To freeze or not to freeze: insect overwintering strategies

Perhaps winter hasn’t quite yet crawled up your windowpanes or stretched its fingers across your favorite pond, but it’s certainly making its presence known at latitude 64°N. I’ve been pulling out extra quilts, wrapping up in scarves for my morning bike commute, and making more baked goods to keep up with my hot chocolate habit.

As a graduate student, I study the molecular story behind arctic ground squirrel hibernation at the University of Alaska Fairbanks. I’m the first to admit I’m a mammal kind of gal⎯I gravitate towards the furry and fuzzy and revel in soft fur, large eyes, and squeaky-cute chirps. However, every now and then I step outside of my mammalian bias and remember that there is a world of tiny, crawling, wiggling creatures that are surviving the cold in ways that are equally as extraordinary as the strategies employed by my favorite hibernating rodent.

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Arctic ground squirrel hibernating in the lab. So cute. Copyright © 2013 Øivind Tøien/Institute of Arctic Biology.

I don’t think I’m alone in my mammalian predisposition. It can be easy to overlook insects, especially the more inconspicuous and less flashy species. However, during the Alaskan spring and summer, it is impossible to ignore the state’s most infamous insect: the mosquito.

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Mosquito (Culex quinquefasciatus) larva. Image courtesy of the CDC.

Growing up in Alaska, I never thought about what happened to mosquitoes during the winter. Perhaps I was simply happy they were gone, or maybe my gravitation towards the furry was present from a tender age. In any case, it wasn’t until I was in my late twenties that I learned there are two general types of Alaskan mosquitoes. One variety⎯affectionately called “snow mosquitoes”⎯overwinter in adult form. When temperatures start to drop, they tuck away in tree bark or bury themselves in the leaf litter and begin the process of supercooling.

You may have heard of supercooling, the process by which a liquid can remain liquid below its usual freezing point. A supercooled liquid must remain completely free of any impurity, as even a speck of dust can serve as a nucleation point for ice crystals to form. After snow mosquitoes rid their blood of impurities, they are able to survive winter temperatures as low as -31°C.

The adults of the other variety of mosquito lay their eggs in the fall. After depositing the next generation of blood-sucking babes, the adults do not attempt to make it through the chilly winter ahead and die an unmourned death. Their progeny hatch in the spring and are considered much more voracious biters than their cousins. (Interested in mosquito matters? Refer to the seminal 1949 book The Natural History of Mosquitoes by Marston Bates.)

(Quick mammalian aside: Arctic ground squirrels are the only known mammal to supercool. Similar to mosquitoes, they are also thought to remove their blood of impurities that would otherwise encourage ice growth. Arctic ground squirrels can lower their body temperature to -2.9°C, an incredible feat for an endotherm.)

Supercooling is an example of a freeze-avoidant strategy, in which an animal shifts its physiology to avoid the buildup of ice crystals in its blood. Yellowjacket queens living in subarctic Alaska also supercool. To avoid touching snow or ice, which can disturb a supercooled insect and promote instantaneous freezing, the queens hang by their mandibles from a twig or leaf stem in the leaf litter. The hollow space occupied by their hanging body creates a buffer of air between them and any dangerous frozen water.

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Vespula vulgaris, or common wasp, or yellowjacket. Image courtesy of JL Boyer.

Another equally impressive strategy employed by overwintering insects is freeze tolerance. Instead of preventing the formation of internal ice, these insects embrace it. There are various means of becoming an insect icicle, and most involve promoting crystallization extracellularly. Encouraging ice to form outside of cells protects the delicate machinery within cells, which carries clear benefits to the animal. One exception to this rule is found in the alpine cockroach (Celatoblatta quinquemaculata), which can survive temperatures down to -9°C and allows for the formation of ice crystals within its gut cells. It isn’t entirely clear how they achieve this feat, but it could be via thermal hysteresis proteins (also known as antifreeze proteins). These proteins widen the gap between water’s melting point and freezing point by shaping ice into protein-sheathed, faceted ice crystals. Employing a thermal hysteresis strategy decreases the insect’s lower lethal temperature. Other freeze-tolerant insects include the Isabella tiger moth (Pyrrharctia isabella) and the flightless midge (Belgica antarctica).

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The mechanism of thermal hysteresis via antifreeze proteins. Figure courtesy of Davies 2014.

It’s incredible to think about anything staying warm during a Fairbanks winter, much less a tiny mosquito or a wee wasp queen. To maintain my own endothermic heat through Alaska’s longest season, I use a variety of items and strategies, including down jackets, mittens, extra socks, toe warmers, heating oil, gasoline, wood stoves, hot chocolate, soup, quilts, and dog snuggles. Not nearly as efficient as some of my insect friends, but they will have to do.

