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Research Article
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Estimation of Genetic Parameters and Selection Effect on Genetic and Phenotype Trends in Silkworm Commercial Pure Lines |
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Alireza Seidavi
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ABSTRACT
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This study was conducted to analyze genetic parameters in six commercial Iranian silkworm pure lines including Japanese origin pure lines of 31, 103 and 107 and Chinese origin ones of 32, 104 and 110. All stages of rearing, recording and collecting data were performed over four rearing periods. Quantitative traits of Cocoon Weight (CW), Cocoon Shell Weight (CSW) and Cocoon Shell Percentage (CSP) were evaluated in this study. Covariance components of the characters are estimated by means of REML method. It is estimated heritability, phonotypical, environmental and genetical correlations using DFREML software package. From obtained results, genetic parameters including heritability and genetic correlation for economical trait were different significantly; hence it must be applied appropriate breeding strategies in each pure line. Cocoon weight and CSW heritability was higher than CSP one. Additive genetic correlation between CW-CSW, CSW-CSP and CW-CSP was high, medium and low, respectively. From obtained results, response to selection for cocoon weight and cocoon shell weight are higher than cocoon shell percentage since latest trait had lower heritability. Therefore, it is expected correlated traits improve using selection based on cocoon shell weight. Furthermore, genetic trend of traits were negative and significant at non-selected populations. Phenotype trend of traits also were negative and significant at selected and non-selected populations which is cleared environmental conditions decline at improvement generations.
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INTRODUCTION
Researchers are noticed to genetic parameters of live organisms from previous
years. Liu et al. (1995) trait fluorine-resistance
heritability in the silkworm and they found the controller alleles realized
dominance relative to trait this mainly by genes effects are controlled as additive.
They also found dominance effect and realized the impact of parent material,
but they were not identified the other genes and interactions. Finally, heritability
this trait on the fluoride sensitivity and persistence, were calculated 86.08
and 78.40% respectively. Bhargava et al. (1993)
also studied heritability of seventeen traits in the silkworm. Based on their
results, heritability for larval duration, cocoon shell weight, cocoon weight,
fiber length and weight of 10000 matured larva were high amounts (71.40-86.70%)
and for two other traits were average amounts. They stated heritability of these
two traits were 65.30 for cocoon production and 69.80% for percentage of cocoon
shell. These findings indicate that two recent traits are affected by environmental
conditions. Already Rayar et al. (1989) studied
structure of genetic diversity for 18 economical traits in 13 one-way and 16
three-way silkworm varieties. They demonstrated heritability of larva weight,
larval duration, cocoon shell weight and length of fiber is high. They also
report cocoon fiber length have the most improvement under selection process.
Phenotypic variation coefficients of all traits were higher than variance genotype
coefficients. These findings indicated that the effect of environmental factors
is negligible. Also, Lin et al. (1994) were studied
heritability and other genetic characteristics for cocoon weight loss during
degumming process in silkworm.
Kshama et al. (1995) also were reviewed heritability
and genotype and environment correlations in three competence-related traits
including fertility, persistence and rate of growth and four quantitative traits
including cocoon weight, cocoon shell weight, length and diameter of the fiber
in two strains NB7 NB4D2 and their crosses. They reported the amount of heritability
were higher in quantitative traits and they were between 48-64%, while the competence-related
characteristics have less heritability and were about 25-18%. Thus these results
confirm the classic hypothesis that the competence traits have less h heritability
than other traits. They also estimated both genotype and environment correlations.
Based on their results, there are genotype and environment positive correlations
between cocoon production and cocoon single weight. They reported similar results
for cocoon weight and fiber length, cocoon weight and denier yarn. They demonstrated
selection based on the length and fiber denier result to good improvement for
cocoon production. Nagaraja et al. (1996a) studied
genetic parameters of 17 different traits. Their results indicate heritability
are high for fertility, the maximum larval weight, cocoon weight and number
of produced cocoon, effective rearing rate, pupation rate and moth emergence
rate. These findings and Seidavi et al. (2008b)
report demonstrated selection have high efficiency in these cases. Also, many
researches were conducted in order to investigation on silkworm genetic and
breeding (Govindan et al., 1991; Jayswal
et al., 2000; Islam et al., 2005;
Kamrul and Rahman, 2008).
Kumar et al. (1995) calculated genetic characteristics
of quantitative traits in 46 bivoltine varieties. They emphasize that fiber
length, cocoon weight and cocoon shell weight have high heritability and controlled
under gene factors largely. Meanwhile, the correlation between cocoon weight
and cocoon shell weight and also between cocoon shell weight and cocoon shell
percentage were very significant.
