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Heterosis in Normal Versus Dwarf Laying Hens
BREEDING AND GENETICS Heterosis in Normal Versus Dwarf Laying Hens P. MERAT,1 F. MINVIELLE, A. BORDAS, and G. COQUERELLE Institut National de la Recherche Agronomique, Jouy en Josas 78350, France
1994 Poultry Science 73:1-6
INTRODUCTION
tive disadvantage associated to dw in the Leghorn line appeared to be smaller in the In egg lines, the sex-linked dwarfism Fx cross, but heterosis could not be gene, dw, is associated generally with a estimated, as performance of Fayoumi decrease in the number of eggs, this being hens was not measured in that experiespecially so in light body weight Leghorn ment. In the present study, egg production lines (Merat, 1990). However, this disad- traits of normal and dwarf hens of two vantage was decreased in the Fx genera- parental lines, a White Leghorn and a tion as compared with parental lines brown egg line, and their reciprocal segregating for the dwarf gene (Yoo et ah, crosses were compared to determine the 1980). Also, on the average, the depressing influence of heterosis on the dw-associated effect of dw on egg production increases as egg production reduction and the inthe line mean body weight decreases fluence of body weight in accounting for (M6rat, 1990). Therefore, a direct effect of this reduction in production. dw, different in parental lines or in their crosses, can be proven only if line differMATERIALS AND METHODS ences in body size are accounted for or if parental lines have similar body weight. The latter condition was fulfilled in a Lines and Genotypes comparison between a White Leghorn line The White Leghorn line was obtained in segregating for the dwarf gene and its Fj 1974 from the Bundesforschungsanstalt fur cross with a Fayoumi line of similar body Kleintierzucht, Celle, Germany. Since 1980, weight (Nferat and Bordas, 1991). For each generation was obtained from Dw/dw several egg production variables, the rela- sires and dw/w dams. In 1986, females of a brown egg dwarf line under selection for egg production (clutch length) in the authors' laboratory since 1984 (Kipi, 1990) Received for publication January 25, 1993. were crossed to males of a related brown Accepted for publication September 14, 1993. 1 Laboratoire de G6n6tique Factorielle, INRA, Jouy egg line heterozygous for the silver/gold and sex-linked dwarf genes. Each year en Josas 78350, France. 1
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
ABSTRACT The effect of genotype at the sex-linked dwarf locus on heterosis in crosses between a White Leghorn and a brown egg line for body weight, egg production, and related traits was studied. Heterozygous Dw/dw males were used to produce normal and dwarf pullets in each of the pure lines and their reciprocal crosses (eight genotype-line combinations). There were 54 pullets per combination. Line differences were significant for shank length, body weights at 8,17, and 52 wk, age at first egg, egg number, clutch length, rate of lay, and egg weight. Heterosis was observed for all of these traits. Body weight as a covariate was not important in analyses of egg number, clutch length, and egg weight. The egg production reduction associated with the dw gene in pure lines was smaller in Fi hens. This discovery may be adequate to warrant use of dwarf crossbred hens for egg production. {Key words: sex-linked dwarfism, crossing, heterosis, egg production, body weight)
2
MERAT ET AL.
Measurements Traits measured are listed in Table 1. Data associated with egg production were collected up to the age of 51 wk (about 7 mo of production). Mean egg weight was estimated from a 2-wk egg collection beginning at the age of 43 wk. Hens were weighed individually at 8,12, and 52 wk of age. Pauses were taken as periods of at least 2 consecutive d without an oviposition, and the total number of days of pause was expressed as a percentage of total number of days in the laying period. Statistical Analysis For every variable, a two-way analysis of variance with interaction was performed.
