Many species of fungi display an exemplary ability to adapt to their environment, and can carry out their life cycle utilizing a wide array of resources. For this reason, many of us concerned with conservation and environmental rehabilitation look towards our generalist fungal friends to carry out chemical and physical mechanisms to reverse some of the perturbations set in motion by human activities. For example, the oyster mushroom not only can break down large carbon molecules of different hardwoods, but also metabolize petroleum from oil spills into digestible sugars. Generalist fungi have many tools in their arsenal to compete and live in a diversity of ecosystems. Their generalist ecology although seemingly beneficial, sometimes becomes problematic. By carrying out multiple functions, generalist fungi interact with more species compared to specialist fungi. With this in mind, generalists are almost always competing.
Competition is an interaction that decreases the fitness of both parties involved, even the victor. The victor has had to allocate resources towards defenses instead of its own reproductive effort. Generalists have a large niche width, but also compete with more species which may limit their success. In today’s edition of Fungi Friday, I’d like to talk about a species of fungus that has virtually relieved itself from competition, as it is the only macrofungal specialist that can access the unique resource it acquires energy from.
Mycena crocea is a small saprotrophic species that breaks down walnuts and hickory nuts. That is it. By specializing in a specific resource, its fitness is never reduced by competition. There are always tradeoffs, as this fungus has a narrow niche, but if you look closely in areas dominated by walnut and hickory, you can bet to find this species.
Hickory and walnut are two plants within the family Juglandaceae. Today, this plant family is represented by about 60 different species. Experts in the field of paleobiology study fossilized pollen and fruit from ancient plants to better understand the evolution of these distant lineages. With this research, we now know when this family diversified. The genesis of the Juglandaceae most likely stems from the later periods of theCretaceous, 100-80 million years ago. This family became really successful, rapidly diversifying in the Early Tertiary, right after the extinction of non-avian dinosaurs. After most dinosaurs went extinct, small hairy endotherms known as mammals started to fill the niches that were left unoccupied. The radiation of mammals heavily altered the trajectory of many plant families living during this transitional period.
Mammals are busily, smart creatures with exceedingly high caloric demands. Plants that enticed mammals to disperse their seeds proved to be extremely successful. Hickory nuts and walnuts offered an energy dense food supply to these organisms, while those organisms dispersed the plant’s potential offspring to new suitable regions of the forest floor. This mutualism is something we still see today. As mammals entered Earth’s scene after the extinction of most dinosaurs, many plants shifted to a mammalian seed dispersal syndrome. Walnuts and hickory nuts were this new nutrient rich resource that was soon exploited by Mycena crocea.
I can’t find a publication that delves into the time period Mycena crocea originated, but with the information I do have access to, I can hypothesize. I predict that this fungal specialist started fruiting shortly after these plants within the Juglandaceae started to engage in mutualisms with mammals. Ever since the late Paleocene, nearly 60 million years ago, these plants started putting out large energy rich seeds. I expect the ancestors of this fungal specialist to start fruiting from walnuts and hickory nuts around 40 million years ago. By being specialists, Mycena crocea have their resources to themselves, and will do just fine, given that their nut producing plants survive this modern, largescale human disturbance.