Breeding Perspective for Silk Yield and Quality in Indian Tropical Tasar Silkworm, Antheraea mylitta Drury (Lepidoptera: Saturniidae)
R. Manohar Reddy,
Though, India enjoys the availability and practice of mulberry, tasar (tropical/temperate), eri and muga silks, the utilization of tropical tasar seri-biodiversity, however requires appropriate breeding methods so to exploit the global demand of this vanya silk, besides reforming tribals, weaker sections and landless rural populace on economic front. Among the existing forty four ecoraces of Antheraea mylitta Drury, only Daba and Sukinda are commercially applied for cocoon production and want is in situ conservation and ex situ stabilization of additional ecoraces, particularly the Shorea robusta (sal) based, as amenable parental base for hybridization and silk production. The breeding for disease and adversity resistance, correlation among the trait(s) of commercial value as the tasar silkworm being an eco-insect reared outdoor and exploring the biotechnological tools for transgenic application appears pragmatic and worth. The management of genotype and environment interaction through multilocational breeding stations irrespective of rearing seasons applying compatible ecorace; breed or line(s) and involving the beneficiaries for their indigenous knowledge along with trained breeders is the indispensable strategy to achieve the tasar raw silk productivity and quality. The review has dealt with feasible breeding scenario of tasar silkworm in attaining the qualitative yield for the commercial sustenance of tropical tasarculture.
Received: March 04, 2010;
Accepted: May 08, 2010;
Published: July 10, 2010
India stood second largest producer of silk, after China, with 15610, 2038,
603 and 119 MTs of mulberry, eri, tasar and muga raw silk respectively during
2008-09, provided employment for 2.50 lakh Below Poverty Line (BPL) families
and annual foreign exchange of Rs 3165 crores to the country (Anonymous,
2009). Among silks, the tropical tasar, an important vanya silk is produced
by the wild silkworm of Antheraea mylitta Drury of order Lepidoptera
and family Saturniidae (Jolly et al., 1974; Suryanarayana
and Srivastava, 2005). It is polyphagous and feeds primarily on sal (Shorea
robusta), asan (Terminalia tomentosa) and arjun (Terminalia arjuna)
and secondarily on Zizyphus mauritiana, Terminalia paniculata,
Anogeissus latifolia, Syzigium cumini, Careya arborea,
Lagerstroemia parviflora and Hardwickia binata (Suryanarayana
et al., 2005). The tasarculture is a base livelihood for the tribals
of Jharkhand, Chhattisgarh, Orissa, Madhya Pradesh, Uttar Pradesh, West Bengal,
Bihar, Maharashtra and Andhra Pradesh states either through the collection of
nature grown cocoons or by rearing in forests or on raised economic plantation
(Thangavelu et al., 2002; Ojha
et al., 2009). The objective of this activity is to utilize economic
wild life, conserving the associated environment for sustainable rural and tribal
development (Raffi and Ramanujam, 2001; Gill
and Lal, 2002; Mahapatra, 2009).
Though, the tasar silk insect has forty four ecoraces, only the semi-domesticated
Daba and Sukinda are contributing for countrys tasar raw silk, besides
few other wild ecoraces like Raily from Chhattisgarh; Modal and Jata from Orissa;
Sarihan and Laria from Jharkhand; Bhandara from Maharashtra and Andhra from
Andhra Pradesh (Rao et al., 2004; Suryanarayana
et al., 2005; Hansda et al., 2008;
Ojha et al., 2009; Reddy,
2010). The varied voltinism and higher interference of environment on crop
performance with tentative returns made the industry unreliable over the alternative
agro based enterprises (Thangavelu, 2002). Though, the
tasarculture is an important co-discipline of applied forest biology, needs
special understanding and addressing towards breeding perspective to promote
the sustainable utilization of this precious natural resource (Mahapatra,
2009; Reddy et al., 2010a). The basic information
on ecology, environmental factors, climatology, flora, fauna and their inter-relationship,
the life cycle, diapause, reproductive biology and voltinism and population
dynamics of tasar insect reveal its critical requirements to handle for breeding.
