Fitness components of bivoltine breed CSR2 after introgression of multivoltine thermotolerance character of silkworm Bombyx mori L

Bivoltine rearing in tropical regions is highly unpredictable due to fluctuating temperature and humidity conditions. It is therefore extremely imperative to develop a stable bivoltine cocoon crop under adverse environment of the tropices. Improvement of the existing bivoltine lines through introgression of true multivoltine tolerant factors by conventional backcross breeding strategy leading to the development of Near Isogenic Lines (NILs) can bring sustained boon in the sericulture industry. A targeted breeding approach was initiated towards the improvement of productive bivoltine breed ‘CSR2’, developed in South India. In this regard, ‘Nistari’ and ‘Sarupat’ (indigenous multivoltine thermotolerant breeds of West Bengal and Assam respectively) were selected as donor parents for introgression of thermostable factors. Protein profiling of haemolymph indicated the presence of 102-kDa, 89 KDa and 86 KDa polypeptide in the indigenous multivoltines, but absent in developed bivoltine ‘CSR2’. These proteins, as characteristic feature of thermotolerant multivoltine breed, have been successfully integrated in NILs. Esterase isozyme analysis revealed the presence of thermostable esterases (both specific and non-specific) in NILs of ‘CSR2’. In case of -esterase three bands (Est1,Est2 and Est3) are present in multivoltine parent but in CSR2 Est3 is absent . Heat stable Est1 and Est2 have successfully integrated in all NILs of which Est2 is heat sensitive in CSR2. This marks the introgression of multivoltine thermostable factors into the improved NILs of bivoltine Silkworm breed ‘CSR2’ leading to improved survivability measured in terms pupation percentage that will help generate bivoltine seed cocoons in this zone for sustaining the silk industry.