Caviar on the 4th of July
February 7th, 2017
The Caviar Fungus that Kills Big Crane FliesAnn Hajek
Species in the fungal order Entomophthorales are mostly obligate pathogens of insects; they are difficult to grow in the lab but are excellent at causing massive mortality in the field. These fungi grow throughout the insect, kill it and then use the entire insect body as nutrients. A dead insect is usually used to make one or the other of two types of spores. Conidia are shorter-lived infective spores produced externally and often shot off to increase spore dispersal; when these fungi are going to produce conidia, they often zombify the insect, taking over its behavior just before death and forcing it to climb to elevated locations as a ‘zombie’ insect, where conidia that are shot off will disperse further to find new hosts. Alternatively, resting spores are made within dead insects, often later in the insect’s life cycle, and these spores can survive for several years in the environment.
Before use of DNA-based methods, taxonomy of the species of Entomophthorales was often principally based on the morphology of the conidia. But, then there is a problem if a dead insect containing only resting spores is found; there are no conidia to use to identify the fungus. So, long ago a kind of temporary genus was formed for this situation: Tarichium. This genus was temporary because the conidia were not present to figure out the correct genus and species.
Finding dead insects only containing resting spores has happened many times and it happened to us. While on a hike on the 4th of July in central New York, we found a dead female of the crane fly Tipula submaculata attached to a leaf about 4 feet above the ground. This was pretty obvious because you don’t usually see these large crane flies in such an exposed location in the middle of the day. When we looked at the dead crane fly under a microscope, the cadaver was filled with small black balls, which looked something like caviar. Looking closer, we found that these were entomophthoralean resting spores. Over 5 more years, we visited this same site in late June or early July. We never found a cadaver with conidia and, each year, we found a few of the adults containing resting spores.
This created the first opportunity to use molecular methods to figure out the species of Entomophthorales killing an insect, with the identification based only on resting spores. We found that this ‘caviar’ species is closely related to a fungal species described in 1942 from Tipula colei collected in Knoxville, Tennessee. But, it was different enough that we described it as a separate fungal species but within one of the ‘proper’ genera in the Entomophthorales (and not the ‘temporary’ genus Tarichium): Zoophthora independentiae (we chose this species name because this was first collected on July 4).
It has been common to find different species of entomophthoralean-killed insects that only contain resting spores. We think that our DNA-based approach can now be put to use to identify these collections of zombifying fungi.
Hajek, A.E., Gryganskyi, A., Bittner, T., Liebherr, J.K., Liebherr, J.H., Moulton, J.K., Jensen, A.B., Humber, R.A. 2016. Phylogenetic placement of two species known only from resting spores: Zoophthora independentia sp. nov. and Z. porteri comb nov. (Entomophthorales: Entomophthoraceae). J. Invertebr. Pathol. 140: 68-74.