Mechanisms Of Freezing Tolerance Research Articles

Hatchling turtles survive freezing during winter hibernation.

Hatchlings of the painted turtle (Chrysemys picta marginata) are unique as the only reptile and highest vertebrate life form known to tolerate the natural freezing of extracellular body fluids during winter hibernation. Turtles survived frequent exposures to temperatures as low as -6 degrees C to -8 degrees C in their shallow terrestrial nests over the 1987-1988 winter. Hatchlings collected in April 1988 had a mean supercooling point of -3.28 +/- 0.24 degrees C and survived 24 hr of freezing at -4 degrees C with 53.4% +/- 1.98% of total body water as ice. Recovery appeared complete after 20 hr of thawing at 3 degrees C. However, freezing at -10.9 degrees C, resulting in 67% ice, was lethal. A survey of possible cryoprotectants revealed a 2- to 3-fold increase in glucose content of liver and blood and a 3-fold increase in blood glycerol in response to freezing. Although quantitatively low, these responses by spring turtles strongly indicate that these may be the winter-active cryoprotectants. The total amino acid pool of blood also increased 2.25-fold in freezing-exposed turtles, and taurine accounted for 52% of the increase. Most organs accumulated high concentrations of lactate during freezing, a response to the ischemic state imposed by extracellular freezing. Changes in glycogen phosphorylase activity and levels of glucose 6-phosphate and fructose 2,6-bisphosphate were also consistent with a dependence on anaerobic glycolysis during freezing. Studies of the molecular mechanisms of natural freeze tolerance in these turtles may identify protective strategies that can be used in mammalian organ cryopreservation technology.

Freezing-tolerant insects with low supercooling points

1. 1. Pytho deplanatus [Coleoptera: Pythidae (Salpingidae)] is an alpine species that overwinters as a larva under the bark of fallen spruce in the Rocky Mountains. It belongs to a small group of insects which not only is tolerant of freezing but also develops the capacity to supercool to low sub-zero temperatures (−54°C), indeed in this case the lowest ever recorded for this category. 2. 2. The freezing tolerance profile of P. deplunatus is similar to that of many other insects in that glycerol is accumulated to moderately high levels during low temperature acclimation, while water content and glycogen concentrations are considerably reduced. Blood sugar levels, mainly trehalose. also accumulate during this period. 3. 3. The most feasible explanation for the co-existence of freezing tolerance and low supercooling points is that P. deplanatus is physiologically incapable of synthesizing haemolymph nucleating agents and that nucleators in the gut and other fluid compartments are absent in the overwintering larvae. The adaptive advantage of this strategy is that larvae would normally overwinter in a supercooled state, but if freezing did occur (e.g. by inoculation from contact water in the hibernaculum) then the antifreeze function of glycerol and blood sugars would assume a cryoprotective role. 4. 4. The freezing tolerance mechanisms of P. deplanatus larvae are compared and contrasted with those of (a) the congeneric P. americanus which also overwinters under the bark of fallen trees but has both an alpine and arctic distribution, and (b) Xylophagus sp. (Diptera: Xylophagidae) which has a different phylogenetic background but occupies a similar niche in the Arctic.

Lipid participation in intracellular freezing avoidance mechanisms of lettuce seed

Freeze-fracture electron microscopy was used to study water content related freezing resistance in Grand Rapids lettuce seeds. Consistent and recognizable conformational changes occurred in lipid-water phases of lettuce seeds at different moisture contents. In air-dry lettuce seed cotyledons, the lipids lying in spherical lipid bodies near the cell wall appeared amorphous, while the structure was crystalline above 20% water content. The lipid bodies interassociated into membrane bilayers in seeds containing 20 to 25% water. Such lyotropic phase transitions in membrane lipids during lettuce seed hydration are believed to contribute to the biphasic freezing behavior observed in lettuce seeds at different moisture contents and to provide a natural freezing tolerance mechanism for highly desiccated plant tissues such as seeds.