), and turkeys (Nestor and Bacon, 1972) selected for increased weight. The decline in egg production of Line HP was similar to that of Line HW but was not associated with either an increase in the production of abnormal eggs (unpublished data) or a consistent increase in ovarian follicle production (Bacon and Nestor, 1983). Egg production of Line LW exhibited a significant quadratic response. There was no change in egg production during the first eight generations of selection for body weight in Line LW, after which egg production declined greatly. Females of Line LW lay few abnormal eggs, but have smaller clutches than Line Rl hens (unpublished data). Reddy and Siegel (1976) reported that a line of chickens selected for LW produced a smaller percentage of abnormal eggs than a line selected for HW. The LW females appear to be approaching the lower limit of body weight for sustaining egg production. Anthony et al. (1986) reported that LW birds did not reach asymptotic values of the growth curve prior to egg production and
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first 7 generations of selection realized h2 was .34 in both lines (Nestor et al, 1982). Plasma P in Line Rl increased significantly with generations. Parameters for the regression line were: a = 694, (3 = 6.04, with a mean of 742 |xg/ml. The reason for the change in Line Rl is unknown. Increases in inbreeding occurring in Lines HP and LP during the 15 generations were 12.7 and 12.8%, respectively. Weighted selection differentials increased with generation in Line HP (nonsignificant) and decreased with generation in Line LP (P<.01). This suggests that the variance is correlated with the mean. For Generations 1 to 5, 6 to 10, and 11 to 15, respective average selection differentials were 168, 231, and 230 (jig/ml in Line HP and -107, -80, and -53 (xg/ml in Line LP. The asymmetry in response observed in Lines HP and LP (Figure 2) can be largely explained by the difference in selection differentials. The plasma P level of Line LP might be reaching a biological limit where further reductions would result in a rapidly diminishing rate of egg production. When blood samples are collected for P analysis, the hen must be laying prior to and immediately after the sample collection. Thus the lowest plasma P level possible in Line LP would be equal to the lowest level observed in laying females. The plasma P level of lightstimulated Line Rl males was 219 (xg/ml (unpublished data). Genetic changes in plasma P both in the upward and downward direction resulted in small, but significant, decreases in body weight at 4 weeks of age (Figure 1, Table 1). As the change in body weight was in the same direction in both lines, it is unlikely that the change was due to a genetic correlation between plasma P and body weight. Genetic drift in body weight might be occurring in Lines HP and LP. Plasma P increased significantly in Line LW, but did not change significantly in Line HW (Figure 2, Table 1). Data from Line LW suggest that a negative genetic relationship exists between the two traits, whereas no genetic relationship is indicated by data from Line HW. When data from all lines are considered, it appears that there is little or no genetic association between body weight during the growing period and plasma P at the beginning of lay. Egg production of Lines HW, LW, and HP declined, relative to Line Rl, with generations of selection (Figure 3, Table 1). There was no significant change in egg production in Line LP. The egg production of Line Rl did not change
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TABLE 2. Response1 to selection in sublines of the high weight line (HW) selected for increased 4-week body weight (males only) and high (HW-HP) or low (HW-LP) levels of total plasma phosphorus in females HW-HP
HW-LP
4-week Body weight
Phosphorus level
Egg production
4-week Body weight
Phosphorus level
Egg production
(g) 34" 35" 38" 33" 35" 35" 44* * 1.08 .22 + .17
(jug/ml)
(no./hen)
(g)
(Mg/ml)
(no./hen)
Base 1 2 3 4 5 6 b2 Heritability
-43 34 112* 26 84** 214** 234** 41.8* .34+ .09
-6 -5 1 -9 -13* -18** -22** -4.2**
34** 33** 38** 35** 39** 35** 45** 1.57 .32 + .17
-43 -2 -28 -89* -79* -63* -111** -18.2* .18 + .05
-6 -2 -1 -13* -3 -14** -14* -2.5
1
Expressed as a deviation from the randombred control line (Rl).
2
The linear regression coefficient of response on generations of selection for Generations 1 to 6.