Main effects were genotype at the sexlinked dwarfism locus (normal or dwarf) and line (Leghorn, brown egg, Leghorn x brown egg, brown egg x Leghorn). Variables expressed as percentage also were transformed with the arc sine square root transformation and then analyzed without changing the significance of the effects. Therefore, only results on untransformed data are given. Means of lines within each genotype were compared by the StudentNewman-Keuls procedure. A covariance analysis with body weight as a covariable was done on egg number, clutch length, and mean egg weight. As it did not alter appreciably the results, it is not detailed herein. Heterosis was estimated within genotype (normal and dwarf), both overall, as difference between crossbreds and purebreds, and, for each reciprocal cross, from appropriate linear combinations of least squares means for lines. Significance of heterosis was evaluated by the Student's t test with n - r degrees of freedom, with n being the total number of observations (around 430 depending on the variable) and r the total number of subclasses (8). In the same way, heterosis for dwarf and for normal hens was compared by a F test with 1 and n - r degrees of freedom. All analyses were performed using the General Linear Models (GLM) procedure (SAS Institute, 1988). RESULTS AND DISCUSSION Table 1 lists least squares means for all traits, by combination of genotype and line, with the level of significance for the main effects and for the interactions. Heterotic effects within genotype, overall and for each reciprocal cross, with the associated level of significance, are presented in Table 2. In general, the degrees of heterosis did not differ when normal and dwarf crossbred hens were compared (Table 3). Heterotic effects did differ for the two phenotypes for rate of lay (P < .05), but differences for 8-wk shank length (P < .01) and body weight (P < .05) were present in specific, but different, reciprocal cross populations.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
thereafter, male progeny heterozygous at these two loci were backcrossed to females of the selection line to produce double heterozygote males with the silver phenotype and normal and dwarf brown egg females with, respectively, the silver and gold phenotypes, in the absence of recombination. The dwarf mutation occurred independently in the Leghorn line (USA) and in the Brown Egg line (Jouy en Josas, in 1959). In 1990,11 Dw/dw males from each line were mated to 4 Leghorn and 3 brown egg females each, so that both purebred and reciprocal crossbred full-sib and half-sib progeny were obtained. In each sib group, the frequency of normal and dwarf was expected to be 50%. All progeny were obtained in a single hatch. Shank length at 8 wk of age, confirmed by a second measurement at 17 wk, was used to differentiate dwarf from normal birds. The two phenotypes were represented by equal numbers within both full-sib and half-sib families prior to being tested for egg production and related traits. Numbers of hens tested per line and genotype (Dw/w or dw/w) are presented in Table 1. Female chicks were group reared under standard conditions. At 18 wk of age, they were transferred at random into individual cages. They were housed in a single unit (14 h light:10 h dark) at 22 ± 1 C and were provided ad libitum access to a commercial egg layer diet (15.5% total protein, 2,600 kcal ME/kg, 3.4% calcium).
BE
5
10
12
1
54 71.2' 421c 339.1 157.7" 141.2b 81.0<: 4.06b 13.19 .24 b .34 b 1.05b 47.29b 1,294' 5
52 77.6" 545b 1,080b 142.0= 175.9a 87.4" 7.27" 6.94 1.57" 1.73" 1.50* 52.90" 1,754"
L x BE
5
54 75.7b 640b 1,037c 147.0b 163.0" 84.7b 6.61" 9.03 .77b .47b 2.35" 52.40" 1,585b
BE x L
Dwarf
"-'•Means for lines within genotype with no common superscript differ significantly (P < .05). 1L = White Leghorn line; BE = brown egg line. •P < .05. **P < .01. ***P < .001.
