Impact of Trait Selection in Optimizing the Egg and Silk Yields of Daba Ecorace of Tropical Tasar Silkworm, Antheraea mylitta Drury for Seed and Commercial Crop Seasons
R. Manohar Reddy
Vanya silk production in Daba ecorace of Antheraea mylitta Drury (Lepidoptera: Saturniidae) often ends up with tentative yields and the present study aimed to optimize its seed (egg) and silk productivity through varied parental combine based on their commercial traits for optimal sustenance. The prospects of varied parental combinations were assessed through consecutive rearing seasons (RS1 = July/Aug, 06; RS2 = Sept/Dec, 06; RS3 = July/Aug, 07; RS4 = Sept/ Dec, 07; RS5 = July/Aug, 08) to optimize the commercial trait output. The high pupal weight female and high shell weight male combination (C4) has recorded gain in fecundity (+8.6%), shell weight (+22.2%), silk ratio (+13.9%) and silk yield (+12.1%) with minor reduction in egg hatching (-2.4%) and cocoon yield (-6.6%) over the randomly mated parental combination (control-C1), when data of five successive rearing seasons were pooled. The high shell weight parental combination (C3) has recorded higher gain in shell weight (+12.7%) and silk ratio (+9.8%), marginal progression in fecundity (+1.2%) and regression in egg hatching (-7.8%), cocoon yield (-24.1%) and silk yield (-17.8%) when compared to control. The high pupal weight parental combination (C2) has exhibited maximum gain in fecundity (+12.2%) and loss in hatching (-19.6%), cocoon yield (-35.5%), shell weight (-12.7%), silk ratio (-10.6%) and silk yields (-45.4%) in comparison to control. The parental combinations have expressed dissimilar performance in individual rearing seasons and average of five successive seasons, though they originated from same parental stock. The study suggested that same ecorace can have different parental combinations based on magnitude of commercial traits with associated crop rearing seasons and these can be utilized for evolving breeds of commercial worth with higher fecundity and shell weight to supplement the commercially important vanya silk yield.
The tasar silk is produced by Antheraea mylitta Drury, a wild tropical
sericigenous insect (Lepidoptera: Saturniidae) available in central India having
its distribution (11-31°N latitude and 72-96°E longitude), touching
the Himalayan range in North, Kerala in South, Rajasthan in West and Nagaland
in East. The polyphagous insect feeds primarily on Terminalia tomentosa
(Asan), Terminalia arjuna (Arjun) and Shorea robusta (Sal) in
addition to several secondary and tertiary food plants (Suryanarayana
et al., 2005).
In spite of wider distribution of the species over diverse ecological niche
as 44 ecoraces (Suryanarayana and Srivastava, 2005), only
few are applied commercially and the need is to exploit other prospective races
(Hansda et al., 2008; Ojha
et al., 2009). The bivoltine tasar insect has two annual lifecycles,
the first being the seed crop (July-August) with shorter larval span yielding
non-diapausing cocoons with thin shell and simultaneous moth emergence followed
by egg laying, while the subsequent commercial crop (September-December) is
with longer larval span yielding cocoons with thicker shell undergo prolonged
pupal diapause of 6-7 months (Suryanarayana and Srivastava,
2005). The potential phenotypic expression of a genotype desires suitable
environment (Srivastava et al., 2004; Mulder
and Bijma, 2005) and the environment being exogenic factor, it influences
the expressivity of gene by generating different phenotypes under different
environments (Jong and Bijma, 2002; Zhao
et al., 2007; Kumar et al., 2008).
The variations in climate, nutrient status, feeding duration and larval crowd
along with environmental stimuli will influence the insect body size (Davidowitz
et al., 2004; Miller, 2005; He
and Wang, 2006). The varied mating systems can balance desired traits in
selected race combination and target of such stock continuation is for high
egg recovery and silk yields (Nakada, 1992; Yamaguchi,
2001; Nagaraju, 2002; Reddy
et al., 2009b). The race maintenance, selection methods, crossing
techniques, understanding of ecological requirements during insects life
cycle have great influence on trait expression during current and successive
progenies (Moghaddam et al., 2005; Rajnarain
et al., 2008; Reddy et al., 2009c).
