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Components of Specific Combining Ability Estimated from Strain and Breed Crosses in Chickens1,2
Components of Specific Combining Ability Estimated from Strain and Breed Crosses in Chickens1'2 S. WEARDEN3, J. V. CRAIG4 AND D. TINDELL 5 Departments of Statistics and of Dairy and Poultry Science, Kansas State University, Manhattan, Kansas 66502 (Received for publication February 27, 1967)
INTRODUCTION
H
EXPERIMENTAL
Samples of 3 WL and 3 RIR "closed flock" strains were obtained from leading commercial breeders in 1956. They were
crossed in a full diallel pattern during the following two years. Two "shifts" of males and approximately 90 females per strain were used in producing the 36 strain combinations each year. Each shift involved a different sample of males with 9 males per strain and shift the first year and 6 males per strain and shift during the second year. Traits studied both years were: sexual maturity (age at first egg), hen-day percentage egg production after first egg and hen-housed egg production from housing at about 150 days of age to 260 days of age, 5- and 10-month body weight and laying house livability. Additionally, hen-day and hen-housed egg production to 470 days of age were measured during the second year. All hens were trap-nested 3 days per week. Details such as number of pullets per strain combination and management procedures may be obtained from Wearden et al. (1965). Number of pullets housed by shifts and years, classified according to mating systems, are shown in Table 1. STATISTICAL METHODS
'This investigation is part of the Kansas contribution to the NC-47 Regional Poultry Breeding Project. 2 Contribution No. 109, Department of Statistics, and No. 656, Department of Dairy and Poultry Science, Kansas Agricultural Experiment Station, Manhattan, Kansas. 3 Department of Statistics, Kansas State University, Manhattan. Present address: Division of Statistics, West Virginia University, Morgantown. 4 Department of Dairy and Poultry Science, Kansas State University, Manhattan. 5 Coordinator, Southern Regional Poultry Breeding Project (S-S7), A.R.S., U.S.D.A., Athens, Georgia.
Each shift was considered a replicate of a full diallel cross, and the analysis referred to by Wearden (1964) as the Henderson analysis was conducted on the unweighted means. Interactions between shifts and genetic terms in the diallel analysis were assumed to be zero. That assumption was substantiated by lack of significance in tests of heterogeneity among the six mean squares involving interactions of terms with replicates. The interaction mean squares all appeared to estimate the same variance so they were pooled and used as a measure of ran-
1398
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ETEROSIS, the increase in performance of crosses over parental strains, has generally been found in fowl for chick livability, early growth rate, age at sexual maturity, egg production and at times for adult livability. Degree of heterosis seems to be positively associated with genetic diversity of the parental strains. A full diallel crossing experiment involving 3 White Leghorn (WL) and 3 Rhode Island Red (RIR) "closed flock" strains has been analyzed and reported by Wearden et al. (1965) with particular emphasis on estimating the relative importance of general and specific combining ability and maternal effects. This follow-up study attempts to elucidate the components of specific combining ability.
1399
SPECIFIC COMBINING ABILITY
Yijk = n + s; + Sj' + h k + c + Ci + q + c u + r;j + e ijk ' and for the ith pure strain it is: Yiik = M + Si + Si' + h k - (p - l)c - (p - 2) C i + e iik where JX = general mean Si = paternal effect of pure strain i (includes general sex linked effects of line, Si = gi+zj); i = j = l, • • • ,P Sj = effect of dam strain j (includes maternal effect s'j = gj-|-mj) hk = random effect of kth replicate; k = 1, • • • , n c = mean dominance or general heterosis Ci = differential heterosis due to sire strain i Cj = differential heterosis due to dam strain j Cij = genetic effect peculiar to ijth cross; Cij = Cji r ; j = additional effect of using ith strain as male parent and jth strain as female parent; rij = — rji eijk = random effect peculiar to ijth cross in replicate k
TABLE 1.—Number of pullets housed per
mating system by shifts and years Shift Mating system Pure strain WL RIR Strain crosses WL RIR Breed crosses WL
First year
Second year - Total
1
2
1
2
133 130
115 U9
213 106
155 74
616 429
112 61
117 129
184 114
103 61
516 365
122 145
212 205
234 270
133 125
701 745
The computational methods, sources of variation and degrees of freedom are given in Table 2. Because females are the heterogametic (ZW) sex, and because of the relatively large Z chromosome in chickens, paternal or sex-linked effects are possible. Likewise, maternal effects commonly occur in vertebrates. The Henderson Analysis permits examination of mean squares for Strain of Sire and Strain of Dam. However, additive genetic variance is confounded with that of sex-linked effects in Strain of Sire mean square and with the variance due to maternal effects in Strain of Dam mean square. Maternal effects were generally found lacking in the earlier study by Wearden et al. (1965). Hence, Strain of Dam mean square was used as the error term in testing Strain of Sire mean square to determine whether sex-linked effects were present. However, livability data were not previously analyzed (ibid) so that sex-linked or maternal effects may be included in Strain of Sire or Strain of Dam mean squares, respectively. Hence a two-sided F-test was made to compare the importance of maternal effects relative to those of sex-linkage on livability. With a model allowing for reciprocal effects, the Henderson Analysis has some shortcomings, but it still has a term that permits variance of residual reciprocal effects to be detected. Thus, the analysis
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dom variation for further testing. Wearden (1964) showed that the Crosses mean square of the Henderson Analysis is synonymous with the b mean square of the Hayman Analysis. Therefore the Crosses sum of squares was partitioned according to the method outlined by Hayman (1954) and yielded a sum of squares due to general heterosis (bi), a second due to strain differences in heterotic response (b2), and a third sum of squares due to fortuitous combinations of genes produced by crossing particular strains (6 3 ). The model for the ijth cross in replicate k is:
(p - l)(p - 2)/2 n —1 (p2 - l)(n - 1)
Residual reciprocal Replicates Error
= = =
E (Yu.)Vn E (Yi..)Vnp E (Y.i.)Vnp E (Y..k)2/P2
M' - I
P' - I
Sum of squares
2n Ecu2 P(P - 3) i
Error M.S.