*P<.05. **P<.01.
probably continue to grow after sexual maturity. The HW and Rl birds achieve their asymptotic weight prior to sexual maturity. As body weight during the growing period and plasma yolk precursor level at the beginning of lay are not strongly correlated genetically, and selection for increased value of both traits results in decreased egg production, sublines of Line HW were developed by selecting for both traits. Responses to selection in Lines HW-HP and HW-LP are shown in Table 2. Selection for HW in both sublines did not result in any change in body weight until the 6th generation of selection, when both sublines showed major increases in body weight. Linear regression coefficients of response on generations of selection, although positive, were not significant. Realized h2 for 4-week body weight were .22 ± .17 and .32 ± . 17, respectively, in Lines HW-HP and HWLP. The corresponding realized h2 in Line HW was similar (.26 ± .09) during the same generations of selection. Response to selection for plasma P level was greater in Lines HW-HP than in HW-LP (Table 2). Total realized gain in HW-HP was 234 jjig/ml while in Line HW-LP the gain was -111 |xg/ml when measured as deviations from the Line Rl control. Realized h2 for total plasma P were .34 ± .09 in Line HW-HP and .18 ± .05 in Line HW-LP. During the first seven generations of selection, the realized h2 for total plasma P levels
in Lines HP and LP was .34 (Nestor et al., 1982) when levels are measured as deviations from the Line Rl base population. Egg production of Line HW-HP declined greatly in the last three generations of selection (Table 2). Relative to Line Rl there was a decline of 4.2 eggs (P<.01) per hen per generation. Egg production of Line HW-LP was less than that of Line Rl in three of the last four generations of selection but the linear regression coefficient (-2.5) of response on generations was not significant. In Line HW, during the same generations of selection, the regression coefficient of egg production (expressed as a difference from Line Rl) on generation was -2.2 (nonsignificant) and the difference in egg production from Line Rl was -14 eggs per hen, identical to that observed in line HW-LP. These results suggest that selection for both HW and HP is more detrimental to egg production than selecting for either trait alone. Genetically reducing the level of yolk precursor along with selecting for HW had no beneficial effect on egg production. A subline (LP-HW) of Line LP was initiated during the eighth generation at a time when a plateau was being reached in response to selection in Line LP. There was no significant change in P level of Line LP-HW (Table 3). Realized h2 was .05 ± .12. Selection for HW in Line LP-HW was successful. Body weight of Line
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Generation of selection
395
SELECTION FOR BODY WEIGHT AND YOLK PRECURSOR
TABLE 3. Response1 to selection in a subline of the low plasma phosphorus (P) line (LP-HW) in which males were selected for increased 4-week body weight and females for decreased P level Generation of selection
(g) .7* -1 3* 2 4* 6* p** 1.74** .30+ .05
Plasma P level
Egg production
0/g/ml)
(no./hen)
-292** -313** -312** -356** -336* -353** -313** -2.9 .05
-8 0 3 -4 0 -8 -12 -2.5* .12
1
Expressed as a deviation from the randombred control (Rl).
2
The linear regression coefficient of response on generations of selection for Generations 1 to 6.
*P<.05. **P<.01.
LP-HW increased by a total of 16 g over the six generations of selection. Realized h2 was .30 ± .05. Although egg production of Line LP-HW did not differ significantly from that of Line Rl in any generation, the linear regression coefficient of response on generation was -2.5 eggs/hen (P<.05), indicating that the genetic increases in body weight were associated with a decline in egg production. In summary, divergent selection for 4-week body weight of Japanese quail resulted in lower egg production in both lines. Selection for HP also resulted in a lowering of egg production, but genetically lowering the level of yolk precursor had no effect on egg production. Even though body weight during the growing period and plasma yolk precursor are not strongly related genetically, egg production declined to a greater extent when both were selected for simultaneously than when selection was made for either trait alone. REFERENCES Anthony, N. B., K. E. Nestor, and W. L. Bacon, 1986. Growth curves of Japanese quail as modified by divergent selection for 4-week body weight. Poultry Sci. 65:1825-1833. Bacon, W. L., and K. E. Nestor, 1983. Divergent selection for body weight and yolk precursor in Corturnix coturnix japonica. 5. Correlated response in adult body weight, liver weight, ovarian follicle production, and carcass composition of laying hens. Poultry Sci. 62:1876-1884.
