270 P
. Sellier et al. Livestock Production Science 63 2000 265 –274 Table 6
a
Estimates of phenotypic and genetic correlations among fat androstenone levels BCAND1, BCAND2, male genital tract measurements TBUG1, LBUG2, WBUG2, WTES2, gilt puberty status GPUBS, average daily gain ADG and backfat thickness BFAT
Trait BCAND1
BCAND2 TBUG1
LBUG2 WBUG2
WTES2 GPUBS
ADG BFAT
BCAND1 –
0.97 0.65
0.34 0.58
0.47 0.22
0.04 0.11
0.02 0.06
0.08 0.06
0.08 0.09
0.14 0.12
BCAND2 0.71
– 0.66
0.54 0.68
0.41 –0.08
–0.16 0.11
0.06 0.10
0.07 0.08
0.13 0.12
0.09 TBUG1
0.37 0.38
– 0.83
0.93 0.54
0.41 –0.30
–0.07 0.07
0.03 0.07
0.12 0.09
0.08 LBUG2
0.30 0.42
0.60 –
0.90 0.26
0.12 –0.30
–0.23 0.03
0.11 0.17
0.11 0.15
WBUG2 0.35
0.51 0.80
0.84 –
0.46 0.34
–0.33 –0.14
0.07 0.15
0.13 0.13
WTES2 0.18
0.22 0.26
0.32 0.37
– 0.45
–0.29 –0.34
0.10 0.09
0.11 GPUBS
– –
– –
– –
– –0.58
0.02 0.10
0.09 ADG
0.13 20.06
20.16 20.15
20.22 20.13
20.16 –
–0.16 0.12
BFAT 0.12
0.18 20.19
20.10 20.13
20.12 0.04
0.29 –
a
Phenotypic correlations below the diagonal, genetic correlations SE between brackets above the diagonal.
2
the h estimates for WBUG2 and TBUG1 were of ment comprised a control line, a line selected for low
the same order and the two traits were highly fat AND level and small testis size ‘agonistic’
correlated. Heritability of testes weight WTES2 selection, and a line selected for low fat AND level
was similar to that of WBUG2, whereas the genetic and large testis size ‘antagonistic’ selection. The
association of WTES2 with BUG size measurements responses found in the latter line, i.e. a significant
was of medium magnitude. The genetic correlation decline of fat AND level and no genetic change in
of fat AND level BCAND1 or BCAND2 with weights of testes and bulbo-urethral glands, sug-
BUG size TBUG1 or WBUG2 was of the same gested that it would be feasible by breeding to reduce
order whatever the respective stages of measurement fat AND level without adversely affecting sexual
2
of the two traits. The h estimate for gilt puberty
maturity status in boars. The study reported here was status GPUBS was about 0.50, and this trait
designed for confirming this finding over a greater showed positive genetic correlations with TBUG1,
number of generations of selection while replacing WBUG2 and WTES2. There was a significant nega-
testis size by the development of accessory sex tive genetic relationship between GPUBS and ADG.
glands BUG for appraising the stage of sexual The phenotypic and genetic correlations of fat AND
maturation attained by young boars at a fixed body levels with ADG and BFAT were of small mag-
weight. nitude. The two latter traits showed moderate nega-
tive genetic correlations with male genital tract 4.1. Responses to selection
measurements. 4.1.1. Direct responses
Using a restricted selection index has resulted in
4. Discussion realized genetic trends for each component trait
which disagreed with expectation. Such unexpected The primary incitement to perform the present
patterns of responses to ‘antagonistic’ selection are experiment came from the fairly encouraging results
not unusual in the studies relying on selection indices obtained by Sellier and Bonneau 1988 in a single-
with constraint. Some restricted index selection generation selection experiment. This earlier experi-
experiments have been successful in mice, and
P . Sellier et al. Livestock Production Science 63 2000 265 –274
271
showed both significant changes in the trait selected with Large White purebred boars. This has resulted
for and no or little change in the restricted trait e.g. in a relatively small selection pressure on this trait,
Atchley et al., 1997. However, there are several except in the first generation.
studies in which unintended responses in the com- ponent traits of the restricted selection index
4.1.2. Correlated responses occurred for references, see Eisen, 1992. The
The trends found in the select line for fat andros- present experiment exemplifies a typical case, i.e. the
tenone level and bulbo-urethral gland development trait selected downward AND did not noticeably
were similar for both stages of measurement, in change while the restricted trait BUG size showed
agreement with the very high genetic correlations a significant positive response to selection.
between homologous traits. The correlated response Restricted index selection appears to be very
observed in females of the select line, i.e. an sensitive to the degree of accordance of expected
increased percentage of gilts showing first oestrus with true genetic parameters involved in the selection
prior to 116.8 kg BW, is in line with the earlier index. Here, the genetic parameters as estimated by a
sexual maturity of the males of this line as demon- REML analysis of data from the whole experiment
strated by the greater development of BUG. This somewhat differed from those inferred for construct-
suggests that, provided that the weightings of ing the selection index. In particular, the REML-
TBUG1 and LAND2 are correctly balanced in the estimated heritability of TBUG1 was noticeably
selection index see above, it is probably possible to
2
higher than the expected one h 50.63 vs. 0.40, reduce fat androstenone levels with no adverse effect
which has ‘favoured’ the response in this trait in on sexual maturity of both males and females by
comparison with LAND2. In addition, the two index using BUG development as an estimate of sexual
traits were revealed as exhibiting a slightly closer maturity status. On the contrary, selection against
genetic correlation than expected 0.66 vs. 0.50. androstenone with no consideration of sexual de-
This discrepancy between expected and true values velopment Willeke et al., 1987 or using testes
of genetic correlation is likely to have reduced the development as an estimate of sexual maturity status
negative response in LAND2 due to a larger than Sellier and Bonneau, 1988 resulted in a delay in
expected counteracting effect of the upward selection the sexual maturity of related females.
