Abstract: In spite of huge availability of nature grown tropical tasar silkworm food plants and rural tribal man power, the tasarculture and raw silk production is yet to attain the potential. The reason being the inadequacy of prospective commercial silkworm seed and breed options, and the urgent need is a coherent application of existing parental races by effective selection. The global demand for vanya silks in general and tasar silk in particular, call-for sustainable utilization of country’s seri-biodiversity potential. Viability and productivity proportion of tasarculture in terms of seeds, cocoons and essentially raw silk, need attention for its vital role in reforming the livelihood and economic condition of rural, backward and tribal farmers. The conventional approaches on basic stock maintenance, commercial seed production, selective use of parental races or parents for heterosis and heterobeltiosis, method of backcrossing to exploit the traits of commercial importance and applying the advantage of GenotypexEnvironment (GxE) interactions are indispensable. In spite of current knowledge on sophisticated transgenic silkworm, appropriate application of on-hand parental resource material and methodologies can expedite tasar silk productivity improvement in addition to up-keep the agro based cottage industry’s cost-effectiveness and biodiversity conservation. The review deals with the current situation and probable strategies for enhancing the productivity and quality of tasar raw silk.
INTRODUCTION
Tropical tasarculture is the rearing of wild silkworms of Antheraea mylitta Drury for production of tasar silk known for aesthetic look and distinctive feel. Over generations, wild sericigenous insect species have adapted to different environments and 44 ecoraces with phenotypic variations are reported in tropical India (Suryanarayana and Srivastava, 2005). They feed mainly on primary food plants viz., Shorea robusta (Sal), Terminalia arjuna (Arjun) and Terminalia tomentosa (Asan) besides other secondary and tertiary food plants (Suryanarayana et al., 2005). Among the ecoraces, only Daba and Sukinda along with a little of Jata are semi domesticated and commercially applied in India (Hansda et al., 2008; Ojha et al., 2009). The intensification of activity needs in situ conservation, ex situ acclimatization and appropriate application of ecorace resources as it got ample prospects in up-liftment of rural tribals, by generating employment along with upkeep of seri-biodiversity. The selection of parental races should be precise as per commercial requirement and productivity (Nagaraju et al., 1996; Raju, 1999; Nagaraju, 2002; Munoz et al., 2004; Mulder and Bijma, 2005; Reddy et al., 2008; Manohar et al., 2009c). The diverse mating systems can balance the traits in selected line for high cocoon, silk yield (Yamaguchi, 2001; Manohar et al., 2009a) and the selection of bigger female cocoon/pupa improves fecundity in Bombyx mori L. (Singh et al., 1994), in Antheraea proylei (Ghosh et al., 1996) and in other lepidopterans (Badhera, 1992; Miller, 2005; Manohar et al., 2009b). The parental selection with specific traits lead to explicit individuals in their progeny (Darlington and Mather, 1952) and when the silkworm strain of low fecundity crossed with better strain, it regains fecundity levels (Aruga, 1994). The fecundity, cocoon and shell weights have higher correlations and co-heritability with silk yield (Siddiqui et al., 1989; Naqvi et al., 2004). The males of different breeds influence the grainage performance of B. mori L. (Rajanna et al., 1999; Krishnaprasad et al., 2002) and the age of mother moths in A. mylitta D (Rath et al., 2007). Superior silkworm varieties can be evolved using repeated backcrossing with choice of donor and recipient parents (Raju and Krishnamurthy, 1993; Das, 2001; Moghaddam et al., 2005: Reddy et al., 2009). The introgressive hybridization with parents of genetic diversity offer heterosis with dominance of genes (Verma et al., 2005). Silkworm hybrids have shown improved reeling performances over pure races (Takabayashi et al., 1994) and the silk filament length is positively correlated with cocoon weight and negatively with silk filament denier (Sekharappa et al., 1999). Although, the semi-domestication of ecoraces, inadequate seed production, unpredictable outdoor environment and erratic cocoon yields affects consistency in silk production, the suitable application of on-hand parental materials and proven methodologies can substantiate the tasar silkworm productivity and quality.