1
BE x L
4 5 4 5 4 90.6" 91.8" 91.8" 854' 914b 980a 1,432b 1,542b 1,735a 147.3b 146.3b 140.8b 175.6" 173.5" 173.8" 90.9" 90.1" 89.8" 7.41" 7.72" 5.94b 9.70 8.92 10.94 6.18" 5.11* 5.47* 6.09 4.36 4.86 5.49 4.57 7.71 56.14" 57.09" 55.98" 2,443b 2,421 b 2,684"
5
Number tested 8-wk shank length, m m 8-wk body weight, g 17-wk body weight, g Age at first egg, d Egg number Egg laying rate, % Clutch length, d Days of pause, % Number of double-yolk eggs Soft-shelled eggs, % Broken eggs, % Egg weight, g 52-wk body weight, g Number dead during egg production test
5 4 85.6b d 720 1,222' 155.6" 151.2b 86.5b 5.45b 14.44 3.07b 5.25 5.04 52.16b 1,941'
L x BE
Trait
Normal
TABLE 1. Least squares means of body weights and egg production traits according
om http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
4
MERAT ET AL. TABLE 2. Crossbred superiority (percentage) for normal and dwarf hens evaluated for combined and separate reciprocal crosses1 Dwarf
Normal L x BE
BE x L
Both
L x BE
BE x L
Both
2 o*** 2.6* .9 -.6 8.1* 3.1** 30.2* -22.7 -14.1 3.8*** 5.6
3.1*** 7.6*** 4.3* -1.2 6.8+ 2.2* 35.4** -24.0 -28.3 5.6*** 4.7*
2.6*** 5.1*** 2.6* -.9 7.5** 2.7** 32.8** -23.4 -21.2 4.7*** 5.2**
3.8*** 5.5** 5.7** -4.6*** 14.7*** 7.4*** 80.0*** -32.2 -13.5 6.2*** 13.7***
3.0** 5.0** 1.5 -1.2 6.3+ 41*»* 63.6*** -12.9 -70.7 5.1** 2.8*
3.4*** 5.3*** 3.6* -2.9* 10.5*** 5.7*** 71.8*** -23.1 -42.1 5.2*** 8.1***
*L = White Leghorn line; BE +P < .10. *P < .05. **P < .01. ***P < .001.
brown egg line.
Line Differences
White Leghorn line. The present data also suggest that mortality was higher in the Differences between White Leghorn and brown egg line, a difference not reported brown egg lines expected from previous knowledge were confirmed (Table 1). In- previously. These differences were obdeed, the brown egg line had higher body tained for both normal and dwarf hens. weights and mean egg weight, commenced However, the percentage of soft-shelled lay 2 wk earlier, laid more eggs, had fewer eggs was higher only in the normal hens of pauses, and a higher rate of lay than the the brown egg line.
TABLE 3. Comparison of crossbred superiority of normal versus dwarf hens by combined and separate reciprocal crosses1 Value of F and significance of the contrast Trait
Overall1
8-wk shank length 8-wk body weight 17-wk body weight Age at first egg Egg number Egg laying rate Days of pause Number double-yolk eggs Soft-shelled eggs Broken eggs Clutch length Egg weight 52-wk body weight
1.08 .38 .00 1.01 .39 4.55 .08 1.25 .25 1.32 1.33 .09 .02
*L = White Leghorn line; BE +P < .10. *P < .05. **P < .01.
L x BI
BE x L NS NS NS NS NS
* NS NS NS NS NS NS NS
brown egg line.
1.71 4.10 2.49 .00 .03 1.04 .37 .84 .38 1.83 .24 .23 .92
NS
*
NS NS NS NS NS NS NS NS NS NS NS
8.93 .00 2.15 2.70 1.42 6.09 .02 .83 .03 .26 1.93 .95 1.47
»* NS NS t NS
**
NS NS NS NS NS NS NS
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
Trait Shank length 8-wk body weight 17-wk body weight Age at first egg Egg number Egg laying rate Clutch length Days of pause Broken eggs Egg weight Adult body weight (52 wk)
HETEROSIS IN NORMAL VERSUS DWARF LAYING HENS
Heterosis
Effects Related to dw Globally, often-mentioned effects of the sex-linked dwarf gene (Merat, 1990) were also obtained in this experiment. Adult body weight was reduced by about a third and mean egg weight by about 5 g in the Leghorn line and 4 g in the brown egg line. Egg number also decreased in the two lines, by 10 and 7 eggs, respectively, as did rate of lay, clutch length, and percentages of broken, soft-shelled, and double-yolk eggs. Finally, the often observed delaying effect of dw on the age at first egg was not observed in this experiment. Genotype by Line Interaction The genotype by line interaction was highly significant (P < .01) only for the shank length and the three body weights. It was also significant (P < .05) for