X
Preface
Since the dawn of civilization, continuous improvement of commercial
qualities is in vogue to reap maximum economic benefits in several plants and
animals. The mulberry silkworm, Bombyx mori L. is known to produce the
finest fibre of grace and elegance, the ‘silk’. By virtue of its economic
importance, silkworm has attracted the attention of mankind in several silk
producing countries and has undergone several changes in its inherent
capacity to produce the silk. Sericulture with its vast potential for income and
employment generation especially in rural areas plays an important role in
alleviating rural poverty and identified crop enterprise as most labour intensive
cottage industry. More than 58 counties practice sericulture through out the
World with China (93100 MT) occupying the top position followed by India
(16525MT).
Sericulture in India dates back to antiquity and is closely related with
the rich heritage of Indian civilization. India is the only country in the world that
enjoys the status of producing all the four varieties of silk viz., Mulberry,
Tasar, Muga and Eri with Assam being the only state producing muga silk.
Among these, mulberry silk which is produced by rearing of the silkworm, B.
mori on mulberry leaves, constitutes about 92% of the silk production in India.
The chief silk producing states in India include traditional states i.e.,
Karnataka, Andhra Pradesh, Tamil Nadu, West Bengal and Jammu &
Kashmir and introduced of late in several states such as Maharashtra,
Madhya Pradesh, Jarkhand, Chattisgarh, Uttar Pradesh, Gujarat, Kerala, etc.
Andhra Pradesh stands second in terms of silk production but first with regard
to silk productivity with an annual production of 5526MT and a productivity of
> 58Kg/100 dfls.
Andhra Pradesh is one of the major silk producing states and initiated
its roots during 1950s especially in Anantapur and Chittoor districts and later
spread to Kurnool, Cuddapah and other districts. The major share of cocoon
u
production in the state is from Rayalaseema region and partly from Telangana
and Coastal Andhra.
At present, 96% of the total silk production in Andhra Pradesh is mainly
contributed by the crossbreed, polyvoltine (Pure Mysore) x bivoltine (NB4D2 or
CSR2) which is popularly called ‘crossbreed’ as in other southern states. The
silk produced from this hybrid is of inferior quality with lower silk productivity.
This results in low financial returns to the silk rearers and silk reelers. The
rearing of bivoltine hybrids which produce high quality silk is largely
unsuccessful due to susceptibility of these hybrids to viral infections and
higher levels of temperature.
Hence, adequate technology support to
sericulture industry through intensive research and development of productive
and silkworm hybrids producing high quality silk is crucial factor to enhance
the sericulture activity. Many silkworm breeds/hybrids were developed with
productive traits and tested in the field with mixed results by different research
institutes. Semi-arid tropical environment characterized by low humidity and
high temperature conditions of Andhra Pradesh requires suitable crossbreeds
with high survival rate and moderate productivity.
The general practice in commercial silkworm seed production centers
is mating of polyvoltine female parent with bivoltine male parent to prepare
desired crossbreed layings, discarding the other sex of both parents. During
the silkworm hybrid preparation, sexing is conducted inevitably to prevent self­
mating. Sex separation or sexing in silkworms could be conducted either at
larval stage or cocoon stage or pupal stage. Separation of sex at larval stage
especially at fifth instar is conducted by observing for Ishiwata and Heralds
glands. The female larva has a pair of small spots called “Ishiwata glands”
situated on the ventral side of each of the 11th and 12th segments. The male
has tiny spot “Heralds gland” situated on the ventral side between 11th and
12th segments. This is practically very difficult during silkworm rearing as it
requires scientific experience and should be conducted at stipulated larval
stage.
Ill
Several efforts were made to sex separate silkworms at cocoon stage
based on the weight differences of male and female cocoons, but the
attempts were not practically feasible due to marginal variation and/or
overlapping weights of opposite sexes. Average weight of the female cocoon
is usually higher than the male cocoons. This difference has facilitated the
“Cocoon sexing machine”, which is generally used in Japan, China and
Russia (Strunnikov, 1960). But this method is error prone and it is not possible
to separate male and female cocoons precisely when the breed has
overlapping cocoon weights for male and female sexes.
