Live imaging of momentary sporangiospore extrusion from trichospores and its morphological changes in Zancudomyces culisetae (Harpellales)

Harpellales, an order of Kickxellomycotina, are commensalistic fungi that live in the digestive tracts of arthropods. They create specialized spores, called trichospores, which are asexual, deciduous, monosporous sporangia that infect individuals in the host population. The trichospores do not produce germ tube(s) and expel the sporangiospore inside very quickly. This germination style is referred to as sporangiospore extrusion. In this study, we were able to record live imaging of the sporangiospore extrusion from trichospores and subsequent morphological changes. Using live image analysis, the rates of spore extrusion, elongation and contraction were determined. When the pH shifted from 10 to 5 in a liquid medium containing potassium, the sporangiospore burst through the sporangial wall in less than 30 s, and rapidly tripled in length (56 μm) from the initial trichospore length (18 μm). During the next 60 s, the spore gradually contracted to be similar to the original length (22 μm) of the trichospore. The mechanisms underlying spore membrane elongation and contraction were revealed by fluorescent staining of the cell wall and plasma membrane. Momentary sporangiospore extrusion from trichospores and its contraction are thought to be an adaptation for quick attachment to the gut cuticle of host insects.