rate of lay (Table 1). The interaction tested in Table 1 may result from the differential effect of the dwarf gene in the two lines or from some interference between this gene and heterosis. Some effects related to dw and dependent on the body weight likely correspond to the former hypothesis: generally, the smaller the mean body size of the line, the larger the dw-mduced reduction of egg number (Merat, 1990). Indeed, in the present experiment the number of eggs and the mean egg weight appear to have been reduced more in dwarf Leghorn hens. On the other hand, the dwarf gene has less effect on some traits in Fa than in pure lines. A higher heterosis for rate of lay for dwarf hens is shown in Table 3. Table 2 suggests similar observations for clutch length, percentage days of pause, 52-wk
body weight, and age at first egg. For these traits, either heterosis is significant for dwarf hens only (age at first egg) or its level of significance is larger for this genotype (egg number, rate of lay, clutch length, 52-wk body weight). The influence of body weight differences on the effect of the dwarf gene on egg production traits cannot explain the above phenomenon, as the ¥x birds are intermediate in body size between the pure lines. Statistical analyses of egg number, clutch length, and mean egg weight (data not shown) with body weight as a covariable did not alter the results. The effect of sex-linked dwarfism on heterosis occurred mainly in one reciprocal cross. Heterosis in White Leghorn x brown egg crossbred dwarf hens was, respectively, 14.5, 7.5, and 13.4% for egg number, egg laying rate, and adult body weight (Table 1). In this instance, heterosis of various characters, particularly of rate of lay, was suggested to be larger in dwarf hens than in normal-sized females of the same genetic background. In a previous comparison by Yoo et al. (1980), a larger heterosis was found to be associated with the dw gene and body weight and egg mass. In the report of M6rat and Bordas (1991), a similar trend was suggested for age at first egg, egg number, and rate of lay till 51 wk of age. In these cases, the disadvantage for egg production associated with dw in egglaying lines was reduced in the crossbreds. This indicates that using appropriate dwarf line crosses may be of some interest for egg production. In the present experiment, the origin of the sex-linked dwarf gene, different for the two lines, may account for the different effects on the traits. However, egg production performances of crossbreds with the White Leghorn dwarf gene were more like those of the brown egg line. Therefore, some kind of interaction between dw and background genes, possibly including sex-linked genes, appears to provide a better explanation of present results. ACKNOWLEDGMENTS The technical assistance of the staff at the poultry experimental unit, La Miniere, and of M. Boitard with data transfers, is greatly appreciated.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
In this study, positive values for heterosis for successive body weights, shank length, mean egg weight, egg number, rate of lay, and clutch length, were obtained for both dwarf and normal hens (Table 2). Heterosis was not significant for pauses, apparently because this trait is highly variable. However, percentage days of pause was consistently lower in the crosses. Finally, the percentage of broken eggs did not show any heterosis.
5
6
MERAT ET AL.
REFERENCES Kipi, A., 1990. Etude de la r6ponse a la selection dans deux lignges de poules pondeuses nanifiees, de type oeuf brun, portant ou non le gene cou nu. M&noire, Dipldme d'Etudes Approfondies, Institut National Agronomique, Paris-Grignon, France. M6rat, P., 1990. Pleiotropic and associated effects of major genes. Pages 429-467 in: Poultry Breeding and Genetics. R. D. Crawford, ed. Elsevier, Amsterdam, The Netherlands.
