14 Hot and Cool Bugs: Energetics and Thermal Tolerances of

AAAS, Pacific Division
2015 San Francisco Meeting
Symposium Abstracts
14 Hot and Cool Bugs: Energetics and Thermal Tolerances of Insects in an Ecological Context
Cold Adaptation Remodels Energetic Pathways in Drosophila melanogaster, CAROLINE M. WILLIAMS1*, NISHANTH
E. SUNNY2, ARTHUR S. EDISON3, MARSHALL D. MCCUE5, THEODORE J. MORGAN6, and DANIEL A.
HAHN4 (1Department of Integrative Biology, University of California, Berkeley, CA USA 94705; 2Department of Medicine,
3
Department of Biochemistry and Molecular Biology, and 4Department of Entomology and Nematology, University of
Florida, Gainesville FL USA 32601; 5Department of Biological Sciences, St Mary’s University, San Antonio TX USA
78228; 6Division of Biology, Kansas State University, Manhattan KS USA 66505; cmw@berkeley.edu).
Ectotherms must maintain energy homeostasis in rapidly changing thermal conditions; a considerable challenge given
that metabolism comprises a complex suite of processes that differ in thermal sensitivities. Cold-adaptation on a broad
phylogenetic scale involves changes to enzymes and pathways that allow them to function more effectively at low
temperatures, but we lack an understanding of the microevolutionary variation in energy metabolism segregating within
populations that may contribute to cold-stress tolerance. Without an integrative understanding of this naturally segregating
variation, from the genomic through the physiological to the organismal levels, we cannot predict the evolvability of coldstress hardiness, an important component of predicting impacts of global climate change. We hypothesize that susceptibility
to cold-stress is set by an imbalance between energy supply and demand incurred at low temperatures, and that hardiness may
be conferred by reorganizing metabolic networks to maintain energy balance more effectively at low temperatures. Using
complementary resources of the Drosophila melanogaster Genetic Reference Panel, and Drosophila lines selected in the
laboratory for fast or slow recovery from a cold-induced coma, we show that cold-hardy flies have higher metabolic rates,
maintain metabolic homeostasis more effectively during cold exposure, and show considerable restructuring of metabolic
networks. Using stable isotopes, we demonstrate higher rates of substrate oxidation in hardy flies and great plasticity in
metabolic flux in response to cold exposure. These alterations to nutrient flux may rebalance energy supply and demand in
the cold, and assist in maintaining energy balance in the face of changing temperatures.
Mitonuclear Interactions Influence Cold and Heat Tolerance Along Elevation Gradients in a Montane Insect, NATHAN E.
RANK1,3* and ELIZABETH P. DAHLHOFF2,3 (1Department of Biology, Sonoma State University, Rohnert Park
California 94928 USA; 2Department of Biology, Santa Clara University, Santa Clara, California 95053 USA, 3White
Mountain Research Center, Bishop CA 93514; rank@sonoma.edu).
Montane insects often live in small fragmented populations are expected to change dramatically with climate change.
The ability of populations to persist in montane habitats depends partly on whether they possess genetic variation in their
physiological capacity to respond and adapt to changes in climate. In the Eastern Sierra Nevada mountains of California, the
willow leaf beetle Chrysomela aeneicollis occurs at high elevations just below tree line. Genetic marker loci in the
mitochondrial and nuclear genome show significant variability among drainages along a 75 km transect from Taboose Pass in
the south to Rock Creek in the north. Geographic variation along this transect is much greater for the allozyme locus
phosphoglucose isomerase (PGI) than for other nuclear marker loci. In prior studies, we have described functional,
physiological, and reproductive differences among PGI genotypes that correspond to their changes in frequency over a
latitudinal temperature transect. Here, we show that mitochondrial genotype mediates response to cold and heat. Performance
and fitness characters at high elevation show that the best combination of mitonuclear genotypes corresponds to the natural
distribution of PGI and COII alleles (northern mitochondrial genotypes perform best when paired with northern PGI
genotypes). In contrast, at lower elevations performance is best when a mismatch is present between mitochondrial and
nuclear genotypes (northern mitochondrial genotype performs best in combination with the southern PGI genotype). Natural
selection may thus act jointly on COII and PGI and this genetic variability may contribute to population persistence in the
face of anticipated rapid environmental change.
Adaptation to Thermal Stress in a Finnish Butterfly Threatened by Climate Change, ELIZABETH P. DAHLHOFF1,2*,
SUVI IKONEN2, VICTORIA C. DAHLHOFF2, and I. HANSKI2 (1Department of Biology, Santa Clara University, Santa
Clara, California 95053 USA; 2Metapopulation Research Group, Department of Biological Sciences, University of Helsinki,
Finland; edahlhoff@scu.edu).
