Stability and path analysis for yield and related traits in silkworm, (Bombyx mori L.) reared under stress conditions

The silkworm, Bombyx mori L. is a poikilothermic organism whose growth and development is adversely affected by high temperatures. Rearing of silkworm at high temperature conditions of tropical countries like India causes extensive crop loss by reducing cocoon yield. Breeding new silkworm breeds tolerant to high temperatures is the most effective and economical method to increase cocoon yield. Introduction of stable genotypes in such breeding programs and knowledge on the relationships between cocoon yield and other traits would be more advantageous in increasing cocoon yield at high temperatures. Therefore, in this study twenty silkworm genotypes were reared in five artificially simulated environments with different temperatures ranging from 25°C to 38°C. Based on coefficient of variation (%) values, the traits cocoon yield / 10,000 larvae by weight (CY) and cocoon weight showed maximum and minimal variation among the traits in all the five environments. Analysis of variance revealed a substantial genetic variability among the genotypes for most of the traits studied. The results of combined analysis of variance showed significant differences over traits indicating that the traits are influenced by genotype × environment interactions. Stability analysis with respect to CY showed higher mean value compared to grand mean, regression coefficient equal to unity and less deviation from regression in ATR16, CSR17, BHR2, S-5 and D13 genotypes, suggesting that they are consistent with the CY over the different environments. Clustering based on the stability parameters resulted in separation of twenty silkworm genotypes into 5 clusters based on their ability to tolerate high temperatures. Correlation studies demonstrated positive and significant (P < 0.05) correlations between CY and other traits. Pupation rate was most positively associated with CY followed by larval weight, cocoon weight, cocoon shell weight and cocoon shell percent. Path coefficient analysis also revealed that the pupation rate had the maximum direct effect on CY followed by cocoon shell weight and larval weight. Cocoon shell percent and cocoon weight exhibited negative direct path effect on cocoon yield / 10,000 larvae by weight. Therefore, the selection based on pupation rate, larval weight and cocoon shell weight can be effective in improving cocoon yield in silkworm at different environments with varying temperature.