As was mentioned in Chapter 1, the extent of fungal diversity is vast yet lies in highly uncharted territories; there is simply so much we still don’t know! Although there are millions of undiscovered species, there are also species that have such different sexual and asexual forms that they are wrongly labeled as two separate entities. Scientists are improving their methods, leading to a better understanding of how they should be classified. The chapter goes through the major players in the fungal kingdom, showing what differentiates the phyla.

First, they look at Chytridiomycota, or the chytrids, which occupy a somewhat basal position in the fungal kingdom. These are the only true aquatic fungi, as they usually need water to properly disperse, and assume a comparatively simple morphology. They reproduce using zoospores, or motile spores, with a single flagellum in the rear. They have two options when it comes to spores– they can be eucarpic, or having a portion of their thallus turn to the sporangium and make rhizoids, or holocarpic, or the entire thallus turns into the sporangium. They can also be monocentric, where the thallus produces a single sporangium, or polycentric, where multiple form on the rhizoids (called rhizomycelium). (Do note, by the way, that anaerobic rumen chytrids, despite their name, are not classified with chytrids given they are multiflagellate.) The chytrids have many important ecological roles, from decomposition to acting as a food source to zooplankton. There even is a chytrid that is parasitic towards vertebrates, and that is Batrachochytrium dendrobatidis, which causes a skin infection in various amphibians. Another interesting thing to note is that to isolate them for scientific purposes, you can flood a soil sample and provide their preferred food to lure them to the surface. Overall, they are small and have gone through many taxonomic changes recently, but nevertheless play a large role in our ecosystems. 

The next phylum discussed is Neocallimastigomycota, which often present themselves as anaerobic chytrids. They live in terrestrial and aquatic environments, but are most commonly known for living inside the hindgut of mammals, helping breakdown substances in the food they eat, such as lignocellulose. This gives them economic importance as they are necessary for livestock nutrition. The anaerobic chytrids don’t have mitochondria. Instead, they have hydrogenosomes, which are degenerate mitochondria lacking a genome that produce hydrogen, in addition to the ATP and other compounds. Finally, they have no known sexual stage, and they can be either mono or polycentric.

Blastocladiomycota are saprotrophic or parasitic fungi that often have the ability to be facultatively anaerobic. In addition to all having a ribosome-filled cap around their nucleus, they also have a fascinating, albeit sometimes puzzling, reproductive cycle. Alternation of generations describes the back and forth from asexual to sexual stages. Rather than describe it in great detail, I provided the graphic from the book. Essentially, if you were to draw a line through the circle starting at “plasmogamy” and ending before “meiosis in thick walled…” you would find that the top right half is all just n and the bottom left half is all just 2n, thus showing the back and forth between sexual and sexual. And for some vocab, note that gametothallus is haploid while sporothallus is diploid.

Continuing about Blastocladiomycota, the Allomyces genus is anisogamous (AKA the females are slow while the males are active) and use pheromones. The females produce a hormone called sirenin to help the male find them. The males are extremely sensitive to this, as they use an intracellular signaling cascade to amplify, and it is needed if they are in an aquatic or semi-aquatic ecosystem where gametes could easily get lost. The sperm also produce a hormone called parisin, but its function is not entirely known. Compared to plants and animals, these fungi utilize a vastly different method of cell division for its zoospores. Rather than constricting at the center or forming a new cell wall for division, fungi use cytoplasmic vesicles that join together to separate the cells.

Next up is the former Zygomycetes. They mostly have multinucleate mycelium and reproduce through the fusion of hyphal branches (gametangia). This creates a zygote which will go on to make spores. Interestingly, their resting spores use sporopollenin, which is a protective, durable polymer usually found on pollen grains. It is important to note that resting spores can stand the test of time (at least some of it) while dispersal spores can stand the test of space (again, at least some of it). Recently this phylum was split into the Zoopagomycota and Mucoromycota. The book dives into information about the subphyla, but I will only summarise Glomeromycotina here. Essentially, this subphylum was originally its own phylum (Glomeromycota), but was put under Mucoromycotina. There is no proof of any sexual stage in these fungi, yet they contain genes that would imply it is possible, thus suggesting that there may be more we don’t know about these fungi. They form AM (arbuscular mycorrhizas) and make up a large percent of the carbon sinks in non-aquatic environments. It is believed that the help of these fungi allowed plants to become terrestrial.

Ascomycota is the largest phylum in the kingdom. They’re filamentous fungi who’s mycelium have septate hyphae. When they have sexual spores, they are formed in sacs called “ascus.” The cool thing about these fungi is that you, reader, already know them. They made your bread, your beer, your penicillin, your statins, and more. Using genetic markers, we found that humans have been using and domesticating these fungi for over 4,000 years, all taking advantage of the Crabtree effect, or their ability to ferment even in aerobic conditions.

Another large and diverse phylum is Basidiomycota. They all have sexual exospores that form on their basidium, hence the name. Also, if you were wondering, basidium are these little structures that usually grow on the gills of a fungus that hold the spores. Most of these fungi are smut or rust diseases, often affecting plants via white or brown rot. Those that form white rot have the special ability to break down lignin and cellulose of leaf litter. Some also form relationships with other species, such as having a mutualistic relationship with leaf-cutter ants that provide the fungus leafs and protection in return for food for their larvae. Additionally, basidiomycetes make up the fungi we commercially sell as food and drugs, either through farming them or, in the case of truffles and some other species, apply silviculture.

All this talk of species begs the question: what constitutes a species? The unfortunate answer is that there are no clear rules. The more common methods use either morphological, biological, ecological/physiological, or evolutionary/phylogenetic traits. These, respectively, are based on their physical appearance, their sexual capabilities, their host specificity or habitat, and genetic material and lineage. The lattermost of these is now the preferred method, especially with fungi, given their “easy-going” nature in most aspects of their existence. Fungi can easily look different or only fruit during very specific times, as well as separately evolve useful features, such as gills, since it is the best way to increase the surface area to volume ratio. These new considerations are what spawned the efforts to reclassify fungal species. Water moulds, slime moulds, and some other species were removed from the fungal kingdom and given the name “untrue fungi.” While they share many characteristics with true fungi, untrue fungi ultimately have to go once discovered. My takeaway from this chapter is that fungal taxonomy/phylogeny is confusing but that fungi have a lot of unusual, fascinating traits.

Happy reading,
-Beppa