The commercial attributes of tasar insect, viability in the offered eco-climatic
condition suggests their biotic and economic potential and the commercial feasibility
of ecorace/ breed/ line. Hence, the coordinating of adoptable breeding strategies
for tasar silk yield and quality found vital for its indispensible role in generating
rural livelihood, employment and foreign revenue.
APPROACH FOR SUSTAINABLE YIELDS
The utilization of biodiversity either by choice or by suitability must match
with yields and quality of end produce for commercial sustenance. The process
of applying insects in a generation to become parents of next progeny is selection,
either to upgrade genetic configuration or to construct an end product in the
desired direction was reported by Basavaraja et al.
(2005), Reddy et al. (2008, 2009).
The selection, though in simple is just choosing parents based on ideal phenotypic
character; the extent of genetic or end product improvement, however depends
on their variability, level of selection, heritability rate, correlation among
the traits chosen; as artificial selection acts only an added force on natural
selection (Yamaguchi, 2001; Miller, 2005; Chandrasekhar
and Basavaraja, 2008; Reddy et al., 2010a).
As the availability of basic parental material being the constraint in tropical
tasar silkworm, the focus priority should be on ecorace domestication for amenability
and application. Further, the role of environment on genotype was proven evident,
the productive potential of progeny on the chosen commercial trait(s) need compatibility
of crop season. The wider selection with more number of traits reduces clarity
of phenotype on the targeted trait of economic importance. The limited availability
of domesticated ecoraces and their pupal diapause need a different and coherent
approach, while exploiting parental variation for qualitative output as was
reported by Hansda et al. (2008), Ojha
et al. (2009) and Reddy et al. (2009a-c,
2010c). The in situ conservation, ex situ
stabilization, basic stock maintenance based on the magnitude of commercial
trait(s) and their combine for heterobeltiosis, evolving disease and adversity
resistant breeds, backcrossing for traits of qualitative advantage and utilization
of compatibility of genotype x environment (G x E) relations for optimal commercial
trait expressivity for rearing season(s) are the explorable breeding avenues
for sustainable and qualitative silk yields of tasar silkworm, A. mylitta.
ORGANIZING ADDITIONAL ECORACES
Basically, the conservation and management of any wildlife should encompass
whole spectrum of biota and activities ranging from ecosystems at macro level
(in situ conservation) to micro level (ex situ conservation),
yet, the conservation priority of tasar insect must be in situ, as thereby
protection is accorded not only to the insect species and their productivity
but also the habitats, ecosystems and biodiversity. However, Hansda
et al. (2008), Rajnarian et al. (2008), Ojha
et al. (2009) and Reddy et al. (2010b)
view that the tasar insect wildlife must be conserved ex situ as valuable
genetic resource and promising livelihood of tasar cultivators. The performance
of ecoraces vary with their origin, genetic diversity and habitat as their distinction
reflects through phonotypical trait was reported by Yamaguchi
(2001), Nagaraju (2002), Miller
(2005) and Reddy et al. (2009a, d).
The tasar races of each ecozone needs systematic classification to utilize their
diversity for commercial efficiency in terms of silk yield and quality as the
individual variety can be a potential resource in building up variation among
new population through hybridization. The trivial deviation of any character
from in situ under ex situ is said to be stabilized, which vary
among tasar ecoraces or within the ecorace population. The positive inclination
of wild ecorace under captivity with human interference ranks the feasibility
and enhances its utility either for breeding or for commercial application aiming
silk productivity and quality.