cr2 + - ^ - E gi2 + - ^ r E mi2 p - 1 p-1 (T2 + np2(p — l)c 2 np(p - 2)
a? o-B2
Ci2
ct =
c =
(g,
(zi
6i = ( Y . . . - p E Y i i . ) 2 / n p 2 ( p - l ) 62 = E(Y... + Y.i. - pY«.)Vnp(p - 2) - (2Y... 6 = C'-G' + l
i>i
D = E (Yii. - Y ii .) 2 /2n
R = E ( Y . . - Y.i.) 2 /2np
C = E (Yii. + Y n .) 2 /2n + E Y u . V n
Error M.S. Error M.S.
Error M.S.
Error M.S.
Error M.S.
Dam M.S.
Denominator for F-test
<72 + - ~ £ gi2 + - ^ - E z i 2 p-1 p-1
Expectation of mean square
D - R a3 + 2n
C - b i - h - 1
G' = E ( Y i . . + Y.i.)V2np
I = ^.../np2
S' P' M' B'
T ' = E ^ iijk
P (P-- 3 ) / 2
h
where
p - 1
1
h
D -
Strain of dam
1
p - 1
Strain of sire
h
d.f.
Source of variation
TABLK 2.—Analysis used in testing for components of specific combining
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SPECIFIC COMBINING ABILITY
used has tests of significance for additive gene action, maternal or paternal effects, three types of genetic interaction, and residual reciprocal effects. However, when the last is present, there is no valid test for additive gene action.
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RESULTS The presence and relative importance of additive genetic variance in the traits studied, except for livability, were discussed in a previous paper (Wearden et al., 1965). Maternal effects appear to be important for laying house livability to 260 days as the Strain of Dam mean square is greater than that associated with Strain of Sire, Table 3. General sex-linked effects are indicated for age at sexual maturity and part year hen-housed production by the Strain of Sire mean square significantly exceeding that of Strain of Dam. This study was to explore the nature of other forms of genetic action, namely general heterosis (Z>i), strain differences in heterotic response (b2), fortuitous combinations of genes found in particular strain crosses (b3), and residual reciprocal effects (d). The traits for which those effects were found significant (Table 3) are given. bt. Heterosis contributed significantly to variation within the diallel cross in six of the nine analyses. Only ten-month body weight and adult livability showed no significant heterotic effect. In the other traits, general heterosis was so consistently large that it must be considered one of the major sources of nonadditive genetic variation. Though only one of the fifteen degrees of freedom due to Crosses is attributed to bi, it explained 43.8% of the Crosses sums of squares in sexual matruity, 27.6% in part-year hen-day percent production, 39.9% in part-year hen-housed production, 26.7% in five-month body weight, 53.0% in full-year hen-day percent production, and 21.9% of the sum of squares due to
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S. WEAEDEN, J. V. CRAIG AND D. TINDELL
In an effort to determine the extent that nonadditive genetic effects were due to type of mating system (pure strain, strain cross, or breed cross), the data were partitioned into six groups: pure strain White Leghorns, pure strain Rhode Island Reds, White Leghorn strain crosses, Rhode Island Red strain crosses, and the two reciprocal breed crosses. Means for those six groups are given in Table 4. An orthogonal set of comparisons to explain the variability among the groups was devised and is presented in Table 5. The first comparison is bx and is retained as a basis for evaluating the relative importance of the other comparisons. The second comparison evaluates additional heterotic effect obtained by making breed crosses rather than strain crosses. The additional effect due to breed crossing is especially noticeable in sexual maturity, as breed crosses laid their first eggs, on the average, 4.75 days earlier than strain cross birds. The additional heterotic effect due to breed crosses is also evident in both partand full-year hen-housed production. Differences exhibited in this trait reflect in part the earlier sexual maturity of breed crosses, but hen-housed egg production is also affected by livability and rate of lay. The third and fourth comparisons evaluate breed differences as reflected in pure strains and in strain crosses. If there were marked differences in those two comparisons, it would indicate that heterosis was of different magnitudes in the two breeds. In general, the Leghorns had a somewhat greater difference between pure strains and strain crosses than did the Rhode Island Red strains, but a separate analysis of strain by bx interaction was nonsignificant even for full-year hen-housed production, which exhibited the greatest difference between the strains in heterotic response. The final orthogonal comparison measured reciprocal crossing effects due to
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Crosses in full-year hen-housed production. b2 . This genetic interaction was important only for five-month body weight. The five degrees of freedom attributable to this effect explained 40.4% of the variability among Crosses. Even so, an F-test conducted on the b2 mean square yielded a variance ratio (F = 2.13; .05 < p < .10) with a chance probability greater than the traditional .05 significance level. b3. This genetic interaction was described by Sprague and Tatum (1942) as specific combining ability. It is a form of genetic interaction that cannot be predicted in advance of the experimental cross between strains producing it. The nine degrees of freedom associated with this source of variation explained 46.4% of the sums of squares due to Crosses in sexual maturity, 52.3% of the part-year hen-housed production and 89.2% of variability due to Crosses in livability to 260 days. However, the Among Crosses term was not a major source of variation in livability. Residual Reciprocal Effects. Yates (1947) gave the method of computation for this particular term in the analysis of variance. In it there are no additive genetic effects, none of the b variability described above, no maternal effects, and no general sex linkage. By general sex linkage, we mean an effect due to the Z-chromosome which is common to all crosses containing the same Z-chromosome. Hence, for the term to be significant, some other effect must be present. Residual reciprocal effects were a significant source of variation in sexual maturity, full-year hen-housed production, and full year livability. A possible explanation of this term is that Z-chromosome inheritance is not the same in some crosses as it is in others, e.g., the sex chromosome of a White Leghorn may give a different result in a cross with another strain of White Leghorn than it does in a cross with a Rhode Island Red strain.
