In the previous tutorial, we outlined the taxonomy of a single basidiomycete, the fly agaric.
Starting from the domain classification and using binomial nomenclature, we learned that this
mushroom’s scientific name is Amanita muscaria. With this understanding of taxonomy, we can now
expand our range of fungi beyond the individual species and peer into the diversity that each
fungal phylum holds. Let’s begin by discussing the different fungi that fall within the phylum
Basidiomycota, commonly known as the club fungi. Currently, within Basidiomycota, there are three
subphyla. Those are Ustilaginomycotina, Pucciniomycotina, and Agaricomycotina.
Let’s first examine fungi within the subphylum, Ustilaginomycotina. Generally,
fungi within this subphylum are known as smuts, or fungi that cause diseases
in plants. These smuts fall into the class Ustilaginomycetes, then the order Ustilaginales.
These fungi mainly infect plants in the Poaceae family, also known as the grass family.
This means that some smuts can adversely affect economically important cash crops such as barley,
wheat, sugar cane, or potatoes. Smuts attack plant fruits and seeds causing them to swell
into structures called galls. These galls are full of spores and burst open when fully
matured. Although many smuts cause issues for us, the corn smut, Ustilago maydis, is an edible
smut that infects corn. It causes the kernels of corn to swell into galls. In Mexico, these
galls from Ustilago maydis are used for fillings in tacos, quesadillas, and several other dishes.
Corn smut has an exciting life cycle consisting of two phases: a single-celled haploid yeast phase
and a dikaryotic mycelia phase. As we recall from our study of biology, haploid means the cells each
have one nucleus and one set of chromosomes. Dikaryotic means there are two nuclei in each
cell, one from each of the parent fungi, a feature that is unique to the fungi kingdom.
We must not confuse dikaryotic with diploid. In diploid organisms the nuclei from parents
fuse into one, whereas dikaryotic fungi have two separate nuclei that do not fuse into one.
Starting at the yeast phase, the cells have two genetic alleles that determine if another yeast
cell is a compatible mate, the “a” locus and “b” locus. If the two yeasts have different
alleles at the “a” locus, conjugation tubes form on the cells and grow toward each other,
fusing at the conjugation tube tips. This connection will allow the cells’ respective nuclei
to be transported from one to the other. However, this is conditional upon the “b” locus. If the “b”
locus differs from one cell to the other, the conjugation tubes will transport the nuclei and
form a dikaryotic cell, which can grow into a filamentous dikaryotic mycelium. This dikaryotic
mycelium will insert itself into the plant tissue and form galls that release the spores.
Originally, species of fungi within the subphylum Ustilaginomycotina were generally
known to be plant pathogens called smuts and fall into the class Ustilaginomycetes.
Ustilago maydis is considered a Ustilaginomycete. Recently, there have been other unique divisions
of the fungal lineage that fall into this subphylum. These are subject to change
due to the ever-expanding knowledge of fungal taxonomy, but along with ustilaginomycetes,
there are three other classes: Exobasidiomycetes, Moniliellomycetes, and Malasseziomycetes.
Species within the class Exobasidiomycetes also act as plant pathogens on grass species
and thus are considered smuts. A notable species is Tilletia indica, or Karnal bunt
which parasitizes wheat, causing kernels of wheat to produce a putrid odor, and making the products
of wheat such as flour taste off-putting. Species within the Malasseziomycetes are known to cause
dermatomycoses or fungal skin infections. A genus within the class Malasseziomycetes,
known as Malassezia, lives on human skin and some species cause problems such as dandruff.
Next, we’ll discuss the subphylum, Pucciniomycotina. Most species within
this subphylum are plant pathogens in the form of rusts, but many species act as parasites that
act on mushrooms, also called mycoparasites. We’ll discuss the rust fungi, which generally
constitute the class Pucciniomycetes and fall into the order Pucciniales.
Rusts get their name from the powdery red fruiting bodies that form on parts of plants.
Wherever there are plants, there are also rusts which can infect a diverse number of plants
from a wide range of ecosystems. Many rust fungi have coevolved with their host plants, which has
led the order to become quite diverse, with about 7,000 described species. A notable rust fungus is
Gymnosporangium virginiana, or Cedar Apple rust, which infects both Juniper trees and Apple trees
to complete its life cycle. When a fungus requires two host organisms to complete its life cycle,
it is considered heteroecious. Rust fungi are known for having a heteroecious life cycle.
Fungi that need only one organism to complete their life cycle are considered autoecious.
Rust fungi can have quite an interesting life cycle with up to five stages and five
different types of spores. A rust is considered macrocyclic if it produces all spore types.
These are urediniospores, teliospores, basidiospores, pycniospores, and aeciospores.
Let’s describe the life cycle and spore types of the well studied Puccinia graminis, or stem
rust. Puccinia graminis is heteroecious and has a primary host where asexual reproduction occurs
and an alternate host where sexual reproduction occurs. Its primary host is wheat and its
alternate host is the barberry plant. Barberry is considered the alternate host because the rust can
reproduce with or without its presence. We’ll begin the life cycle during the summer months
by focusing on the stem of its primary host, wheat. Hyphae have penetrated the wheat stem, and
at the places where the red rust emerges, there are structures called uredinium that are filled
with urediniospores. The urediniospores allow for the rust to asexually reproduce and infect
other plants. As the temperature gets colder, and the fungus recognizes that winter is coming,
the fungus forms a telium instead of a uredinium. The telium houses the production of teliospores,
which are black, thick-walled, and diploid. These teliospores are quite durable and are stable until
spring, when they germinate and transition from a diploid spore to a haploid spore through meiosis.
These new haploid spores are known as basidiospores, and they allow the fungus
to infect its alternate host, barberry leaves. The basidiospores germinate inside the leaves and
produce a haploid mycelium. From this mycelium, structures called pycnia produce receptive
mycelium and haploid pycniospores, which are found in a sticky substance that attracts insects. The
insects will transport pycniospores from one pycnia to another, allowing sexual reproduction.
Once a pycniospore fuses with receptive hyphae, new hyphae produce a mycelium having both nuclei
instead of just one. This new, double nucleus mycelium, known as a dikaryotic mycelium,
grows through the barberry leaf and produces a structure called an aecium which houses dikaryotic
aeciospores. These spores can only infect the primary host, wheat. Aeciospores are separated
from urediniospores because the spore shape and production process is quite different.
Once aeciospores reach the wheat plant, they penetrate and begin producing a mycelium.
Soon, that mycelium will produce dikaryotic urediniospores, and the cycle starts over again.
So that covers two subphyla of the phylum basidiomycota. But there are
many more organisms in there to discuss, so let’s move forward and examine some more.