In general, sex separation is carried out at pupal stage based on the
external markings on the abdomen and it involves cutting open of each and
every cocoon which is quite laborious, expensive, time consuming and also
leads to chances of damage during preservation. The above cited constraints
during sexing operation influences the cost of production of disease free
layings (DFLs) on commercial lines with wastage of unused (either male or
female) cocoons and raising the production costs. Due to the limitations in the
above mentioned methods, most of the crossbreed seed production centres in
India adopt to isolate male moths after light exposure followed by picking of
female moths as bimodal emergence pattern is fairly established in Pure
Mysore.
In this context, development of silkworm strains exhibiting sex-limited
nature is of paramount relevance for commercial hybrid production. Silkworm
breeds with sex-limited character are available in Japan, Russia, China and
South Korea. The sex-limited nature is based on egg color (white eggs are
male and black colour eggs are female), larval (larvae with markings are
females and larvae without any markings are males) or cocoon (females spin
dark yellow colour cocoons and males spin white cocoons) stage.
Sex-limited nature of larval markings in silkworm has been studied as
early as 1941 by Tazima, in the course of his experiments on the mutagenic
effects of ionizing radiations. The females with a translocated dominant gene
+p in the ‘W’ chromosome manifested morphological feature with eye-spot,
IV
crescent mark and star spot (ECS) markings on the body while the males
were plain without any markings. Later Hasimoto (1948) synthesized a sexmarked strain where the female larvae had ‘zebra markings’. Kimura et al.
(1971) translocated yellow blood gene to W chromosome. Since then, many
efforts have been made to produce the ‘zebra’, ‘Black egg’ and ‘Yellow
cocoon” sex limited strains (Tazima 1951, Strunnikov, 1983; Mano, 1984).
The females of sex-limited strains first discovered by Tazima had low viability
and the larval weight was less than the male in the fifth instar. Later, through
repeated x-ray irradiation
experiments these
defects were eliminated.
Consequently, a sex-limited “Moricaud” silkworm with normal viability was
evolved. The fragment carried dominant genes for larval markings and its
attachment to the W chromosome was made discrimination of sexes very
easy by the presence or absence of larval markings. Since then, many efforts
were made to evolve sex limited breeds nationally and internationally. Sexlimited silkworm breeds are also developed in India Viz,, Bivoltine: J112, Cuo
.CCiSL, NB4D2SL, CSR3, CSR12, CSRis and CSR19, CSR8,317 SLO and 318
SLD; Polyvoltine: PM(SL), BL24(SL), BL27(SL), BL45(SL), BL54(SL), Nistid (SL)
SL-KBS, SL-KMS, MY^SL), CC^SL) and SLSY (Sengupta, 1968, Nagaraju et
al. 1989, Mundukur et al. 1989, Rajendra and Sadananda, 1990, Datta et.al.,
2001, Ravindra Singh et al., 2001, Suresh Kumar et al., 2002, Rao et al.,
2003, Basavaraja et.al. 2004, Ajay Kumar Goel et al 2006). However, no
silkworm breed was exploited for commercial crossbreed production.
The present work is aimed to develop the sex-limited silkworm strains
through introgression of sex-limited trait from donor to the identified productive
breeds and development of crossbreed variety utilizing the sex-limited strains.
Further, the study also aimed at understanding their tolerance to high
temperature and nutritional aspects.
V
The present study is fabricated into following five chapters for
convenience.
I. Selection of suitable silkworm breeds for introgression of sex-limited trait
II. Introgression of sex-limited trait into selected breeds
III. Line x Tester analysis for identification of promising sex-limited silkworm
hybrid
IV. Evaluation for high temperature tolerance of developed sex-limited
breeds/hybrids
V. Nutritional studies on the newly developed sex-limited breeds / hybrids
Chapter -1
Selection of suitable silkworm breeds for introgression of sex-limited
trait
Eight polyvoltine breeds with plain larvae and oval cocoons of greenish
yellow colour and five bivoltine breeds including two oval and three peanut
breeds with plain larvae among the germplasm stock maintained at Andhra
Pradesh State Sericulture Research & Development Institute, Hindupur were
chosen as initial breeds. Their performance was evaluated in different
seasons for ten economic characters such as fecundity, hatching percentage,
pupation rate, yield per 10000 larvae by weight and number, single cocoon
weight, cocoon shell weight, cocoon shell ratio, filament length and neatness.
