INHERITANCE IN SELECTED BIVOLTINE SILKWORM BREEDS RESISTANT TO NUCLEAR POLYHEDROSIS VIRUS (BmNPV).

The knowledge of mode of inheritance of resistance in silkworm is essential before utilizing such stocks for breeding work. The popular bivoltine breeds such as CSR18 and 61N and one susceptible bivoltine breed CSR2 were selected for the study and crosses were made between Resistant x susceptible (RX S) and their hybrids. The resistant and susceptible parents are crossed and F, is prepared, their reciprocal F, (R US and S D R), F, and F, hybrids back crossed with resistant (F, U R = BCR) and susceptible (F, J S = BCS) parents were inoculated with a fixed LC50 concentration of BMNPV. In the present study, the results have shown that NPV resistance is controlled by four dominant genes, N1, N2, N3 & N4 and each is contributing about 252% resistance (tolerance). CSR18 and 61N is having genetic constitution of N1, N2, N3 & N4 having 50% resistance and CSR2 has n1, n2, N3 and n4 with 25% tolerance. F, will have a genetic constitution of N1, N2, N3 & n4 with 75% tolerance. In F, and F, the expected survival percentage is calculated as a total survival percentage of the population of resistant breeds CSR18 and 61N and F, population of susceptible breed CSR2. The total observed survival percentage of F, in resistant breeds comes to 74.17 & 66.67 which is very close to the expected survival percentage 75.00. The chi square value calculated based on the observed and expected numbers. Thus, the observed values in F, are insignificant and deviating from the expected values, thus strengthen the hypothesis to be a major dominant gene in the resistant breeds. The present study is an attempt to determine the mode of inheritance of resistance to BmNPV in Indian silkworm stocks for using them in future breeding programme.