Parasite networks may be key to host manipulation

It is well-established that some parasitic microorganisms manipulate the behavior of their hosts, but the mechanisms by which they accomplish this remain a mystery, especially given the parasites’ small size in relation to their hosts. One possible means for overcoming this size discrepancy is for individuals to coordinate their behavior. Fungi from the species complex Ophiocordyceps unilateralis infect ants of the tribe Camponotini, inducing aberrant host behaviors that precisely place host cadavers in areas suited for parasite dispersal. Fungi are thought to enter the host as individual cells, which proliferate in the hemocoel for two to three weeks. This implies that the timing of host manipulation depends on the parasite reaching an optimal stage of development and distribution within the host. In this study, we examine the distribution of the fungus and its interaction with host tissues at the time of manipulation. O. unilateralis fungi were visualized inside manipulated Camponotus castaneus ants using Serial Block-Face Scanning Electron Microscopy (SBF-SEM). SBF-SEM images and 3D reconstructions of the fungal cells and ant muscle fibers reveal that this parasite exhibits coordinated behavior by forming fungal networks through cell-cell fusions. Similar fusions in plant-pathogenic fungi have been shown to facilitate colony formation, nutrient sharing, and colony homeostasis. The adaptive function of the fungal networks in this study are unknown, but they could play an important role in the parasite’s ability to manipulate its host.