Abstract
The purpose of this report was to determine which strain should be used as male parent, multior bivoltine strain, in the first hybridization for cross breeding of a robust silkworm race suitable for summer and autumn rearing seasons in a low latitude area in China. Offsprings (F2 and F3) of reciprocal crosses between Cambodge and C. 124 and between Daizo and C. 124 were exposed to various adverse environmental conditions sach as high temperature and humidity, sub-standard food, lack of food and samithion (a kind of insecticide) contamination. Resistance to the adverse environmental conditions (vigour) was evaluated based on the survival rate at the pupal stage. Experimental results showed significant differences in the survival rate and cocoon shell weight depending on the crossing and selection methods applied. These findings suggest that the resistance to the adverse environments is controlled by a system of polygenes on autosornes as well as by sexual genes (possioly maturation genes). The resistance was usually greater in the crossing form of the bivoltine (female)×multivoltine strain (male) than in the reverse crossing form. Therefore, when performing cross breeding for a robust silkworm race suitable for the hot rearing season in southern China, it might be preferable to use multivoltine strains as male parent in the first hybridization.
