Symposium 5: Animal-Associated Fungi -- From Parasitism to Mutualism

Entomopathogenic fungi routinely kill their hosts before releasing infectious spores, but select species keep insects alive while sporulating, which enhances dispersal. Transcriptomics and metabolomics studies of entomopathogens with post-mortem dissemination from their parasitized hosts have unraveled infection processes and host responses, yet mechanisms underlying active spore transmission by Entomophthoralean fungi in living insects remain elusive. Here we report the discovery, through metabolomics, of the plant-associated amphetamine, cathinone, in four Massospora cicadina-infected periodical cicada populations, and the mushroom-associated tryptamine, psilocybin, in annual cicadas infected with Massospora platypediae or Massospora levispora, which appear to represent a single fungal species. The absence of some fungal enzymes necessary for cathinone and psilocybin biosynthesis along with the inability to detect intermediate metabolites or gene orthologs are consistent with possibly novel biosynthesis pathways in Massospora. The neurogenic activities of these compounds suggest the extended phenotype of Massospora that modifies cicada behavior to maximize dissemination is chemically-induced.

Certain fungal entomopathogens have evolved to adaptively manipulate their host’s behavior as a strategy to increase transmission. One enigmatic example are the fungi within the complex Ophiocordyceps unilateralis that infect and manipulate ants of the tribe Camponotini. Infected ants are made to leave the nest and latch on with their mandibles at elevated positions to facilitate spore dispersal. We use an integrative approach to unravel how these fungi are able to establish the altered ant behaviors we observe. Studies that range from -omics approaches and forward genetics, to behavioral analyses and ecological surveys are moving us closer to understanding how Ophiocordyceps fungi alter the behavior of their ant hosts. We ask is what effectors these fungi secrete to manipulate ant behavior and how these effectors impact the ant pathways involved. We address this question with controlled infection experiments followed by behavioral analyses and comparative mixed transcriptomics. This work is resulting in candidate manipulation compounds that we aim to further characterize using forward genetics. We also ask how seasonal and daily fluctuations in light, humidity and temperature contribute to the disease dynamics of Ophiocordyceps-ant systems. Field studies demonstrate that manipulated biting behavior is centered around solar noon and suggest that illumination levels form an important factor for ant cadaver placement and fruiting body development. Moreover, transcriptomics data show daily fluctuations in Ophiocordyceps gene expression in cultures entrained by light. Combined, these approaches will reveal ultimate and proximate drivers involved in fungal manipulation of ant behavior.

Adult ambrosia beetles carry co-adapted fungal cultivars (ambrosia fungi) in storage organs called mycangia, which occur in various body parts and vary greatly in complexity. In general, ambrosia fungus clades in the Ceratocystidaceae (Microascales) radiated after the evolutionary origin of their associated mycangia, and these mycangium-cultivar combinations have remained stable through evolutionary time. However, the symbioses of the ambrosia beetle tribe Xyloterini were understudied.  We collected representatives of the three Xyloterini genera and characterized their mycangia and mycangial symbionts. As reported earlier, Trypodendron consistently yielded ambrosia fungi in the genus Phialophoropsis, including three new putative species, though some fungal species were shared among beetle species. Unexpectedly, mycangia of the previously-unstudied Indocryphalus pubipennis are smaller and differently-shaped than Trypodendron mycangia, and they carry a different genus of Ceratocystidaceae: Toshionella, which are the ambrosia fungi of Asian Scolytoplatypus (Scolytoplatypodini), many of which are sympatric with I. pubipennis. Xyloterinus politus was known to have two different types of mycangia with uncharacterized symbionts. A new putative species of the ubiquitous ambrosial genus Raffaelea (Ophiostomatales) was found in the paired oral mycangia of both sexes, and a new putative genus and species with affinity to the Ophiostomatales was found in the reduced prothoracic mycangia of females. These findings suggest that de novo symbioses and ancient horizontal transfers were associated with morphological changes in mycangia in the Xyloterini. This further supports the theory that developments of novel mycangium types are critical events in the evolution of ambrosia beetles and their co-adapted fungal mutualists.