PARENTAL SELECTION APPROACH FOR PRODUCTIVITY IMPROVEMENT
Selection is the process of deciding superior quality animals to become parents of next generation and is a basic tool for improving the genetic structure and productivity status of any stock in the direction desired. Though the simplest form of selection is choosing parents based on preferred phenotypic traits, the degree of improvement depends on variability among parents, extent of selection pressure induction, heritability of trait; as the induced artificial selection acts only as additional force to enhance the natural processes of selection. The role of environment on genotype found apparent, the productive potential of progeny needs to be attained with matching seasons and specifically for the trait of commercial importance. Further, the selection of more traits reduces the expressiveness of phenotype on trait of economic importance. Unlike fully domesticated mulberry silkworm, B. mori, the wild/semi-domesticated tropical tasar silkworm, A. mylitta, with pupal diapause, needs coherent application of parental variation available among ecoraces or within the ecorace, to optimize the productivity. The ex situ stabilization, correct combining of male, female components, application of suitable varieties for heterosis and operation of backcrossing to elevate viable traits, importantly the exploitation of genotype x environment interactions are the vital areas to be tapped in attaining productivity potential.
RESULTS AND DISCUSSION
Basic Seed Stock Maintenance and Multiplication
The maintenance and multiplication of basic seed, periodical replenishment
with breeder’s stock are essential for utilizing optimal vigor of ecorace
in meeting the quality needs of commercial tasar seed. The performance of Daba
ecorace of A. mylitta is comparatively inferior under ex situ
(commercial rearings) than its in situ (natural rearings) habitat and
on orderly maintenance of ecorace for five successive generations, revealed
positive improvement (Table 1). The improved performance in
fecundity from 257 to 309 eggs in T2 indicates the role of specific
parents in improving trait of commercial importance (Singh
et al., 1994; Ghosh et al., 1996;
Raju, 1999). Also, different types of parental mating
can balance desired traits leading to high egg recovery, cocoon and silk yield
(Yamaguchi, 2001; Manohar et
al., 2009b, c) and same way, T3 and
T4 could clearly augment shell weight and fecundity cum shell weights,
respectively. The improvement in commercial traits among treatments, though
all lines originate from same stock, clearly indicates the role of parental
selection in improving breed with desirable traits. The higher fecundity along
with better hatching found advantageous, as they together can contribute more
number of brushed larvae (which was all along a persistent problem in tasarculture),
which can assure improved cocoon yield, in spite of larval loss under fluctuating
outdoor rearing environment. The improvement of total silk yield (main commercial
parameter) at G5 although, generation was seed crop season, could
compete with commercial crop season at G4. This is one of the salient
impacts of selected parents and could occur with contribution of associated
traits like fecundity, egg hatching and shell weights (Siddiqui
et al., 1989; Manohar et al., 2009a).
The least variation in control (T1) compared to other treatments
(T2, T3 and T4) specify the role of parents
selected over generations in modifying breed for productivity improvement. The
enhancement in shell weight was minimum in T2, reflecting same trend
in total silk yield and was vice versa in T3, where the total silk
yield (78.4 g), improved over T2 in spite of un-changed fecundity
(261). But in T4, where the parents are mixture of high pupal and
high shell weights, fecundity, hatching and shell weights improved and contributed
highest for total silk yield (91.8 g).
| Table 1: | Impact of parental selection on commercial traits of Daba ecorace during basic seed stock maintenance over five generations |
Values are Mean±SE. T1: Control: Parents randomly mated, T2: High pupal femalexhigh pupal male, T3: High shell femalexhigh shell male, T4: High pupal femalexhigh shell male, G1: July/August’ 06, G2: Sept/Nov’ 06, G3: July/August’ 07, G4: Sept/Nov’ 07, G5: July/August’ 08 | |
The highest silk ratio (17.7%) recorded in T4 at G4 was due to significant improvement of shell weight (1.9 g) and highest total silk yield in T4 at G4 (110 g) and G5 (91.8 g) levels was due to overall influence of fecundity, hatching and shell weights. This infers the importance of parental selection in basic seed stock maintenance of Daba ecorace for genetic recombination and gain in desired traits of productivity.
Optimization of Commercial Seed Production
The performance of silk insect reproduction varies with extraneous climatic
factors in addition to physiological status of the parents involved. The correlation
between the sizes of parental cocoons, pupae, moths, etc. and reproduction of
economic insects is of vital importance to make their cultures commercially
viable. The significant variations among different groups of males versus mid
pupal group of females and vice versa in both Daba and Jata ecoraces (Table
2, 3) indicate the importance and impact of parental selection.
