| Abstract |
Object: For ectothermic insects, thermal tolerance is one of the most essential properties for survival in ambient environments. However, the molecular mechanisms involved in thermal tolerance in the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae) have not been fully elucidated. The present study investigated mechanisms that provide heat tolerance in embryos of Bombyx mori. Materials and Methods: Eggs of the bivoltine silkworm strain p50 were exposed to different temperatures, to determine the lethal threshold temperature and to assess the effects of mild and low temperature incubation on tolerance to heat shock and on embryonic protein profiles. Protein levels were measured by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Results: When eggs were exposed to transient heat shock for one hour, the lethal threshold temperature was between 47.0°C and 48.0°C. However, exposure to 40.0°C for four hours, substantially elevated tolerance to the threshold heat shock and simultaneously increased levels of 70 and 27 kDa proteins in eggs, whereas exposure to 35.0°C did not. Exposure to 10.0°C for four hours lowered heat tolerance and did not alter the expression of 70 and 27 kDa proteins. Conclusion: The present findings indicated that hardening silkworm eggs at mild temperatures increases heat tolerance in embryos. To our knowledge this is the first observation of heat hardening in silkworm embryos. These putative heat-shock proteins of 70 and 27 kDa might be involved in the effect of mild temperature hardening on heat tolerance. Cold stress might deprive embryos of energy reserves available for protection against heat damage. Protective mechanisms against heat and cold stress are probably different in this species. Further studies of the molecular mechanisms of heat tolerance should provide insight into the development of novel, high-yield silkworm strains in tropical environments.
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