Silkworm thermal biology: a review of heat shock response, heat shock proteins and heat acclimation in the domesticated silkworm, Bombyx mori

Abstract

Heat shock proteins (HSPs) are known to play ecological and evolutionary roles in this postgenomic era. Recent research suggests that HSPs are implicated in cardiovascular biology and disease development, proliferation and regulation of cancer cells, cell death via apoptosis, and several other key cellular functions. These activities have generated great interest amongst cell and molecular biologists, and these biologists are keen to unravel other hitherto unknown potential functions of this group of proteins. Consequently, the biological significance of HSPs has led to cloning and characterization of genes encoding HSPs in many organisms including the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae). However, most of the past investigations in B. mori were confined to expression of HSPs in tissues and cell lines, whereas information on their specific functional roles in biological, physiological, and molecular processes is scarce. Naturally occurring or domesticated polyvoltines (known to be the tropical race) are more resistant to high temperatures and diseases than bi- or univoltines (temperate races). The mechanism of ecological or evolutionary modification of HSPs during the course of domestication of B. mori - particularly in relation to thermotolerance in geographically distinct races/strains - is still unclear. In addition, the heat shock response, thermal acclimation, and hardening have not been studied extensively in B. mori compared to other organisms. Towards this, recent investigations on differential expression of HSPs at various stages of development, considering the concept of the whole organism, open ample scope to evaluate their biological and commercial importance in B. mori which has not been addressed in any of the representative organisms studied so far. Comparatively, heat shock response among different silkworm races/strains of poly-, bi-, and univoltines varies significantly and thermotolerance increases as the larval development proceeds. Hence, this being the first review in this area, an attempt has been made to collate all available information on the heat shock response, HSPs expression, associated genes, amino acid sequences, and acquired/unacquired thermotolerance. The aim is to present this as a valuable resource for addressing the gap in knowledge and understanding evolutionary significance of HSPs between domesticated (B. mori) and non-domesticated insects. It is believed that the information presented here will also help researchers/breeders to design appropriate strategies for developing novel strains for the tropics.

Figure 1.

Protein profile of Bombyx mori larvae, heat shocked (HT) at 40°° C and untreated control (C). (A) The first instar larvae of Nistari strain; (B) The second instar larvae of P2D1 strain. Arrows denote expression of the 90 kDa heat shock protein. M indicates molecular weight marker. (Only representative images have been presented for different silkworm strains; for details refer to Vasudha et al. 2006). High quality figures are available online.

Figure 2.

Protein profile derived from the fifth instar Bombyx mori larvae of CSR2 strain, heat shocked (HT) at 40°° C and untreated control (C). Arrows indicate expression of 84, 60, 62, 47, 42, and 33 kDa heat shock proteins. M indicates molecular weight marker. High quality figures are available online.

Figure 3.

The hypothetical correlation of thermotolerance in different silkworm strains/races of Bombyx mori is presented. Based on the researchers' observation and other literature, non-lethal, threshold, and lethal temperatures for B. mori were determined as 30–35°° C, 40-45°° C, and >45°° C, respectively. L1, L2, L3, and L4 refer to different levels of thermotolerance, resulting in 100, 75, 50, and 25% survival rate at threshold temperatures of 40–45°° C respectively during the process of acclimation and adaptation to heat in subsequent generation. This population would be designated as thermotolerant silkworm strains suitable for tropics. High quality figures are available online.