The front ranking breeders in the field agrees to the fact that it is a difficult task to breed such bivoltine breeds, which are suitable to high temperature environment and yet productive. Therefore means other than the conventional breeding methods are to be adopted to attain the goal. With the aid of modern biotechnological tools it may be possible to quantify the factors responsible for the expression of temperature tolerance. Resistance to high temperature has been recognized as a heritable character in silkworm and the possibility for temperature tolerant silkworm races were suggested by Kato as early as 1989. Thorough understanding of the phenomenon of temperature tolerance in silkworm is an essential pre requisite for attaining any results in this direction.
Heat Shock Proteins
It is known that rapid heat hardening can be elicited by a brief exposure of cells to sub-lethal high temperature, which in turn provides protection from subsequent and more severe temperature. In 1962, Ritossa reported that heat and the metabolic inhibitor dinitrophenol induced a characteristic pattern of puffing in the chromosomes of Drosophila. This discovery eventually led to the identification of the heat-shock proteins (Hsp) or stress proteins whose expression these puffs represented. Beginning in the mid-1980's, investigators recognized that many Hsps function as molecular chaperones and thus play a critical role in protein folding, intracellular trafficking of proteins, and coping with proteins denatured by heat and other stresses. Accordingly, the study of stress proteins has undergone explosive growth.
Heat-shock proteins are classified into families on the basis of sequence homology and typical molecular weight as Hsp 110, Hsp 100, Hsp 90, Hsp 70, Hsp 40, Hsp 10 and small heat- shock protein families. In eukaryotes many families comprise multiple members that differ in inducibility, intra cellular localisation and function.
Extensive studies have been conducted on the heat- shock response in insects such as Drosophila, Chironomous, Lymantria dispar, the tobacco hornworm-Manduca sexta, the desert ant-Cataglyphis, the fleshfly-Sarcophaga crassipalpis, the locust Locusta migratoria etc. There are reports on the activity of hest shock proteins in silkworm. Evegnev et. al. (1987) studied heat shock response in Bombyx mori cells. Temperature elevation induced active transcription of heat shock mRNAs in infected cells. But at the level of translation headstock treatment failed to induce hsp synthesis and was not able to inhibit production of polyhedrin in such cells.