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SPECIFIC COMBINING ABILITY TABLE 4.—Mean performance levels of productivity traits for particular mating systems Egg production Mating system
Breed crosses W W X RIR 9 RIRcfXWL? 1
5 month
10 month
(kg.)
(kg.)
1.4 1.8
Sexual maturity
Hen-day after 1st egg
Henhoused1
Laj dng hoiuse livab ility
260 days
470 days
260 days
470 days
(days)
(%)
(%)
(number)
(number)
(%)
(%)
1.9 2.5
178.7 183.5
71.9 69.3
59.0 54.5
22.5 19.8
66.2 59.7
90.9 86.7
77.8 71.5
1.4 1.8
1.9 2.5
175.2 180.5
73.3 72.0
61.8 58.2
24.6 20.7
73.5 58.2
92.9 87.7
78.0 64.8
1.6 1.6
2.2 2.2
170.1 176.1
73.8 73.1
61.7 61.7
26.1 24.5
70.0 73.6
88.9 93.0
67.7 78.2
260 days
470 days
On basis of 3 days trap-nesting per week.
breeds. Differences were significant with respect to sexual maturity and livability. Crossbred pullets with WL sires came into production 5.92 days sooner, but had 4.03 and 10.49 percent greater mortality to 260 and 470 days of age, respectively. Differences in hen-housed egg production between the reciprocal breed crosses did not differ significantly, indicating a cancelling out effect of advantages and disadvantages associated with the 2 components. DISCUSSION The relative performance levels of pure strains, strain crosses and crossbreds, indicating heterosis to be present for sexual maturity and egg production traits, generally agree with those reported by previous workers (see review of Tindell, 1961). Differences between crossbreds and strain crosses were significant and the sum of squares comparable to that for over-all heterosis (bi) for sexual maturity and both part- and full-year hen-housed production. The advantage of crossbreds over strain crosses, within breeds, for sexual maturity and egg production, raises the possibility that breeders should not abandon cross-
breeding entirely in favor of straincrossing. The heavier adult body weights and tinted egg shell color of the crossbreds, commonly cited as disadvantages on a commercial basis may be changed by selective breeding and/or backcrossing techniques that are well understood by breeders. Evidence for differential heterosis associated with particular strains (b2) was consistently lacking. The third type of genetic interaction (b3) was significant for sexual maturity and part-year hen-housed production, indicating that some crosses are superior to others, but such superiority cannot be predicted on the basis of pure strain or average performance of a strain in crosses. Experiments involving reciprocal breed crosses were conducted by Dudley (1944), Dickerson et al. (1950), Warren and Moore (1956), Nordskog and Phillips (1960), and Eisen et al. (1967). Except for Dickerson et al. (1950) they found, as we did, that WL male by heavy breed female hybrids had poorer laying house livability than reciprocal cross birds. Dudley (1944), Dickerson et al. (1950), Nordskog and Phillips (1960) and Eisen et al. (1967) found, which our results confirm,
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Pure strain WL RIR Strain crosses WL RIR
Body weights
S. WEARDEN, J. V. CRAIG AND D. TINDELL
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that crossbreds sired by WL males mature earlier. Nordskog and Phillips (1960), from reciprocal crosses of the WL and Fayoumi (F) breeds, found that WL c? X F ? progeny had poorer livability than reciprocal cross birds. No difference in adult livability of reciprocal crosses involving the F and heavy breeds was indicated. They postulated that differences in adult livability of reciprocal crosses involving the WL breed were associated with the WL sex chromosome and not with maternal effects. Results of our study, however, suggest that maternal strain effects are significant for livability at least to 260 days of age. Sex-linked effects of the type postulated by Nordskog and Phillips (1960) may also be involved in full year livability as residual reciprocal effects were present. Wearden et al. (1965) estimated maternal effects to be absent or unimportant for sexual maturity and other egg production traits. Contrarily for five-month body weight, we estimated maternal effects from variance component analyses to make up 16-17 percent of the total variance. Although we estimated maternal effects to be absent for sexual maturity, reciprocal breed cross differences were present. Best explanation of these differences seems to be sexlinked effects. The very large sire mean square in the analysis for sexual maturity adds credibility to that explanation. Hen-housed egg production takes into account, as mentioned previously, sexual maturity, rate of lay and livability and therefore may involve maternal and/or sex-linked effects, but not necessarily acting in the same direction. Reciprocal crossbred differences in sexual maturity and livability were in favor of the WL
140S
SPECIFIC COMBINING ABILITY