M6rat, P., and A. Bordas, 1991. Caract6ristiques de ponte comparers pour des poules naines (dw) et de taille normale (Dw+) dans une lign£e Leghorn blanche et dans un croisement de lere g^n6ration Leghorn x Fayoumi. Genet. Sel. Evol. 23:455-460. SAS Institute, 1988. SAS® User's Guide: Statistics. SAS Institute Inc., Cary, NC. Yoo, B. H., B. L. Sheldon, and R. N. Podger, 1980. Effects on performance of the dwarf gene in three layer genetic backgrounds. Pages 59-64 in: Proceedings of the 1980 South Pacific Poultry Science Convention. Auckland, New Zealand. Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
1994 Poultry Science 73:1-6
INTRODUCTION
tive disadvantage associated to dw in the Leghorn line appeared to be smaller in the In egg lines, the sex-linked dwarfism Fx cross, but heterosis could not be gene, dw, is associated generally with a estimated, as performance of Fayoumi decrease in the number of eggs, this being hens was not measured in that experiespecially so in light body weight Leghorn ment. In the present study, egg production lines (Merat, 1990). However, this disad- traits of normal and dwarf hens of two vantage was decreased in the Fx genera- parental lines, a White Leghorn and a tion as compared with parental lines brown egg line, and their reciprocal segregating for the dwarf gene (Yoo et ah, crosses were compared to determine the 1980). Also, on the average, the depressing influence of heterosis on the dw-associated effect of dw on egg production increases as egg production reduction and the inthe line mean body weight decreases fluence of body weight in accounting for (M6rat, 1990). Therefore, a direct effect of this reduction in production. dw, different in parental lines or in their crosses, can be proven only if line differMATERIALS AND METHODS ences in body size are accounted for or if parental lines have similar body weight. The latter condition was fulfilled in a Lines and Genotypes comparison between a White Leghorn line The White Leghorn line was obtained in segregating for the dwarf gene and its Fj 1974 from the Bundesforschungsanstalt fur cross with a Fayoumi line of similar body Kleintierzucht, Celle, Germany. Since 1980, weight (Nferat and Bordas, 1991). For each generation was obtained from Dw/dw several egg production variables, the rela- sires and dw/w dams. In 1986, females of a brown egg dwarf line under selection for egg production (clutch length) in the authors' laboratory since 1984 (Kipi, 1990) Received for publication January 25, 1993. were crossed to males of a related brown Accepted for publication September 14, 1993. 1 Laboratoire de G6n6tique Factorielle, INRA, Jouy egg line heterozygous for the silver/gold and sex-linked dwarf genes. Each year en Josas 78350, France. 1
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
ABSTRACT The effect of genotype at the sex-linked dwarf locus on heterosis in crosses between a White Leghorn and a brown egg line for body weight, egg production, and related traits was studied. Heterozygous Dw/dw males were used to produce normal and dwarf pullets in each of the pure lines and their reciprocal crosses (eight genotype-line combinations). There were 54 pullets per combination. Line differences were significant for shank length, body weights at 8,17, and 52 wk, age at first egg, egg number, clutch length, rate of lay, and egg weight. Heterosis was observed for all of these traits. Body weight as a covariate was not important in analyses of egg number, clutch length, and egg weight. The egg production reduction associated with the dw gene in pure lines was smaller in Fi hens. This discovery may be adequate to warrant use of dwarf crossbred hens for egg production. {Key words: sex-linked dwarfism, crossing, heterosis, egg production, body weight)
2
MERAT ET AL.
Measurements Traits measured are listed in Table 1. Data associated with egg production were collected up to the age of 51 wk (about 7 mo of production). Mean egg weight was estimated from a 2-wk egg collection beginning at the age of 43 wk. Hens were weighed individually at 8,12, and 52 wk of age. Pauses were taken as periods of at least 2 consecutive d without an oviposition, and the total number of days of pause was expressed as a percentage of total number of days in the laying period. Statistical Analysis For every variable, a two-way analysis of variance with interaction was performed.