Bacon, W. L., K. E. Nestor, D. W. Long, and D. H. Ohm, 1982. Divergent selection for body weight and yolk precursor in Coturnix coturnix japonica. 2. Correlated responses in plasma lipoproteins and lipids. Poultry Sci. 61:161-165. Bacon, W. L., K. E. Nestor, and P. A. Renner, 1973. Ovarian follicular development in egg and growth lines of Japanese quail. Poultry Sci. 52:1195-1199. Christie, W. W., and J. H. Moore, 1972. The lipid components of the plasma, liver and ovarian follicles in the domestic chicken (Gallus gallus). Comp. Biochem. Physiol. 41B:287-295. Falconer, D. S., 1964. Introduction to Quantitative Genetics. Oliver and Boyd, Edinburgh, Scotland. Hillyard, L. A., H. M. White, and S. A. Pangburn, 1972. Characterization of apolipoproteins in chicken serum and egg yolk. Biochemistry 11:511-518. Jaap, R. G., and J. A. Clancy, 1968. Reproductive idiosyncrasies of the broiler pullet. Proc. 3rd Europ. Poult. Conf., Jerusalem, Israel. Jaap, R. G., and F. V. Muir, 1968. Erratic oviposition and egg defects in broiler-type pullets. Poultry Sci. 47:417-423. Mclndoe, W. M., 1959. A Hpophosphoprotein complex in hen plasma associated with yolk production. Biochem. J. 72:153-159. Nestor, K. E., 1977. The use of apaired mating system for the maintenance of experimental populations of turkeys. Poultry Sci. 56:60-65. Nestor, K. E., and W. L. Bacon, 1972. Production of defective eggs by egg and meat type turkey hens. Poultry Sci. 51:1361-1365. Nestor, K. E., and W. L. Bacon, 1982. Divergent selection for body weight and yolk precursor in Corturnix coturnix japonica. 3. Correlated responses in mortality and reproduction. Poultry Sci. 61:2137-2142. Nestor, K. E., W. L. Bacon, and A. L. Lambio, 1982. Divergent selection for body weight and yolk precursor
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Base 1 2 3 4 5 6 b2 Heritability
4-week Body weight
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in Coturnix coturnix japonica. 1. Selection response. Poultry Sci. 61:12-17. Nestor, K. E., W. L. Bacon, and P. A. Renner, 1970. Ovarian follicular development in egg and meat type turkeys. Poultry Sci. 49:775-780. Reddy, P.R.K., and P. B. Siegel, 1976. Selection for body weight at eight weeks of age. 11. Ovulation and
oviposition patterns. Poultry Sci. 55:1518-1530. Snedecor, G. W., 1959. Statistical Methods. Iowa State Coll. Press, Ames, IA. Strong, C. F., Jr., K. E. Nestor, and W. L. Bacon, 1978. Inheritance of egg production, egg weight, body weight and certain plasma constituents in Corturnix. Poultry Sci. 57:1-9.