SUMMARY AND CONCLUSIONS Adult body weights, laying house livability and egg production traits of pullets were analyzed from a full diallel crossing experiment involving 3 White Leghorn and 3 Rhode Island Red commercial strains. Maternal influence was of borderline significance for part-year laying house livability. General sex-linked effects were found for age at first egg and part-year hen-housed egg production. Other forms of genetic action explored were: general heterosis (61), strain differences in heterotic response (b2), fortuitous gene combinations found in particu-
lar strain crosses (b3), and residual reciprocal effects (d). Though only one of fifteen degrees of freedom due to Crosses was associated with 61, it accounted for 43.8, 27.6, 39.9, 26.7, 53.0 and 21.9 percent of sums of squares in age at sexual maturity, part-year hen-day production, part-year hen-housed production, five-month body weight, full-year henday production and full-year hen-housed egg production, respectively. Strain differences in heterotic response, b2, were of borderline significance for fivemonth body weight only. Age at sexual maturity and part-year hen-housed egg production were significantly affected by bs with nine degrees of freedom explaining, respectively, 46.4 and 52.3 percent of the sum of squares due to Crosses. Residual reciprocal effects, d, were a significant source of variation in sexual maturity, full-year hen-housed production, and full-year livability. Orthogonal comparisons of mating systems indicted that: general heterotic effects were of major importance in 6 of the 9 traits analysed; strain crosses between breeds were superior to strain crosses within breeds for sexual maturity and henhoused egg production. Reciprocal crossing effects between breeds were large for sexual maturity and laying house livability. The livability results are consistent with the significant residual reciprocal effects found and are interpreted as indicating that sex chromosomes of the White Leghorn and Rhode Island Red breeds have different effects in the within- and between-breed strain crosses. REFERENCES Dickerson, G. E., Q. B. Kinder, W. F. Krueger and H. L. Kempster, 1950. Heterosis from crossbreeding and from outbreeding. Poultry Sd. 29: 756. Dudley, F. J., 1944. Results of crossing the Rhode
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the reciprocal breed crosses were inconsistent in studies previously cited and we found no significant differences for henhoused egg production. It appears that the relative advantage of earlier sexual maturity resulting from using WL as the male parent in crossing with heavy breed females, compared with the reciprocal cross, is offset to different degrees depending on factors particular to each study that affect livability. Hence, in situations with generally low levels of mortality WL sires would be more desirable. Contrarily, the reciprocal crossbreds would presumably be superior under conditions where higher levels of mortality were usually encountered. Residual reciprocal effects found for age at sexual maturity, full-year hen-housed egg production and full-year livability indicate non-additive genetic variability contributed by the sex chromosome. This term, when significant, indicates an interaction of the Z-chromosome with the autosomal chromosomes received from the strain of the other parent. As there were also significant differences between strain and breed crosses for sexual maturity and henhoused production, a breed by Z-chromosome interaction is suspected.
1406
S. WEARDEN, J. V. CRAIG AND D. TINDELL of corn. J. Amer. Soc. Agron. 34: 923-932. Tindell, D., 1961. General and specific combining abilities, maternal effects and genotype-environmental interactions as estimated from strain and breed crosses of chickens. Ph.D. Dissertation. Kansas State University, Manhattan. Warren, D. C., and C. H. Moore, 1956. Adult mortality in reciprocal crosses of Leghorns and heavy breeds. Poultry Sci. 35: 1178. Wearden, S., 1964. Alternative analyses of the diallel cross. Heredity, 19: 669-680. Wearden, S., D. Tindell and J. V. Craig, 1965. Use of a full diallel cross to estimate general and specific combining ability in chickens. Poultry Sci. 44: 1043-1053. Yates, F., 1947. The analysis of data from all possible reciprocal crosses between a set of parental lines. Heredity, 1: 287-302.
Performance of Hens Molted by Various Methods12 H. R. WILSON, J. L. FRY, R. H. HARMS AND L. R. ARRINGTON Department of Poultry Science, University of Florida, Gainesville, Florida 32601 (Received for publication February 27, 1967)
F
ORCED molting has been studied for many years as a possible method of rejuvenating hens to increase egg production and egg quality. Although recommended by a few poultrymen, Rice (1905) was skeptical that the practice would be profitable. More recent results have been variable and the advantages and disadvantages have been summarized by Cox (1964) and Bell (1965). In addition, force molting has been reported to improve poor fertility in turkeys (Moyer et al., 1966) and to increase the DDT depletion rate in hens (Wesley et al, 1966). The method most commonly used for inducing a molt has been some type of feed and/or water restriction such as that sug1
Florida Agr. Exp. Sta. Journal Series No. 2643. Supported in part by NIH grant AM 08760.
