Terrestrial fungi predate the first arthropods and insects to walk on land. With this in mind, one can easily fathom how ancient these fungal parasites actually are. Not much time passed until early terrestrial fungi started to parasitize insect hosts. Since then, there has been an evolutionary arms race between these two groups of organisms. The selection of insect traits has allowed these animals to cope with these fungal parasites. By detecting fungal exudates which indicate a fungal infection, insects can then discharge a defensive chemical response. Insect lineages with these traits have a much better chance of surviving and spreading their genes to the next generation. However, it is not that simple. Parasitic fungi too adapt over time and this evolutionary arms race ensues. A new paper published last year helps elucidate these complex interactions.
Andreas Vilcinskas from the University of Giessen in Germany wanted to look more closely at these fungi and their insect counterparts. To analyze these interactions, he used a model insect, the greater wax moth (Galleria mellonella). When spores from parasitic fungi land on the outer surface of wax moth larvae, they germinate and start burrowing into the developing larvae. These fungi release enzymes that digest chitin and other proteins and lipids. These larvae detect these digestive compounds and emit a cascade of responses which leads to the formation of melanin, a dark pigment that surrounds and isolates the infection. If detected early enough, the melanized tissue can then shed and the fungal infection is stopped dead in its tracts.
"Learning from insects means learning to prevail." Professor Andreas Vilcinskas.
Unfortunately for the moth, this initial defensive response rarely prevents fungal invasion. Once inside the hemocoel, specialized receptors recognize specific sugar-containing molecules present in both bacterial and fungal pathogens. When Vilcinskas injected moth larvae with fungal cells, Galleria mellonella induced the expression of numerous proteins with direct antifungal activities as well as indirect antifungal properties. He realized that some of these proteins inhibit the enzymes produced by these parasites that aid in their virulence. Early work carried out by Vilcinskas identified one of these defensive molecules as a c-type lysozyme that actually causes fungal apoptosis.
Over the past two decades, a ridiculous diversity of proteins and enzymes with anti-fungal properties have been isolated from different insects within the Lepidoptera. These wide-ranging defensive molecules truly highlight the premise of this article; the evolutionary arms race between parasitic fungi and their insect host. A defensive molecule an insect produces will only work for so long. Its fungal nemesis soon comes up with a new strategy, evolutionary speaking of course. For this reason, that is why we have isolated hundreds of defensive molecules instead of just a few.
More recently, many researchers interested in these interactions have shifted their focus to the fungi. Historically, these reciprocal fungal adaptations are much more difficult to examine because researchers have been reliant on in vitro studies. However, growing parasitic fungi on agar plates with dissolved antifungal peptides derived from infected insects has shed new light. The particular substances used in this study include the peptide metchnikowin, and the enzyme lysozyme. To the researcher’s amazement, these fungi upregulated the synthesis of chymotrypsin and metalloproteases, compounds that can digest the insect’s defensive molecules.
Metarhizium anisopliae. A fungus, capable of killing more than 200 insect species including cockroaches (pictured here) and lepidopteran insects used in this study .
Another indication that these interactions have been around for hundreds of millions of years is that there is yet another level of this arms race. This recent study also reveals that these moths can actually detect the fungal compounds that digest the initial defensive compounds. When the insect’s immune system perceives fungal derived chymotrypsin and metalloproteases, these moths synthesize different antimicrobial compounds.
Organisms in a parasitic relationship like this are driven to adapt to their counterpart’s offensive or defensive strategies. Given long enough time, you get these ridiculously complex cascades of compounds that offer success, until that strategy is masked by yet another evolutionary step from its counterpart. With these outright absurd interactions out there, it makes even more sense why certain invasive species do so well. They come in contact with species that haven’t taken part in this arms race, and literally have no chance at dealing with offensive compounds. After learning about this particular arms race, I think novel weapons hypothesis provides more invasive species success than we have initially thought.
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