Silkworm egg production process includes three stages i.e., 3P (pure line),
2P (ancestors) and P (parents). All breeding programs and selection must apply
in 3P generation and silkworm pure lines are reared under limited size and high
pressure selective breeding pressure.
There is little information about genetic parameters of Iranian commercial
pure lines. Therefore, this experiment was conducted for study and estimation
of genetic parameters (co-variance components of additive and environmental
genetic, heritability, additive genetic correlation, genetic improvement and
genetic trend) for characteristics of cocoon weight, cocoon shell weight and
percentage of cocoon shell. Also, it is estimated the effect of individual selection
based on cocoon weight on phenotypic and genetic trends in six Iranian commercial
silkworm pure lines.
MATERIALS AND METHODS
This study was conducted at Islamic Azad University, Rasht Branch during 2009
using six silkworm varieties data that they are conserved as Iran commercial
pure lines. Three pure lines have Japanese origin (31, 103 and 107) and three
pure lines have Chinese origin (32, 104 and 110) and all these six pure lines
reproduced and preserved in Iran Silkworm Research Center. All rearing stages
including larval rearing, feeding, cocoon production, silkworm egg preparation
and conservation, hatching and related ancillary activities such as pebrin microscopic
experiment, investigation on fetal development, recording and collecting data
was conducted in the Iran Silkworm Research Center under standard conditions.
Studied quantitative characteristics included cocoon weight (g), cocoon shell
weight (g) and cocoon shell percentage (%). Recorded characteristics of cocoon
weight and cocoon shell weight were performed using a precision digital balance.
Data are recorded during four consecutive generations or rearing duration.
First, it was established base population for each pure line from 3P (pure lime)
populations. Each pure line was contained two groups as selected and random
groups. For each pure line, it is selected in selected group 40 male and 40
female cocoons which had superior amounts of the cocoon weight in their populations
in each sex. These selected individuals recorded based on three individual trait
included cocoon weight, cocoon shell weight and cocoon shell percentage and
then mated together randomly. Furthermore, in each pure line, a randomized or
control group was constructed using 40 male and 40 female cocoons which they
are assigned randomly without any selection and they had cocoon weight equal
to population average. These randomized individuals also recorded for three
individual trait included cocoon weight, cocoon shell weight and cocoon shell
percentage and then mated together randomly. Therefore, there were 40 silkworm
egg batches in each pure line for each selected and randomized groups. Totally,
there were 960 records in base population for each trait (cocoon weight, cocoon
shell weight and cocoon shell percentage). Rearing and silkworm egg production
in future generations for selected and control groups were followed in separate
paths.
At the first generation or 3P population from 40 batches in each group and
pure line, it is hatched 8 silkworm egg batches which had superior hatchability
and fecundities. Consequently, each group in each pure line was contained eight
full-sib families. In the end of the rearing duration and cocoon production
and after determining of the individuals' sexuality, in order to pedigree construction
25 male and 25 female cocoons recoded in the each family based on cocoon weight,
cocoon shell weight and cocoon shell percentage. Totally first generation data
included 4800 records for each trait and included 800 records for each pure
line. Finally, 40 male and 40 female cocoons selected randomly among the eight
families from each pure line and group and mated randomly for silkworm egg construction
of next generation.
Stages of rearing and producing raw data in second (2P) and third (P) generations
were conducted as same as the first generation. Totally data files include 15360
records for each of the three studied traits (2560 records in each pure line).
It should be noted that individual selection was applied only in the base population
and all mates were conducted in next generations as randomly.
Estimation of genetic parameters, heritability, genetic correlation, environmental
correlation and phenotypic correlation were conducted based on randomized data.
However, genotype and phenotypic trends were estimated based on total data included
both selected and randomized groups.
Additive and environmental covariance components (residual effects) were estimated
for three studied characteristics using Restricted Maximum Likelihood (REML)
by means of Derivative-Free REML (DFREML) based on animal model (three- traits
in Henderson mixed equations, using full-sib records (Meyer,
1997). Environmental and phenotypic characteristics were estimated. It was
estimated heritability, genetic correlation, environmental correlation and phenotypic
correlation between traits. It were used the DXMUX program, POWELL Procedure
and DFREML software version 3.1 with convergence 10-8.
