Subterranean habitats are environmentally stable with respect to temperature, humidity, and the absence of light. The transition to a subterranean lifestyle might therefore be expected to cause considerable shifts in an organism's physiology; here, we investigate how subterranean colonisation affects thermal tolerance. Subterranean organisms might be at an increased risk of decline in the face of global temperature rises, but robust data on the fauna is lacking, particularly at the molecular level. In this study we compare the heat shock response of two species of diving beetle in the genus Paroster: one surface-dwelling (P. nigroadumbratus), the other restricted to a single aquifer (P. macrosturtensis). P. macrosturtensis has been previously established as having a lower thermal tolerance compared to surface-dwelling relatives, but the genomic basis of this difference is unknown. By sequencing transcriptomes of experimentally heat-shocked individuals and performing differential expression analysis, we demonstrate both species can mount a heat shock response at high temperatures (35C), in agreement with past survival experiments. However, the genes involved in these responses differ between species, and far greater genes are differentially expressed in the surface species, which may explain its more robust response to heat stress. In contrast, the subterranean species significantly upregulated the heat shock protein gene Hsp68 in the experimental setup under conditions it would likely encounter in nature (25C), suggesting it may be more sensitive to ambient stressors, e.g. handling. The results presented here contribute to an emerging narrative concerning weakened thermal tolerances in obligate subterranean organisms at the molecular level.
Stressed out underground?: Transcriptomics illuminates the genome-wide response to heat stress in surface and subterranean diving beetles