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1987 Poultry Science 66:390-396 INTRODUCTION
Growth-selected strains of chickens (Jaap and Clancy, 1968), turkeys (Nestor et al., 1970), and Japanese quail (Bacon et al., 1973) accumulate more yolk material in their ovaries than do their egg production-selected counterparts. A large percentage of ovarian follicles are lost in growth strains than in normal strains due to ovulation into the body cavity, follicular atresia, production of defective eggs, or to a combination of these factors. Lipids synthesized in the liver of laying females are transported by the plasma in the form of lipoprotein complexes to the ovary where they are deposited intact into the follicles (Mclndoe, 1959; Christie and Moore, 1972; Hillyard etal., 1972). Bacon etal. (1982) found
Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Journal Article Number 102-86. 2 Present address: Department of Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
that the phenotypic correlation coefficient between total plasma phosphorus (P) and total plasma lipid from the yolk low density fractions of laying female quail was .96, indicating that P is a good measure of yolk precursor in the blood. Strong et al. (1978) also reported that the genetic correlation between plasma P and the plasma concentration of a lipophosphoprotein complex was large and positive. Nestor et al. (1982) developed lines of Japanese quail from a randombred control population (Rl) by divergent selection for 4-week body weight and total plasma P. Based on 7 generations of selection, realized heritabilities (h2) of 4-week body weight were .37 and .44 in the high (HW) and low (LW) weight lines, respectively. Respective realized h2 of high (HP) and low (LP) yolk precursor lines were .32 and .34. No significant changes occurred in plasma yolk precursor of weight lines or in body weight of yolk precursor lines. Egg production declined in HW and HP but not in LW and LP lines (Nestor and Bacon, 1982). The number of ovarian follicles in rapid development in the ovary increased in HW relative to the control Rl, but no consistent changes occurred in the other lines
390
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ABSTRACT Japanese quail lines were started from a randombred control by selecting for high (HW) or low (LW) 4-week body weight and high (HP) or low (LP) total plasma phosphorus (P), a measure of yolk precursor. In the 9th generation, 2 sublines were initiated from Line HW based on high (HW-HP) or low (HW-LP) level of P in females while males were continually selected for HW. A subline (LP-HW) was started from Line LP in the 9th generation by selecting the males for HW while continuing to select the females for LP. All lines were maintained with 36 parental pairs. Realized heritability (h ) values for 4-week body weight were .38 ± .02 in Line HW and .32 ,02 in Line LW for 15 generations. Respective h were .32 ± .0! and .34 ± .01 for P in Lines HP and LP. Although P increased in Line LW, there was no change in Line HW. Body weight of Lines HP and LP declined slightly, but significantly. Egg production decreased significantly in Lines HW, LW, HP, HW-HP, and LP-HW, but did not change in Lines LP and HW-LP. Decrease in egg production of Line HW-HP was greater than that of Lines HW or HP. These results suggest that if both body weight and P are selected for simultaneously, then egg production will decrease more than if selection pressure is applied on either trait alone, even though the two traits do not appear to be genetically closely correlated. (Key words: body weight, yolk precursor, egg production, quail, plasma)
SELECTION FOR BODY WEIGHT AND YOLK PRECURSOR
MATERIALS AND METHODS
Details on the origin and maintenance of Rl, HW, LW, HP, and LP lines were given in Nestor et al. (1982). Individual selection of both sexes was used in HW and LW lines and for the females of HP and LP lines. Males of HP and LP lines were selected on the basis of their sisters' performance. In the 9th generation of selection of Line HW, sublines were initiated by mass selecting the females for high (HW-HP) and low (HW-LP) plasma yolk precursor as measured by P, whereas males of both lines were selected for HW. Another subline (LP-HW) of Line LP was started in the 9th generation by selecting males for HW and females for LP. All selected lines and the randombred control were maintained with 36 pairs of parents (Nestor, 1977). Increase in inbreeding was measured by one-half the reciprocal of the effective population size (Falconer, 1964). Realized h2 of the selected traits was estimated by regressing accumulated gains (deviation from Rl) on the accumulated selection differential. The standard error of this regression coefficient provided an approximate standard error of the heritability estimates. Prior to the selection of breeders in each line, one female, if available, was randomly selected from each family each generation for measurement of correlated response in egg production and P. A few extra females were randomly selected to replace those families with no females. Egg production was recorded for 120 days measured from the date of first egg for each female except in the first generation of Lines HW, LW, HP, and LP, wherein recording was started when approximately 50% of the females had laid their first egg.
