The Abiotic Environment

extracellular fluid freezes, the osmotic pressure of the remaining liquid will increase, so 661

that water will be drawn out of the cells by osmosis.

Freezing tolerance is generally found in insects living in extremely cold environments. The general strategy used by freezing-tolerant species is to synthesize ice-nucleating proteins in late fall/early winter (i.e., at temperatures above —10°C) that initiate freezing of extracellular fluids. This early induction of ice formation is advantageous because the rate of ice formation is less than at lower temperatures, thus allowing water to move out of cells to maintain osmotic equilibrium and reduce the likelihood of intracellular freezing (Baust and Rojas, 1985). Through the winter both intra- and extracellular polyhydroxyls are generated. With their ability to bind extensively with water the extracellular cryoprotec-tants will retard the rate at which freezing spreads, while the intracellular cryoprotectants will hold water within cells, to counteract the outwardly pulling osmotic force. It has also been suggested that the cryoprotectants may bind with plasma membranes to reduce their permeability to water. The role of the antifreeze proteins in freezing-tolerant insects is less clear (Bale, 2002). An early suggestion was that they may protect insects from freezing in early fall, before the ice-nucleating agents have been synthesized. A more likely function is that they prevent "secondary recrystallization" (refreezing) in the spring, when polyhydroxyls are being degraded under the influence of rising temperatures, yet the insect must be safeguarded against unexpected freezing temperatures.

Of interest is the evolutionary selection of glycerol as the dominant cryoprotectant because in high concentration this molecule is toxic at above-freezing temperatures. Storey and Storey (1991) suggested that at least three factors have been critical. First, two molecules of glycerol are produced from each molecule of its precursor hexose phosphate, important where colligative properties are concerned. Second, the synthesis of a triol (3-carbon-containing polyol) from a 6-carbon precursor conserves the carbon pool compared to synthesis of 4- or 5-carbon polyols (when the extra carbons are lost as carbon dioxide). Third, the pathways for glycerol synthesis and breakdown already exist in the fat body as part of lipid metabolism. Insects that use glycerol have biochemical pathways for synthesizing it in increasing amounts as the temperature falls progressively below 0°C and, equally, for degrading it when the temperature increases. Such has been shown to be the case in Pteros-tichus brevicornis, an Arctic carabid beetle that overwinters as a freezing-tolerant adult. In P. brevicornis glycerol synthesis begins when an insect is exposed to a fall temperature of 0° C, and by the following December-January the concentration of this molecule may reach or exceed 30 g/100 ml, sufficient to enable an insect to withstand the -40°C to -50°C temperatures to which it may be exposed at this time. Conversely, as temperatures increase toward 0°C with the advent of spring, the glycerol concentration falls and the cryoprotectant disappears from the hemolymph by about the end of April, coincident with the return of above-freezing average temperatures (Baust and Morrissey, 1977). A comparable situation is observed in Eurosta solidaginis, a gall-forming fly that overwinters as a freezing-tolerant third-instar larva. The larva has a three-phase cryoprotectant system that comprises glycerol, sorbitol, and trehalose. Production of the molecules begins somewhat above 0°C but is probably triggered by declining temperatures. At temperatures below 0°C, production of glycerol and sorbitol is greatly enhanced. With the return of warm weather in spring, the concentration of the three molecules rapidly declines (Baust and Morrissey, 1977).

Cold-hardiness and overwintering diapause (Section 3.2.3) frequently occur together, and the question of whether the phenomena are physiologically related has been widely debated (Denlinger, 1991). As noted above, studies have correlated the synthesis of cryopro-tectants with lowered temperatures, and vice versa. However, only a handful of examples are

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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