How does the ice fraction and glass transition relate to insect freezing survival?
The paper published in the Journal of Experimental Biology focuses on how the amount of internal ice formation at different temperatures and glass transition influence insect survival in the frozen state at extremely low temperatures.
The left figure shows a larva of C. costata frozen in liquid nitrogen. The right figure shows the amount of ice which forms at a given temperature in C. costata, and the relationship between survival in liquid nitrogen and glass transition.
Many temperate and polar insects have a remarkable ability to survive at low temperatures. While most species rely on a so-called supercooling strategy (whereby they avoid formation of internal ice even at very low temperatures), some can survive internal freezing. In our study, we focused on how the amount of internal ice at different temperatures and glass transition (vitrification; known in some insects) is related to the ability to survive in the frozen state. We used larvae of two species of fruit flies, Chymomyza costata (temperate origin) and Drosophila melanogaster (tropical origin). While D. melanogaster has only limited ability to survive freezing, C. costata can survive even at extremely low temperatures and represents one of the most complex organisms known to survive in liquid nitrogen (-196°C) in a fully hydrated state. Variously acclimated larvae of both species allowed us to study the whole spectrum of survival, from 0 to 100%. Using differential scanning calorimetry (DSC), we found that the amount of ice did not significantly differ between acclimation variants of both species, and thus cannot explain observed differences in survival. Then we focused (again using DSC) on glass transition known to occur in larvae of C. costata. We found that glass transition tightly correlates with the ability to survive in liquid nitrogen. During cold acclimation, the larvae of C. costata synthesize a relatively large quantity of the amino acid proline. After most of the body water freezes, the remaining solution reaches higher concentrations, containing relatively more proline. Such a concentrated solution does not freeze but instead turns into glass which is more favorable than ice for the preservation of various cellular and molecular structures, and it is most likely what allows the larvae to survive even at extremely low temperatures.
Rozsypal J., Moos M., Šimek P., Košťál V. (2018) Thermal analysis of ice and glass transitions in insects that do and do not survive freezing. Journal of Experimental Biology 221: 1-11.
DOI: 10.1242/jeb.170464
