UC Berkeley

Climate change is one of the top causes of biodiversity loss. Organisms will experience many pressures associated with climate change, one of the most obvious being increased temperature. It is therefore important to understand how animals will react to this stress. Ectotherms, such as ants, are especially sensitive to the climate as they rely on environmental temperature for everything from optimal foraging to development time. In this dissertation, I explore the individual and population level reactions to thermal stress of a cold-specialist, the winter ant, Prenolepis imparis. I also identify the role past climatic fluctuations have had in shaping this species’ current distribution.

In my first dissertation chapter, I conducted a RNA-seq analysis to identify stress-induced genes in P. imparis individuals at the transcriptome level. To identify candidate genes involved in the stress response, I induced stress by placing the ants at a low or high temperature. Then, I sequenced the transcriptome of these stressed individuals. The genes that show an increase during transcription are candidates for allowing individuals to recover from the stress. I identified a total of 709 differentially expressed genes. In the cold-stressed ants, I did not identify a strong response, indicating that the temperature we chose for trials was not cold enough. Conversely, I found a strong response to the heat. Those transcripts we found highly induced include protein folding genes, heat shock proteins, proteins associated with heat shock proteins, Ca2+ ion transport, and a few unknown genes. I also found functional categories relating to protein folding, muscles, and temperature stimulus increased in the heat-stress response.

In my second dissertation chapter, I measured the short-term acclimation ability of high- and low-elevation populations of P. imparis across California. In addition, I also characterized the thermal environment both above and below ground. I found that the high-elevation sites showed increased tolerance and reduced capacity in acclimation ability relative to the low-elevation counterparts at their lower limits, suggesting an evolutionary trade-off between tolerance and acclimation ability. In addition, I found less acclimation capacity across all populations in their upper limits. I also found that the high-elevation sites experience cooler temperatures both above and below ground. The greater acclimation response at lower limits in high-elevation populations could suggest that they are better physiologically prepared to survive cooler temperatures.

In my final dissertation chapter, I used phylogenetic and population genetic analyses to identify population genetic structure and historical demographic patterns across the range of P. imparis. I relate the genomic patterns to those expected as seen with in situ diversification, or maintained connectivity. I recovered five well-supported genetically isolated clades across the distribution. I also investigated gene flow between these major genetic clades and did not find evidence of gene flow between clades. High support for five major geographic lineages and lack of evidence of contemporary gene flow indicate in situ diversification across the species’ range, probably influenced by glacial cycles of the late Quaternary.

Overall, the results from this dissertation give insight into plasticity as well as the evolutionary processes that have shaped this species. My results suggest molecular pathways by which phenotypic plasticity will allow individuals to overcome heat-stress: the candidate genes provided here are a valuable resource in understanding pathways and proteins necessary for survival at unfavorable temperatures. I also report that individuals from different populations show different levels of thermal tolerance and plasticity. All the populations show less tolerance and reduced plasticity to the heat. This is troubling in the face of climate change, this limited acclimation response at the upper thermal limits suggests evolutionary constraints in heat tolerance, so major changes at the molecular level will be needed for these populations to persist in warmer environments. Finally, the entire range of this species has been profoundly affected by climatic fluctuations during the Quaternary. These fluctuations led those individuals to have separate evolutionary histories and raises the possibility that there are several unique species of P. imparis.