gested by the Poultry Council of the State College of Washington (1947). Noles (1966) reported that feed and water restriction is an effective method of force molting. Progesterone has been shown to be effective in causing a cessation of lay accompanied by a molt (Adams, 1955, 1956; Shaffner, 1955; Himeno and Tanabe, 1957; and others). The rest period can be extended with multiple injections of this hormone (Harris and Shaffner, 1957). Hansen (1960) reported higher egg production subsequent to molts induced by feed and water restriction as compared with those induced by progesterone. Gard et al. (1965) reported that egg production of hens molted with an oral progestin, 6 chloro Ae-17-acetoxy progesterone (CAP), was equivalent to that of birds molted by feed and water restriction. High levels of dietary iodine have been
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Island Red and White Leghorn breeds of poultry. J. Agr. Sci. 34: 76-81. Eisen, E. J., B. B. Bohren, H. E. McKean and S. C. King, 1967. Genetic combining ability of light and heavy inbred lines in single crosses of poultry. Genetics, 55: 5-20. Goto, E., and A. W. Nordskog, 1959. Heterosis in poultry. 4. Estimation of combining ability variance from diallel crosses of inbred lines in the fowl. Poultry. Sci. 38: 1381-1388. Hayman, B. I., 1954. The analysis of variance of diallel crosses. Biometrics, 10: 235-244. Nordskog, A. W., and R. E. Phillips, 1960. Heterosis in poultry. 5. Reciprocal crosses involving Leghorns, heavy breeds and Fayoumi. Poultry Sci. 39: 257-263. Sprague, G. F., and L. A. Tatum, 1942. General vs. specific combining ability in single crosses
INTRODUCTION
H
EXPERIMENTAL
Samples of 3 WL and 3 RIR "closed flock" strains were obtained from leading commercial breeders in 1956. They were
crossed in a full diallel pattern during the following two years. Two "shifts" of males and approximately 90 females per strain were used in producing the 36 strain combinations each year. Each shift involved a different sample of males with 9 males per strain and shift the first year and 6 males per strain and shift during the second year. Traits studied both years were: sexual maturity (age at first egg), hen-day percentage egg production after first egg and hen-housed egg production from housing at about 150 days of age to 260 days of age, 5- and 10-month body weight and laying house livability. Additionally, hen-day and hen-housed egg production to 470 days of age were measured during the second year. All hens were trap-nested 3 days per week. Details such as number of pullets per strain combination and management procedures may be obtained from Wearden et al. (1965). Number of pullets housed by shifts and years, classified according to mating systems, are shown in Table 1. STATISTICAL METHODS
'This investigation is part of the Kansas contribution to the NC-47 Regional Poultry Breeding Project. 2 Contribution No. 109, Department of Statistics, and No. 656, Department of Dairy and Poultry Science, Kansas Agricultural Experiment Station, Manhattan, Kansas. 3 Department of Statistics, Kansas State University, Manhattan. Present address: Division of Statistics, West Virginia University, Morgantown. 4 Department of Dairy and Poultry Science, Kansas State University, Manhattan. 5 Coordinator, Southern Regional Poultry Breeding Project (S-S7), A.R.S., U.S.D.A., Athens, Georgia.
Each shift was considered a replicate of a full diallel cross, and the analysis referred to by Wearden (1964) as the Henderson analysis was conducted on the unweighted means. Interactions between shifts and genetic terms in the diallel analysis were assumed to be zero. That assumption was substantiated by lack of significance in tests of heterogeneity among the six mean squares involving interactions of terms with replicates. The interaction mean squares all appeared to estimate the same variance so they were pooled and used as a measure of ran-
1398
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ETEROSIS, the increase in performance of crosses over parental strains, has generally been found in fowl for chick livability, early growth rate, age at sexual maturity, egg production and at times for adult livability. Degree of heterosis seems to be positively associated with genetic diversity of the parental strains. A full diallel crossing experiment involving 3 White Leghorn (WL) and 3 Rhode Island Red (RIR) "closed flock" strains has been analyzed and reported by Wearden et al. (1965) with particular emphasis on estimating the relative importance of general and specific combining ability and maternal effects. This follow-up study attempts to elucidate the components of specific combining ability.
1399
SPECIFIC COMBINING ABILITY
Yijk = n + s; + Sj' + h k + c + Ci + q + c u + r;j + e ijk ' and for the ith pure strain it is: Yiik = M + Si + Si' + h k - (p - l)c - (p - 2) C i + e iik where JX = general mean Si = paternal effect of pure strain i (includes general sex linked effects of line, Si = gi+zj); i = j = l, • • • ,P Sj = effect of dam strain j (includes maternal effect s'j = gj-|-mj) hk = random effect of kth replicate; k = 1, • • • , n c = mean dominance or general heterosis Ci = differential heterosis due to sire strain i Cj = differential heterosis due to dam strain j Cij = genetic effect peculiar to ijth cross; Cij = Cji r ; j = additional effect of using ith strain as male parent and jth strain as female parent; rij = — rji eijk = random effect peculiar to ijth cross in replicate k
TABLE 1.—Number of pullets housed per
mating system by shifts and years Shift Mating system Pure strain WL RIR Strain crosses WL RIR Breed crosses WL
First year
Second year - Total
1
2
1
2
133 130
115 U9
213 106
155 74
616 429
112 61
117 129
184 114
103 61
516 365
122 145
212 205
234 270
133 125
701 745
The computational methods, sources of variation and degrees of freedom are given in Table 2. Because females are the heterogametic (ZW) sex, and because of the relatively large Z chromosome in chickens, paternal or sex-linked effects are possible. Likewise, maternal effects commonly occur in vertebrates. The Henderson Analysis permits examination of mean squares for Strain of Sire and Strain of Dam. However, additive genetic variance is confounded with that of sex-linked effects in Strain of Sire mean square and with the variance due to maternal effects in Strain of Dam mean square. Maternal effects were generally found lacking in the earlier study by Wearden et al. (1965). Hence, Strain of Dam mean square was used as the error term in testing Strain of Sire mean square to determine whether sex-linked effects were present. However, livability data were not previously analyzed (ibid) so that sex-linked or maternal effects may be included in Strain of Sire or Strain of Dam mean squares, respectively. Hence a two-sided F-test was made to compare the importance of maternal effects relative to those of sex-linkage on livability. With a model allowing for reciprocal effects, the Henderson Analysis has some shortcomings, but it still has a term that permits variance of residual reciprocal effects to be detected. Thus, the analysis
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dom variation for further testing. Wearden (1964) showed that the Crosses mean square of the Henderson Analysis is synonymous with the b mean square of the Hayman Analysis. Therefore the Crosses sum of squares was partitioned according to the method outlined by Hayman (1954) and yielded a sum of squares due to general heterosis (bi), a second due to strain differences in heterotic response (b2), and a third sum of squares due to fortuitous combinations of genes produced by crossing particular strains (6 3 ). The model for the ijth cross in replicate k is:
(p - l)(p - 2)/2 n —1 (p2 - l)(n - 1)
Residual reciprocal Replicates Error
= = =
E (Yu.)Vn E (Yi..)Vnp E (Y.i.)Vnp E (Y..k)2/P2
M' - I
P' - I
Sum of squares
2n Ecu2 P(P - 3) i
Error M.S.
cr2 + - ^ - E gi2 + - ^ r E mi2 p - 1 p-1 (T2 + np2(p — l)c 2 np(p - 2)
a? o-B2
Ci2
ct =
c =
(g,
(zi
6i = ( Y . . . - p E Y i i . ) 2 / n p 2 ( p - l ) 62 = E(Y... + Y.i. - pY«.)Vnp(p - 2) - (2Y... 6 = C'-G' + l
i>i
D = E (Yii. - Y ii .) 2 /2n
R = E ( Y . . - Y.i.) 2 /2np
C = E (Yii. + Y n .) 2 /2n + E Y u . V n
Error M.S. Error M.S.
