Asian Journal of Applied Sciences

Year: 2020 | Volume: 13 | Issue: 2 | Page No.: 84-93
DOI: 10.3923/ajaps.2020.84.93

Evaluation of Mutant Silkworm Genetic Resources for Important Morphological and Quantitative Characters

G. Lokesh , M. Maheswari, Ritwika Sur Chaudhuri, D.S. Somaprakash, S. Sekar and R.K. Mishra

Abstract: Background and Objective: Genetic diversity and variability in the population are pre-requisite for the crop improvement programme. Collection and maintenance of genetic diversity is a fundamental component in long-term management strategies for genetic improvement of silkworm. The silkworms mutant were evaluated for morphological characters and rearing traits during two crop seasons i.e., winter and summer to analyze the performance and to identify better mutant accessions. Materials and Methods: Total 23 mutant silkworm accessions were considered for the study, the important morphological characters during egg, larva and cocoon stages were recorded and compared with the catalogue data. Total 12 important silkworm quantitative traits were studied and analysed using multi trait analysis package to identify better accessions. Results: Significant amount of variations were observed among different accessions for different traits. Among 23 mutant silkworms 08 were qualified in the cumulative Evaluation Index (EI) (>50). The cluster analysis showed heterogeneity among the silkworm accessions based on the grouping. The Principal Component Analysis (PCA) indicated the grouping of 06 mutants along with commercially important silkworm races Multivoltine Pure Mysore (PM) and bivoltine (CSR-2). Conclusion: Since these genotypes were considered most suitable for basic genetic studies rather its usefulness in silk production. The correlation studies using PCA revealed that some of the mutant silkworms conserved in the germplasm showed on par with commercially important silkworm races and can be used to explore the combining ability studies for further commercial exploitation.

Keywords: evaluation, germplasm, genetic resources, Silkworm, mutant, Bombyx mori and genotypes

INTRODUCTION

Study about genetic diversity are an important tool that enables breeders to make good selection of parents to ensure genetic variability. Heterosis between genotypes is often enhanced when the two parents are genetically diverse¹. The silkworm Bombyx mori. L is one of most important economic insect species which is exploited for the production of natural silk. The species has wide distribution found both in temperate as well as tropical regions with greater genetic diversity in morpho-biochemical and biometric characters². It has been estimated that, more than 3000 silkworm strains are available worldwide which developed through breeding^3,4. The temperate silkworm strains generally have Uni and bi-voltinism are quantitatively as well as qualitatively superior races compared to tropical polyvoltine silkworm races. On the other hand polyvoltines are better in terms of survival rate, hardiness and resistance to biotic and abiotic challenges⁵. The silkworms has been used as a model for genetic studies because of its large size, ease of rearing in laboratory and short life cycle. The existence of more than hundreds of geographical races and genetically improved strains used for commercial silk production which differ not only qualitative traits but also in quantitative traits such as body size, feeding duration, thermal tolerance and disease resistance. These traits remain to be subjected to systematic analysis using modern genetic tools⁶.

The principal aims of crop improvement is to develop silkworm breeds with superior multiple traits including improved silk productivity, adaptability, disease tolerance and other commercially important characters. Before any breeding tasks, it is imperative to understand the behavior, performance in different life stages and quality and quantity of the silk produced by the silkworm breed/race which is pre-requisite for the selection as parents. The domesticated mulberry silkworm, Bombyx mori L. represents itself as various mutants evolved both from spontaneous and induced mutation. These mutants are maintained by fanciers and breeders in the closed line culture system for many years and serve as a basic tool for genetic analyses including phylogenetic, physiological, ethological, biochemical and molecular studies since systematic linkage studies have been successfully carried out⁷. More than 400 mutations have been mapped corresponding to 230 genes with 28 linkage groups^8-10. The mutant silkworm races shows different phenotypic characters, such as variation in egg color, larval duration, larval marking, cocoon shape, cocoon color and hemolymph colour. Morphological characterization has direct or indirect relation with various quantitative and qualitative traits¹¹. These races also show wide diversity in the yield, economic parameters and exhibit considerable variations for several heritable characters viz., egg colour, larval markings, cocoon colour and cocoon shape. Further, morphological traits along with correlation parameters help to identify and group similar performing germplasm for effective conservation in the gene bank. As the mutant silkworm genetic stocks, it is possible to use directly in silkworm breeding for evolving new races¹² and characterizations of morphological mutant traits were utilized as a basic tool for genetic analysis and were used to study the genetic diversity and distance among the population. In current research, the performance of mutant silkworm genetic resources conserved in the centre are evaluated for 11 rearing and grainage parameters and also compared with the popular commercially exploited bivoltine silkworm race to examine its potentialities for commercial cocoon production.