The healthy genetic resource of tasar silkworm with 44 ecoraces and the available
genotypic and phenotypic variations in their natural population can very well
serve the fundamental needs in evolving breeds/lines/varieties of better commercial
value (Thangavelu, 2002). The better fecundity, egg
fertility and amenability during rearing and grainage activities, improved effective
rate of rearing (ERR), cocoon yield, larval survivability and disease resistance
with wider adaptability to growing areas indicates the status of tasar ecorace
on domestication and economic utilization was reported by Hansda
et al. (2008), Ojha et al. (2009),
Reddy et al. (2010c). In contrary, the wild ecoraces
posses better fecundity, higher cocoon and shell weights, longer and finer silk
filaments, but, their low egg fertility, hatching, cocoon yields and mainly
non-amenability and non-adaptability to ex situ environments are the
inadequacies for their commercial application. However, in-depth study on ecoraces
under various zones and attempts for in situ conservation and ex situ
stabilization can widen the parental base for their commercial use under various
tasar practicing locations. The choice of amenable ecoraces provides advantage
to mix feasible characters (available with wild and semi-domesticated) through
hybridization, either to enhance productivity or quality and even both collectively
(Reddy et al., 2008, 2009d).
Further, the mid parent heterosis itself can elevate silk yield, filament length
and refines filament denier at F1 level (Reddy
et al., 2010c) and intercrossing among parents of same ecorace generated
under different ecozones can yield better in terms of quantity and quality.
The conservation and utilization of ecoraces to produce viable basic and commercial
seed to contribute for qualitative silk yields was reported by Hansda
et al. (2008), Rajnarain et al. (2008)
and Reddy et al. (2009c). In spite of vast availability
of S. robusta flora and its based ecoraces compared to T. arjuna
and T. tomentosa, they are yet to be explored commercially. However,
these ecoraces are contributing for countrys raw silk as nature grown
cocoons and few of them are known for the silk filament of very fine denier
(Suryanarayana and Srivastava, 2005), though they show
heavy mortality during late larval stages and do not permit human handling either
during rearing or seed production. Their conservation not only generates the
amenable tasar genetic resource for breeding but also saves them from extinction,
as their current level of decline is alarming with deforestation and over exploitation
(Mahapatra, 2009). Thus, the strategic approach of conserving
the tasar silk insect in situ and ex situ is must to retain
the bio-variability (both inter and intra-population) for current and future
MULTILOCATIONAL BREEDING STATIONS
The reproductive potential of different forms of tasar ecoraces has the influence
of environmental factors and physiological status of parents involved as the
parental moth correlation and their origin is vital to make them commercially
viable (Reddy et al., 2008, 2009c,
2010b). The reports of Rajnarain
et al. (2008) and Reddy et al. (2009c,
e) reveal that the quality of tasar silkworm seed and
optimal vigor of ecorace are mainly depends on the parental stocks used, the
system of basic stock maintenance (breeders stock) and their replenishment
periodicity. The breed should not undergo inbreeding depression, genetic drift
with un-scientific selection methods and not to lessen the adoptability tolerance
to meet the quality needs of commercial silkworm seed (Yamaguchi, 2001). The
performance of tasar ecorace is comparatively inferior under ex situ
(commercial rearings) than its in situ habitat (natural rearings) and
hence, it requires the orderly maintenance of basic stock (during multiplication)
to retain its level of vigour for sustainable commercial performance.
The potential of a race can be seen mostly in its native position as the phenotypical
expression being a collective outcome of genotype in a given environment and
this holds very true for tasar ecoraces (Hansda et al.,
2008; Ojha et al., 2009). Unlike the domesticated
silk insect Bombyx mori, which reared indoor with controlled conditions
of environment and feed to regulate its potential (Yamaguchi, 2001; Basavaraja
et al., 2005; Chandrasekhar and Basavaraja, 2008),
the performance of the wild tasar insect A. mylitta, depends on its growing
surroundings (in situ or ex situ) and variety and status of its
food plant. Further, the altitude and photoperiod of the ecological niche of
a particular ecorace can influence its phenotype and voltinism. Though, the
inherent potential of any tasar ecorace in respect of fecundity, cocoon weight,
shell weight and silk ratio traits can be expressed superior under its in
situ habitat, they can be retained or improved by applying selective parents
(based on pupal or shell weights) at basic seed stock level was reported by
Reddy et al. (2009c, e).
