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The objective of the present study was to estimate the level of inbreeding coefficient during inbreeding of the pedigree of random mating and selected populations of two distinct races of mulberry silkworm, Bombyx mori (Lepidoptera: Bombycidae), in the silkworm germplasm. The six generation data of the two races, namely multivoltine Pure Mysore and bivoltine NB4D2, were studied for inbreeding depression coefficient using the residual maximum likelihood method, utilizing two statistical models by analyzing six quantitative traits, namely, larval weight, cocoon weight, shell weight, shell ratio, pupation rate, and filament length. The results of the present experiment demonstrated that the inbreeding coefficient was significant in Model 1 for most of the economic traits in the random mating populations of both the races compared to those of selected populations. These results suggest that during stock maintenance, application of rigid selection for increased numbers of generations helps to retain original characteristics of the pure races while reducing the deleterious effects of inbreeding. The significance of inbreeding coefficient is discussed with reference to the inbreeding of silk moths in the silkworm germplasm.

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Three different races of lepidopteron silk moth Bombyx mori were used in reciprocal and inter se crosses to determine heterosis effects at F1 and recombination loss at the F2 generation for three fitness traits (fecundity, larval duration, survival rate) and four productivity traits (larval weight, cocoon weight, shell weight, filament length). Eleven mating types were represented in the present study, including three pure breeds and a variety of F1 and F2 populations arising from regular and reciprocal crosses, respectively. Equations were derived to evaluate heterosis, maternal and overdominance effects for the above traits. Estimates of heterosis and overdominance effects revealed significant heterosis effects for all the traits, but overdominance was only seen for larval duration (favorable effect) and survival rate (unfavorable effect). Maternal effects were significant for the majority of the traits under study. The results revealed significant reduction for all the quantitative traits from F1 to F2, except for larval duration. The most obvious explanation for the reduction of fitness parameters and productive traits is the reduction in heterozygosity from F1 to F2 (it is expected that one half of the heterozygosity of F1 is lost in F2). For larval duration this explanation seems insufficient and breakdown of epistatic gene effects (i.e. recombination loss) has been suggested.

Newly emerged male moths of silkworm Bombyx mori were treated with in 24 h of eclosion irradiated with two independent doses of 50 Gy and 100 Gy X-rays. The effects of X-rays in the parental generation have indicated significant increase of unfertilized and unhatched eggs followed by significant reduction in the hatchability in the treated batches compared to control. The inheritance of induced sterility was examined in the succeeding generations by rearing the F1, F2 and F3 silkworm progenies and by crossing the progenies of treated males with untreated female moths. It is evident from the results that the number of hatched eggs gradually increased from F1–F3 generations. Thus, the results showed that the egg hatchability do not remain constant at every generation and hatching tendency of eggs increase in the progenies of treated batches. The mechanism of inherited sterility was discussed.