Strain- and temperature-dependent outcomes in the environmentally acquired bug-Caballeronia symbiosis

Leaf footed bugs (Leptoglossus, Coreidae) acquire their bacterial Caballeronia symbionts from the environment – usually from local soil. Despite large contributions to insect fitness, nymphs must re-acquire Caballeronia every generation. Acquiring symbionts from the environment presents a unique set of potential challenges, including acquisition of a sub-optimal strain or no partner at all. However, it might benefit hosts by enabling acquisition of a locally-adapted partner. To understand how environmental symbiont acquisition may influence insect fitness, we (1) characterized spatial turnover in insect- and soil-associated Caballeronia in the field, (2) tested whether different strains of Caballeronia confer different benefits to Leptoglossus, and (3) tested whether the benefits conferred by different strains vary with temperature.

We show that the location where a bug develops influences which symbiont strain it acquires, consistent with a process of passive spatial turnover in hosted symbiont strains. This matters because we find that strains can vary in quality, with some consistently promoting higher host fitness than others across two species of Leptoglossus. Furthermore, we find that some strains are more beneficial at cooler temperatures, while others are more beneficial at warm temperatures. To our knowledge, this represents the first demonstration of temperature dependence in the outcomes of an environmentally-acquired insect symbiosis. In sum, our results suggest that environmental symbiont acquisition could buffer host populations from environmental stressors in ways inaccessible to hosts with vertically acquired symbionts.

We show that the location where a bug develops influences which symbiont strain it acquires, consistent with a process of passive spatial turnover in hosted symbiont strains. This matters because we find that strains can vary in quality, with some consistently promoting higher host fitness than others across two species of Leptoglossus. Furthermore, we find that some strains are more beneficial at cooler temperatures, while others are more beneficial at warm temperatures. To our knowledge, this represents the first demonstration of temperature dependence in the outcomes of an environmentally-acquired insect symbiosis. In sum, our results suggest that environmental symbiont acquisition could buffer host populations from environmental stressors in ways inaccessible to hosts with vertically acquired symbionts.