Main effects were genotype at the sexlinked dwarfism locus (normal or dwarf) and line (Leghorn, brown egg, Leghorn x brown egg, brown egg x Leghorn). Variables expressed as percentage also were transformed with the arc sine square root transformation and then analyzed without changing the significance of the effects. Therefore, only results on untransformed data are given. Means of lines within each genotype were compared by the StudentNewman-Keuls procedure. A covariance analysis with body weight as a covariable was done on egg number, clutch length, and mean egg weight. As it did not alter appreciably the results, it is not detailed herein. Heterosis was estimated within genotype (normal and dwarf), both overall, as difference between crossbreds and purebreds, and, for each reciprocal cross, from appropriate linear combinations of least squares means for lines. Significance of heterosis was evaluated by the Student's t test with n - r degrees of freedom, with n being the total number of observations (around 430 depending on the variable) and r the total number of subclasses (8). In the same way, heterosis for dwarf and for normal hens was compared by a F test with 1 and n - r degrees of freedom. All analyses were performed using the General Linear Models (GLM) procedure (SAS Institute, 1988). RESULTS AND DISCUSSION Table 1 lists least squares means for all traits, by combination of genotype and line, with the level of significance for the main effects and for the interactions. Heterotic effects within genotype, overall and for each reciprocal cross, with the associated level of significance, are presented in Table 2. In general, the degrees of heterosis did not differ when normal and dwarf crossbred hens were compared (Table 3). Heterotic effects did differ for the two phenotypes for rate of lay (P < .05), but differences for 8-wk shank length (P < .01) and body weight (P < .05) were present in specific, but different, reciprocal cross populations.
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
thereafter, male progeny heterozygous at these two loci were backcrossed to females of the selection line to produce double heterozygote males with the silver phenotype and normal and dwarf brown egg females with, respectively, the silver and gold phenotypes, in the absence of recombination. The dwarf mutation occurred independently in the Leghorn line (USA) and in the Brown Egg line (Jouy en Josas, in 1959). In 1990,11 Dw/dw males from each line were mated to 4 Leghorn and 3 brown egg females each, so that both purebred and reciprocal crossbred full-sib and half-sib progeny were obtained. In each sib group, the frequency of normal and dwarf was expected to be 50%. All progeny were obtained in a single hatch. Shank length at 8 wk of age, confirmed by a second measurement at 17 wk, was used to differentiate dwarf from normal birds. The two phenotypes were represented by equal numbers within both full-sib and half-sib families prior to being tested for egg production and related traits. Numbers of hens tested per line and genotype (Dw/w or dw/w) are presented in Table 1. Female chicks were group reared under standard conditions. At 18 wk of age, they were transferred at random into individual cages. They were housed in a single unit (14 h light:10 h dark) at 22 ± 1 C and were provided ad libitum access to a commercial egg layer diet (15.5% total protein, 2,600 kcal ME/kg, 3.4% calcium).
BE
5
10
12
1
54 71.2' 421c 339.1 157.7" 141.2b 81.0<: 4.06b 13.19 .24 b .34 b 1.05b 47.29b 1,294' 5
52 77.6" 545b 1,080b 142.0= 175.9a 87.4" 7.27" 6.94 1.57" 1.73" 1.50* 52.90" 1,754"
L x BE
5
54 75.7b 640b 1,037c 147.0b 163.0" 84.7b 6.61" 9.03 .77b .47b 2.35" 52.40" 1,585b
BE x L
Dwarf
"-'•Means for lines within genotype with no common superscript differ significantly (P < .05). 1L = White Leghorn line; BE = brown egg line. •P < .05. **P < .01. ***P < .001.
