Twenty-six bivoltine silkworm (Bombyx mori L.) genotypes were tested along with two check varieties (NB 4 D 2 and SH 6) for their performance. PDF | On Feb 27, , Zafar Iqbal Buhroo and others published Rearing Performance of Some Popular Bivoltine Silkworm (Bombyx mori L.). multiplication of silkworm breeds st P4lP3lPZ leaels and maintenance of . The bivoltine silkworms produce cocoons with high raw silk recovery and bivoltine.
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BIVOLTINE SILK WORM REARING
The hot climatic conditions of tropics prevailing particularly in summer are contributing to the poor performance of the bivoltine breeds and the most important aspect is that many quantitative characters such as viability and cocoon traits decline sharply when temperature is high.
Therefore, the acceptance level of this hybrid with the farmers was not up to the expected level. However, though, these hybrids are tolerant to high temperature environments, they are not tolerant to many of the silkworm diseases. Keeping this in view, an attempt is made to develop silkworm hybrids tolerant to high temperature environments.
Psyche: A Journal of Entomology
Silkworm breeding aims to achieve superior performances in respect of egg yield, cocoon raw silk yield, cocoon stability, and production followed by expansion to new areas besides others. Silkworm breeders continue to strive for an inherent gain in resistance by incorporating resistant genes into the genetic backgrounds of high yielding temperate bivoltines.
Besides this, the cocoon crop stability also relies more on improving other production technologies which have to be explored. It is interesting to note that in inbreeding experiments, besides choice of parents, selection and inbreeding the hybrids are very important which have to be carefully executed since both inbreeding and hybridization are forms of nonrandom mating or selective mating, but operate in opposite ways.
Inbreeding is a kind of genetic assortative mating as compared with phenotypic assortative mating in hybridization. The major effect of inbreeding which is most apparent in the reduction of mean performance of the population is in question. While gene frequencies do not change on the whole, genotypic frequencies do change towards the production of more homozygotes and fewer hetrozygotes.
Thus, any change in the population mean as a result of inbreeding must be related to difference in genotype value between homozygote and heterozygote [ 1 ].
India enjoys the patronage of second position for the production of silk in the world next only to China.
Sericulture in India is practiced predominantly in tropical environmental regions such as Karnataka, Tamil Nadu, Andhra Pradesh and West Bengal and to a limited extent in temperate environment of Jammu and Kashmir. But its quality is at low ebb when compared to the existing international standard. Considering these drawbacks, adoption of bivoltine sericulture became imperative and imminent considering its potentiality even under Indian tropical conditions.
Keeping this in view, breeding experiments were initiated at Central Sericultural Research and Training Institute, Mysore to evolve hardy bivoltine silkworm races suited to tropical conditions for achieving the primary objective of establishing bivoltine hybrids as a concept among sericulturists. Accordingly, many productive and qualitatively superior bivoltine hybrids have been developed by utilizing Japanese commercial hybrids as breeding resource material [ 2 ].
However, the hot climatic conditions prevailing particularly in summer are not conducive to rear these high yielding bivoltine hybrids throughout the year. It is as well-established fact that under tropical condition, unlike polyvoltines, bivoltines are more vulnerable to various stresses, that is, hot climatic conditions of tropics, poor leaf quality, and improper management during summer which are not conducive for bivoltine rearing.
In order to select efficiently the breeds with high temperature tolerance, it is important to analyse the impact of high temperature on many silk yielding attributes of silkworm races and their heritability. The success of sericulture industry depends upon several factors of which the impact of the environmental factors such as biotic and abiotic factors is of vital importance.
Among the abiotic factors, temperature plays a major role on growth and productivity of silkworm, as it is a poikilothermic cold blooded insect [ 3 ]. It is also known that the late age silkworms prefer relatively lower temperature than young age and fluctuation of temperature during different stages of larval development was found to be more favourable for growth and development of larvae than constant temperature.
However, polyvoltine races reared in tropical countries are known to tolerate slightly higher temperature [ 5 ], which is also true with crossbreeds, that have been evolved specially for tropical climate.