Phenotypic trend of cocoon weight, cocoon shell weight and cocoon shell percentage
in each pure line is calculated in each selected and randomized group separately
using the following statistical model:
aij = bSi + eij
where, aij in above equation is individual phenotypic value, b is
regression of phenotypic value to generation (trait phenotypic trend), Si
is ith generation and eij is the residual effects.
Additive genetic trend of cocoon weight, cocoon shell weight and cocoon shell
percentage in each pure line is calculated in each selected and randomized group
separately using the following statistical model:
aij = bSi + eij
where, aij in above equation is individual additive genetic value,
b is regression of additive genetic value to generation (trait genetic trend),
Si is ith generation and eij is the residual effects.
The DFREML and SAS statistical software were used for estimation of genetic
and phenotypic trends their significant using DNMRT method (Duncan,
1951; SAS, 1997).
RESULTS
Table 1 shows trait heritability. Heritability limits of
cocoon weight were between 0.408 (pure line 31) and 0.473 (pure line 103) and
for cocoon shell weight were between 0.348 (pure line 107) and 0.486 (pure lines
103 and 104) and for percentage of cocoon shell were between 0.099 (pure line
32) and 0.402 (pure line 31), respectively. From obtained results, heritability
of cocoon shell percentage was less than the two other traits. Totally heritability
of three traits including cocoon weight, cocoon shell weight and cocoon shell
percentage were 0.496, 0.499 and 0.313, respectively.
Table 2 shows genetic correlation coefficients between cocoon
traits. There are high correlations between cocoon weight and cocoon shell weight.
These findings indicate that these two traits affected by common major genes
and environmental factors. This phenomenon is explained easily because of cocoon
shell weight is constituted a part of cocoon weight.
Table 1: |
Heritability (± Standard error) of cocoon weight, cocoon
shell weight and cocoon shell percentage in studied pure lines |
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Table 2: |
Genetic correlation between cocoon weight, cocoon shell weight
and cocoon shell percentage in studied pure lines |
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Table 3: |
Environmental correlation between cocoon weight, cocoon shell
weight and cocoon shell percentage in studied pure lines |
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Additive genetic correlation between these two traits were between 0.702 (Pure
line 31) and 0.935 (Pure line 110). Genetic correlation coefficient between
these two traits was 0.831 in all pure lines. From obtained results, there are
low genetic correlations between cocoon weight and cocoon shell percentage (-0.369
in pure line 107 and 0.177 in pure line 110) that showed it is difficult improve
these two traits simultaneously via breeding programs. This coefficient was
negative in pure lines of 31, 107 and 32 and was 00.039 in the total pure lines.
Each of these two traits shared in one of sectors of sericulture industry,
since cocoon weight is effective and importance for silkworm rearers (farmers),
but cocoon shell percentage is effective and importance for silk factories.
Considering the results, if it is definite economic efficiency function for
each section separately, the average increscent in each of these characteristics
solely can be decreased efficiency of reciprocal economic sector largely. But
it must be pointed that the all components of a industrial system have some
interactions with each other and it should be definite economic efficiency function
based on considering the total profit and subsidiary components (egg production,
cocoon, silk fiber and silk textile) at national level. Furthermore, any system
planning for trait genetic improvement must be applied based on the relative
economic values.
From obtained results, it was achieved positive and moderate additive genetic
correlation between cocoon weight and cocoon shell percentage between 0.268
in pure line 32 and 0.640 in pure line 104, which was 0.514 in all pure lines.
Since, the most important economic trait in silkworm is cocoon shell weight
and this trait also possessing a high correlation with cocoon weight and cocoon
shell percentage, with regard to its high heritability and response to selection,
it is necessary to emphasize breeding programs must be concentrated on the this
trait.
Table 3 shows environmental correlation coefficients between
cocoon traits. From obtained results, it was achieved positive and moderate
environmental correlation between cocoon shell weight and cocoon shell percentage
between 0.521 in pure line 31 and 0.641 in pure line 107, which was 0.577 in
all pure lines.
Table 4 shows phenotype correlation coefficients between
cocoon traits. From obtained results, it was achieved positive and moderate
phenotype correlation between cocoon shell weight and cocoon shell percentage
between 0.488 in pure line 103 and 0.617 in pure line 104, which was 0.546 in
all pure lines.
Table 5 shows genetic trend (genetic value regression to
generation) for pure lines separately. Genetic trend of cocoon weight, cocoon
shell weight and selection index in non-selected group were negative only in
pure line 31 (-0.016 g, -0.0003 g and -527 Rials per generation, respectively).
