Molecular adaptation to low-temperature in New Zealand stick insects

Abstract

Variation in environmental temperature affects numerous biochemical processes in poikilotherms, which in turn influences their biogeography. The physiological response to low-temperature has received much attention, but extending genomic-based studies beyond Diptera is needed to fully comprehend the molecular mechanisms underpinning this adaptation. The unusual incidence of alpine sticks insects within New Zealand suggests these species have suitable cold-tolerant phenotypes and genotypes warranting investigation. The objectives of this thesis were to: (i) verify the application of RNA-Seq to identify mild cold-shock responsive genes in an alpine stick insect (Micrarchus nov. sp. 2); (ii) further exploit RNA-Seq to assess intra- and inter-specific variation in this response among alpine and lowland (Micrarchus hystriculeus) species; (iii) assess the extent that energy pathways evolve aiding adaptation to low-temperature and other metabolically stressful life-history traits in Australasian stick insects. Alpine Micrarchus regularly experience sub-zero temperature in the wild and qPCRverified 454-based RNA-Seq identified three novel cold-responsive loci, emphasising the varied response to low-temperature across insects. Subsequent qPCR-verified Illumina-based RNA-Seq showed divergent interspecific transcriptional responses to cold-shock between the alpine and lowland species. Background genetic variation caused and maintained through a combination of reduced gene flow, genetic drift and local adaptation is partly responsible for the extensive intraspecific and reduced within-population, variation in expression response. Local adaptation in the nuclear genome is preserved despite a complex pattern of unidirectional mitochondrial introgression between species. The Lanceocercata stick insect radiation includes transitions from tropical to temperate climates, lowland to alpine habitats and winged to wingless forms. This diversity in lifestyle is predicted to afford equally variable metabolic demands. In response, a signature of positive selection was detected in five core-glycolysis genes, with Likelihood methods identifying two (phosphoglucose isomerase and glyceraldehyde 3-phosphate dehydrogenase) that encode branch-point enzymes linking glycolysis to the pentose phosphate pathway as under positive selection. These results indicate adaptation to stressful lifestyles requires a balance between maintaining cellular energy production while compensating for stressinduced oxidative damage. These studies illustrate that adaptation to low-temperature, and other stressful lifestyles, is a complex interaction between DNA sequence evolution and gene expression changes, which can evolve differentially in isolated species and populations.