The quantity and quality are fundamentally important even for economics of commercial
seed production and use of right parents and combinations can optimize productivity
improvement. In spite of better female component, the optimal reproductive success
can be attained with availability of appropriate male counterpart. The fertility,
vital character of egg hatching, depends on potency of male in transferring
sperms along with secretions of accessory glands (Ravikumar
et al., 1995; Krishnaprasad et al., 2002).
| Table 2: | Reproductive performance of different pupal weight group of females against standard pupal weight group of males |
Values are Mean±SD and % change over control | |
| Table 3: | Reproductive performance of different pupal weight group of males against standard pupal weight group of females |
Values are Mean±SE and % change over control | |
| Table 4: | Impact of different male components on the grainage parameters |
| Table 5: | Performance levels of parents and hybrids during commercial crop season |
| Values are Mean±SE, % of mid parent heterosis | |
Likewise, in spite of better male component, the optimal reproductive success depends on the healthiness and size of female counterpart in laying more of fertile eggs (Singh et al., 1994; Ghosh et al., 1996; He and Wang, 2006; Reddy et al., 2008; Manohar et al., 2009c). The optimal fecundity and fertility is possible only with proper mating combination of parents from middle to low pupal weight groups in males and middle to high in females (Singh et al., 1994; Calvo and Molina, 2005) in attaining productivity and sustainability of commercial tasar seed. The pattern of reproductive performance with varied male components (Table 4) indicates higher moth coupling in DabaxJata combination specify higher mating propensity of Jata race and role of a male breed (Krishnaprasad et al., 2002). The poor coupling in DabaxRaily, performance at par with control in Dabax(JataxDaba) and least in Dabax(RailyxDaba) indicates the role of male parent in moth coupling. The higher fecundity and better hatching in DabaxJata indicates the potency of Jata male in transferring adequate sperms, spermatic fluid along with Fecundity Enhancing Substances (FES) (Kishorekumar and Paul, 1993; Singh et al., 1994; Ravikumar et al., 1995; Rajanna et al., 1999), in addition to heterotic effect on egg hatching at F1 level (Naqvi et al., 2004). The results reveal that there is an ample scope of applying different parents in improving the tasar seed productivity and quality.
Conservation and Utilization of Parent Ecoraces
The performance of parental ecoraces and their hybrids (Table
5) clearly indicate the role of their origin and genetic diversity (Moghaddam
et al., 2005; Tuteja et al., 2006;
Ojha et al., 2009) on commercial out-put. The
higher number of fertilized eggs, highest silk yield in Daba race proves its
commercial superiority and economic viability in spite of lower shell weight.
In contrary, the other two wild ecoraces, Jata and Raily in spite of having
better shell weight and longer filament length, their low egg fertility and
lesser silk yield made them un-suitable for commercial rearings. However, the
in-depth study on ecoraces under various zones and their in situ conservation
and ex situ stabilization can wider the parental base to use under various
tasar practicing locations for productivity improvement. The mixing of advantage
characters of both domesticated and wild ecoraces through hybridization (Aruga,
1994; Moorthy et al., 2007; Manohar
et al., 2009a) contributes for enhancing productivity.
| Table 6: | Levels of heterobeltiosis (better parent heterosis in %) in general and specific F1 hybrids |
The mid parent heterosis in DabaxJata has shown strong hybridization force with elevated leap in silk yield and filament length (Siddiqui et al., 1989; Sekharappa et al., 1999; Verma et al., 2005) indicating its commercial prospective. The study infers the need of parental ecorace conservation and their effective utility in producing hybrid seed and in optimizing productivity improvement.
Exploitation of Heterobeltiosis through F1 Hybrids
The commercial utilization of heterosis is based on the excess over middle
parent i.e., relative heterosis and excess over better parent i.e., heterobeltiosis.