Error M.S.
Error M.S.
Error M.S.
Dam M.S.
Denominator for F-test
<72 + - ~ £ gi2 + - ^ - E z i 2 p-1 p-1
Expectation of mean square
D - R a3 + 2n
C - b i - h - 1
G' = E ( Y i . . + Y.i.)V2np
I = ^.../np2
S' P' M' B'
T ' = E ^ iijk
P (P-- 3 ) / 2
h
where
p - 1
1
h
D -
Strain of dam
1
p - 1
Strain of sire
h
d.f.
Source of variation
TABLK 2.—Analysis used in testing for components of specific combining
ded from http://ps.oxfordjournals.org/ at New York University on April 11, 2015
1401
SPECIFIC COMBINING ABILITY
used has tests of significance for additive gene action, maternal or paternal effects, three types of genetic interaction, and residual reciprocal effects. However, when the last is present, there is no valid test for additive gene action.
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RESULTS The presence and relative importance of additive genetic variance in the traits studied, except for livability, were discussed in a previous paper (Wearden et al., 1965). Maternal effects appear to be important for laying house livability to 260 days as the Strain of Dam mean square is greater than that associated with Strain of Sire, Table 3. General sex-linked effects are indicated for age at sexual maturity and part year hen-housed production by the Strain of Sire mean square significantly exceeding that of Strain of Dam. This study was to explore the nature of other forms of genetic action, namely general heterosis (Z>i), strain differences in heterotic response (b2), fortuitous combinations of genes found in particular strain crosses (b3), and residual reciprocal effects (d). The traits for which those effects were found significant (Table 3) are given. bt. Heterosis contributed significantly to variation within the diallel cross in six of the nine analyses. Only ten-month body weight and adult livability showed no significant heterotic effect. In the other traits, general heterosis was so consistently large that it must be considered one of the major sources of nonadditive genetic variation. Though only one of the fifteen degrees of freedom due to Crosses is attributed to bi, it explained 43.8% of the Crosses sums of squares in sexual matruity, 27.6% in part-year hen-day percent production, 39.9% in part-year hen-housed production, 26.7% in five-month body weight, 53.0% in full-year hen-day percent production, and 21.9% of the sum of squares due to
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1402
S. WEAEDEN, J. V. CRAIG AND D. TINDELL
In an effort to determine the extent that nonadditive genetic effects were due to type of mating system (pure strain, strain cross, or breed cross), the data were partitioned into six groups: pure strain White Leghorns, pure strain Rhode Island Reds, White Leghorn strain crosses, Rhode Island Red strain crosses, and the two reciprocal breed crosses. Means for those six groups are given in Table 4. An orthogonal set of comparisons to explain the variability among the groups was devised and is presented in Table 5. The first comparison is bx and is retained as a basis for evaluating the relative importance of the other comparisons. The second comparison evaluates additional heterotic effect obtained by making breed crosses rather than strain crosses. The additional effect due to breed crossing is especially noticeable in sexual maturity, as breed crosses laid their first eggs, on the average, 4.75 days earlier than strain cross birds. The additional heterotic effect due to breed crosses is also evident in both partand full-year hen-housed production. Differences exhibited in this trait reflect in part the earlier sexual maturity of breed crosses, but hen-housed egg production is also affected by livability and rate of lay. The third and fourth comparisons evaluate breed differences as reflected in pure strains and in strain crosses. If there were marked differences in those two comparisons, it would indicate that heterosis was of different magnitudes in the two breeds. In general, the Leghorns had a somewhat greater difference between pure strains and strain crosses than did the Rhode Island Red strains, but a separate analysis of strain by bx interaction was nonsignificant even for full-year hen-housed production, which exhibited the greatest difference between the strains in heterotic response. The final orthogonal comparison measured reciprocal crossing effects due to
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Crosses in full-year hen-housed production. b2 . This genetic interaction was important only for five-month body weight. The five degrees of freedom attributable to this effect explained 40.4% of the variability among Crosses. Even so, an F-test conducted on the b2 mean square yielded a variance ratio (F = 2.13; .05 < p < .10) with a chance probability greater than the traditional .05 significance level. b3. This genetic interaction was described by Sprague and Tatum (1942) as specific combining ability. It is a form of genetic interaction that cannot be predicted in advance of the experimental cross between strains producing it. The nine degrees of freedom associated with this source of variation explained 46.4% of the sums of squares due to Crosses in sexual maturity, 52.3% of the part-year hen-housed production and 89.2% of variability due to Crosses in livability to 260 days. However, the Among Crosses term was not a major source of variation in livability. Residual Reciprocal Effects. Yates (1947) gave the method of computation for this particular term in the analysis of variance. In it there are no additive genetic effects, none of the b variability described above, no maternal effects, and no general sex linkage. By general sex linkage, we mean an effect due to the Z-chromosome which is common to all crosses containing the same Z-chromosome. Hence, for the term to be significant, some other effect must be present. Residual reciprocal effects were a significant source of variation in sexual maturity, full-year hen-housed production, and full year livability. A possible explanation of this term is that Z-chromosome inheritance is not the same in some crosses as it is in others, e.g., the sex chromosome of a White Leghorn may give a different result in a cross with another strain of White Leghorn than it does in a cross with a Rhode Island Red strain.
