Mechanisms underlying insect freeze tolerance

Abstract

Freeze tolerance - the ability to survive internal ice formation - has evolved repeatedly in insects, facilitating survival in environments with low temperatures and/or high risk of freezing. Surviving internal ice formation poses several challenges because freezing can cause cellular dehydration and mechanical damage, and restricts the opportunity to metabolise and respond to environmental challenges. While freeze-tolerant insects accumulate many potentially protective molecules, there is no apparent 'magic bullet' - a molecule or class of molecules that appears to be necessary or sufficient to support this cold-tolerance strategy. In addition, the mechanisms underlying freeze tolerance have been minimally explored. Herein, we frame freeze tolerance as the ability to survive a process: freeze-tolerant insects must withstand the challenges associated with cooling (low temperatures), freezing (internal ice formation), and thawing. To do so, we hypothesise that freeze-tolerant insects control the quality and quantity of ice, prevent or repair damage to cells and macromolecules, manage biochemical processes while frozen/thawing, and restore physiological processes post-thaw. Many of the molecules that can facilitate freeze tolerance are also accumulated by other cold- and desiccation-tolerant insects. We suggest that, when freezing offered a physiological advantage, freeze tolerance evolved in insects that were already adapted to low temperatures or desiccation, or in insects that could withstand small amounts of internal ice formation. Although freeze tolerance is a complex cold-tolerance strategy that has evolved multiple times, we suggest that a process-focused approach (in combination with appropriate techniques and model organisms) will facilitate hypothesis-driven research to understand better how insects survive internal ice formation.

Transcriptional dynamics following freezing stress reveal selection for mechanisms of freeze tolerance at the poleward range margin in the cold water intertidal barnacleSemibalanus balanoides

“…−10 °C), but are unable to survive temperatures below −16 °C (Boardman, Terblanche, & Sinclair, 2011; Layne & Kuharsky, 2000). Being able to survive freezing is therefore a physiological trait that is likely mechanistically distinct from being able to survive low temperature (reviewed in Toxopeus & Sinclair, 2018).…”

“…They need to be able to manage potential mechanical damage from ice crystal formation, cellular dehydration as water leaves the cell to join the growing extracellular ice lattice, and sustained periods of anoxia since oxygen cannot perfuse easily to frozen tissue (Lee, 2010). Many biochemical correlates to survival of these challenges have been described; most commonly this involves the mass biosynthesis of low molecular weight polyols and sugars that act as osmoprotectants and cryoprotectants that can also decrease the melting (and therefore freezing) point of cells for avoiding intracellular ice formation (Koštál, Korbelová, Poupardin, Moos, & Šimek, 2016; Toxopeus & Sinclair, 2018). However there is no correlation between quantity of low molecular weight cryoprotectants and degree of freeze tolerance—for instance, woolly bear caterpillars P. isabella accumulate circa 2.5 M glycerol as a cryoprotectant but have a lower lethal temperatures of only circa −16 °C (Boardman et al, 2011; K.…”

“…In the extracellular spaces, they may inhibit recrystallization, keeping ice crystals small enough to avoid mechanical damage. However they have been described from only a few freeze tolerant species, and their role in freeze tolerance remains controversial (Duman, Bennett, Sformo, Hochstrasser, & Barnes, 2004; Toxopeus & Sinclair, 2018). Aquaporins facilitate the movement of water out of the cell during freezing and glycerol transporters to facilitate the movement of sugars into the cell have been reasonably well-explored in plant freeze tolerance (Peng, Arora, Li, Wang, & Fessehaie, 2008; Philip, Yi, Elnitsky, & Lee, 2008; Storey & Storey, 2013; Yi et al, 2011).…”

“…The ability to survive internal ice formation is a paraphyletic trait broadly distributed across the animal kingdom, with freeze-tolerant species from multiple phyla including arthropods, molluscs, and vertebrates (Sinclair, Addo-Bediako, & Chown, 2003; Storey & Storey, 1996; Toxopeus & Sinclair, 2018). Freeze tolerance appears to have evolved independently on at least 20 occasions, and this is almost certainly a large underestimate (Sinclair et al, 2003).…”

“…The repeated emergence of this trait suggests that the underlying mechanisms must be readily evolvable and that freeze tolerance is possible in a variety of underlying physiological conditions. Despite the ubiquity of the trait, and over 200 years of research, the physiological mechanisms underlying animal freeze tolerance remain poorly understood (Lee, 2010; Sømme, 2000; Teets & Denlinger, 2014; Toxopeus & Sinclair, 2018).…”