Means were compared within generations using a one-way analysis of variance and Tukey's test (Snedecor, 1959). Linear regression coefficients of means or response on generations were used to estimate trends across generations. Curvilinear response was estimated by fitting the data to quadratic equations. RESULTS AND DISCUSSION
Response to selection for HW was linear in Line HW (Figure 1, Table 1). The realized h2 for 4-week body weight in Line HW was .38 ± .02, which is nearly equal to the estimate (.37 ± .05) reported by Nestor et al. (1982) from the first 7 generations of this study. Response to selection for LW in Line LW appears to be declining in later generations. A significant quadratic trend existed in the response over generations. Realized h2 in Line LW was .32 ± .02 for the 15 generations versus .44 ± .28 (Nestor et al., 1982) from the first 7 generations of selection. Selection differentials, weighted by the number of offspring produced, averaged slightly higher in Line LW than in Line HW (9.5 vs. 8.9 g) over all generations. Selection differentials appear, however, to be declining in Line LW. For Generations 1 to 5, 6 to 10, and 11 to 15, average selection differentials for body weight were -10.4, -9.4, and -8.6 g, respectively. A part of the decline in selection
Body Weight
Generation of Selection
FIG. 1. Influence of selection for high (HW) and low (LW) 4-week body weight and high (HP) and low (LP) plasma phosphorus at the beginning of lay on 4-week body weight expressed as a deviation from a randombred control (Rl).
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(Bacon and Nestor, 1983). These results suggest that the significant decline in egg production of HW and HP lines may have resulted through two different unknown physiological pathways. The purpose of this study was to summarize data collected through 15 generations of selection in HW, LW, HP, and LP lines. In addition, sublines of HW were developed to determine if the effects on egg production of selection for increased growth rate could be increased or decreased by modifying yolk precursor level. A subline of LP was also developed to investigate the possibility that the average body weight of the LP line could be increased without reduction in egg production.
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TABLE 1. Characteristics of the regression of traits on generations of selection Regression coefficient3 1
a2
Line
Body weight, g
HW LW HP LP
Total plasma P, 4 figlmX
HP LP HW LW
41 35 53 76*
Egg production, number/hen
HW LW HP LP
-1.8 -2.4 -2.0 -1.7
1.99 -.37 .98 -.19
Linear
Quadratic
3.34** -5.48*** -.39*** -.37**
NS 5 .16** NS NS
46.1*** -48.0*** 1.26 10.2*
1.59** 1.36*** NS NS
-.99** .65 -.67*** .06
NS -.10* NS NS
1 HW = High body weight line; LW = low body weight line; HP = high phosphorus line; and LP = low phosphorus line. 2 3
Intercept at generation zero. Only the linear regression values are given if the quadratic regression was not significant.
4
Phosphorus.
5
NS = Nonsignificant.
* = P<.05. ** = P < . 0 1 . *** = P<.001.
differentials in Line LW can be attributed to an increased rate of mortality from hatch to four weeks of age (Nestor and Bacon, 1982). The decline in genetic variation for 4-week body weight of Line LW cannot be attributed to inbreeding, because the accumulated inbreeding was slightly greater in Line HW (17.7%) than in Line LW (15.0%). Inbreeding in R1 increased by 7.1% during the 15 generations. There was no significant trend in 4-week body weight with generations in Line Rl. Average 4-week body weight of Line Rl was 88.9 g. Response to selection for either HP or LP, as measured by total plasma P at the beginning of lay, exhibited quadratic trends (Figure 2, Table 1). In Line HP, response to selection for P was greater than that expected due to a significant quadratic trend in later generations of selection. A significant quadratic trend is also evident in Line LP, indicating a plateau was reached in selection response in that line. Realized h2 for total plasma P were .32 ± .01 and .34 ± .01, respectively, in Lines HP and LP for the 15 generations of selection. In the
T o t o l Plasmo
Phosphorus
/ /
S
X4
/A
S ss
-^1-
2
3
4
5
6
7
8
9
10
12
13
14
IS
Generation of Selection
FIG. 2. Effect of selecting for high (HW) and low (LW) 4-week body weight and high (HP) and low (LP) total plasma phosphorus at the beginning of lay on total plasma phosphorus at the beginning of lay as expressed as a deviation from a randombred control (Rl).