The data analyzed for genetic variability and genotype environment
interaction. The superiority of the breeds was established by ranking based on
“Multiple Trait Evaluation Index” and “Sub-ordinate Function". Based on
overall performance and merit, three breeds each of polyvoltine and bivoltine
viz., APM3, APM2 and APM1 and APSa, APS12, APDR105, respectively were
identified as basic resource materials for introgression of sex-limited trait.
VI
Chapter - II
Introgression of sex-limited trait into selected breeds
Introgression of sex limited trait of larval marking into selected
multivoltine breeds viz., APM3, APM2 and APMi and bivoltine breeds viz.,
APDR-ios, APSa and APS12 was carried out utilizing sex-limited polyvoltine
breed, PMSL and bivoltine breeds, 317SLO and 318SLD, as donors. The
donors were available at germplasm stock of Andhra Pradesh State
Sericulture Research & Development Institute, Hindupur. Six sex-limited
strains for larval marking viz., APM3SL, APM2SL and APM1SL (polyvoltine)
and APSi2SL, APSaSL and APDR105SL (bivoltine) were developed as per
targeted traits through recurrent back crossing coupled with selection.
Chapter - III
Line x Tester analysis for identification of promising sex-limited
silkworm hybrid
Newly synthesized sex-limited lines viz., APM3SL, APM2SL and
APIVhSL (polyvoltine) and APS12SL, APS8SL and APDR108SL (bivoltine) and
their nine hybrid combinations were subjected to Line x Tester analysis. The
general combining ability (gca) and specific combining ability (sea) pertaining
to ten economic traits viz., fecundity, hatching percentage, cocoon yield per
10,000 larvae by number, cocoon yield per 10,000 larvae by weight, survival
rate, cocoon weight, cocoon shell weight, cocoon shell ratio, filament length
and neatness during pre-monsoon, monsoon and post-monsoon seasons
were estimated for their consistency and adaptability. The data was analyzed
for expression of heterosis and over dominance in hybrids for the above ten
economic traits. APM3SL x APSASL and APM1SL x APS8SL were found to be
promising based on overall performance, degree of manifestation of heterosis
and over dominance, general and specific combining ability of the lines and
hybrids.
Vll
Chapter - IV
Evaluation for high temperature tolerance of developed sex-limited
breeds/hybrids
The two promising hybrids viz., APM3SL x APS12SL and APM1SL x
APSsSL and their parents along with control hybrid (PM x CSR2) were
evaluated for tolerance to high temperature (36±1°C) and high humidity
(85±5%) for 6 hours at 5th instar. The data was collected on mortality,
survival rate, cocoon weight, shell weight and shell ratio. APM3SL x
APS12SL exhibited tolerance to high temperature over the control hybrid (PM
x CSR2) and found suitable for rearing all around the year in Andhra
Pradesh.
Chapter - V
Nutritional studies on the newly developed sex-limited breeds / hybrids
The identified multivoltine parents (APM3SL and APM^L) and bivoltine
parents (APS12SL and APSsSL) with their hybrids (APM3SL x APSSSL and
APM^L x APSsSL) along with control (PM x CSR2) and parental breeds were
subjected for nutritional efficiency studies with 5th instar larvae. The nutritional
indices like ingesta, digesta, approximate digestibility and reference ratio and
nutritional efficiency parameters like ECI to cocoon, ECD to cocoon, ECI to
shell, ECD to shell, l/g to cocoon, D/g to cocoon, l/g to shell and D/g to shell
were recorded. APM-iSL, APSSSL, APM3SL x APSSSL showed better
nutritional indices and efficiency among the tested polyvoitine, bivoltine and
crossbreeds.
The present study was undertaken to develop productive and adaptive
crossbreed utilizing silkworm breeds with sex-limited trait for larval marking,
which has several advantages over the existing hybrid, PM x CSR2.
Utilization of sex-limited strains for larval marking for commercial hybrid
production avoids the wastage of seed cocoons (male cocoons in polyvoitine
vm
and female cocoons in bivoltine) and thereby reducing the cost of production
along with production of correct or true hybrid silkworm eggs. The crossbreed
developed under the present study was tested with regard to stability and
viability, dietary and economic efficiency and its adaptation to the prevailing
eco-climatic conditions of Andhra Pradesh.