Heterogeneity in infection outcome: lessons from a bumblebee-trypanosome system

Interactions between insect hosts and their parasites are significant because their parasites can also be parasites of humans and of species that we utilize. Host-parasite interactions are complex, even in insects, and there can be heterogeneous outcomes in infection success, load, virulence and transmission, with consequences for the evolution of hosts and their parasites, and also for epidemiology. A comprehension that the triad of host, parasite and environment interact to dictate infection outcome is key for anyone interested in host-parasite research. Studies in model systems used to good effect to characterize insect immunity and infection rarely scrutinize such heterogeneity. Evolutionary ecology studies addressing natural variation offer a window on the causes and consequences of such heterogeneity. A system at the forefront in this area is that of bumblebees and their trypanosome parasite Crithidia. Placing results and interpretations in a broader context we synthesize the plethora of work on bumblebee immunity and parasite interactions. We describe and discuss the sources of heterogeneity that should also be considered in human-relevant insect-parasite systems, including genotypic variation in both parasites and hosts, the mediating role of the environment, and explore the emerging evidence for microbiota modulating defence against parasites.