Further, the basic stock if maintained under in situ conditions can exhibit
optimal potential on commercial trait(s), mostly true to breed vigour, which
can contributes subsequently to improve the commercial seed. The approach of
maintaining individual ecorace and basic stock under its native areas (multilocational
basic seed breeding stations) will lead to true to type ecorace stocks to utilize
the original potential and vigour of tasar ecoraces even on their commercial
application. This approach for semi-domesticated and commercially exploited
Daba and Sukinda ecoraces can further enhances their potential because of better
inherent performance levels under in situ (Suryanarayana
and Srivastava, 2005) over the current basic and commercial stocks maintained
APPROPRIATE BREEDING METHODS
Heterobeltiosis in commercial trait(s): The commercial utilization of
heterosis is based on excess over middle parent i.e., relative heterosis and
excess over better parent i.e., heterobeltiosis. The potential of response in
chosen character can be seen in offspring generation as heterotic effect; however
it depends on genetic variation and selection accuracy among parents (Basavaraja
et al., 2005). The selection of commercial character must based on
breeding value and is essential for productivity improvement as like female
moth emerges from heavier pupae lay more eggs and male moth of particular ecorace
show higher mating potency as was observed by Reddy et
al. (2008, 2009a, b,
2010c). The parental nativity, phenotypic variability
and genetic diversity are imperative reasons for better heterobeltiosis either
in fecundity, egg hatching, shell weight and silk ratios or collectively in
more traits, might be with trait specific combining ability. Such parents of
any hybrid can certainly express character wise heterobeltiosis with prospect
of productivity and quality of silk. The objective of improving either fecundity
and fertility or shell weight and silk ratio is only to attain overall gain
in total silk yield, which is important for commercial sustenance (Yamaguchi, 2001; Nagaraju, 2002). Though, the positive heterosis
in shell weight can attain gain in silk yield, the fecundity, egg fertility,
Effective Rate of Rearing (ERR) and cocoon yields are the other associated factors
responsible for overall improvement in silk yield. Also, the heterobeltiosis
in reciprocal hybrids can augment productivity with an additional scope of applying
parental material in full for seed production with reduced cost of production.
Correlation of productive trait(s): Though, the sustenance of tasarculture
depends mainly on the silk production, the quality of raw silk is also equally
important and both of them are normally influenced by the breed, feed and growing
environment. The tasar rearing being an outdoor practice mostly on the nature
grown food-plants, the success of tasar silk production go with the race or
breed applied for commercial rearing (Rao et al.,
2004; Hansda et al., 2008; Reddy
et al., 2009a, b, 2010c).
The generation of raw silk mainly took place under commercial crop rearing season
(September-December), where the environmental conditions and quality of the
feed are superior and hence the role of breed is paramount in achieving the
higher quantity (yield) with better quality (denier) of silk. The tasar silkworm
is destined to undergo diapause after commercial rearing season and it develops
thicker cocoon shell to combat the environmental adversities ahead with severe
winter and summer while completing its life cycle. Hence, by nature the silk
insect produce more silk during the commercial crop season than seed crop (July-August).
The reports of Petkov et al. (2000), Radhakrishna
et al. (2001), Davidowitz et al. (2004),
Zhao et al. (2007) and Reddy
et al. (2009c, e) indicates that the production
of higher silk yield during commercial crop rearing season was with better genotype
and environment (G x E) interaction. Hence, correlating the productive trait(s)
appropriate to crop seasons and their response to heterosis or heterobeltiosis
and survival of the breed as ERR or cocoon yield over the existing race option(s)
amounts to yield elevation and commercial prospective (Sekharappa
et al., 1999; Petkov et al., 2003;
Reddy et al., 2009c; Seshagiri
et al., 2009). The influence of hybridization as positive heterosis
in F1 hybrids though indicates the commercial advantage, it may suite
for productivity and not quality as the positive heterosis leads to thicker
silk filament with high denier. Further, the denier of filament is more of racial
character and attaining higher silk yield with filament of lower denier looks
intricate as the productivity and quality are negatively correlated as observed
by Sekharappa et al. (1999), Petkov
et al. (2000), Verma et al. (2003)
and Reddy et al. (2009d, 2010a).
