Spatial analysis of gene regulation reveals new insights into the molecular basis of upper thermal limits

Abstract

The cellular stress response has long been the primary model for studying the molecular basis of thermal adaptation, yet the link between gene expression, RNA metabolism and physiological responses to thermal stress remains largely unexplored. We address this by comparing the transcriptional and physiological responses of three geographically distinct populations of Drosophila melanogaster from eastern Australia in response to, and recovery from, a severe heat stress with and without a prestress hardening treatment. We focus on starvin (stv), recently identified as an important thermally responsive gene. Intriguingly, stv encodes seven transcripts from alternative transcription sites and alternative splicing, yet appears to be rapidly heat inducible. First, we show genetic differences in upper thermal limits of the populations tested. We then demonstrate that the stv locus does not ubiquitously respond to thermal stress but is expressed as three distinct thermal and temporal RNA phenotypes (isoforms). The shorter transcript isoforms are rapidly upregulated under stress in all populations and show similar molecular signatures to heat-shock proteins. Multiple stress exposures seem to generate a reserve of pre-mRNAs, effectively 'priming' the cells for subsequent stress. Remarkably, we demonstrate a bypass in the splicing blockade in these isoforms, suggesting an essential role for these transcripts under heat stress. Temporal profiles for the weakly heat responsive stv isoform subset show opposing patterns in the two most divergent populations. Innate and induced transcriptome responses to hyperthermia are complex, and warrant moving beyond gene-level analyses.

Adaptation to heat stress reduces phenotypic and transcriptional plasticity in a marine copepod

“…Recent work in a variety of taxa provides evidence for substantial heritable variation in patterns of gene expression (Telonis‐Scott et al . ; Leder et al . ), so it is not surprising that transcriptional plasticity can respond to selection.…”

“…Observed reductions in the phenotypic plasticity of heat tolerance in selected lines were mirrored by reductions in transcriptional plasticity. Recent work in a variety of taxa provides evidence for substantial heritable variation in patterns of gene expression (Telonis-Scott et al 2014;Leder et al 2015), so it is not surprising that transcriptional plasticity can respond to selection. Recent work in Arabidopsis has also demonstrated that genes with genetically variable expression responses to environmental stress play an important role in adaptation to local environments (Lasky et al 2014), providing an important link between the evolution of transcriptional plasticity and adaptation to abiotic stress.…”

Evidence for lower plasticity in CTMAX at warmer developmental temperatures

“…It is possible that the temperature chosen to harden the flies (37 °C) may influence our capacity to detect significant hardening responses. 37 °C was chosen as this temperature induces a significant hardening response in D. melanogaster (Telonis‐Scott et al ., ), although 34 and 35 °C did not induce a hardening response in the more thermally sensitive D. birchii and D. sulfurigaster (Mitchell et al ., ). Although 37 °C is higher than temperatures commonly experienced in tropical environments (frequently observed maximum temperature ~35 °C, http://www.bom.gov.au), this is not outside the realm of future climate change projections which predict an increase in temperature in the Australian tropics of 1–2 °C by 2030, under an intermediate emissions scenario (RCP4.5) (http://www.climatechangeinaustralia.gov.au).…”

“…Shorter exposures (minutes-hours) at more stressful temperatures are often referred to as hardening. Hardening responses in heat resistance have been extensively studied in Drosophila and are linked to the transient upregulation of heat-shock proteins (Telonis-Scott et al, 2014;Willot et al, 2017). In ectothermic species, temperature acclimation (both developmental and adult) and adult hardening treatments increase heat resistance with increasing temperature (Chown, 2001;Hoffmann et al, 2003).…”

Constraints, independence, and evolution of thermal plasticity: Probing genetic architecture of long- and short-term thermal acclimation

“…Long-term acclimation may provide the additional time needed to enhance both the physiological and transcriptional milieu further for a larger-magnitude acclimation response than seen with short-term acclimation. Splicing has been shown to be up-regulated in response to artificial selection on cold-tolerance traits (50), and alternatively spliced isoforms of a thermally responsive gene (stv) have been associated with a protective response after multiple heat exposures (51).…”