Fungi produce an incredible array of chemical compounds. Over the past 50 years, we have uncovered that some of these compounds act as antimicrobial substances, while others induce plants roots to organize their cortical cells which allows mycorrhizal inoculation. Although these secondary compounds play imperative ecological roles helping fungi compete in their environment, we have only isolated and studied and miniscule subset of fungal compounds. The way these molecules interact with the natural world may be as diverse as the fungal Kingdom itself, so researchers Florian P. Schiestl and his team set off to better understand fungal secondary compounds. Through their analyses, they concluded that some metabolites actually attract insects similar to flowering plants. These findings depict not only an analogous ecology to floral sent, but how the vastly different Kingdoms share a similar evolutionary trajectory.
I have written about stinkhorn species (Phallaceae) twice now, so as you may know, these fungi produce a carrion smell that mimics dead flesh to attract diverse assemblages of insects. Though, other species of fungi produce fragrances that attract more specialist insect counterparts. Take for example the Ascomycete fungi within the genus Epichloë. These fungi at first glance look like plant parasites as they infect different species of grasses. Upon closer investigation however, it was found out that these fungi produce alkaloids that deter herbivory. If a grazing mammal eats enough grass infected with an Epichloë fungus, it may become ill, so grass with the symbiote are best avoided.
Epichloë fungi are self-incompatible, and can only reproduce when the conidia from one mating type meets stromata from the other mating type. These fungi don’t just leave this meeting of gametes up to low probability wind events, but actually have evolved a relationship with insects that play an active role in conidia dispersal. The stroma actually attracts conidia covered female flies of the genus Botanophila, which then that oviposit a clutch of eggs beneath the fungal structures. The hatched larvae then feed on the now fertilized tissue, and soon enough, leave the confines of the infected grass to find a mate.
The metabolite chokol K was isolated from the fungus and experiments using the compound where conducted to examine its ecological function. In the bioassays conducted by these researchers, it was confirmed that chokol K does, without a doubt, attract these insects. Chokol K lined fly traps gathered several Botanophila flies while control traps without the compound collected no flies. Besides attracting its insect counterpart, this metabolite also has antifungal properties, that ultimately enhance its own fitness helping it compete against antagonistic parasitic fungi.
Not only does chokol K attract Botanophila flies to Epichloë stromata, but these results suggest that the evolution of these compounds may have had followed a similar path to flower sent. Olle Pellmyr and Leonard Thien back in 1986, assessed the evolution of floral sent. They proposed that initially, before attracting pollinators, the chemicals associated with these aromas may have warded off herbivores and deterred fungal and bacterial growth on the vulnerable reproductive organs. Now, it’s difficult to untangle what function came first in Epichloë fungi, but since fungi live in places chock-full of diverse communities of opportunistic fungi, and antifungal compounds within the Kingdom are extremely widespread, it seems that like angiosperms, this specific fungal aroma initially evolved to reduce the growth of competitor fungi. Over millions of years, the interactions with Botanophila flies transformed the structure of these antifungal compounds, allowing these aromas to be detected by their insect counterpart.