The continued efforts for the improvement of cocoon characters of domesticated silkworm were aimed at increased quality silk production. The main objective of silkworm rearing is to produce qualitatively and quantitatively superior cocoons, which in turn will have a direct bearing on the raw silk production. Sericulture, the viable agro-based industry aptly matches the socioeconomic backdrop of rural India.
Silkworm breeds that are reared over a series of environment exhibiting less variation are considered stable. The climatic conditions prevailing in the tropics are most unpredictable and the problems of tropical sericulture are occurrence of aggravated silkworm diseases, unsuitable mulberry leaf for bivoltine silkworms, and lack of sustainable silkworm breeds for effective selection of desirable characters.
In order to introduce bivoltine races in a tropical country like India, it is necessary to have stability in cocoon crop under high temperature environment. The prerequisite of summer hybrid is healthiness and adaptability to adverse conditions of high temperature, low food quality, relatively higher economic traits, with potential for increased cocoon production.
Considering the poor performance of productive bivoltine hybrids during summer season, emphasis was given to evolve bivoltine silkworm breeds suitable to tropical conditions for achieving the primary objective of establishing bivoltine sericulture with quality raw silk among sericulturists. Though, this hybrid was authorized by Central Silk Board for commercialization, large-scale testing in the field is yet to take momentum due to its low productivity.
Therefore, attempts are being made to develop bivoltine hybrids tolerant to high temperature conditions. In oval lines, CSR46 and CSR50 are characterized by plain larvae while CSR18 is characterized by marked and plain sex limited larvae where female is marked and male is plain, similarly in dumbbell lines, CSR19 is characterized by sex limited marked and plain larvae and CSR47 and CSR51 are characterized by marked larvae.
By utilising the breeding resource material three dumbbell lines, namely, HH8, HH10, HH12, tolerant to high temperature and high humidity conditions were developed, the parentage of which are depicted in Table 1. Silkworm rearing was conducted following the standard method under the recommended temperature and relative humidity till 2nd day of 5th instar. During the process of breeding composite layings were prepared by utilized fifteen to twenty disease-free layings to ensure large population size with wide genetic base from F1 to F5, and progenies were raised by conducted mass rearing.
Environmental chamber with precise and automatic control facilities for uniform maintenance of temperature and humidity from 3rd day of 5th instar and were fed fresh mulberry leaves twice a day. From the base population of F1, larvae were also counted larvae and inbreeding was done for each breed and reared at room temperature up to F12, these room temperature reared batches were considered as control batches.
Owing the thermal effect in successive generations, it was observed that after 5th generation both qualitative and quantative characters have declined sharply. So the experiment was modified in such a way that with every alternate generation from F6 onwards to F12 both high temperature lines were brought to room temperature conditions and reared continuously till spinning to recoup the lost vitality under stress conditions.
The breeding plans of the three dumbbell lines are depicted in Figures 1 to 3. Generation wise mean performance for rearing of HH8 is presented in Table 2. The highest cocoon weight 1. Generation wise mean performance for reeling of HH8 is presented in Table 3. Lowest renditta of 6. Thinner filament size of 2. Generation wise mean performance for rearing of HH10 is presented in Table 4. Generationwise mean performance for reeling of HH10 is presented in Table 5.
Generation wise mean performance for rearing of HH12 is presented in Table 6. Generation wise mean performance for reeling of HH12 is presented in Table 7.
The breeding of silkworm since long has been aimed towards evolving of superior and hardy breeds either by means of selection alone or by combining outcrossing or backcrossing with selection in the subsequent generations. The final aim of the breeder is primarily to evolve a breed which can give rise to stabilized crops and secondly to improve both quantity and quality of silk [ 11 ].
The breeding of silkworm races probably dates back to the beginning of the history of silkworm rearing, but it has made great progress rather recently [ 12 ]. Sericulturally advanced countries like Japan has achieved remarkable progress by executing systematic breeding plans for the development of productive races. In silkworms, studies carried out for various characters have shown that the characters could be changed to suit the breeders choice, since selection for one trait has correlation with genetic change of other characters.
The correlation for few traits is negative and for some it is positive [ 13 — 16 ]. Therefore, during the course of breeding of new breeds, the breeder has to be aware of the response of certain characters in selection and its correlated changes with other economic traits.