MATERIALS AND METHODS

Study area: The present study was conducted during 2018-2019 at Central Sericultural Germplasm Resources Centre (CSGRC), Hosur, Tamilnadu state, India.

Research procedure: Silkworm Bombyx mori mutant genetic resources which were collected and maintained at CSGRC, India are selected (Table 1) for the evaluation of important morphological characters, rearing parameters and grainage performance as per the standard descriptor (Table 2, 3). The rearing of 23 silkworm mutant accessions was conducted during June-July and December-January. Three replications of each mutant accession were maintained in a separate 2’×3’ Plastic perforated trays arranged on rearing stand. The rearing was conducted as per the Standard silkworm rearing procedure¹³ and Standard Operational Procedure for conservation of silkworm germplasm. G-2 mulberry variety for chawki rearing and V-1 mulberry variety leaves was used for rearing of late age silkworms. The morphological data was collected based on the physical observation and compared with passport data available at the centre. The data for 12 important rearing and grainage parameters was collected for 5 years (2 crops/year). Mutant silkworm data was compared with commercially important mutivoltine (PM) and bivoltine (CSR-2) silkworm breeds using PCA analysis for better understanding of the possibilities of mutant silkworms for commercial exploitation.

Statistical analysis: Mean data of 5 years for 23 mutants from 12 important quantitative characters were considered. The number of rearing conducted during the period was treated as replications.

Multivariate analysis was conducted to analyze the variability in the parameters among different accession. WINDOWSTAT statistical package was used for ANOVA to compare the performance of the accessions in two different seasons. PAST3 statistical software package was used for Principal Component Analysis (PCA) to compare the mutant accessions with commercially important silkworm races and to generate cluster grouping.

RESULTS AND DISCUSSION

The evaluation of morphological characters of the mutant silkworm genetic resources in the present study was recorded in egg, larva and cocoon stages and compared with the catalogue data (Table 4). The present observations are on par with the catalogue data which is maintained at the Silkworm Germplasm Information System (SGIS). Maintenance of original characters of the silkworm genotypes over the generations is one of the prime objectives of the silkworm germplasm.

Quantitative trait analysis: The data of 23 mutant accessions recorded for 5 years was analyzed for 12 important characters showed significant variations among different mutant genotypes which were evident from the calculation of Coefficient Variation (CV%). The higher variability was recorded with single shell weight, minimum value recorded 0.097 g in BBE-0318 and maximum recorded in BBE-0392 (0.217 g). In contrast, lower variations recorded in total larval duration 533.85 h in BBE-0309 and 566.4 h in BBE-0391 (Table 5). The multiple trait evaluation indices assessment of all the accessions by taking into consideration of all the parameters recorded in Table 5. High variability was observed in the parameters such as fecundity (10.158), larval weight (16.687), survivability /ERR by weight (11.76), cocoon weight (12.47), shell weight (23.49) and SR% (13.17) recorded significant variations among the mutant silkworm genotypes conserved in the germplasm (Table 6).

Multiple trait evaluation method is being utilized for testing large number of silkworm germplasm and based on the performance for important economic characters and promising genotypes are selected^14-18. In the present study also the 23 mutant accessions were evaluated based on the multiple trait evaluation to understand the better performing accessions. More than 21 traits contribute to silk yield and there exists an inter-relationship between multiple traits in silkworm¹⁹. The precision of selection of breeds among many numbers of breeds can achieved through the evaluation index method that gives priority to all yield component traits²⁰. Based on the performance of the silkworm genotypes, individual indices were calculated for each of the 10 parameters. Since in larval duration the desirability is lower values, hence this character was not considered for calculation of Evaluation Index (EI). The EI values were calculated for each of the genotype in all the 10 parameters and ranking was assigned based on the qualifying average EI-value>50. It was found that eight accessions such as BBE-0306 (EI = 61.83), BBE-0392, 0307, 0391, 0323, 0320, 0319 and 0333 (EI = 55.57) qualified the EI value >50 with 9-10 qualifying parameters in each accessions. Remaining accessions were also performed better but not qualified the bench mark of EI>50. In the recent past evaluation index method developed by Mano et al.²¹ has been utilized for short listing better performing silkworm genotypes/hybrids for commercial exploitation^18,22 and the same has been utilized in the present study as well for evaluating 23 mutant silkworm Bombyx mori L. genotypes in respect of different traits viz., fecundity, hatching, larval weight, ERR by no. and ERR by Wt., Pupation, cocoon weight, shell weight, shell ratio, cocoon yield by number and by weight. The ranking was assigned to those genotypes which qualifies with Cumulative EI>50 (Table 7). Similar works also reported earlier wherein the identification of top performing silkworm breeds in different seasons was achieved²³. Evaluation index is one such method that increases the precision of selection of breed among an array of breeds by a common index giving due to weight-age to all the yield component traits²⁰.