The approach of breeding with specific parents is a tool for genetic improvement
towards fecundity, shell weight and total silk yield in tropical tasar silkworm
(Reddy et al., 2009a). The parental selection
as per commercial requirement lead to explicit individuals in their progeny
and different mating systems can balance the traits in selected line for higher
silk productivity and quality (Benchamin, 2002; Reddy
et al., 2009d). The selection of bigger females improves silkworm
fecundity and parents with higher shell will contribute better shell weight
and silk ratios (Yadav et al., 2001; Calvo
and Molina, 2005; Chandrasekhar and Basavaraja, 2008;
Reddy et al., 2008). However, the lack of information
on impact of rearing of different parental combinations under varied rearing
seasons on the versatility and expression of commercial traits in semi-domesticated
Daba ecorace made to choose the present study.
MATERIALS AND METHODS
Collection and segregation of parental seed cocoons: The cocoons of tropical tasar Daba ecorace of Antheraea mylitta Drury used for study were obtained during May, 2006 from stabilized stocks of Central Tasar Research and Training Institute (CTR and TI), Ranchi. Sex wise assessment and assortment of cocoons were done based on high pupal weight and high shell weight groups. After making the groups based on high pupal and shell weight (above average), regrouping was done for mating of specified parents (males and females). The groups are: C2: females and males having high pupal weight, C3: females and males having high shell weight, C4: females with high pupal weight and males with high shell weight and C1: control parents where no sorting and grouping was made.
Preservation of seed cocoons and preparation of Dfls: The seed cocoons selected under parental combinations were maintained separately in tasar grainage house following integrated package of tasar seed cocoon preservation. The Disease free layings (Dfls) were prepared separately under each group following integrated package of tasar seed production. The cocoon stocks of subsequent progenies of these four combinations (C1 to C4) were maintained separately following selection over average performance and prepared Dfls during the five successive rearing seasons (RS2 to RS5) Sep/Dec, 06, Jul/Aug, 07, Sep/Dec, 07 and Jul/Aug, 08 to continue the cocoon stocks.
Rearing of different parental combinations: The Dfls prepared under four different parental combinations were reared following integrated tasar silkworm rearing package with Complete Randomized Block Design (CRBD) for five generations during Jul/Aug, 06 (RS1), Sep/ Dec, 06 (RS2), Jul/Aug, 07 (RS3), Sep/ Dec, 07 (RS4) and Jul/Aug, 08 (RS5). The cocoons thus, produced were utilized to continue their progenies successively from rearing season RS1 in Jul/Aug, 06 to RS5 in Jul/Aug, 08, of-course inducing above average selection pressure in each subsequent generation. Three replications have been maintained in each parental combination (C1 to C4) considering larvae of one Dfl as one replication during all rearing seasons (RS1 to RS5) and the observations were recorded for fecundity, egg hatching, cocoon yield, single shell weight, silk ratio percentage and absolute silk yield. The data recorded on different commercial parameters in the study were subjected to statistical analysis.
Analysis of variance: The ANOVA for commercial traits under different parental combinations, varied successive rearing seasons and parental combinations versus rearing seasons of Daba ecorace (Table 1) indicate their levels of significance. The variance among four different parental combinations (C1 to C4) and varied successive rearing seasons (RS1 to RS5) has shown significance at 0.1% level except for fecundity among rearing seasons which recorded non-significant. The variance in parental combinations versus rearing seasons has also shown significance at 0.1% level in respect of egg hatching, cocoon yield, shell weight, silk ratio and silk yield, while the fecundity found significant at 5% level.
