Introduction

An important response of animal tissues to environmental cold is to increase the proportion of unsaturated fatty acids in the membrane phospholipids. This response leads to a decrease in the structural order of the hydrocarbon interior of membranes and thereby offsets the ordering effects of cold (Cossins, 1994; Hazel & Williams, 1990). It has been termed ‘homeoviscous adaptation’ (HA) in recognition of its homeostatic and adaptive significance (Sinesky, 1974), and has been widely observed not only in animals but also in plants and microorganisms during both phenotypic and genotypic responses to altered environmental temperature. HA appears to play one of two important adaptive roles. Firstly, it may ensure that the fine physical structure of cellular membranes, and hence those functions which are dependent on structure, are preserved when growth temperature is altered despite the intrinsic temperature dependance of these phenomena. This is a capacity adaptation in the sense of Precht (Cossins & Bowler, 1987). Secondly, it may alter the sensitivity of cell, tissue and whole-organism function to disruption at extreme temperatures, thereby improving the match between thermal resistance and environmental temperature. This is a resistance adaptation.

This general scheme is now supported by a considerable amount of evidence linking changes in lipid unsaturation during thermal acclimation to changes in membrane physical structure and, in a few cases, to changes in membrane-associated function (Cossins, 1994). The linkages have been established both in direction and in time course. However, it should be recognized that they constitute correlations, and despite their plausibility do not constitute unequivocal proof of a causal relationship.