Since, the rearing of mutant silkworms were conducted in two different seasons. The comparative analysis of the performance of these silkworms was made by ANOVA, which revealed significant difference between the seasons, between the accessions and between the accessions and the seasons. Highly significant values were recorded in the important commercial characters of the silkworm such as fecundity (p<0.01), survivability (p<0.01), cocoon weight (p<0.01), shell weight (p<0.05) and total cocoon yield (p<0.01). However, there was no significant difference in values were also recorded with larval duration, larval hatching and in shell ratio during the two seasons. However, the analysis of data indicates that all the genotypes utilized in the study vary significantly with respect to most of the parameters studied during winter and summer seasons (p<0.001, Confidence Distribution (CD) value 40.73).

Higher mean values were recorded for all the parameters during winter season (Table 8) compared to the multi-trait values recorded in the summer season (Table 9). This shows the favorable season for better expression of quantitative traits in mutant silkworm accessions. Since, these mutant silkworms are temperate in origin, the silkworm rearing performance always better due to congenial and less fluctuations in the environmental temperature during winter season (Table 10)²⁴.

A detailed analysis was undertaken to test the efficacy of hierarchical agglomerative clustering (UPGMA method) in grouping the different mutant silkworm accessions based on quantitative trait analysis (Fig. 1). The results indicate that all the mutant accessions were grouped together in two major groups. Further, sub-grouping under these 2 groups highlights genetically different association with the differentiation of various groups. The cluster analysis provides scope for adopting a re-combinational breeding program using distant cluster members. Thus, the sub-grouping of high yielding bivoltine strains offers an opportunity to exploit the genetic differences between high yielding strains^25,26. The clustering also indicates the possibility for recombining low and high-yielding members from genetically distant clusters. The results presented here establish its usefulness in realizing a better projection of the genetic difference between silkworm strains of different yield potentials²⁷. Recently the cluster analysis of 369 bivoltine silkworm accessions was done using Ward’s minimum variance cluster analysis method for 11 economic traits revealed the diversity among the bivoltine genotypes that were grouped into 20 clusters based on the degree of variations²⁸. The maintenance of heterogeneity in the germplasm is important and is necessary for better maintenance without any inbreeding depression²⁹. The inclusion of genotypes of the same origin in different clusters clearly indicates the presence of considerable genetic diversity among the population used in this study.

The PCA was conducted for multivariate traits and analyzed for correlation of the performance of mutant silkworm accessions along with the commercially exploited silkworm races such as Bivoltine CSR-2 (BBI-0290) and Multivoltine Pure Mysore (PM) (BMI-0001) indicated that the performance of mutant accessions viz., BBE-0306, 0307, 0319, 0320, 0323 and 0333 are positively correlated with the performance of BBI-290 (CSR-2) and BMI-0001 (PM) and grouped under one group (Fig. 2). Whereas, accessions such as BBE-0308, 0310, 0314, 0316 and BBE-0317 are negatively correlated compared to the BBI-0290 and BMI-0001 and grouped separately. The grouping of silkworm genotypes indicates the higher similarity with respect to multiple traits³⁰. Similarly, 6 mutant accessions showed similarity with CSR-2 and PM silkworm races for some of the commercially important characters. This indicates the possibilities of using mutant silkworm genetic resources for commercial exploitation through crop improvement programme.

CONCLUSION

The extent of variability among different genotypes is essential and is basis for the germplasm collection for their effective utilization in the future. The concerted efforts are utmost necessary for the characterization, evaluation and identification of trait specific genotypes using standard and reliable methods. The present study is an attempt to showcase the potentialities of the mutant silkworm accessions for their utilization in the commercial exploitation. Since these genotypes were considered most suitable for basic genetic studies rather its usefulness in silk production. The correlation studies using PCA with commercially important silkworm races PM and CSR-2 revealed that some of the mutant silkworms conserved in the germplasm can be used to explore the combining ability studies for further commercial exploitation.

SIGNIFICANCE STATEMENT

This study emphasizes the importance of the mutant silkworm germplasm and its proper maintenance for their sustainable utilization in the future. This study showcases the genetic potentialities of the mutant silkworm genetic resources which can be utilized by the silkworm breeders to develop trait specific breeds. Also, this study give a light on the efficient management of silkworm germplasm and also their evaluation to understand the genetic potentialities of silkworm genetic resources to exploit for commercial silk production through appropriate silkworm breeding strategies.

REFERENCES