Molecular Mechanisms

The environmental cues that regulate diapause have been well defined, and there is also a fairly good understanding of the downstream hormonal signals that serve to coordinate diapause. But, the molecular underpinning of diapause remains poorly understood. Is diapause simply a shutdown in gene expression or does it represent the expression of a unique set of genes? An examination of the synthesis of brain proteins in flesh flies suggests that far fewer proteins are synthesized in the brain during diapause but, in addition, the brains of diapausing flies synthesize a set of proteins not observed in brains of nondiapausing flies. This suggests that diapause represents both a shutdown in gene expression and the expression of a unique set of genes.

One of the most conspicuous groups of genes that are diapause up-regulated is that of the heat-shock proteins. Both heat-shock protein 70 (Hsp70) and one of the small heat-shock proteins (Hsp23) are up-regulated in flesh flies during diapause. The Hsps are up-regulated upon entry into diapause, remain elevated throughout diapause, and then drop sharply at diapause termination. But, not all heat-shock proteins are up-regulated during diapause. Hsp90, by contrast, is actually down-regulated. The Hsps may offer protection from environmental stresses during diapause and possibly contribute to the cell cycle arrest that characterizes diapause.

Genes that are diapause down-regulated are potentially of equal interest. Among the genes in this category is the gene that encodes proliferating cell nuclear antigen, a cell cycle regulator. The down-regulation of this gene during diapause may be important in bringing about the cell cycle arrest. As more genes are examined, it is evident that certain genes are expressed throughout diapause, others are turned off during diapause, while still others are expressed only during early or late diapause or may be expressed intermittently during diapause.

It is still too early to know if common sets of genes are expressed during diapause in different species and different life stages, but preliminary data suggest that the expression patterns of at least some of the genes, those that encode Hsp70, may be shared across species and life stages.

See Also the Following Articles

Aestivation • Bombyx mori • Cold/Heat Protection • Dormancy • Juvenile Hormone

Further Reading

Danks, H. V. (1987). "Insect Dormancy: An Ecological Perspective."

Biological Survey of Canada, Ottawa. Denlinger, D. L. (1972). Induction and termination of pupal diapause in

Sarcophaga flesh flies. Biol. Bull. 142, 11-24. Denlinger, D. L. (1985). Hormonal control of diapause. In "Comprehensive Insect Physiology, Biochemistry, and Pharmacology" (G. A. Kerkut and L. I. Gilbert, eds.), Vol. 8, pp. 353-412. Pergamon Press, Oxford. Denlinger, D. L. (2002). Regulation of diapause. Annu. Rev. Entomol. 47, 93-122.

Denlinger, D. L., Giebultowicz, J. M., and Saunders, D. S. (eds.) (2001). "Insect Timing: Circadian Rhythmicity to Seasonality." Elsevier, Amsterdam.

Lee, R. E., Jr., and Denlinger, D. L. (eds.) (1991). "Insects at Low

Temperature." Chapman & Hall, New York. Saunders, D. S. (2002). "Insect Clocks," 3rd edition. Elsevier, Amsterdam. Tauber, M. J., Tauber, C. A., and Masaki, S. (1986). "Seasonal Adaptations of Insects." Oxford University Press, Oxford.

Walter R. Terra

University of Sdo Paulo, Brazil

Digestion is the process by which food molecules are broken down into smaller molecules that are able to be absorbed by the gut tissue. Most food molecules requiring digestion are polymers such as proteins and starch, and are sequentially digested through three phases (Fig. 1). Primary digestion is the dispersion and reduction in molecular size of the polymers and results in oligomers. During intermediate digestion, these undergo a further reduction in molecular size to dimers, which in final digestion form monomers. Digestion usually occurs under the action of digestive enzymes from the midgut, with minor or no participation of salivary enzymes. In most insects, midgut pH is either mildly acidic or neutral. Lepidopteran and trichopteran larvae, scarabaeid beetles, and nematoceran flies have alkaline midguts, whereas cyclorrhaphous flies have a very acidic section in the middle of the midgut. The midgut is, as a rule, an oxidizing site, although in some wool-digesting insects it is a reducing site, a condition necessary to break disulfide bonds in keratin, thus facilitating enzymatic hydrolysis.

FIGURE 1 Digestion of important nutrient classes. Arrows point to bonds cleaved by enzymes. (A) Protein digestion; R, different amino acid moieties. (B) Starch digestion. (C) p-linked glucoside. (D) Lipid digestion; PL, phospholipase; R, fatty acyl moieties.
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