ROLE OF INTER BATCH CROSSING ON MAINTAINING THE RACIAL CHARACTERS OF BIVOLTINE SILKWORM BOMBYX MORI L., AT P3 LEVEL
To fulfill the needs of commercial rearers, continuous supply of hybrid layings has to be met by the National Silkworm Seed organization without any hurdles. To meet the demand of the farmers, maintenance of pure races gained major impetus and P3 level maintenance centre was given the major responsibility to cater the needs of P2 multiplication centres of Karnataka, Andhra Pradesh and Tamil Nadu. It is very well established that continuous maintenance of silkworm races over years will result in inbreeding depression causing decline in the expression of quantitative traits at hybrid level. To overcome inbreeding depression, inter batch crossing technique was adopted from 2010 till date and adoption of inter batch crossing at every cycle helped to maintain the race characters without any deterioration over the years.
Comparative performance of the productive races from past eight years at P3 level for the expression of characters viz., fecundity, yield/ 100dfls by number and by weight, single cocoon weight, single shell weight, cocoon shell percentage and pupation rate revealed consistency in the expression of quantitative traits without any inbreeding depression. Among the seven quantitative traits, cocoon shell percentage is an important trait that represents productivity of a race which does not change with the season. Hence, the consistency of a race over years can be measured by this trait. The expression of cocoon shell percentage in all races during 2017-18 when compared to 2011-12 data, showed improvement ranging from 2.10 ~ 3.56%. Mean rearing performance of five races showed marginal variation for cocoon shell percentage over the years and it ranged from 22.9 to 24% in CSR2, 20.8 to 22.1% in CSR4, 20.9 to 22.1% in CSR6, 20.4 to 22% in CSR26 and 24.1 to 24.8% in CSR27. The importance of systematic silkworm race maintenance at P3 level in the expression of quantitative characters is discussed.
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