However, the breeding approach for attaining better productivity together with
quality over the existing options looks rational and hence attaining silk productivity
keeping silk quality either marginally changed either way or unchanged or vice
versa must be a success and commercial advantage in tropical tasarculture.
Disease and adversity tolerant breeds: The induction of resistance to
disease is an important objective in silkworm breeding as it complements both
yield and quality of the end produce. The health is the most vital character
and it besets with quantitative and qualitative characters and if a breed fails
to tolerate the diseases resulting poor cocoon, it cannot be considered as promising
(Watanabe, 2002; Basavaraja et
al., 2005). The healthiness might be due to resistance to diseases in
which the disease cannot infest it, while the other is adaptability against
pathogens and changed rearing environment. However, these two aspects are controlled
by genetical, physiological and pathological apart their rearing environment
and the rate of disease prevalence, yet, vary among disease resistant and susceptible.
The resistance to disease is strain specific by gene action, while the tolerance
is strain un-specific by minor gene or polygenic action, which is biologically
common. The disease resistance to a particular strain of pathogen is vertical,
while the other is horizontal, which universally resists all strains of pathogens.
The reports of Nagaraju (2002) and Basavaraja
et al. (2005) reveal that the disease resistance mainly revolves
around the immune response, which in general very poor in insects and more specifically
in silkworm. The knowledge on the principles of disease tolerance and resistance
through interactions of host and pathogen are vital to evolve a breed survives
against the pathogenic action for a disease. The defense systems through haemocytes,
humoral immunity and anti-viral activity of digestive juice needs exploitation
besides the hypothetical genetics and biotechnological advancements in linking
host pathogen interaction (Zhang et al., 2008;
Lie et al., 2010).
Backcrossing for quality trait(s): The selection of breeding method
is to develop a breed with stability and productivity in terms of silk quantity,
the method of repeated backcrossing among appropriate donor and recipient parents
looks apt for attaining the quality of commercial traits was reported by Petkov
et al. (2000), Nagaraju (2002), Verma
et al. (2003) and Reddy et al. (2010a).
The parental selection is always crucial in commercial trait precision and hence
silk yield and filament quality variation apart from varied shell weight, silk
ratio and filament length are important while selecting parents, even if they
are divergent ecoraces (Verma et al., 2003; Basavaraja
et al., 2005). To introgress the quality associated commercial trait
like filament denier, the repeated backcrossing found significant, if the parental
selection is based on quantitative in recipient and qualitative in donor (Reddy
et al., 2009d). In general, domestication and acclimatization of
tasar ecoraces to new environment (ex situ) decline their performance
in commercial traits and are not effective as donors to introgress economic
trait(s). In view of this, selection of wild ecorace with superior traits as
recipient parent and infusion of compatible trait(s) from domesticated with
continued selection for desired trait(s) in following generations appears rational.
At times, the opposite trend in silk yield, filament length illustrates the
correlation among quantity and quality. However, the interaction of high heterogenousity
and introgressive hybridization of parents involved can influence silk yield
positively. The introgression of finer filament denier with retained or improved
silk yield through application of suitable parental ecoraces applying the backcross
breeding proven prospective to promote silk quality at least with the retained
levels of silk yield. In the same way, Watanabe (2002)
and Basavaraja et al. (2005) have suggested that
the application of backcross method using parental resource of disease resistance
are of helpful to evolve the breed survives better against the pathogens.
Biotechnology application: The silkworm genetic resources with reference
to the tropical tasar need to be characterized using molecular tools to identify
and understand the border line among the ecoraces and their populations (Petkov
et al., 2000; Nagaraju, 2002; Moghaddam
et al., 2005). This knowledge on population structure can provide
the basic clues for evolving appropriate conservation and management strategies
of tasar genetic resource. Zhang et al. (2008)
and Lie et al. (2010) have observed that the
quantitative trait loci (QTLs) linked with the trait(s) of yield, quality and
disease resistance can establish the linkages either to manage productivity
or quality. This may provide insight for a very important aspect of regulating
the diapause either to make the tasar insect multivoltine or to budget higher
food allocation for qualitative silk productivity.
