Biochemical correlates to cold hardening in insects

Abstract

A review of each species discussed or of those reported by other investigators would not at this time provide greater insights into those questions fundamental to understanding low temperature tolerance. The strategies of freezing tolerance/avoidance are heterogeneous and rarely lend themselves to a uniform hypothesis. A number of landmarks in this area have been identified. They include (1) the discovery of glycerol as a natural cryoprotectant and its relationship to frost hardening, (2) studies on the diversity of species demonstrating chill and frost hardiness, (3) cryoprotectant modulation in the frozen state and the attendent maintenance of life processes while frozen, (4) the complexity of multicomponent cryoprotectant systems, and (5) the influence of proteinaceous thermal hysteresis and nucleator agents. The field of insect cold hardiness is proceeding beyond the realm of descriptive studies and will focus more sharply on deductive approaches. Fundamental questions relating to the induction of cold hardiness and the reverse, warm acclimatization, (triggers) require study. Four environmental factors, including temperature, state of hydration, nutrient input (balance), and photoperiod appear critical to hardening. Many species may rely on one or more of these factors and elucidation of trigger mechanisms is essential. In order to understand the functions of the diversity of cryoprotectants and their respective sources, an array of descriptions of biochemical phenomenon in the frozen state will be required. It is known that some species can modulate cryoprotectant levels while frozen. The site(s) of synthesis are unknown as are the mechanisms of transport within the “frozen” system. Would passive distribution phenomena be adequate to explain the bulk transport of cryoprotectants in frozen systems? During the past 5 years, the concepts of proteinaceous agents playing critical roles in the hardening process have been identified. Little is known, however, about the structure of these agents, their mechanisms of action, and the manner in which they integrate with low-molecular-weight protective compounds. Overriding each of the above, are the questions of gene regulation and endocrine control. Is the same endocrine control of carbohydrate metabolism found at 20–30 °C functional at 0, −5, or −40 °C. Interestingly, if asked how a given species of insect survives following a freezing encounter, one can only present a list of theories and correlative data supporting those theories. The puzzle has many more pieces than we provide by measurements of supercooling points and glycerol content.