Facing lethal temperatures: heat shock response in desert and temperate ants

Several genera of desert ants have adapted to endure prolonged exposure to high temperatures. The study of these ants is essential to unravel how species respond and adapt to thermal stress. We investigated the thermal tolerance and the transcriptomic heat stress response of three desert ant genera (Cataglyphis, Melophorus and Ocymyrmex) and two temperate genera (Formica and Myrmica) to explore convergent and specific adaptations. We found a variable transcriptomic response among desert species exposed to similar levels of physiological heat-stress: Cataglyphis holgerseni and Melophorus bagoti differentially regulated very few transcripts, 0.12% (54/44,525) and 0.14% (53/38,726) respectively, while Cataglyphis bombycina and Ocymyrmex robustior showed greater expression alterations affecting 0.6% (253/41,912) and 1.53% (698/45,701) of their transcriptomes, respectively. These two responsive mechanisms - reactive and constitutive - were related to desert species thermal tolerance survival pattern and convergently evolved in distinct desert ant genera. By comparison, the two temperate species differentially expressed thousands of transcripts more than desert ants in response to heat stress (affecting 8% and 12,71% of F. fusca and Myr. sabuleti transcriptomes), suggesting that keeping restrained gene expression is an important adaptation in heat adapted species. Finally, we found a significant overlap of the molecular pathways activated in response to heat-stress in temperate and desert species, and our data revealed that larger gene expression responses also affected a greater number of taxonomically restricted genes. These results suggest that the molecular processes involved in heat-stress response are mostly evolutionary conserved in ants, but new genes may also play a role.