So often, I find fungi to write about that have a large ecological impact. Organisms that interact with ecosystems in stunningly detrimental ways or overwhelmingly positive ways entice readers to visit this website, and are generally easy to write about. However, it would be a disservice to you if I didn’t mention the commensals; the species that skate by without having truly negative or positive impacts. I just recently learned of a group of Ascomycete fungi called the laboulbeniomycetes that do just that.
The genus Laboulbenia contains roughly 600 species, but that number is more than likely to grow. These species have an insane degree of host specificity, with individual species only attaching to one or two species of insect they’ve co-evolved with. Some Laboulbenia species grow on living termites, cockroaches, mites and millipedes, but the vast majority fruit from the exoskeleton of beetles.
If you are to know just one fact about beetles, it should be that there is an insane number of them. There are around 400,000 described beetle species! This fact becomes even more staggering once one realizes that there are only about 250,000 species of plants described to date! Darwin himself was astonished at the rapid diversification of this largely successful insect lineage. Because of the ridiculous number of beetles that have evolved on Earth thus far, and because this small commensal fungal tends to favor growing from beetle exoskeletons, we are likely just scratching the surface of the total diversity of the laboulbeniomycetes.
The ecological term “commensal,” is not mentioned much in these articles, so it’s time for a little species interaction run-down. Two species that benefit the fitness of each other are said to be in a symbiotic, or mutualistic relationship (+,+). Mycorrhizal fungi and the plants they engage with are a perfect example of a mutualistic or symbiotic relationship. Oppositely, two species fighting for the same limiting resources are in a competitive relationship (-,-). The fitness of both species in this relationship is reduced. Take two species of saprophytic fungi for example. Both may be trying to decompose the same fallen tree, but because of the presence of the other decomposer, their pool of potential resources is greatly reduced. More than one interaction can signify a (+,-) relationship. Through predation, one species’ fitness is reduced (by being eaten), while the other species is enhanced (by eating). Herbivory and parasitism are analogous to predation, with the fitness of the feeding herbivore or parasite being enhanced, while the fitness of the plant being eaten or host is reduced. Though, the fitness of the plant being eaten may be increased if the feeding animal is dispersing the plant’s seeds. Finally, we have commensalism. This (+,0) species interaction ensues when one species benefits while the other species’ fitness remains the same.
Now you might be wondering how these Laboulbenia species grow on insects without reducing the insect’s fitness. The main attribute these fungi have that minimizes negative interactions on their insect counterpart is there small size and low energy demand. These fungi penetrate the exoskeleton of their host and grow only a few filaments of hyphae. Although they do incorporate nutrients from their host, this really is a miniscule amount of resources, even from an organism as small as a beetle. When they absorb enough energy, these fungi produce a small fruiting body with asci, that will eventually release sexual spores into the environment.
So on paper, following the exact definition of these aforementioned species interactions, this is a parasitic relationship. But in retrospect, it is more commensal than anything. These commensal/parasitic interactions are going to vary depending on the fungal and insect species involved, but really this interaction is reminiscent of another fungus that invades our day to day life. Athlete’s foot is a fungal infection that affects 15% of the entire human population! In many cases these fungal growths go unnoticed. If severe fungal growth ensues, the dermal layers of one’s foot may crack enough to allow bacteria to colonize, but most of the time, this interaction stays on the commensal side of the spectrum. A person with athlete’s foot isn’t really going to have a more difficult time passing their genes to the next generation.
Laboulbenia species that grow on the exoskeletons of certain insects represent a commensal relationship that is analogous to athlete’s foot. Both interactions are similar in the fact that the growing fungus flourishes with a miniscule amount of nutrients.Laboulbenia species are largely overlooked, and because of this this, there are more than likely thousands of undescribed Laboulbenia species. I myself has had athlete’s foot, and not to be gross but I more than likely have a unique microcosm of bacteria and fungi growing on my feet. Like Laboulbenia species, these organisms largely go unnoticed.