The potential of selection response in chosen character was found improved in
offspring generation; however, it depends on genetic variation, selection accuracy
among parents (Darlington and Mather, 1952). Selection
of parental commercial characters based on breeding value is very much essential
for productivity improvement (Yamaguchi, 2001; Rao
et al., 2003; Reddy et al., 2008;
Manohar et al., 2009a, b)
and silk moth off heavier pupae lay more eggs is of great concern in breeding
(Singh et al., 1994). The ecorace with better
cocoon characters have low fecundity and egg fertility or ecorace with higher
fecundity and egg fertility have poor cocoon commercial characters are the main
constraints in tasarculture. The general and specific F1 hybrids
of wild and semi-domesticated parents have shown heterosis and heterobeltiosis
in egg and cocoon characters (Table 6). Parental nativity,
phenotypic variability and genetic diversity found to be imperative reasons
for better heterobeltiosis in fecundity, hatching, shell weight and silk ratios
(Sengupta et al., 1987: Siddiqui,
1997; Hansda et al., 2008; Reddy
et al., 2008). The better positive heterobeltiosis for fecundity
in JataxDaba [PxP] and JataxDaba [PxS] combinations and positive heterobeltiosis
for shell weight, silk ratio in combinations of JataxDaba [SxS] and RailyxDaba
[SxS] indicate specific combining ability for fecundity and shell weight cum
silk ratios, respectively ( Siddiqui, 1997; Naqvi
et al., 2004). The involvement of specific parents is the reason
seen expressing character wise heterobeltiosis among different hybrid combinations
with prospect of productivity improvement through fecundity and shell weights.
Utilization of Heterosis through Reciprocal F1 Hybrids
The heterosis in commercial characters of reciprocal hybrids (Table
7) denotes scope of total usage of parental cocoon material in commercial
seed production and to cut-off seed production cost. The high heterosis potential
recorded for fecundity in selective reciprocal hybrid, DabaxJata [PxP] indicates
the selection response between the chosen traits of divergent geographic ecoraces.
This is of large applicability against persistent problem of low fecundity in
many ecoraces, or otherwise they are good in all cocoon commercial characters.
Also, the positive relative heterosis showed by DabaxJata [SxS] and DabaxRaily
[SxS] in respect of shell weight and shell ratio indicates the selection response
of parents. The most important objective of improving shell weight is to attain
overall gain in silk yield and improvement in shell weight and silk ratio indicate
positive heterotic effect and combinability of parents for yield contributing
traits (Siddiqui et al., 1989; Naqvi et al.,
2004), while, the positive heterotic gain among random coupling
of parents Daba, Jata show epistasis effect (Sengupta et
al., 1987: Siddiqui, 1997; Reddy et al.,
2008; Manohar et al., 2009a). The positive
heterosis for egg fertility, shell weight and silk ratio in DabaxRaily indicates
its contribution for better silk yield as egg fertility and shell weights are
main factors responsible for productivity improvement. The selective parents
as reciprocal hybrids have heterosis potential to augment productivity improvement
through fecundity and shell weights.
Backcrossing to Improve Egg Hatching, Silk Yield and Quality
Silkworm breed with better fecundity and egg hatching can achieve viable
cocoon production on commercial scale. Jata ecorace with high fecundity (315
eggs) and low egg hatching (45%) as recipient and Daba which is vice versa,
with low fecundity (200 eggs) and high egg hatching (80%) as donor parent were
chosen to introgress egg hatchability trait through repeated backcrossing. The
subsequent selection with better fecundity, egg hatching, at every stage of
breeding and high pupal weight for specific hybrids, the backcrossing resulted
(Table 8) to introgress the desired character of egg hatching
(Singh et al., 1994; Ghosh
et al., 1996; Raju and Krishnamurthy, 1993;
Das, 2001; Moghaddam et al.,
2005) (with an improvement of +19.9% over ex situ stabilized) into
Jata ecorace at BC2 level.