1
BE x L
4 5 4 5 4 90.6" 91.8" 91.8" 854' 914b 980a 1,432b 1,542b 1,735a 147.3b 146.3b 140.8b 175.6" 173.5" 173.8" 90.9" 90.1" 89.8" 7.41" 7.72" 5.94b 9.70 8.92 10.94 6.18" 5.11* 5.47* 6.09 4.36 4.86 5.49 4.57 7.71 56.14" 57.09" 55.98" 2,443b 2,421 b 2,684"
5
Number tested 8-wk shank length, m m 8-wk body weight, g 17-wk body weight, g Age at first egg, d Egg number Egg laying rate, % Clutch length, d Days of pause, % Number of double-yolk eggs Soft-shelled eggs, % Broken eggs, % Egg weight, g 52-wk body weight, g Number dead during egg production test
5 4 85.6b d 720 1,222' 155.6" 151.2b 86.5b 5.45b 14.44 3.07b 5.25 5.04 52.16b 1,941'
L x BE
Trait
Normal
TABLE 1. Least squares means of body weights and egg production traits according
om http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
4
MERAT ET AL. TABLE 2. Crossbred superiority (percentage) for normal and dwarf hens evaluated for combined and separate reciprocal crosses1 Dwarf
Normal L x BE
BE x L
Both
L x BE
BE x L
Both
2 o*** 2.6* .9 -.6 8.1* 3.1** 30.2* -22.7 -14.1 3.8*** 5.6
3.1*** 7.6*** 4.3* -1.2 6.8+ 2.2* 35.4** -24.0 -28.3 5.6*** 4.7*
2.6*** 5.1*** 2.6* -.9 7.5** 2.7** 32.8** -23.4 -21.2 4.7*** 5.2**
3.8*** 5.5** 5.7** -4.6*** 14.7*** 7.4*** 80.0*** -32.2 -13.5 6.2*** 13.7***
3.0** 5.0** 1.5 -1.2 6.3+ 41*»* 63.6*** -12.9 -70.7 5.1** 2.8*
3.4*** 5.3*** 3.6* -2.9* 10.5*** 5.7*** 71.8*** -23.1 -42.1 5.2*** 8.1***
*L = White Leghorn line; BE +P < .10. *P < .05. **P < .01. ***P < .001.
brown egg line.
Line Differences
White Leghorn line. The present data also suggest that mortality was higher in the Differences between White Leghorn and brown egg line, a difference not reported brown egg lines expected from previous knowledge were confirmed (Table 1). In- previously. These differences were obdeed, the brown egg line had higher body tained for both normal and dwarf hens. weights and mean egg weight, commenced However, the percentage of soft-shelled lay 2 wk earlier, laid more eggs, had fewer eggs was higher only in the normal hens of pauses, and a higher rate of lay than the the brown egg line.
TABLE 3. Comparison of crossbred superiority of normal versus dwarf hens by combined and separate reciprocal crosses1 Value of F and significance of the contrast Trait
Overall1
8-wk shank length 8-wk body weight 17-wk body weight Age at first egg Egg number Egg laying rate Days of pause Number double-yolk eggs Soft-shelled eggs Broken eggs Clutch length Egg weight 52-wk body weight
1.08 .38 .00 1.01 .39 4.55 .08 1.25 .25 1.32 1.33 .09 .02
*L = White Leghorn line; BE +P < .10. *P < .05. **P < .01.
L x BI
BE x L NS NS NS NS NS
* NS NS NS NS NS NS NS
brown egg line.
1.71 4.10 2.49 .00 .03 1.04 .37 .84 .38 1.83 .24 .23 .92
NS
*
NS NS NS NS NS NS NS NS NS NS NS
8.93 .00 2.15 2.70 1.42 6.09 .02 .83 .03 .26 1.93 .95 1.47
»* NS NS t NS
**
NS NS NS NS NS NS NS
Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015
Trait Shank length 8-wk body weight 17-wk body weight Age at first egg Egg number Egg laying rate Clutch length Days of pause Broken eggs Egg weight Adult body weight (52 wk)
HETEROSIS IN NORMAL VERSUS DWARF LAYING HENS
Heterosis
Effects Related to dw Globally, often-mentioned effects of the sex-linked dwarf gene (Merat, 1990) were also obtained in this experiment. Adult body weight was reduced by about a third and mean egg weight by about 5 g in the Leghorn line and 4 g in the brown egg line. Egg number also decreased in the two lines, by 10 and 7 eggs, respectively, as did rate of lay, clutch length, and percentages of broken, soft-shelled, and double-yolk eggs. Finally, the often observed delaying effect of dw on the age at first egg was not observed in this experiment. Genotype by Line Interaction The genotype by line interaction was highly significant (P < .01) only for the shank length and the three body weights. It was also significant (P < .05) for rate of lay (Table 1). The interaction tested in Table 1 may result from the differential effect of the dwarf gene in the two lines or from some interference between this gene and heterosis. Some effects related to dw and dependent on the body weight likely correspond to the former hypothesis: generally, the smaller the mean body size of the line, the larger the dw-mduced reduction of egg number (Merat, 1990). Indeed, in the present experiment the number of eggs and the mean egg weight appear to have been reduced more in dwarf Leghorn hens. On the other hand, the dwarf gene has less effect on some traits in Fa than in pure lines. A higher heterosis for rate of lay for dwarf hens is shown in Table 3. Table 2 suggests similar observations for clutch length, percentage days of pause, 52-wk