1403
SPECIFIC COMBINING ABILITY TABLE 4.—Mean performance levels of productivity traits for particular mating systems Egg production Mating system
Breed crosses W W X RIR 9 RIRcfXWL? 1
5 month
10 month
(kg.)
(kg.)
1.4 1.8
Sexual maturity
Hen-day after 1st egg
Henhoused1
Laj dng hoiuse livab ility
260 days
470 days
260 days
470 days
(days)
(%)
(%)
(number)
(number)
(%)
(%)
1.9 2.5
178.7 183.5
71.9 69.3
59.0 54.5
22.5 19.8
66.2 59.7
90.9 86.7
77.8 71.5
1.4 1.8
1.9 2.5
175.2 180.5
73.3 72.0
61.8 58.2
24.6 20.7
73.5 58.2
92.9 87.7
78.0 64.8
1.6 1.6
2.2 2.2
170.1 176.1
73.8 73.1
61.7 61.7
26.1 24.5
70.0 73.6
88.9 93.0
67.7 78.2
260 days
470 days
On basis of 3 days trap-nesting per week.
breeds. Differences were significant with respect to sexual maturity and livability. Crossbred pullets with WL sires came into production 5.92 days sooner, but had 4.03 and 10.49 percent greater mortality to 260 and 470 days of age, respectively. Differences in hen-housed egg production between the reciprocal breed crosses did not differ significantly, indicating a cancelling out effect of advantages and disadvantages associated with the 2 components. DISCUSSION The relative performance levels of pure strains, strain crosses and crossbreds, indicating heterosis to be present for sexual maturity and egg production traits, generally agree with those reported by previous workers (see review of Tindell, 1961). Differences between crossbreds and strain crosses were significant and the sum of squares comparable to that for over-all heterosis (bi) for sexual maturity and both part- and full-year hen-housed production. The advantage of crossbreds over strain crosses, within breeds, for sexual maturity and egg production, raises the possibility that breeders should not abandon cross-
breeding entirely in favor of straincrossing. The heavier adult body weights and tinted egg shell color of the crossbreds, commonly cited as disadvantages on a commercial basis may be changed by selective breeding and/or backcrossing techniques that are well understood by breeders. Evidence for differential heterosis associated with particular strains (b2) was consistently lacking. The third type of genetic interaction (b3) was significant for sexual maturity and part-year hen-housed production, indicating that some crosses are superior to others, but such superiority cannot be predicted on the basis of pure strain or average performance of a strain in crosses. Experiments involving reciprocal breed crosses were conducted by Dudley (1944), Dickerson et al. (1950), Warren and Moore (1956), Nordskog and Phillips (1960), and Eisen et al. (1967). Except for Dickerson et al. (1950) they found, as we did, that WL male by heavy breed female hybrids had poorer laying house livability than reciprocal cross birds. Dudley (1944), Dickerson et al. (1950), Nordskog and Phillips (1960) and Eisen et al. (1967) found, which our results confirm,
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Pure strain WL RIR Strain crosses WL RIR
Body weights
S. WEARDEN, J. V. CRAIG AND D. TINDELL
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that crossbreds sired by WL males mature earlier. Nordskog and Phillips (1960), from reciprocal crosses of the WL and Fayoumi (F) breeds, found that WL c? X F ? progeny had poorer livability than reciprocal cross birds. No difference in adult livability of reciprocal crosses involving the F and heavy breeds was indicated. They postulated that differences in adult livability of reciprocal crosses involving the WL breed were associated with the WL sex chromosome and not with maternal effects. Results of our study, however, suggest that maternal strain effects are significant for livability at least to 260 days of age. Sex-linked effects of the type postulated by Nordskog and Phillips (1960) may also be involved in full year livability as residual reciprocal effects were present. Wearden et al. (1965) estimated maternal effects to be absent or unimportant for sexual maturity and other egg production traits. Contrarily for five-month body weight, we estimated maternal effects from variance component analyses to make up 16-17 percent of the total variance. Although we estimated maternal effects to be absent for sexual maturity, reciprocal breed cross differences were present. Best explanation of these differences seems to be sexlinked effects. The very large sire mean square in the analysis for sexual maturity adds credibility to that explanation. Hen-housed egg production takes into account, as mentioned previously, sexual maturity, rate of lay and livability and therefore may involve maternal and/or sex-linked effects, but not necessarily acting in the same direction. Reciprocal crossbred differences in sexual maturity and livability were in favor of the WL
140S
SPECIFIC COMBINING ABILITY
SUMMARY AND CONCLUSIONS Adult body weights, laying house livability and egg production traits of pullets were analyzed from a full diallel crossing experiment involving 3 White Leghorn and 3 Rhode Island Red commercial strains. Maternal influence was of borderline significance for part-year laying house livability. General sex-linked effects were found for age at first egg and part-year hen-housed egg production. Other forms of genetic action explored were: general heterosis (61), strain differences in heterotic response (b2), fortuitous gene combinations found in particu-
