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Abstract
<p class="first" id="d3593320e62">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.
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