body weight, and age at first egg. For these traits, either heterosis is significant for dwarf hens only (age at first egg) or its level of significance is larger for this genotype (egg number, rate of lay, clutch length, 52-wk body weight). The influence of body weight differences on the effect of the dwarf gene on egg production traits cannot explain the above phenomenon, as the ¥x birds are intermediate in body size between the pure lines. Statistical analyses of egg number, clutch length, and mean egg weight (data not shown) with body weight as a covariable did not alter the results. The effect of sex-linked dwarfism on heterosis occurred mainly in one reciprocal cross. Heterosis in White Leghorn x brown egg crossbred dwarf hens was, respectively, 14.5, 7.5, and 13.4% for egg number, egg laying rate, and adult body weight (Table 1). In this instance, heterosis of various characters, particularly of rate of lay, was suggested to be larger in dwarf hens than in normal-sized females of the same genetic background. In a previous comparison by Yoo et al. (1980), a larger heterosis was found to be associated with the dw gene and body weight and egg mass. In the report of M6rat and Bordas (1991), a similar trend was suggested for age at first egg, egg number, and rate of lay till 51 wk of age. In these cases, the disadvantage for egg production associated with dw in egglaying lines was reduced in the crossbreds. This indicates that using appropriate dwarf line crosses may be of some interest for egg production. In the present experiment, the origin of the sex-linked dwarf gene, different for the two lines, may account for the different effects on the traits. However, egg production performances of crossbreds with the White Leghorn dwarf gene were more like those of the brown egg line. Therefore, some kind of interaction between dw and background genes, possibly including sex-linked genes, appears to provide a better explanation of present results. ACKNOWLEDGMENTS The technical assistance of the staff at the poultry experimental unit, La Miniere, and of M. Boitard with data transfers, is greatly appreciated.
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In this study, positive values for heterosis for successive body weights, shank length, mean egg weight, egg number, rate of lay, and clutch length, were obtained for both dwarf and normal hens (Table 2). Heterosis was not significant for pauses, apparently because this trait is highly variable. However, percentage days of pause was consistently lower in the crosses. Finally, the percentage of broken eggs did not show any heterosis.
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REFERENCES Kipi, A., 1990. Etude de la r6ponse a la selection dans deux lignges de poules pondeuses nanifiees, de type oeuf brun, portant ou non le gene cou nu. M&noire, Dipldme d'Etudes Approfondies, Institut National Agronomique, Paris-Grignon, France. M6rat, P., 1990. Pleiotropic and associated effects of major genes. Pages 429-467 in: Poultry Breeding and Genetics. R. D. Crawford, ed. Elsevier, Amsterdam, The Netherlands.
M6rat, P., and A. Bordas, 1991. Caract6ristiques de ponte comparers pour des poules naines (dw) et de taille normale (Dw+) dans une lign£e Leghorn blanche et dans un croisement de lere g^n6ration Leghorn x Fayoumi. Genet. Sel. Evol. 23:455-460. SAS Institute, 1988. SAS® User's Guide: Statistics. SAS Institute Inc., Cary, NC. Yoo, B. H., B. L. Sheldon, and R. N. Podger, 1980. Effects on performance of the dwarf gene in three layer genetic backgrounds. Pages 59-64 in: Proceedings of the 1980 South Pacific Poultry Science Convention. Auckland, New Zealand. Downloaded from http://ps.oxfordjournals.org/ at Purdue University Libraries ADMN on May 24, 2015