Comparison of different parental combinations: The performance of different
parental combinations over five successive rearing seasons and their average
performance as against their respective controls, C1 x RS1
to C4 x RS1 (Table 2) show clear variations
in respect of egg and cocoon related traits. The optimal trait expression has
been recorded in fecundity of high pupal combination (C2) with an
average of 286 eggs over five successive rearing generations, ranging between
286 and 309 eggs during different rearing seasons. A specific trend was also
noticed in uniformity of shell weight under different rearing seasons. The high
pupal and high shell combination, C4 has recorded better average
fecundity next to C2, with an average of 277 eggs (ranging between
273 and 294) while the average egg hatching was 66.4% (ranging between 64.2
and 73.7). The 73.7 % of egg hatching at fifth rearing generation recorded in
C4 was the highest among all the parental combinations and crop rearing
||ANOVA for egg and silk traits of Daba ecorace under varied
parental combinations (C1 to C4) and crop rearing
seasons (RS1 to RS5)
|*Significant at 5%; ***Significant at 0.1%; NS: Non significant
|| Performance of varied parental combinations (C1
to C4) of Daba ecorace under crop rearing seasons (RS1
|Combinations, C1: Control: Parents mated randomly,
C2: High pupal femalexhigh pupal male, C3: High shell
femalexhigh shell male C4: High pupal femalexhigh shell male,
Rearing Seasons, RS1: July/Aug, 06, RS2: Sept/Dec,
06, RS3: July/Aug, 07, RS4: Sept/Dec, 07 and RS5:
July/Aug. 08. Values are Mean±SE, average and % change over control
However, the shell weight average of 1.54 g (ranging between 1.4 and 1.9),
silk ratio percentage average of 13.9 (ranging between 13.4 and 17.7) and silk
yield average of 76.1 g (ranging between 60.6 and 110) are the highest recorded
among all parental combinations. The performance of high shell combination,
C3 was moderate, though it could out-do the C2 combination
in egg hatching average of 62.7% (ranging between 59.8 and 68.8), shell weight
of 1.42 g (ranging between 1.4 and1.6 g) and silk yield of 55.8 g (ranging between
37.7 and 78.4), it has shown less in average fecundity with 258 eggs (ranging
between 256 and 262). Though, the control (C1) has recorded better
cocoon yield average (52.7 no.) over C4, C3 and C2
respectively, the highest cocoon yield of 62.3 no. has been recorded by C4
at fifth rearing generation. The performances of C2 and C3
parental combinations are inferior to C4 in all traits, except for
fecundity of C2. The impact of five different and successive rearing
seasons on egg, cocoon yield and silk traits of Daba ecorace indicates optimal
yield in fifth Rearing Season (RS5) than the first season (RS1),
except for egg hatching (55.7 %) and silk yield (45.3 g) of C2. The
expression of cocoon related characters were found better in rearing seasons
of commercial crop (RS2 and RS4) over the rearing seasons
of seed crop (RS3 and RS5) as well as over the first Rearing
The tropical tasar silkworm, Antheraea mylitta, with pupal diapause,
limited annual life cycles and rearings in outdoor need coherent application
of parental variation available among or within the ecorace, to augment seed
(egg) and silk production competency. The inadequacy of race option in general
and for a rearing season or region in specific is the main constraint in utilizing
the potential of tasarculture and the critical need is to exploit appropriate
breed options (Hansda et al., 2008; Ojha
et al., 2009). In view of very limited applicable tasar ecoraces,
the maintenance of vigor in existing choice is one of the essential requisites
to attain possible productivity and commercial sustenance (Rajnarain
et al., 2008; Reddy et al., 2009c).
The continuation of same parental stock through generations with repeated and
traditional crossing system leads to inbreeding depression and to regain the
race potential, application of parental recombination is essential (Yamaguchi,
2001; Benchamin, 2002; Chandrasekhar
and Basavaraja, 2008; Reddy et al., 2009b).
Further, the selection of parents must be based on phenotypic traits of productive
and commercial value, besides their better heritability and season compatibility
for commercial advantage.
Performance of different parental combinations: The continuation of
rearing under different parental combinations (C1 to C4)
of Daba ecorace over five successive crop rearing seasons (RS1 to
RS5) has shown enhancement of commercial trait(s) like fecundity
and shell weight in relation to the parents applied. The average fecundity of
286 eggs over five generations in high pupal combination (C2) indicates
the role of parents involved in enhancement of fecundity trait compared to 255
eggs in first generation. The higher fecundity and lower performance in other
traits of C2 in all generations, irrespective of rearing season,
further supports the positive co-relation of pupal weight and fecundity traits
are corroborating with earlier reports (Yadav et al.,
2001; Calvo and Molina, 2005; Chandrasekhar
and Basavaraja, 2008; Reddy et al., 2008).
However, the decline in egg hatching, cocoon yield, shell weight and silk ratio
of C2 has resulted to low silk yield. The high shell combination
(C3) has inherited higher shell weight in successive generations
with an average of 1.42 g compared to 1.00 g in first generation. The expression
of shell weight trait has been consistent irrespective of the rearing season
supports the role of selected parents in enhancement of associated trait as
reported by Chandrasekhar and Basavaraja (2008) and
Reddy et al. (2009a). But, this combination could
surpass the control and C2 in respect of shell weight and silk ratio,
though improvement in fecundity was marginal and silk yield was low in comparison
to the control. Interestingly, both the combinations (C2 and C3)
have exhibited same trend of improvement in traits associated with parental
selection with lesser impact on other commercially important traits, irrespective
of the rearing seasons. This indicates the principal role of high pupal and
high shell parents on the improved fecundity and shell weights respectively
with no significant change in silk productivity over different rearing seasons,
in spite of variations in rearing climate, feed quality and larval feeding duration.
