Physiological and genetic basis of responses to winter in the Sierra Willow Leaf beetle (2014 – present)
Sierra Nevada populations of the willow leaf beetle Chrysomela aeneicollis overwinter beneath snow in the soil as dormant adults for most of their lives (8 out of 12 months). Snow cover is highly variable in the Eastern Sierra Nevada mountain ranges these beetles call home, and recent studies have suggested that snow cover and temperature strongly influence population dynamics. The thermal environment shapes genetic diversity of C. aeneicollis populations, which have stable latitudinal allele frequency clines at phosphoglucose isomerase (PGI) and at (mitochondrial) cytochrome oxidase II (COII) (Dahlhoff et al. 2008, Dahlhoff & Rank, 2000, Rank, 1992, Rank & Dahlhoff, 2002). Allele frequencies at PGI fluctuate both within and among years in response to changes in temperature (Dahlhoff et al. 2008, Rank & Dahlhoff, 2002). PGI variation is associated with growing season fitness components such as metabolic rate, growth, fecundity, and mating success (Dahlhoff et al. 2008, Dick et al. 2013, McMillan et al. 2005), and it also relates to cold hardiness (Dahlhoff & Rank, 2007, Rank et al. 2007). Recent findings suggest that mito-nuclear interactions strongly influence fitness components (survival and mating success) after exposure to stressful cold or heat (Rank et al. 2013, Sayre et al. 2014). These interactions suggest that northern mitochondrial haplotypes generally perform best when they co-occur with northern PGI genotypes (Rank et al. 2013, Zavala et al. 2013). We aim to address a major gap in our understanding by investigating the physiological basis of overwintering success, ultimately identifying candidate genes that underlie adaptation to winter. Kevin Roberts and Lisa Treidel are leading this project.
As a first step, Andre Szejner used a microCT scanner, in collaboration with Kendra Greenlee and Bryan Helm at U North Dakota, to scan the tracheal system of beetles from low and high altitude to see if altitude affects the respiratory system. Here is one of his beautiful images. Data to come!
Life history evolution in Gryllus crickets (2014 – present)
In collaboration with Anthony Zera
Crickets have a life history polymorphism whereby morphs are specialized for flight or reproduction; activities with highly divergent energy demands that operate on different timescales. This polymorphism provides an ideal opportunity assess the extent to which metabolic plasticity has diverged between morphs during life history evolution. The first focus in this project is to investigate how mitochondrial function has been altered by natural selection to meet the different energy demands of the two morphs. Lisa Treidel and Rebecca Clark are leading this project.
Biochemical and physiological architecture of cold hardiness in Drosophila melanogaster (postdoctoral research, University of Florida, 2012- 2014)
In collaboration with Daniel Hahn (advisor), Ted Morgan, Art Edison, and David Allison (PIs, NSF grant 1051890), as well as Arezue Boroujerdi, Maria-Brigida Ferraro, Mario Guarracino, Brittany Lee, Marshall McCue, James Rocca, Nishanth Sunny, Andre Szejner (undergraduate researcher), Glenn Walter, and Miki Watanabe.
Bioenergetics, or energy flow within organisms, interacts with temperature to constrain species distributions and influence global patterns in species abundance. Normal physiological function requires organisms to precisely and dynamically balance energy supply and demand, but fluctuating temperatures make this difficult by changing rates of biochemical reactions within cells. Species distributions can thus be driven not only directly by exposure to lethal temperatures, but also indirectly through repeated or long-term exposures to sub-lethal temperatures that lead to energetic failures.
At low temperatures, insects enter into a cold-induced coma (chill-coma), during which mating and foraging is precluded and predators cannot be avoided. The rate of recovery from chill coma is a commonly used metric of cold tolerance and corresponds to the environment that the insect inhabits (insects from colder locations tend to recover more quickly. This suggests an adaptive response, but the mechanisms underlying variation in chill-coma recovery times are largely unknown. Differences in ionic regulation in the cold are clearly important at the proximate level (MacMillan and Sinclair 2011, Andersen et al. 2013, Findsen et al 2013), but the mechanisms underpinning evolutionary variation in ionic homeostasis are less clear. We hypothesized that evolutionary variation in chill-coma recovery times are driven by differences in the ability to maintain key catabolic processes in the cold, resulting from alterations to intermediary metabolism.
We addressed this hypothesis at the whole organism, pathway flux, and metabolite level using respirometry, growth rate assays, in vivo nuclear magnetic resonance spectroscopy, kinetic studies using stable isotope tracers, and metabolomics. We used a combination of experimental evolution lines and natural isolates with variation in chill coma recovery times (Drosophila genetic reference panel). We demonstrated that cold tolerant flies had higher whole-organism metabolic and growth rates, better maintenance of metabolic homeostasis and more robust metabolic networks in the cold (Williams et al. in press Evolution), and higher rates of glycolysis before and during cold exposure. This supports our hypothesis that remodelling of metabolic pathways underpins cold adaptation, leading to an improved ability to maintain metabolic homeostasis in the cold. Moreover, the metabolic changes were associated with changes in other important life-history traits (such as larval growth rate), suggesting that selection on recovery time from acute low temperature exposure can fundamentally reshape organismal life-histories. Papers resulting from this work will come out over the coming months, watch this space!