| Table 7: | Levels of heterosis (mid parent heterosis in %) in general and specific reciprocal hybrids |
| Table 8: | Performance levels of backcross hybrids (F1, BC1 and BC2) |
Values are Mean±SE and % change over donor and recipient parents | |
| Table 9: | Performance levels of backcross hybrids (F1, BC1 and BC2) |
Values are Mean±SE and % change over donor and recipient parents | |
The high pupal parents selected because of their correlation with fecundity and to retain same while improving the egg hatching. The intensity of introgression was additional in selective high pupal hybrids indicate the role of parental selection in target oriented breeding (Aruga, 1994; Chattopadhyay et al., 2001; Moghaddam et al., 2005; Moorthy et al., 2007; Manohar et al., 2009c). The egg hatching and fecundity improvement was marginal in general hybrids while the high pupal hybrid could surpass even recipient parent, authenticates the role of selected parents in productivity improvement through improved egg hatching and regained fecundity. The selection of breeding method is to develop a breed with stability and productivity in terms of quantity and quality of silk and for such aspire the apt method is repeated backcrossing (Tazima, 1984). The parental selection is always crucial and hence, the variation in total silk yield and filament denier apart from varied shell weight, silk ratio and filament length (Table 9) are the main reasons for selection of divergent races as parents. To introgress the silk associated commercial characters, total silk yield and finer filament denier in to Jata and Raily races, the backcrossing has been repeated with males of Daba race and the approach found momentous as parental selection was based on their quantitative and qualitative traits. While domestication and ex situ acclimatization, the performances of ecoraces will come down and superior traits found in natural habitat will recede and hence, are not effective as donor to introgress desired economic trait(s). In view of this, selection of nature grown ecorace as recipient parent for its superior characters and to infuse additional compatible trait(s) from domesticated Daba race with continued selection for desired trait(s) in the following generations found logical. The induced selection on parents at every stage of breeding for desired trait(s) and the repeated backcrossing with Daba resulted to better viability in terms of total silk yield (Raju and Krishnamurthy, 1993; Moghaddam et al., 2005; Tuteja et al., 2006) into Jata race. The opposite trends in respect of total silk yield, filament length by the same hybrid at F1 and BC2 levels indicate the correlation between quantity and quality of silk (Siddiqui et al., 1989; Sekharappa et al., 1999; Verma et al., 2003; Reddy et al., 2009). But, the non improvement of silk yield in Raily race, coincides with its ex situ performances of non amenability and acclimatization to the changed environment and food plant, as against its superior survival under natural habitat on S. robusta. However, the improvement in shell weight and silk ratios of Jata and Raily races at F1 and BC2 levels signifies the interaction of high heterogenousity and introgressive hybridization (Verma et al., 2003; Reddy et al., 2008; Manohar et al., 2009a). This also infers better silk trait expressiveness of phenotype under commercial crop season, which have optimal genotypexenvironment relations with longer larval feeding period, better leaf quality and congenial climate. The introgression of finer filament denier with improved silk yield indicates the prospects of Jata and Daba ecoraces as parental resource for improving productivity.
EXPLOITATION OF GENOTYPExENVIRONMENT INTERACTION
Phenotype is the combined produce of genotype and environment (Kumar et al., 1999; Jong and Bijma, 2002; Munoz et al., 2004; Srivastava et al., 2004; Mulder and Bijma, 2005; Zhao et al., 2007) and the tasarculture being an outdoor practice and providing the suitable environment is not under control, it will be more logical to choose an ecorace which performs better in foreseen seasons. Environmental conditions have a great influence on effectiveness of parental selection, so do the selections that take an advantage of season’s different characteristics (Kumar et al., 1999, 2008; Munoz et al., 2004; Mulder and Bijma, 2005). In commercial crop season (Sep-Nov), selection of silk yield and filament length, the traits of quantitative nature should be stressed because of congenial weather and quality of feed in addition to longer larval feeding period (Jong and Bijma, 2002). While during seed crop season (Jul-Aug), the fluctuating weather and quality of feed with intermittent rainfall, the priority should be on higher fecundity and better egg hatching to provide extra population to compensate the larval loss (Zhao et al., 2007). The superior performance of parental lines, general and specific hybrids under commercial crop (favourable seasons) indicate the attachment and interactions of genotype with environment. However, the tasarculture need separate breed options for different crop seasons due to varied behavior of non-diapause and diapause destined silkworms with varied climatic conditions. The cocoons of seed crop with thin shell and low silk; commercial crop cocoons with thick shell and high silk denotes their specific contribution towards seed and silk along with genotype x environment interactions in tasar productivity improvement.
EPILOGUE
Adoption of well-proven genetic principles, use of resourceful material by selection, testing and multiplication procedures, genetic correlations certainly augment silk insect productivity. The need based application of parents and combinations by selection; suits combating irregular reproductive behavior, inadequate basic and commercial seed, semi domestication and low race option for rearings and unstable silk productivity and quality. The yield exploitation through heterosis, quality advancement through backcrossing and yield stability through integration of physiological, nutritional and ecological (GxE) essentials, no doubt requires choice of parents. In the years to come, these traditional methods will continue with adoption of advanced biotechnological tools to make the selection of parents more effective in achieving higher productivity with effective cost benefit ratio. This success generates confidence and involvement among rural, backward and tribal rearers in sustainable expansion of tasarculture and conserving endangered wild seri-biodiversity for future generations.