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Dynamic telomeres and the aging process

My name is Sara Wilbur. I’m a third-year masters student in biology at the University of Alaska Fairbanks.

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Me and my dog Junie biking the White Mountains trail. Photo credit: Jason Clark.

I’ve written for NiB before, about work-life balance in academia, and yesterday I was introduced as the newest contributor to NiB. I’m very excited to write for this wonderful project! You can expect future articles to focus on telomeres, arctic ground squirrels/hibernation, and scientific life in Alaska.

Aging, DNA structure, and the dynamic telomere

The mind simplifies difficult concepts to support graspability. One example of this tendency is found in our attempts to define the aging process. Aging is complex, nuanced, and expressed differently across individuals. It would be quite useful if there was a quantifiable “thing” in the body that indicated how long an organism had left to live. In the mid-1970s, a discovery came that presented itself as a solution to the problem of measuring age: protective, terminal chromosome sequences known as telomeres.

2930423615_5320362dea_o.jpgAging is complex and nuanced. Photo credit: Flickr.

As is widely understood, DNA provides the molecular “blueprint” for all organisms, influencing what they look like and how they behave. The particular nucleic acid sequences (the Ts, As, Gs, and Cs) of an individual’s DNA codes for specific proteins, which are involved in virtually every cellular process. However, of all the DNA you have, only 1% of DNA contains coding sections. Initially considered “junk DNA,” the remaining 99% of noncoding DNA fulfills many important functions, including transcriptional regulation (turning genes “up” to make more of a particular protein or “down” to lessen protein production) and DNA protection, a duty fulfilled by the dynamic telomere.

Telomeric duties

Telomeres have two main purposes. One is to maintain chromosome integrity. If you’re a molecule of DNA, a double-strand break is cause for alarm. Fortunately, DNA repair enzymes are recruited to double-strand breaks, allowing DNA to replicate properly and be transcribed faithfully. However, if you think about it, a chromosome end could be seen as a double-strand break. What prevents chromosome ends from being unnecessarily repaired? It turns out that telomeres aren’t simply naked DNA sequences, but are instead intimately associated with several proteins in a complex known as shelterin. Shelterin proteins help the telomere fold back and associate with itself. This forms a “t-loop” to essentially hide and protect the chromosome end.

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Telomeric structure and associated proteins. Figure credit: Blackburn et al. 2015.

Perhaps more famously, telomeres also act as a buffer to prevent coding DNA erosion during cell replication. An important consequence of the evolution of linear chromosomes (found in all eukaryotes, from yeast to elephants) is that a few nucleotides are lost with each round of cell division. The DNA replication machinery cannot fully replace the outermost nucleotides, so the DNA strand gets shorter over time. As it is the telomere sequence that caps chromosomes, it is these sequences—rather than the DNA in between—that take the hit.

How are telomeres implicated in the aging process?

Telomere shortening over time is thought contribute to the aging process. Before I describe why this might be, let’s explore a more fundamental idea: what is aging, anyway? Basically, it’s a loss of physiological—or bodily—function. A proposed root case of declining functionality in the body is cellular senescence, or when a cell ceases dividing; a buildup of these cells within a tissue is associated with aging. Telomere shortening is one cause of cellular senescence: when telomeres reach a critically short length, cells cease to divide. This is a mechanism to prevent cells from becoming cancerous. However, there is a tradeoff: a buildup of senescent cells that can no longer induce tumor growth could be driving the aging process.

Telomere length does change with time, but shortening is also influenced by lifestyle and genetics. Some species have “mega-telomeres” (including mice, which are a common model for in vivo telomere length research), which have a different biology than more run-of-the-mill telomeres (as we humans possess). To further complicate matters, some species possess the enzyme telomerase in their body cells. This enzyme replaces lost nucleotides, essentially preserving telomere length over time. However, telomerase isn’t the answer to short telomere’s prayers: 80 to 90% of all cancers are associated with over-active telomerase activity.

The future of telomere research

The initial excitement surrounding telomeres’ discovery forty years ago and the potential for its use as a simple biomarker of aging and disease are still with us today. However, like any biological process, telomere dynamics are much more complicated than we first thought. For instance, while there is overwhelming evidence from the past few decades that telomeres do decline with age across species, it is still unclear if telomere length can accurately predict calendar age. The future of telomere research will continue to evolve away from cell culture work into living systems, and from common laboratory animals to a wider species diversity, including ectotherms (“cold-blooded” animals), plants, and hibernators. Stay tuned for more on telomere dynamics in these “non-traditional” organisms!

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What’s happening in hibernator telomeres? Juvenile arctic ground squirrel hiding in some willows. Photo credit: the author.