Expression of Heat Shock Proteins During Development in Drosophila

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Abstract
Historically, heat shock proteins (Hsp) were defined to be proteins induced by heat or by various types of chemical stresses. However, numerous early observations suggested that heat shock genes could be expressed without any stress either during normal development and/or during cellular differentiation. Mirault et al. (1978) first suggested that some of the Hsps in Drosophila melanogaster, namely Hsp83 and Hsp70, were made at a low level under normal conditions in the absence of cellular stress. This observation which was made in cultured cells was also found to be applicable to animals as shown by the early demonstration of Chomyn and Mitchell (1982) that Hsp83 was expressed in various tissues of larvae and pupae. Using gene probes, Mason et al. (1984) showed the presence of heat shock transcripts at various stages of development of D. melanogaster. More recently, studies using specific antibodies and/or P-element-mediated transformation have shed some new light on the cell-, tissue-and development-specific expression of the var,ious Hsps in the absence of stress. It is becoming evident that most heat shock genes are expressed at some time or in specific tissues during development and cellular differentiation. The inducibility of the heat shock response in Drosophila has been known to be stage-dependent as shown by Dura in 1981. Thus in terms of inducibility, the fruit fly has a behavior similar to that observed in many other organisms (for reviews see Bond and Schlesinger 1987 and this Vol.). Here, we review studies on the expression of the Hsps in the absence of stress with special emphasis on the small heat shock proteins of D. melanogaster.