Silkworm is one of the most thermal-sensitive organisms. Intensive and careful domestication over centuries has apparently deprived the insect of opportunities to acquire
thermo tolerance. Among many factors attributed to poor performance of the
bivoltine strains under tropical conditions the major aspect is that many quantitative characters decline sharply when temperature is higher than 28°C. The risk of hybridization of
polyvoltine to
bivoltine could not be taken due to the delay in fixation of economic characters. The long and hard struggle to evolve robust-productive silkworm hybrids has not so far met with satisfactory results.
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 be
en 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.
Joy and Gopinathan in 1995 reported the appearance of 93, 70, 46 and 28 kDa protein bands consequent to high temperature exposure in Bombyx mori. in both bivoltine and multivoltine strains, but with variying kinetics. Lee et.al., in 2003 cloned a genomic DNA fragment containing a promoter region for the gene encoding an HSC70-4 homologue, the structure of which was deduced from the partial cDNA sequences that were registered in a Bombyx mori EST date base. The deduced amino acid sequence with 649 residues was 89% and 96% identical to those from Drosphilla melanogaster hsc-4 and Manduca sexta HSC-70-4 respectively. The expression analysis by reverse transcription PCR demonstrated that mRNA transcription occurred in all tissues examined and was not stimulated by heat shock. Thus HSC70-4, the molecular chaperon is ubiquitously expressed in every tissue of Bombyx mori.
Considering the enormous investigations conducted on HSPs in a plethora of organisms ranging from bacteria to man, it is felt that there is an acute shortage of literature on the heat shock response of the silkworm Bombyx mori. There is dire necessity for 1. Understanding the molecular mechanism of temperature tolerance in silkworm. 2. Identification of the various families of HSPs synthesized and the threshold temperature, which induce their expression. 3. Understanding the differential expression pattern of various HSPs in bivoltine and polyvoltine races and 4. To locate the genes responsible for the heat inducible HSPs and subsequent steps to introgress the same into the bivoltine genome either by conventional breeding or by use of molecular techniques.
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