Inbreeding of hybrids to stabilize silkworm breeds which bred true is well documented [ 1216 — 26 ]. Similarly, Kovolov [ 27 ] is of the opinion that improvement of silkworm races is possible by outbreeding with exotic races and improvement of cocoon quality by repeated backcrossing [ 28 ].
According to Allard and Bradshaw [ 29 ], performance of the strain itself in a given environment indicates its superiority. During evaluation, emphasis was given on the phenotypic expression of traits of economic importance under different temperature conditions. However, as the objective of the study was for greater viability and high productivity merits, equal importance was given on these two traits during selection of parents.
The significant variations observed in the phenotypic manifestation for the traits analyzed can be attributed to the genetic constitution of the breeds and their degree of expression to which they are exposed during their rearing. Such variations in the manifestation of phenotypic traits of the breeds studied can be ascribed to the influence of environmental conditions. Variable gene frequencies at different loci make them to respond differently.
The results are in line with the findings of [ 29 — 39 ]. In the present breeding programme, which envisages evolution of new hardy bivoltines, the aim was to develop more resistant bivoltines that can give rise to stable cocoon crops with better viability, even though productivity is low compared to the existing productive bivoltine breeds that are currently used in the field.
In silkworms, the correlation for some characters is positive and for some is negative [ 1516 ]. Such a negative correlation is observed for the traits productivity and viability and hence the attempt made was to increase the viability of the developed breeds. Moreover as suggested by [ 4041 ], the selection parameters were primarily aimed at improving the viability character such as yielded by number without sacrificing much of the productivity traits like cocoon weight, cocoon shell weight, and yield by weight.
In addition, during later generations of inbreeding, selection was applied to select desired genotypes to improve the traits of commercial importance like viability and productivity as suggested by [ 4243 ] to improve the yield of bivoltines.
The imposition of exposure to high temperature levels in 5th instar and the resultant low pupation rate could be attributed to the low feeding activity of the silkworm resulting in the physiological imbalance ailkworm poor health of the larvae and an increased number of nonspinning worms in the mountages.
The work in [ 44 ] demonstrated that silkworms are more sensitive to high temperature during 4th silkwirm 5th instars. The productive bivoltine breeds are reported to be susceptible to high temperature; the authors of [ 845 bivkltine noticed higher survival in the hybrids than the pure races under high temperature conditions. In the present investigation, when lines are exposed to high temperature continuously there is a drastic reduction in the pupation rate and cocoon traits.
Such drastic change is usually obtained as it is low heritable trait in the silkworm and is prone to large variations in environment and management [ 47 ]. The work in [ 48 ] observed that the pupation rate in Indian popular bivoltine breed, NB4D2, is significantly influenced by both low and high humidity. Silkworm bivoltin which are reared over a series of environments exhibiting less variation are considered stable. One of the objectives of the breeder is to recommend stable breeds to the farmers for rearing under different environmental conditions.
Effect of high temperature and low humidity in terms of cocoon crop depends on several slikworm that operate within and outside the body of the silkworm. In the present study, it was observed that apart from the temperature, humidity also influences the silkwirm pattern in the silkworm and is in agreement with [ 4950 ]. The work in [ 51 ] reported the deleterious effect of high temperature and high humidity on quantitative traits of parents, foundation crosses, and single and double hybrids of bivoltine silkworm breeds of Bombyx mori L.
A Journal of Entomology. Subscribe to Table of Contents Alerts. Table of Contents Alerts. Abstract The hot climatic conditions of tropics prevailing particularly in summer are contributing to the poor performance of the bivoltine breeds and the most important aspect is that many quantitative characters such as viability and cocoon traits decline sharply when temperature is high.
Keeping this in view, an attempt is made to develop silkworm hybrids tolerant to high temperature environments 1. Introduction Silkworm breeding aims to achieve superior performances in respect of egg yield, cocoon raw silk yield, cocoon stability, and production followed by expansion to new areas besides others.
Generation wise mean performance for rearing of HH8 at two temperature conditions. Generation wise mean performance for reeling of HH8 at two different temperatures. Generation wise mean performances for rearing of HH10 at two temperature conditions.
Generation wise mean performance for reeling of HH10 at two different temperatures. Generation wise mean performances for rearing of HH at two temperature conditions.