Exploitation of rearing seasons: The tropical tasar silkworm, A.
mylitta, with limited annual life cycles needs exploitation of compatibility
with all possible rearing season(s), to augment silk yield. The inadequacy of
tasar breeds for a season or region also being the constraint in utilizing the
existing tasar flora, the critical need is to specify ecorace options for regional
and seasonal compatibility. Yamaguchi (2001), Nagaraju
(2002), Moghaddam et al. (2005), Rajnarain
et al. (2008) and Reddy et al. (2009e)
have observed that the maintenance of breed vigor is essential through parental
recombine to regain the race potential and compatibility for seasons. The compatibility
classification of parents or their combinations to crop seasons specifying the
silk yield and quality is essential in view of the vast availability of nature
grown tasar flora (Hansda et al., 2008; Ojha
et al., 2009; Reddy et al., 2010a).
The current approach of applying cocoons of seed crop (with less silk content)
for commercial seed production and cocoons of commercial crop (with more silk
content) for basic seed or silk production, needs management for parent season
compatibility to attain silk productivity and quality. The highly organized
sensory and neuro-motor systems of insect (more comparable to those of vertebrates)
and their response and persistence against environmental change as was earlier
reported by Davidowitz et al. (2004), Miller
(2005), Zhao et al. (2007), Chandrasekhar
and Basavaraja (2008) and Reddy et al. (2009c,
2010b) which found advantageous in exploiting qualitative
yields in wild silk moths. However, during commercial crop season the selection
of quantitative traits like silk yield and filament length should be stressed
because of congenial weather and feed quality in addition to longer feeding
period. While in seed crop season, the fluctuations in temperature, humidity,
rainfall and feed quality are high and hence priority should be on fecundity
and hatching to have more population to compensate larval loss and to attain
the qualitative silk productivity.
HUMAN RESOURCE APPLICATION
As like the specific application need of breeding material and methods, the
human resource application is also significant in attaining the potential of
silk yields in tasarculture. The cause and impacts on extensive collection of
wildlife from core habitats, disease out breaks and control, management of predators
attack, besides the species suitability is worth sharing with native beneficiaries
for effective ex situ conservation was suggested by Madhu
(2001), Kler et al. (2002) and Carlsson
and Berkes (2005). The wild nature of tasar insect and the mechanism of
its adaptation as ecorace to different ecological niches can suggest indication
in attaining better silk output. In view of inadequate information on such lines,
the participation of native tasar rearers becomes vital for their close association
with this insect wildlife in in situ for very long. However, the involvement
of landless rural people, self help groups and the Non Government Organizations
(NGOs) in tasarculture is because they understood its potential for rural and
tribal upliftment (Balaji, 2001; Sethi
and Singh, 2001; Pande et al., 2002; Dhirendra,
2009; Satyanarayana et al., 2009; Sinha
et al., 2009). Further, the requirement of precise breeding approach
for handling unique nature of tasar insect (though, the genetic principles remain
same) unlike other known silkworm varieties, demands the trained breeders. Hence,
the human resource participation as native tasar cultivars and trained tasar
breeders and their coordinated efforts can yield qualitative silk for the sustainable
The acceptance of well-proven genetic principles, use of competent parent material, testing procedures and genetic correlations, though complement tasar silk productivity; such application and amalgamation needs wider parental base, which is possible only on organizing additional ecoraces. The yield exploitation by heterobeltiosis and commercial trait correlation, breeding for adversity and disease resistance, quality advancement through backcrossing and yield targeting biotechnological tools, integration of physiological and ecological basics through multilocational breeding stations with compatibility of breed and season looks realistic for the sustainable silk yields. The involvement of aboriginals for native knowledge and trained breeders for methodical handling of the material are indispensable in achieving the task. It is the time for combining the conventional and modern techniques to address the breeding needs in commercial tropical tasarculture to bring the mandatory change towards sustainability.
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