pressure achieved on TBUG1. Given the REML- estimated genetic parameters of the two index traits,
4.2. Genetic parameters the selection index to be used should have put less
emphasis on TBUG1 for having an effective restric- 4.2.1. Fat androstenone level
tion of zero genetic gain in bulbo-urethral gland Regarding fat AND level, the genetic parameters
development. estimated in the present study are in general agree-
As already mentioned, another factor which has ment with those published earlier Jonsson and
probably contributed to the unexpected responses in Andresen, 1979; Bonneau and Sellier, 1986; Willeke
the index traits refers to the realized selection et al., 1987; Sellier and Bonneau, 1988; Willeke and
2
differentials for each trait. Considering the ‘average’ Pirchner, 1989. The h estimates for BCAND1 and
index actually applied over the four generations of BCAND2 are very close to the average literature
selection, the weighting given to TBUG1 relative to value of 0.56 quoted by Sellier 1998. Fat AND
LAND2 was almost 50 higher than expected. The levels and growth rate were poorly associated both at
indices in retrospect greatly varied among genera- the phenotypic and genetic level. It should however
tions. This occurred partly by chance due to the be mentioned that Willeke and Pirchner 1989
small number of sires selected in each generation. found positive realized genetic correlations of about
However, it should be noted that our experimental 0.35 between fat AND levels and body weights at
population whose genetic background was half Large various fixed ages. The genetic independence found
White–half Landrace exhibited a smaller mean value here between fat AND level and backfat thickness is
and a lower phenotypic variability of fat AND level consistent with the lack of any genetic trend for
than could be expected from our previous studies backfat thickness in the select line.
272 P
. Sellier et al. Livestock Production Science 63 2000 265 –274
4.2.2. Sexual maturity of boars androgens and oestrogens, the latter two being
Among the genital tract measurements utilized for responsible for the development of bulbo-urethral
assessing sexual maturation in young boars, testis glands Joshi and Raeside, 1973; Booth, 1980.
size has been by far the most studied on genetic grounds e.g. Legault et al., 1979; Toelle et al., 1984;
4.2.4. Puberty in females
2
Sellier and Bonneau, 1988; Johnson et al., 1994. The current h estimate of 0.53 for the gilt puberty
2
Our heritability estimate for testes weight falls within status variable is in the range of the h
values the range of values previously published for various
previously found for the age at puberty of gilts, traits pertaining to testicular development at fixed
while being noticeably higher than the average body weights or, most often, at fixed ages. It is,
literature value of 0.33 quoted by Rothschild and however, well above the average literature values of
Bidanel 1998. It should however be noted that 0.33–0.44 quoted by Rothschild and Bidanel 1998
heritability was higher for weight than for age at for heritability of testis width, length or weight. The
puberty 0.51 vs. 0.29 in the study of Bidanel et al. same authors concluded from their literature review
1996. A close negative genetic correlation was that a positive genetic correlation of about 0.50
found in the present study between ADG and exists between ADG and testis size when the latter
GPUBS. This result agrees with the finding by trait is measured at constant age, whereas available
Bidanel et al. 1996 of a moderate positive genetic results are more discordant when testis size is
association of growth rate with weight at puberty in measured at constant weight. In the present study, all
gilts. The genetic independence found here between male genital tract measurements performed at fixed
backfat thickness and gilt puberty status is in line BW exhibited a moderate negative genetic associa-
with average literature results Rothschild and tion with ADG. Sexual maturation of the young boar
Bidanel, 1998. therefore appears to be differently associated with
growth rate depending on whether it is assessed at 4.2.5. Genetic association of male with female
constant age or at constant weight. Regarding the puberty traits
genetic relationship of testis size with backfat thick- Fat AND levels in boars were poorly genetically
ness, our study showed a moderate negative correla- linked with gilt puberty status in the present study.
tion between the two traits. This result is in line with As already mentioned, the studies of Willeke et al.
that of Toelle et al. 1984, but not with the very low 1987 and Sellier and Bonneau 1988 concurred to
or slightly positive genetic correlations reported by show a markedly positive genetic association among
Young et al. 1986, Lubritz et al. 1991 and the fat AND level in young boars and sexual
Johnson et al. 1994. precocity in gilts. The genetic relationship between
sexes for attainment of puberty was found to be near 4.2.3. Association of fat androstenone level with
zero in some studies Schinckel et al., 1983; Sellier sexual maturity of boars
and Bonneau, 1988; Johnson et al., 1994, but it was The close genetic relationship found here between
positive in other studies Bates et al., 1986; Young et fat AND level and BUG or testis size confirms the
al., 1986. The present experiment shows a signifi- previous findings of Bonneau and Sellier 1986 and
cant genetic correlation of 0.30–0.40 between sexual Sellier and Bonneau 1988. One may put forward
maturation of males, as assessed by the development that the genetic correlation of fat AND level with the
of testes and accessory sex glands at constant BW, sexual maturity status of the boar is equal or superior
and sexual maturation of females, as assessed by the to 0.50. Highly significant phenotypic correlations,
categorical ‘gilt puberty status’ variable. ranging from 0.30 to 0.70, have been repeatedly
reported among the same traits e.g. Forland et al., 1980; Bonneau and Russeil, 1985; Xue et al., 1996;
5. Conclusion