A consequence of climate change is that organisms living at high latitude may experience increased exposure to thermal
extremes due to warmer summers and drier winters. These environmental factors may adversely affect population persistence
due to alteration of individual performance or effects of stress on reproductive success. In a metapopulation of the Glanville
fritillary butterfly Melitaea cinxia found on the Åland Islands of Finland, extirpation and re-colonization is linked to
differences in dispersal abilities, which vary as a function of genetic variation at the glycolytic enzyme locus phosphoglocuse
isomerase (Pgi). Generally, Pgi heterozygotes have the most robust performance and fitness characters. Here we found that
thermal tolerance, flight performance, metabolic rate and mating success were highest for Pgi heterozygotes after mild heat
stress, consistent with earlier findings; however, after exposure to more extreme thermal stress, individuals homozygous for a
Pgi allele common in Central Europe, but rare in Finland, fared best. Male fitness (mating duration, spermatophore size,
fecundity of mating partner) was highest for males homozygous for this rare, southern Pgi allele after exposure to extreme
thermal stress, suggesting that natural selection might cause an increase in frequency of the southern allele as climate change
proceeds. However, there is also evidence that low frequency of the southern Pgi allele in isolated Finnish populations is due
to linkage to lethal genes not present elsewhere. Thus, allele frequency shifts in response to climate change may be limited,
leaving Finnish populations of the Glanville fritillary especially vulnerable to a warmer, drier climate.
Variation in Transcriptomic Signatures of Thermal Acclimation in Four Key Aquatic Insects in California Riverine Food
Webs, JONATHON H. STILLMAN1,2,3*, SCOTT A. FAY1,2, ALEXANDER R. GUNDERSON1,2,3, and MARY E.
POWER1,2 (1Department of Integrative Biology, University of California, Berkeley, CA USA 94705; 2Berkeley Initiative in
Global Change Biology, University of California, Berkeley, CA, USA; 3Romberg Tiburon Center and Department of
Biology, San Francisco State University, San Francisco CA 94920; jstillman@berkeley.edu).
Predicting changes in trophic ecology of riverine systems in the face of future climate warming requires an
understanding of the thermal performance of aquatic insect larvae. Larvae that differ in key trophic traits (i.e., grazer vs.
predator) may also differ in the efficiency with which they use energy under various thermal regimes. Metabolic energy
efficiency is maximized at optimal temperatures and declines at higher temperatures due to an increase in fermentative
metabolism and an induction of stress responses to cope with thermal or oxidative damage. We compared thermal effects on
gene expression of four aquatic insects (Pteronarcys californica, Calinueria californica, Hesperoperla pacifica and
Dicosmoecus gilvipes) across temperatures of the spring-summer range during late larval instar phase (~12-30°C). Illumina
RNA-seq, de novo transcriptome assembly using Trinity, and analysis using Bowtie2/eXpress and EdgeR were used to
identify differentially expressed genes within each species. Heat sensitive predator taxa exhibited a reduced induction of
cellular stress response genes. Biomarkers of thermal acclimation from one species, D. gilvipes, were used to compare across
a fine-scale range of temperatures, and across populations from sites with differing thermal characteristics (Mendocino,
Sierra Nevada, Big Sur). These data allow a fine-scale analysis of local physiological adaptation to temperature and shifts in
metabolic ecology of streams across much of the central to northern California landscape.
Thermal Ecology of the Migratory Mayfly, Ephemrella maculata, and the Associated Trophic Subsidy to Cold-Adapted
Predators in Rivers, HIROMI UNO (Department of Integrative Biology, University of California, Berkeley CA USA 94720;
hiromiuno1@berkeley.edu).
In spatially and temporally heterogeneous thermal environments, animals can physically move to more preferable
temperature or seasonally shift their life cycles, and that can have significant effect on species interactions. Water
temperatures in rivers are both spatially and temporally heterogeneous. Headwater tributaries are shaded and cool, and wide
sunny mainstems are warmer in summer, while the temperature is colder and more homogeneous in winter. A univoltine
mayfly, Ephemerella maculata in California migrates between mainstems and tributaries of rivers seasonally through their
life cycles. To understand the relationship of their unusual migratory life cycles and the temperature variation, thermal
performance of E.maculata at various life stages, embryos, 1st instar nymphs, and final instar nymphs, and effects on the
water temperature on the life cycle timings were investigated. The thermal performance of E.maculata varied across life
stages, corresponding to the temperature at their habitat and the season of their occurrence. As E.maculata nymphs grow and
emerge from the mainstem rivers, then female adults migrate to cool tributaries, oviposit and die, they transport resource
from warm productive mainstem to cool unproductive tributaries of rivers. With large scale field experiments, I have shown
that because the adults aggregate from large mainstem to small tributaries, the resource subsidy to the tributaries during their
flight season in summer can triple the growth of juvenile steelhead trout rearing in cool but food limited tributaries. The
migration of E.maculata in the river network may buffer the decline of stenothermic predators in warming networks.