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Trait
SELECTION FOR BODY WEIGHT AND YOLK PRECURSOR Ego. Production
Generation of Selection
FIG. 3. Influence of divergent selection for 4-week body weight (HW, LW) and total plasma phosphorus (HP, LP) on 120-day egg production expressed as a deviation from a randombred control (Rl).
significantly with generations. Mean egg production of Line Rl was 113 eggs per hen. Decline in egg production for Line HW was associated with an increase in the production of abnormal eggs (unpublished data) and ovarian follicular production (Bacon and Nestor, 1983). Similar results were observed in chickens (Jaap and Muir, 1968; Reddy and Siegel, 1976), and turkeys (Nestor and Bacon, 1972) selected for increased weight. The decline in egg production of Line HP was similar to that of Line HW but was not associated with either an increase in the production of abnormal eggs (unpublished data) or a consistent increase in ovarian follicle production (Bacon and Nestor, 1983). Egg production of Line LW exhibited a significant quadratic response. There was no change in egg production during the first eight generations of selection for body weight in Line LW, after which egg production declined greatly. Females of Line LW lay few abnormal eggs, but have smaller clutches than Line Rl hens (unpublished data). Reddy and Siegel (1976) reported that a line of chickens selected for LW produced a smaller percentage of abnormal eggs than a line selected for HW. The LW females appear to be approaching the lower limit of body weight for sustaining egg production. Anthony et al. (1986) reported that LW birds did not reach asymptotic values of the growth curve prior to egg production and
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first 7 generations of selection realized h2 was .34 in both lines (Nestor et al, 1982). Plasma P in Line Rl increased significantly with generations. Parameters for the regression line were: a = 694, (3 = 6.04, with a mean of 742 |xg/ml. The reason for the change in Line Rl is unknown. Increases in inbreeding occurring in Lines HP and LP during the 15 generations were 12.7 and 12.8%, respectively. Weighted selection differentials increased with generation in Line HP (nonsignificant) and decreased with generation in Line LP (P<.01). This suggests that the variance is correlated with the mean. For Generations 1 to 5, 6 to 10, and 11 to 15, respective average selection differentials were 168, 231, and 230 (jig/ml in Line HP and -107, -80, and -53 (xg/ml in Line LP. The asymmetry in response observed in Lines HP and LP (Figure 2) can be largely explained by the difference in selection differentials. The plasma P level of Line LP might be reaching a biological limit where further reductions would result in a rapidly diminishing rate of egg production. When blood samples are collected for P analysis, the hen must be laying prior to and immediately after the sample collection. Thus the lowest plasma P level possible in Line LP would be equal to the lowest level observed in laying females. The plasma P level of lightstimulated Line Rl males was 219 (xg/ml (unpublished data). Genetic changes in plasma P both in the upward and downward direction resulted in small, but significant, decreases in body weight at 4 weeks of age (Figure 1, Table 1). As the change in body weight was in the same direction in both lines, it is unlikely that the change was due to a genetic correlation between plasma P and body weight. Genetic drift in body weight might be occurring in Lines HP and LP. Plasma P increased significantly in Line LW, but did not change significantly in Line HW (Figure 2, Table 1). Data from Line LW suggest that a negative genetic relationship exists between the two traits, whereas no genetic relationship is indicated by data from Line HW. When data from all lines are considered, it appears that there is little or no genetic association between body weight during the growing period and plasma P at the beginning of lay. Egg production of Lines HW, LW, and HP declined, relative to Line Rl, with generations of selection (Figure 3, Table 1). There was no significant change in egg production in Line LP. The egg production of Line Rl did not change
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394
TABLE 2. Response1 to selection in sublines of the high weight line (HW) selected for increased 4-week body weight (males only) and high (HW-HP) or low (HW-LP) levels of total plasma phosphorus in females HW-HP
HW-LP
4-week Body weight
Phosphorus level
Egg production
4-week Body weight
Phosphorus level
Egg production
(g) 34" 35" 38" 33" 35" 35" 44* * 1.08 .22 + .17
(jug/ml)
(no./hen)
(g)
(Mg/ml)
(no./hen)
Base 1 2 3 4 5 6 b2 Heritability
-43 34 112* 26 84** 214** 234** 41.8* .34+ .09
-6 -5 1 -9 -13* -18** -22** -4.2**
34** 33** 38** 35** 39** 35** 45** 1.57 .32 + .17
-43 -2 -28 -89* -79* -63* -111** -18.2* .18 + .05
-6 -2 -1 -13* -3 -14** -14* -2.5
1
Expressed as a deviation from the randombred control line (Rl).
