Viral- and fungal-mediated behavioral manipulation of hosts: summit disease

Summit disease, in which infected hosts seek heights (gravitropism), first noted in modern times by nineteenth-century naturalists, has been shown to be induced by disparate pathogens ranging from viruses to fungi. Infection results in dramatic changes in normal activity patterns, and such parasite manipulation of host behaviors suggests a strong selection for convergent outcomes albeit evolved via widely divergent mechanisms. The two best-studied examples involve a subset of viral and fungal pathogens of insects that induce "summiting" in infected hosts. Summiting presumably functions as a means for increasing the dispersal of the pathogen, thus significantly increasing fitness. Here, we review current advances in our understanding of viral- and fungal-induced summit disease and the host behavioral manipulation involved. Viral genes implicated in this process include a host hormone-targeting ecdysteroid UDP-glucosyltransferase (apparently essential for mediating summit disease induced by some viruses but not all) and a protein tyrosine phosphatase, with light dependance implicated. For summit disease-causing fungi, though much remains obscure, targeting of molting, circadian rhythms, sleep, and responses to light patterns appear involved. Targeting of host neuronal pathways by summit-inducing fungi also appears to involve the production of effector molecules and secondary metabolites that affect host muscular, immune, and/or neurological processes. It is hypothesized that host brain structures, particularly Mushroom Bodies (no relation to the fungus itself), important for olfactory association learning and control of locomotor activity, are critical targets for mediating summiting during infection. This phenomenon expands the diversity of microbial pathogen-interactions and host dynamics. KEY POINTS: • Summit disease or height seeking (gravitropism) results from viral and fungal pathogens manipulating insect host behaviors presumably to increase pathogen dispersal. • Insect baculoviruses and select fungal pathogens exhibit convergent evolution in host behavioral manipulation but use disparate molecular mechanisms. • Targets for affecting host behavior include manipulation of host hormones, feeding, locomotion, and immune, circadian, and neurological pathways.