The highest genetic trend of cocoon weight, cocoon shell weight and selection
index belonged to pure line 32 (0.041 g/generation), pure line 107 (0.013 g/generations)
and pure line 107 (2961 rials/generation), respectively. Genetic trend of cocoon
shell percentage in pure line 31 was not significant and these trends were negative
in pure lines of 32 (-0.209 percentage/generation) and 110 (-0.060 percentage/generation).
Table 4: |
Phenotype correlation between cocoon weight, cocoon shell
weight and cocoon shell percentage in studied pure lines |
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Table 5: |
Genetic trend (breeding value regression to generation) of
cocoon weight, cocoon shell weight, cocoon shell percentage and selection
index in studied pure lines* |
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* s5: Significant at 0.05 level; sl: Significant at 0.01 level;
ns: Non significant |
Table 6: |
Genetic trend (breeding value regression to generation) of
cocoon weight, cocoon shell weight, cocoon shell percentage and selection
index in all pure lines* |
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* s5: Significant at 0.05 level; sl: Significant at 0.01 level;
ns: Non significant |
The genetic trends of cocoon shell percentage had the highest amount in pure
line 107 (0.309 percentage/generation) significantly (p<0.05). It is concluded
that if low production pure lines mated randomly, due to higher resistance and
lower mortality, have high genetic stability and maintenance their genetic potential
largely. But since high production pure lines have non-stability genetic structure,
it is necessary to applied appropriate selection and breeding programs to keep
their production high level.
In selected group, genetic trend of cocoon weight in pure lines 104 and 110
were not significant and were negative in pure lines 31 and 103 (-0.033 and
-0.039 g/generation, respectively) and were at the highest level in pure line
107 (0.027 g/generation) significantly (p<0.01). Genetic trend of cocoon
shell weight was not significant in pure line 32. This trends were negative
in pure lines 31 and 103 (-0.013 and -0.008 g/generation, respectively) negative
and was at the highest level in 107 pure line 107 (0.011 g/generation). Trend
of selection index was not significant in the pure line 32. This trends were
negative in pure lines 31 and 103 (-3308 and -1252 rial/generation) and was
at the highest level in pure line 107 (2493 rials/generation) significantly
(p<0.01). Genetic trend of the cocoon shell percentage were negative in pure
lines 31 and 32 (-0.395 and -0.160 percentage/generation, respectively) and
was at the highest level in pure line 104 (0.293 percentage/generation).
Table 6 is presented genetic trend (genetic value regression
to generation) for all pure lines. Genetic trend of total pure lines (Table
6) indicates that the genetic trends were significant and positive in non-selected
population for cocoon weight (0.016 g/generation), cocoon shell weight (0.005
g/generation), cocoon shell percentage (0.071 percentage/generation) and selection
index (1030 rials/generation) (p<0.01); whereas this trend was negative in
selected populations for cocoon weight (-0.005 g/generation) (p<0.01).
Table 7: |
Phenotype trend (phenotype value regression to generation)
of cocoon weight, cocoon shell weight, cocoon shell percentage and selection
index in studied pure lines* |
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* s5: Significant at 0.05 level; sl: Significant at 0.01 level;
ns: Non significant |
Table 8: |
Phenotype trend (phenotype value regression to generation)
of cocoon weight, cocoon shell weight, cocoon shell percentage and selection
index in all pure lines* |
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* s5: Significant at 0.05 level; sl: Significant at 0.01 level;
ns: Non significant |
Also, these trends were not significant in selected populations for cocoon
shell weight, cocoon shell percentage and selection index.
Table 7 shows phenotypic trend (phenotypic value regression
to generation) for pure lines separately. Phenotypic trend of the cocoon weight,
cocoon shell weight and cocoon shell percentage in selected and non-selected
groups were negative and significant (p<0.01) and this trend was not significant
only in pure lines of 107 and 104 for cocoon shell percentage (Table
7). Phenotypic trend of the cocoon weight and cocoon shell weight in selected
group were less than non-selected group. Only phenotypic trend of cocoon shell
percentage in pure lines of 32 and 110 in selected group were higher than non-selected
group. In non-selected group, phenotypic trend of the cocoon weight, cocoon
shell weight and cocoon shell percentage were at the highest level in pure line
32 (-0.036 g/generation), 107 (-0.011 g/generation) and 103 (-0.234 percentage/generation)
respectively. Also, phenotypic trend of the cocoon weight, cocoon shell weight
and cocoon shell percentage in non-selected group were at the lowest level in
pure line 31 (-0.096 g/generation), 31 (-0.026 g/generation) and 32 (-0.659
percentage/generation) respectively. In selected group, phenotypic trend of
the cocoon weight, cocoon shell weight and cocoon shell percentage were at the
highest level in pure line 107 (-0.072 g/generation), 107 (-0.016 g/generation)
and 110 (-0.306 percentage/generation) respectively. Also, phenotypic trend
of the cocoon weight, cocoon shell weight and cocoon shell percentage in selected
group were at the lowest level in pure line 103 (-0.143 g/generation), 31 (-0.043
g/generation) and 31 (-0.884 percentage/generation), respectively.