2
The linear regression coefficient of response on generations of selection for Generations 1 to 6.
*P<.05. **P<.01.
probably continue to grow after sexual maturity. The HW and Rl birds achieve their asymptotic weight prior to sexual maturity. As body weight during the growing period and plasma yolk precursor level at the beginning of lay are not strongly correlated genetically, and selection for increased value of both traits results in decreased egg production, sublines of Line HW were developed by selecting for both traits. Responses to selection in Lines HW-HP and HW-LP are shown in Table 2. Selection for HW in both sublines did not result in any change in body weight until the 6th generation of selection, when both sublines showed major increases in body weight. Linear regression coefficients of response on generations of selection, although positive, were not significant. Realized h2 for 4-week body weight were .22 ± .17 and .32 ± . 17, respectively, in Lines HW-HP and HWLP. The corresponding realized h2 in Line HW was similar (.26 ± .09) during the same generations of selection. Response to selection for plasma P level was greater in Lines HW-HP than in HW-LP (Table 2). Total realized gain in HW-HP was 234 jjig/ml while in Line HW-LP the gain was -111 |xg/ml when measured as deviations from the Line Rl control. Realized h2 for total plasma P were .34 ± .09 in Line HW-HP and .18 ± .05 in Line HW-LP. During the first seven generations of selection, the realized h2 for total plasma P levels
in Lines HP and LP was .34 (Nestor et al., 1982) when levels are measured as deviations from the Line Rl base population. Egg production of Line HW-HP declined greatly in the last three generations of selection (Table 2). Relative to Line Rl there was a decline of 4.2 eggs (P<.01) per hen per generation. Egg production of Line HW-LP was less than that of Line Rl in three of the last four generations of selection but the linear regression coefficient (-2.5) of response on generations was not significant. In Line HW, during the same generations of selection, the regression coefficient of egg production (expressed as a difference from Line Rl) on generation was -2.2 (nonsignificant) and the difference in egg production from Line Rl was -14 eggs per hen, identical to that observed in line HW-LP. These results suggest that selection for both HW and HP is more detrimental to egg production than selecting for either trait alone. Genetically reducing the level of yolk precursor along with selecting for HW had no beneficial effect on egg production. A subline (LP-HW) of Line LP was initiated during the eighth generation at a time when a plateau was being reached in response to selection in Line LP. There was no significant change in P level of Line LP-HW (Table 3). Realized h2 was .05 ± .12. Selection for HW in Line LP-HW was successful. Body weight of Line
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Generation of selection
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SELECTION FOR BODY WEIGHT AND YOLK PRECURSOR
TABLE 3. Response1 to selection in a subline of the low plasma phosphorus (P) line (LP-HW) in which males were selected for increased 4-week body weight and females for decreased P level Generation of selection
(g) .7* -1 3* 2 4* 6* p** 1.74** .30+ .05
Plasma P level
Egg production
0/g/ml)
(no./hen)
-292** -313** -312** -356** -336* -353** -313** -2.9 .05
-8 0 3 -4 0 -8 -12 -2.5* .12
1
Expressed as a deviation from the randombred control (Rl).
2
The linear regression coefficient of response on generations of selection for Generations 1 to 6.
*P<.05. **P<.01.
