In the previous tutorial, we discussed the life cycle of the rust and smut fungi. Now
we are going to introduce the general life cycle of other fungi within the phylum basidiomycota,
specifically the mushrooms that fall into the third subphylum, agaricomycotina. These fungi
primarily include mushroom or basidiocarp-forming species. For example, here is the shaggy mane
mushroom, or Coprinus comatus, a mushroom that has a well-studied life cycle.
Let’s begin with a single spore from this fungus. We call any spore from a basidiomycete
a basidiospore. This basidiospore has a single nucleus, and if it inhabits the correct
environmental conditions, it will germinate and begin to produce filamentous structures,
which we know are called hyphae. As more and more hyphae are formed, the single nucleus
from the basidiospore replicates and a separating wall between the two cloned nuclei forms. We call
this wall a dolipore septum. The term dolipore originates from the central barrel-shaped canal
in the center of each septum. These dolipore septa are partitions along the length of hyphae that
allow for the movement of cytoplasm, but block the movement of nuclei between compartments.
As more and more clones of the original basidiospore nucleus are created and spread
throughout the hyphal network, the resulting structure is known as a homokaryotic mycelium.
Generally, this homokaryotic mycelium will not produce a new basidiocarp unless it fuses
with another genetically compatible mycelium. Hyphae from each of the homokaryotic mycelia
will fuse through a process called plasmogamy,
and form a hyphal compartment that contains a pair of unique nuclei, one from each parent mycelium.
Now, whenever a new hyphal compartment is formed, one of the nuclei will divide with
a dolipore septum formed between each cloned nucleus. However, this process could cause the
nucleus from the other parent to be left behind in that compartment. That nucleus needs to change
compartments, but the dolipore septum is blocking its passage. The hyphae get around this problem,
quite literally, through the creation of clamp connections. As one of the nuclei divides and
forms a dolipore septum, the other nucleus divides and transfers one of the clones to
a small hyphal branch called a clamp. This clamp connects the two compartments but forms a septum
that blocks the cloned nucleus from rejoining the original compartment.
Then, the cloned nucleus is transferred to the new compartment. This process is repeated for each
hyphal compartment, and the resulting structure is known as a dikaryotic or heterokaryotic mycelium.
Depending on the environmental conditions, the mycelium is now ready to produce a basidiocarp.
Generally, the mycelium consists of long and loose hyphal branches. However, whenever the
fungus needs to create a fruiting body, these branches shorten and become more concentrated,
forming hyphal knots. These knots make up the condensed structure that allows for
basidiocarp formation. So, any basdiocarp, or mushroom, is made of dense networks of hyphae,
with some structural differences between species. Along the hymenophore of a basidiocarp,
there is the spore-producing tissue called the hymenium. Coprinus comatus has a gilled,
or lamellate hymenophore, and along the sides of those gills is the spore-producing hymenium.
Currently, Coprinus comatus has a heterokaryotic mycelium with two unique nuclei. In order to
produce spores, the hymenium needs to develop spore-bearing structures called basidia. The two
unique nuclei within cells along the hymenium fuse into a single diploid nucleus through a
process called karyogamy. This is the only period in mushroom reproduction that has a diploid stage.
Immediately after forming this diploid nucleus, the cells undergo meiosis and form four haploid
nuclei. These nuclei are then transferred to structures along the end of the cell,
forming basidiospores. At this point, the cells have fully matured into basidia, the spore-bearing
structures. Basidiomycetes that have spore-bearing surfaces open to the environment, such as gills,
pores, or teeth, have a discharge mechanism known as ballistopory. These spores are connected to the
basidia via cell projections called sterigma. The spores attached to the sterigma are shaped
asymmetrically. There is a prominent bulge at the base of the spores called the Hilar appendix.
Fluid aggregates in the Hilar appendix and the side of the spore. We call the spherical droplet
at the Hilar appendix the Buller’s drop, and the fluid at the side of the spore is the adaxial
drop. The Buller’s drop expands until it touches the adaxial drop, and the two fluid droplets fuse
into one larger drop. At this moment, surface tension is released, and the spore is propelled
off the sterigma into the environment. Hopefully, that spore will find a suitable
location to germinate, and the cycle will repeat. That was a lot of information, so let’s do a quick
review of the whole cycle. Once a basidiospore finds a place where it can germinate, it produces
a homokaryotic mycelium with one nucleus per hyphal compartment. If that homokaryotic mycelium
finds another compatible mycelium, the hyphae from both will fuse through a process called
plasmogamy. This forms a heterokaryotic mycelium that uses clamp connections to keep the two unique
nuclei together. Then, the hyphae within the mycelium form knots and become a dense network
that ultimately forms the fruiting body, or basidiocarp. Along the hymenium, there are
cells called basidia where the two unique nuclei from the heterokaryotic mycelium fuse through
karyogamy. That diploid nucleus then undergoes meiosis forming four haploid nuclei. These
nuclei are transported to the edge of basidia in structures called basidiospores. These spores are
then discharged by the fusion of the Buller’s drop and the adaxial drop through the process called
ballistospory. The spores are dispersed into the environment and the cycle repeats again.
And with that, we should have an enhanced appreciation for the complex mechanisms
responsible for the production of the fungal fruiting bodies we call mushrooms.