Expression of fecundity and shell weight traits: However, the combination
C4, where the parents are of high pupal female and high shell male
could express better average performance both in respect of fecundity and shell
weight, which has contributed for higher silk yield, in spite of marginal reduction
in egg hatching and cocoon yield. The trend of C4 remains as like
C2 and C3 in expression and increase in fecundity and
shell weights all-through the generations and hence it has recorded highest
silk yield. Though, the expression of all the traits are closely associated
and influenced with rearing seasons in control (C1), the selective
parental combinations (C2, C3 and C4) have
clearly influenced the selected trait irrespective of the rearing season. But,
the expression of shell weight has been influenced by the seasons in all the
combinations except for C2, where the selection stress was more on
fecundity. Further, all the combinations have recorded less impact of rearing
seasons in respect of fecundity and egg hatching whereas season influenced the
shell weight and silk ratio significantly are in accordance with earlier reports
(Zhao et al., 2007; Kumar
et al., 2008; Reddy et al., 2009d).
This might be due to the varied feeding duration of larva, climate and feed
quality during seed and commercial rearing seasons and importantly due to diapause
preparation of the silk insect to face severe winter followed by summer.
Performance variation during seed and commercial crops: The seed crop
(July/August) cocoons with less silk content are utilized for seed production
but, all the cocoons so generated may not be considered for seed either due
to disease incidence or the quality requirement in seed cocoons for commercial
grainage activity. The enhanced fecundity and hatching contributes for higher
cocoon yields, while the higher shell weight and silk ratio leads to more silk
yield and the combination of both can fetch better yields and returns. Since,
the cocoon or silk yield are dependent on fecundity, egg hatching, shell weight
and silk ratio and their interaction as reported by Jong
and Bijma (2002), Srivastava et al. (2004)
and Mulder and Bijma (2005) the selected parental combinations
can serve to buffer the total silk yield. This is clear when we see the performance
of C4 at RS5 (seed crop) showing better silk yield over
other combinations except C4 at RS4 (commercial crop).
From the above it becomes clear that the combination of assorted parents has
resulted to yield enhancement in some of the commercial parameters (Davidowitz
et al., 2004; Miller, 2005; He
and Wang, 2006; Reddy et al., 2009b).
Role of environmental conditions of rearing seasons: Miller
(2005), Chandrasekhar and Basavaraja (2008) and
Reddy et al. (2009b) have reported that the environmental
conditions of different rearing seasons also have its influence on expression
of parental combinations. The environmental role can be explained through the
cocoons with thin shell and low silk yield observed during seed crop (July/August)
and thick shell and high silk during commercial crop (September/December) seasons,
which also signifies their role in seed production and silk production. Hence,
for the commercial crop season (Sep-Dec: with temperature of 17-27°C and
RH of 60-80%), selection of quantitative traits like shell weight, silk ratio
and silk yield should be emphasized so as to achieve longer filament, while
during seed crop season (Jul-Aug: with temperature of 23-35°C and RH of
45-75%), the priority should be on fecundity and egg hatching to achieve higher
cocoon yield to support the planned commercial seed production. The tested parental
combinations have shown their contribution for both seed and commercial crop
rearing seasons by expressing increase in fecundity and shell weights and to
finally enlarge the silk yield.
The different parental combinations (C1 to C4) have recorded varied performance in fecundity, egg hatching, cocoon yield, shell weight, silk ratio and silk yield traits over the individual rearing seasons (RS1 to RS5) and average of five successive rearing seasons, but for their beginning from same parental stock. The study infers the obvious role of parental selection and combination over successive rearing seasons in improving the commercial traits of Daba ecorace. The study suggests that same ecorace can have different parental combinations based on magnitude of commercial traits with associated crop rearing seasons and these can be utilized for evolving breeds of commercial worth with higher fecundity and shell weight to supplement the commercially important vanya silk yield.
The author is thankful to the Director, Central Tasar Research and Training Institute, Central Silk Board, Ranchi for providing facilities while making this article.
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