LP-HW increased by a total of 16 g over the six generations of selection. Realized h2 was .30 ± .05. Although egg production of Line LP-HW did not differ significantly from that of Line Rl in any generation, the linear regression coefficient of response on generation was -2.5 eggs/hen (P<.05), indicating that the genetic increases in body weight were associated with a decline in egg production. In summary, divergent selection for 4-week body weight of Japanese quail resulted in lower egg production in both lines. Selection for HP also resulted in a lowering of egg production, but genetically lowering the level of yolk precursor had no effect on egg production. Even though body weight during the growing period and plasma yolk precursor are not strongly related genetically, egg production declined to a greater extent when both were selected for simultaneously than when selection was made for either trait alone. REFERENCES Anthony, N. B., K. E. Nestor, and W. L. Bacon, 1986. Growth curves of Japanese quail as modified by divergent selection for 4-week body weight. Poultry Sci. 65:1825-1833. Bacon, W. L., and K. E. Nestor, 1983. Divergent selection for body weight and yolk precursor in Corturnix coturnix japonica. 5. Correlated response in adult body weight, liver weight, ovarian follicle production, and carcass composition of laying hens. Poultry Sci. 62:1876-1884.
Bacon, W. L., K. E. Nestor, D. W. Long, and D. H. Ohm, 1982. Divergent selection for body weight and yolk precursor in Coturnix coturnix japonica. 2. Correlated responses in plasma lipoproteins and lipids. Poultry Sci. 61:161-165. Bacon, W. L., K. E. Nestor, and P. A. Renner, 1973. Ovarian follicular development in egg and growth lines of Japanese quail. Poultry Sci. 52:1195-1199. Christie, W. W., and J. H. Moore, 1972. The lipid components of the plasma, liver and ovarian follicles in the domestic chicken (Gallus gallus). Comp. Biochem. Physiol. 41B:287-295. Falconer, D. S., 1964. Introduction to Quantitative Genetics. Oliver and Boyd, Edinburgh, Scotland. Hillyard, L. A., H. M. White, and S. A. Pangburn, 1972. Characterization of apolipoproteins in chicken serum and egg yolk. Biochemistry 11:511-518. Jaap, R. G., and J. A. Clancy, 1968. Reproductive idiosyncrasies of the broiler pullet. Proc. 3rd Europ. Poult. Conf., Jerusalem, Israel. Jaap, R. G., and F. V. Muir, 1968. Erratic oviposition and egg defects in broiler-type pullets. Poultry Sci. 47:417-423. Mclndoe, W. M., 1959. A Hpophosphoprotein complex in hen plasma associated with yolk production. Biochem. J. 72:153-159. Nestor, K. E., 1977. The use of apaired mating system for the maintenance of experimental populations of turkeys. Poultry Sci. 56:60-65. Nestor, K. E., and W. L. Bacon, 1972. Production of defective eggs by egg and meat type turkey hens. Poultry Sci. 51:1361-1365. Nestor, K. E., and W. L. Bacon, 1982. Divergent selection for body weight and yolk precursor in Corturnix coturnix japonica. 3. Correlated responses in mortality and reproduction. Poultry Sci. 61:2137-2142. Nestor, K. E., W. L. Bacon, and A. L. Lambio, 1982. Divergent selection for body weight and yolk precursor
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Base 1 2 3 4 5 6 b2 Heritability
4-week Body weight
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in Coturnix coturnix japonica. 1. Selection response. Poultry Sci. 61:12-17. Nestor, K. E., W. L. Bacon, and P. A. Renner, 1970. Ovarian follicular development in egg and meat type turkeys. Poultry Sci. 49:775-780. Reddy, P.R.K., and P. B. Siegel, 1976. Selection for body weight at eight weeks of age. 11. Ovulation and
oviposition patterns. Poultry Sci. 55:1518-1530. Snedecor, G. W., 1959. Statistical Methods. Iowa State Coll. Press, Ames, IA. Strong, C. F., Jr., K. E. Nestor, and W. L. Bacon, 1978. Inheritance of egg production, egg weight, body weight and certain plasma constituents in Corturnix. Poultry Sci. 57:1-9.
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