Most of us would find it pretty hard to live outside all winter anywhere in Canada, let alone in places where temperatures routinely dip below -30ºC. But this is exactly what the mountain pine beetle (and many other insects) does. The question is, of course, how does it pull this off? What is it about mountain pine beetle larval physiology that allows the insects to make it through long months of deep cold?
A paper by Tiffany Bonnett and others, that recently came out of our lab, probes this process in pine beetles in a way that has not been done before. The publication is entitled “Global and comparative proteomic profiling of overwintering and developing mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae), larvae” and is available as an open access publication. We have also published the raw genomics data online at figshare. You can find those data here, here, and here.
What did we do?
Larval mountain pine beetles were collected from trees near to Valemount, BC during the early autumn and late autumn, and then again during the early spring and late spring. The larval beetles were prepared in the lab so that we could use a process called iTRAQ to assess all of the proteins present in the larvae at each of the different collection time points. Essentially we took four snapshots – two in the autumn and two in the spring – an then compared them to each other see what was changing. This gave us a huge amount of data to work with and we used statistics to tell us which proteins increased or decreased in prevalence across either the autumn or the spring.
What did we find?
Among other things:
- Larvae expend a fair amount of energy on detoxification of host resin compounds, both in preparation for the winter, and then during feeding after winter is over.
- Stress physiology plays a large role in this entire process, particularly in the autumn as the larvae are dealing with host tree resin toxins and readying themselves for the upcoming onset of winter.
- We saw evidence for the involvement of several compounds that may play an antifreeze role.
- There is an evident shift between emphasizing overwintering preparations (in the autumn) and emphasizing completing development (in the spring), consistent with expected shifting priorities at different points in the life cycle.
Why is this novel?
The overwintering larvae of the mountain pine beetle remain nestled under the protective bark of their host tree. This makes them quite difficult to work with, and until now not very much information had been generated on this life stage, particularly in the context of winter survival. This work, which has harnessed the power of some very useful genomics databases, has cracked the door (or the bark?) open to allow us to see in broad sweeping terms what is going on in this insect during this vital time in its life cycle. We have seen aspects of larval mountain pine beetle physiology that have never been seen before, and that provides the power to ask new questions and to investigate key genes and pathways in a much more directed manner.
Why is this important?
Up until now, the main known winter survival mechanism for larval mountain pine beetles was the accumulation of glycerol in the autumn. Glycerol acts as a natural antifreeze and is part of the overwintering survival tool kit of many insects. But in most known cases, glycerol is not the only part of the equation, and we didn’t think that it was the sole story in mountain pine beetle either. And it turns out that we were correct with that guess – there are a lot of other things going on as well.
In a larger sense, this means that we now have targets to focus on as we work to understand how deep winter cold can impact populations. Overwintering mortality is one of the major factors contributing to control of bark beetle populations. Now that the mountain pine beetle is moving from the cold interior of British Columbia into even-colder central Alberta, a major research question relates to the climate in its expanding geographical range and how that is going to affect the insect’s potential spread to other regions. Overlay that question with the impacts of climate change, and it should be apparent that understanding mountain pine beetle overwintering physiology is becoming more and more vital.
Where do we go from here?
We now have numerous potential gene targets to look at, any of which is a project unto itself. Because we have shown in other work that larval mountain pine beetles in the late summer are feeding on potentially very toxic food, we are interested in finding out how larval ability to detoxify and digest their food in the autumn can make or break their chances for winter survival. We suspect that certain larvae are better adapted than others at dealing with the nutritional challenges that they face, and thus better able to produce antifreeze compounds and the other components that allow overwintering success.
In other words, we suspect that there is variation in the mountain pine beetle population that results in some larvae surviving the winter while others don’t. We, along with collaborators, hope to determine which genes are important in this process and how selection pressure in their historical and expanding ranges are changing mountain pine beetle populations.
Some of our key questions are:
- How do specific proteins function in protecting larvae from the cold?
- What happens if we “knock out” some of those proteins?
- What characteristics of tree defense and nutrition make some host trees more or less likely to allow the resident larvae to survive a winter?
- Do adult beetle parents choose trees based in any way on how their young may fare?
- Where in the genome should we expect to see natural selection as the insects move into colder and more inhospitable regions? How will these evolutionary shifts be observed in changes in behavior and physiology?
- What are the larger implications of climate change on these processes?
As you can see – and as is the case with science in general – this paper not only provides some answers, but also provides fertile ground for more questions. This work, and other related work in our larger mountain pine beetle system genomics project, has given us the means to chase down some of the answers. We are looking forward to the interesting work ahead. Since this publication and its associated data are all open access, we also look forward to seeing what other people might find to do with our data.
Tiffany R. Bonnett, Jeanne A. Robert, Caitlin Pitt, Jordie D. Fraser, Christopher I. Keeling, Jörg Bohlmann, Dezene P.W. Huber (2012). Global and comparative proteomic profiling of overwintering and developing mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae), larvae Insect Biochemistry and Molecular Biology DOI: 10.1016/j.ibmb.2012.08.003