lar strain crosses (b3), and residual reciprocal effects (d). Though only one of fifteen degrees of freedom due to Crosses was associated with 61, it accounted for 43.8, 27.6, 39.9, 26.7, 53.0 and 21.9 percent of sums of squares in age at sexual maturity, part-year hen-day production, part-year hen-housed production, five-month body weight, full-year henday production and full-year hen-housed egg production, respectively. Strain differences in heterotic response, b2, were of borderline significance for fivemonth body weight only. Age at sexual maturity and part-year hen-housed egg production were significantly affected by bs with nine degrees of freedom explaining, respectively, 46.4 and 52.3 percent of the sum of squares due to Crosses. Residual reciprocal effects, d, were a significant source of variation in sexual maturity, full-year hen-housed production, and full-year livability. Orthogonal comparisons of mating systems indicted that: general heterotic effects were of major importance in 6 of the 9 traits analysed; strain crosses between breeds were superior to strain crosses within breeds for sexual maturity and henhoused egg production. Reciprocal crossing effects between breeds were large for sexual maturity and laying house livability. The livability results are consistent with the significant residual reciprocal effects found and are interpreted as indicating that sex chromosomes of the White Leghorn and Rhode Island Red breeds have different effects in the within- and between-breed strain crosses. REFERENCES Dickerson, G. E., Q. B. Kinder, W. F. Krueger and H. L. Kempster, 1950. Heterosis from crossbreeding and from outbreeding. Poultry Sd. 29: 756. Dudley, F. J., 1944. Results of crossing the Rhode
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the reciprocal breed crosses were inconsistent in studies previously cited and we found no significant differences for henhoused egg production. It appears that the relative advantage of earlier sexual maturity resulting from using WL as the male parent in crossing with heavy breed females, compared with the reciprocal cross, is offset to different degrees depending on factors particular to each study that affect livability. Hence, in situations with generally low levels of mortality WL sires would be more desirable. Contrarily, the reciprocal crossbreds would presumably be superior under conditions where higher levels of mortality were usually encountered. Residual reciprocal effects found for age at sexual maturity, full-year hen-housed egg production and full-year livability indicate non-additive genetic variability contributed by the sex chromosome. This term, when significant, indicates an interaction of the Z-chromosome with the autosomal chromosomes received from the strain of the other parent. As there were also significant differences between strain and breed crosses for sexual maturity and henhoused production, a breed by Z-chromosome interaction is suspected.
1406
S. WEARDEN, J. V. CRAIG AND D. TINDELL of corn. J. Amer. Soc. Agron. 34: 923-932. Tindell, D., 1961. General and specific combining abilities, maternal effects and genotype-environmental interactions as estimated from strain and breed crosses of chickens. Ph.D. Dissertation. Kansas State University, Manhattan. Warren, D. C., and C. H. Moore, 1956. Adult mortality in reciprocal crosses of Leghorns and heavy breeds. Poultry Sci. 35: 1178. Wearden, S., 1964. Alternative analyses of the diallel cross. Heredity, 19: 669-680. Wearden, S., D. Tindell and J. V. Craig, 1965. Use of a full diallel cross to estimate general and specific combining ability in chickens. Poultry Sci. 44: 1043-1053. Yates, F., 1947. The analysis of data from all possible reciprocal crosses between a set of parental lines. Heredity, 1: 287-302.
Performance of Hens Molted by Various Methods12 H. R. WILSON, J. L. FRY, R. H. HARMS AND L. R. ARRINGTON Department of Poultry Science, University of Florida, Gainesville, Florida 32601 (Received for publication February 27, 1967)
F
ORCED molting has been studied for many years as a possible method of rejuvenating hens to increase egg production and egg quality. Although recommended by a few poultrymen, Rice (1905) was skeptical that the practice would be profitable. More recent results have been variable and the advantages and disadvantages have been summarized by Cox (1964) and Bell (1965). In addition, force molting has been reported to improve poor fertility in turkeys (Moyer et al., 1966) and to increase the DDT depletion rate in hens (Wesley et al, 1966). The method most commonly used for inducing a molt has been some type of feed and/or water restriction such as that sug1
Florida Agr. Exp. Sta. Journal Series No. 2643. Supported in part by NIH grant AM 08760.
gested by the Poultry Council of the State College of Washington (1947). Noles (1966) reported that feed and water restriction is an effective method of force molting. Progesterone has been shown to be effective in causing a cessation of lay accompanied by a molt (Adams, 1955, 1956; Shaffner, 1955; Himeno and Tanabe, 1957; and others). The rest period can be extended with multiple injections of this hormone (Harris and Shaffner, 1957). Hansen (1960) reported higher egg production subsequent to molts induced by feed and water restriction as compared with those induced by progesterone. Gard et al. (1965) reported that egg production of hens molted with an oral progestin, 6 chloro Ae-17-acetoxy progesterone (CAP), was equivalent to that of birds molted by feed and water restriction. High levels of dietary iodine have been
Downloaded from http://ps.oxfordjournals.org/ at New York University on April 11, 2015
Island Red and White Leghorn breeds of poultry. J. Agr. Sci. 34: 76-81. Eisen, E. J., B. B. Bohren, H. E. McKean and S. C. King, 1967. Genetic combining ability of light and heavy inbred lines in single crosses of poultry. Genetics, 55: 5-20. Goto, E., and A. W. Nordskog, 1959. Heterosis in poultry. 4. Estimation of combining ability variance from diallel crosses of inbred lines in the fowl. Poultry. Sci. 38: 1381-1388. Hayman, B. I., 1954. The analysis of variance of diallel crosses. Biometrics, 10: 235-244. Nordskog, A. W., and R. E. Phillips, 1960. Heterosis in poultry. 5. Reciprocal crosses involving Leghorns, heavy breeds and Fayoumi. Poultry Sci. 39: 257-263. Sprague, G. F., and L. A. Tatum, 1942. General vs. specific combining ability in single crosses