Table 8 shows phenotypic trend (phenotypic value regression
to generation) for all pure lines. From Table 8 it is showed
that phenotypic trend of cocoon weight, cocoon shell weight and cocoon shell
percentage in non-selected population (-0.061, -0.018 g/generation and -0.314
percentage/generation, respectively) are higher than selected population (-0.108,
-0.029 g/generation and -0.379 percentage/generation), respectively (p<0.01).
DISCUSSION
Ghanipoor et al. (2007) also were demonstrated
lower heritability for cocoon shell percentage. Heritability of cocoon weight
in pure lines of 107 and 32 were higher than compare with the cocoon shell weight.
Lower heritability for cocoon shell percentage means cocoon shell has low genetic
diversity and affected under environmental conditions largely. Therefore improvement
progress in the breeding programs for this trait especially under individual
selection will be difficult. Ghanipoor et al. (2006b)
studied based on the characteristics of the 10000 records belonged some silkworm
pure lines during nine generations and showed that additive genetic variance
for cocoon weight in pure lines of 107, 153 and 154 are higher than other pure
lines. Also additive genetic variance for cocoon shell weight in pure lines
of 110, 101433 and Y were higher than other pure lines. Meanwhile they reported
environmental variance for the cocoon shell percentage was higher than other
cocoon traits. Mirhosseini et al. (2004) were
estimated cocoon weight heritability between 0.374-0.500, cocoon shell weight
heritability between 0.366-0.752 and cocoon shell percentage heritability between
0.192-0.337. Mirhosseini et al. (2005b) estimated
the genetic parameters of economic characteristics for native breeds of silkworm.
They are believed there is high heritability for most economic characteristics.
Previously, Ghanipoor et al. (2007) found that
the selection based on index in pure lines have significant effect on improvement
the cocoon weight and cocoon shell percentage in silkworm hybrids. Also, Ghanipoor
et al. (2007) showed heritability of cocoon shell percentage is lower
than cocoon weight and cocoon shell weight. Ghanipoor et
al. (2005) in another study showed heritability of laying traits in
pure line are between 0.08-0.27, heritability percentage of fertility are between
0.03-0.26 and heritability of hatching percentage are between 0.03-0.16. Mirhosseini
et al. (2004) estimated high heritability for cocoon weight and cocoon
shell weight. They were estimated low heritability for cocoon shell percentage.
They realized that the high genetic correlation between cocoon shell weight
with two other traits, it is expected by selecting of individual based on cocoon
shell weight can improve on correlated characteristics e.g. cocoon weight and
cocoon shell percentage. Mirhosseini et al. (2007c)
investigated on the effect of sexuality characteristics on cocoon trait feasibilities
and they concluded heritability of cocoon weight, cocoon shell weight and cocoon
shell percentage in males were higher than female sex. Thus it cans accelerate
genetic gain in males using high selection pressure. Mirhosseini
et al. (2005b) and Mavvajpoor et al. (2006)
showed that economic characteristics of native silkworm have high heritability
and it is possible their improvement based on appropriate planning in long-term.
Mu et al. (1995) analyzed the eight traits in
two Chinese and three Japanese varieties and realized pupation rate, cocoon
shell weight, cocoon weight, cocoon shell percentage, cocoon production and
weight of 10000 larva controlled by some genes with additive and dominant effects.
But they did not fount any epistatic effect on these traits. They also expressed
cocoon shell weight, cocoon shell percentage and cocoon weight of 10000 larva
controlled mainly by the relative dominance genes, but other characteristics
inherited mainly as dominant. These researchers were also subject to the specific
heritability of cocoon shell weight, cocoon shell percentage and cocoon weight
of 10000 larva are more than other characteristics. Similar reports published
by Chatterjee et al. (1993).
