By Greer Lowenstein, Amanda M. Chandler
Jun 8 2022
Composed of different species each playing a different role, a lichen can be thought of as “an ecosystem in miniature,” a microcosm of the observable world it inhabits (Johnson & Villella 2013). Even within this small environment there exist complex symbiotic relationships, as smaller organisms often find the lichen as a perfect home to take root in. Walking through the forest or city, perhaps you might see a speck of black or a web of bright pink on an otherwise green lichen. Sometimes these color changes can be caused by environmental factors, but other times this could indicate the presence of a lichenicolous fungus.
Lichenicolous fungi are fungal organisms that use a lichen as a host, acquiring fixed carbon from the lichen’s thallus (otherwise called the vegetative body). So, there is the lichen itself, which is a composite organism consisting of algae and/or cyanobacteria in symbiosis with a fungal species, and the lichenicolous fungus - a fungal species living on the lichen. In some cases where these extra fungal organisms are present, discoloration or damage to the lichen’s thallus can be observed, but little is known about the extent to which these fungi affect their hosts. They may be a detriment to the lichen host, though often they are thought to be benign or even commensal, causing little to no effect (Diederich et al. 2018). Many of these fungi (as many as 95%) are, in fact, obligate with their lichen host, meaning that each species is only found with a particular genus or species of lichen and have never been found associated with other taxa (Lawrey & Dierderich 2003). Though they have been collected by a variety of fungal biologists for centuries, there is still very little known about their basic biological requirements (Diederich et al. 2018).
Learning from these lichenicolous fungi, perhaps we can gain further insight into the process of evolution and factors which affect it. According to evolutionary biologist R.D.M. Page, high levels of host-specificity, like with these fungal organisms and their lichen hosts, often “indicate cospeciation resulting from coevolution” (Page 2003). This means that when one partner evolves, it can lead to the formation of another species in tandem. Studies focusing on Biatoropsis usnearum, a lichenicolous fungus that infects lichens of the genus Usnea, found that diversity in lichen parasites may be a “by-product of a number of ecological interactions that have rarely been studied in detail” (Millanes et al. 2014). With further investigations into this lichen-parasite relationship, as well as the general ecology of lichenicolous fungi, we can continue to piece together the puzzle of evolution and increase our knowledge of mutualisms and, in turn, global biodiversity.