Complementary and parallel work in the Morgan lab studied the genetic architecture of cold hardiness, using transcriptomics, next-generation sequencing and mutant assays. Stay tuned for results of these exciting studies!
Overwintering energetics in Lepidoptera (PhD research, University of Western Ontario, 2007-2012)
Winter temperatures are changing both in mean (a generally increasing trend), and in thermal variability (Williams et al. in press Biological Reviews). Although the intuitive outcome of increases in mean temperature are a release from cold-induced stress and mortality, increased temperature also increases the rate of use of stored energy reserves. Many insects that overwinter in temperate environments do so in a state of dormancy, so energy reserves with which they start the winter must see them through to spring and also sometimes fuel reproduction and post-winter performance. Thus, any increase in temperature during the winter can exacerbate energetic stress and decrease performance in winter and subsequent seasons. Negative fitness consequences of increases in winter temperature had been demonstrated for a few insects but not in others (Irwin and Lee 2003, Williams et al. 2003, Mercader and Scriber 2008), so we did not know how widespread these negative effects might be across species, nor whether insects had mechanisms to compensate for energy drain resulting for winter warming. These gaps in knowledge seriously impeded our ability to predict whether negative effects of winter warming were likely to be widespread.
During my doctoral work, I employed a comparative approach to address two main questions: 1) what are the impacts of winter warming or changes in thermal variability on insects at the biochemical, physiological and ecological levels, and 2) can insects modify their metabolism to mitigate any negative effects, either through phenotypic plasticity or genetic adaptation. I found that moderate degrees of winter warming (4 degC) reduced energy reserves in a skipper butterfly that overwinters as a larva and has a restricted distribution, but did not reduce energy reserves in two species of swallowtail butterflies that overwintered as pupae and were widely distributed (Williams et al. 2012 Climate Research). The skipper butterfly, Erynnis propertius, lives along the west coast of North America along a strong thermal variability gradient (the center of its range in Oregon has high daily thermal variability, while the northern range edge populations on Vancouver Island experience little daily variability). Thermal variability can also increase energy use, due to the non-linear shapes of biological rate functions (go here for an excellent explanation of Jensen’s inequality). Therefore I hypothesized that thermally variable environments would be more energy-demanding, and that populations that had evolved in such environments would have been under strong selective pressure to dampen the thermal sensitivity of their metabolism. By rearing larvae in the lab from hatching under conditions designed to mimic Oregon and Vanvcouver Island, I discovered that larvae from Oregon had a suppressed thermal sensitivity of metabolic rate relative to those from Oregon (Williams et al. 2012 PLoS ONE). Interestingly, when the larvae from each population were raised at the opposite to their natal temperature, I found that Vancouver Island larvae could plastically suppress their thermal sensitivity to the same degree as the Oregon population, but that the Oregon population were canalised in their suppression (they did not become more thermally sensitive when raised under less variable conditions). I combined my empirical metabolic rate data with historical climate data to estimate the energetic consequences of the suppression of thermal sensitivity, and found that larvae would save ~20% of their energy use overwinter in a variable environment by suppressing their thermal sensitivity. This was the first evidence for thermal variability driving metabolic suppression, and suggested that changes in thermal variability over winter will be important selective agents.
Finally, I turned to a different system to study the cross-seasonal consequences of changes in the winter thermal environment. Using a lab-based simulated reciprocal transplant, I found that populations of a widespread thermal generalist moth were locally adapted to their winter environment such that the northern range edge population performed better under cooler, northern conditions, while a population from the center of the range performed better under warmer central conditions (Williams et al. in press Functional Ecology). This was mediated through alterations to energy storage that allowed each population to retain energy stores and size better in their natal environment. Both populations suppressed their metabolic rates in warm winter conditions; a pattern I have also observed in a swallowtail (Papilio zelicaon, unpublished data) and which thus appears to be widespread among the Lepidoptera.
Thus, my doctoral research uncovered a large amount of variation in the responses to Lepidopteran species to winter warming. Some species were unaffected, some suffered performance decreases, and for some species the responses were population-specific and depended on the thermal history. I uncovered several systems in which Lepidopteran insects modify their metabolism to compensate for winter warming – a phenomenon that was previously unknown. In the future, I want to delve further into the mechanisms by which metabolism can be modified, including the pathways and enzymes that are modulated and the genetic changes that occur to bring them about.