Temperature is known to affect metabolism and interspecific interactions within hosts. Beneficial bacterial symbionts associated with the gastrointestinal tract often provide resistance to parasitic infections. However, the effect to which temperature alters this symbiont-associated resistance has seldom been studied. Honeybees present a useful system to study these effects by acting as facultative endotherms. Although bees can maintain hive temperatures similar to those of mammals, their symbionts are often subject to seasonal and developmental changes in host temperature which have the potential to alter interspecific interactions with parasites. We investigated the role of temperature on the antiparasitic effects of Lactobacillus gut symbionts from honeybees on two trypanosomatid gut parasites, Crithidia mellificae and Lotmaria passim. We utilized cell cultures and germ-free bees to demonstrate the potential for temperature to shape parasite-symbiont interactions. We found that bacterial symbionts possess greater heat tolerance than parasites and chemically inhibited parasite growth via acid production. Symbiont growth and acid production were accelerated at high temperatures that are typical of those found in honeybee colonies. Symbionts demonstrated progressively stronger antiparasitic effects across the same range, reducing both parasites' peak growth and heat tolerance. In addition, we are investigating pathogen loads of co-inoculated bees across temperatures spanning those characteristically observed in honeybee colonies. Our results suggest that the endothermic behavior of honeybees could potentiate the antiparasitic action of bacterial gut symbionts by limiting parasite fitness at high temperatures. These results implicate thermoregulation as both an integral reinforcer of core symbioses and gut symbiont-mediated antiparasitic defenses.
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