Loss and resiliency of social amoeba symbiosis under simulated warming.

Anthropogenic global change is increasingly raising concerns about collapses of symbiotic interactions worldwide. Therefore, understanding how climate change affects symbioses remains a challenge and demands more study. Here, we look at how simulated warming affects the social amebaDictyostelium discoideumand its relationship with its facultative bacterial symbionts,Paraburkholderia hayleyellaandParaburkholderia agricolaris. We cured and cross-infected ameba hosts with different symbionts. We found that warming significantly decreasedD. discoideum'sfitness, and we found no sign of local adaptation in two wild populations. Experimental warming had complex effects on these symbioses with responses determined by both symbiont and host. Neither of these facultative symbionts increases its hosts' thermal tolerance. The nearly obligate symbiont with a reduced genome,P. hayleyella, actually decreasesD. discoideum'sthermal tolerance and even causes symbiosis breakdown. Our study shows how facultative symbioses may have complex responses to global change.