Symbiont identity impacts host fitness under thermal stress in the bug-Caballeronia system

Microbial symbioses are essential for many animals, but these interactions can shift from mutualistic to parasite across life stages or environments. Analyzing how environmental contexts alter these interactions can strengthen the understanding of how ecological associations are shaped. The effect of high temperatures on symbioses has been of great interest as even brief exposure can disrupt the associations, turning partners into liabilities. In corals, environmentally acquired symbionts with higher thermal optima can better withstand increased temperatures, therefore maintaining the symbiosis during periods of thermal stress. Therefore, we investigated this phenomenon in a terrestrial model system (leaf-footed bugs and their symbiotic bacteria) to identify if environmentally acquired symbionts with higher thermal optima are better able to maintain the symbiosis under prolonged thermal stress. Hosts were provided with one of six phylogenetically distinct symbiont strains with varying thermal tolerances and reared at different temperatures (24 – 40°C). After development to adult, host fitness traits were measured (e.g. mass, development time, and mortality rate) as well as symbiont titer. Prolonged exposure to high temperatures caused hosts possessing a symbiont with lower thermal optima to experience a decline in mass and survival, while development time increased. No differences were observed at lower temperatures. These results suggest that environmental symbiont acquisition might buffer host populations from thermal stress in a way inaccessible to hosts with other modes of symbiont acquisition.