Seidavi et al. (2008a) studied effect of phenotypic
selection for parents based on cocoon weight on the reproductive characteristics
and reminded that there are positive genetic correlation between productive
and reproductive characteristics in the Chinese pure lines and individual selection
cause improvement of reproductive characteristics. Mirhosseini
et al. (2002) showed that the some trait response against direct
selection is caused to high heritability. Also, there are high genetic correlations
between some characteristics of pure line and selection for each one of these
characteristics can improve the expected correlated trait. Ghanipoor
et al. (2006c) in another study were found that in some pure lines
due to negative genetic correlations are likely to reduce reproductive potential
against increscent of selection pressure for cocoon traits. Grekov
(1989) stated there is a positive correlation between the weight of cocoon
shell weight and cocoon weight (+0.659) and thus selection should be applied
based on cocoon shell weight, but it must also be considered the cocoon weight
and fiber length simultaneously. Petkov (1989) also
studied genetic properties of some new silkworm varieties and found correlation
between cocoon shell weight and cocoon shell percentage is 0.528-0.653. He suggested
pure line selection applied based on cocoon shell weight, but it be considered
cocoon weight and fiber length. Rangaiah et al. (1995)
studied fertility, larval growth, larvae weight, cocoon weight, shell cocoon
weight and cocoon shell percentage in 18 silkworm varieties. They calculated
phenotypic and genotype correlations between six traits. They emphasized that
genotypes correlation are higher than phenotypic correlations in all cases.
Cocoon weight, cocoon shell weight and cocoon shell percentage had positive
correlations with fertility and they are therefore proposed breeding programs
must be emphasized on these trait selection.
Obtained results regarding additive genetic correlation between these traits
is accordance with results of Nagaraja et al. (1996b),
Mirhosseini et al. (2004, 2007a,
b). Mirhosseini et al. (2002)
in another study also reported genetic correlation between cocoon weight and
cocoon shell weight are 0.709-0.989, the genetic correlation between cocoon
weight and cocoon shell percentage is -0.124-0.564 and additive genetic correlation
between cocoon shell weight and cocoon shell percentage is 0.270-0.835 in studied
pure lines.
Generally, genetic trend of traits in non-selected groups were higher than
selected groups. It is due to genetic tangible decline of the traits under non-breeding
programs in the successive generations. At this group also it is found low production
pure line (107) had higher genetic stability than high production pure lines
(31 and 103). Mirhosseini et al. (2005a) studied
the genetic trend of biological and quantitative characteristics of silkworm
populations under high selection pressure and found genetic improvement will
decline and decrease in successive generations due to loss of genetic diversity.
Mirhosseini et al. (2008) obtained a positive
genetic trend for the genotype value (genetic competency) during the 8 generations.
Present results have significance outputs and also provided experiment aim
and goals. Based on obtained results, we can use these results and suggestions
for future silkworm breeding programs and performance improvement of silkworm
pure lines.
Finally, obtained results about phenotypic and genotype trends in pure lines
had similar results. Ghanipoor et al. (2006a)
also reviewed phenotypic trend of the production traits in silkworm under genetic
selection condition and demonstrated phenotypic trend and improvement of economical
traits are different in pure lines and affected by various factors like heritability.
Also, differences between studied genotypes and other genotypes in other studies
are possible reason where results differ from previous studies.
CONCLUSION
As conclusions it should be expressed the genetic parameters such as heritability
and genetic correlation between economic traits are different in silkworm pure
lines. Thus, it must be selected appropriate strategy for each pure line. Response
to selection for cocoon weight and cocoon shell weight were high, since these
two traits have higher heritability than cocoon shell percentage. Genetic correlation
between weight cocoon- cocoon shell weight, cocoon shell weight-cocoon shell
percentage and cocoon weight - cocoon shell percentage were high, moderate and
low, respectively. So, using the selection cocoon shell weight can improve the
correlated traits. The genetic trends of traits in the non-selected populations
were negative and significant which indicates there is a kind of un-wanted breeding
program. Finally, phenotypic trend of traits were negative in selected and non-selected
populations which cleared environmental conditions will decline at successive
generations.
ACKNOWLEDGMENTS
This study was supported by the Young Researchers Club, Islamic Azad University,
Rasht Branch, Iran under grant number of 03-16-5-3063. The author also acknowledges
the kind advice of Mr. Moeinoddin Mavvajpour, Alireza Bizhannia and Mani Ghanipoor
for valuable comments and assistances.
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