The life cycle of the Basidiomycota – the mushrooms and their kin – is easily recognizable. Nested within the Kingdom Fungi, the Basidiomycota spend most of their life cycle underground before producing the recognizable fruiting bodies we see above ground.
Basidiomycota: The Mushrooms And Their Kin
As you walk through the woods after a spring rain, you might notice an abundance of mushrooms popping up from the wet soil.But did you know that mushrooms are the reproductive organs of a much bigger fungus, living underground?
The Phylum Basidiomycota, which includes mushrooms and their closest fungus relatives, are very familiar to us.
We recognize them as both a food source, and often as a dangerous toxin. These organisms within the Kingdom Fungi have a unique life cycle, and even their subkingdom designation – Dikarya – illustrates how important their life cycle is to their classification.But what does it mean to be dikaryotic? How does a single cluster of mushrooms form? And just what is the large, branching net of filaments that extends deep underground below the mushrooms that we see, poking up above the soil after it rains?In this lesson we’ll discuss the life cycle of the Basidiomycota, and delve into the strange way that cells split and recombine to form mushrooms.
Basidiomycota Life Cycle
The subkingdom Dikarya contains both the Basidiomycota, and their cousins the Ascomycota, which are the puffy sac fungi. Both of these groups form dikaryotic arrangements of cells, meaning each cell can have not one, but two nuclei, at the same time. An easy way to remember this is the Greek prefix di-, which means two, and the Greek root karyon – which refers to nut, or in this case, nucleus.
The double-nucleus dikaryon cell is a feature that unites these two larger mushroom groups.We’ll end the life cycle with a group of dikaryon cells (plural dikarya), which will form a mushroom. But to begin the Basidiomycota life cycle, let’s take a step back and start with that mushroom.
It Starts With Mushroom Gills
You’ve probably seen the dark frilly gills on the underside of a mushroom’s cap. These gills are also called basidia (singular basidion), and they form and protect the reproductive cells of the mushroom. Because the mushroom is the reproductive body of the larger fungus organism, we also call mushrooms fruiting bodies.When there’s enough moisture and warmth in the environment for a mushroom to trigger its reproduction, the basidia will start to combine their cells. Think of these cells as sort of like sperm and egg cells; they come from the same mushroom, but they only have one set of chromosomes, making them haploid cells – denoted as n.
When the basidia cells combine their nuclei, the process is called karyogamy, and it results in a diploid cell – denoted as 2n. This diploid cell is now called a zygote, just like humans start out as zygotes when egg and sperm meet.
Basidiospores Form Through Meiosis
Then, something strange happens: the zygote splits – twice – through the process of meiosis – resulting in a single cell with four haploid (n) nuclei. These four nuclei bud off into their own spore cells, known as basidiospores. Now the spores are ready to be released from the mushroom gills.
Germination: Hyphae Begin To Grow Underground
Just like the spores that can give us allergies, these basidiospores break away from the basidia gills and float in the air. But they aren’t looking to be inhaled; instead, they want to land on some moist soil to start growing into the mature fungus organism.
Here’s what happens. The basidiospores land on the soil and begin to grow into two distinct mating types: Mating Type + and Mating Type -. You can think of these mating types as almost like male and female, except the differences are on the protein level, rather than having sperm or egg structures.These mating types spread out to form long, thin filaments that extend far underground, forming the mature, haploid body of the fungus. These filaments are called hyphae (HI-fee), and the mass of hyphae are collectively called a mycelium (plural mycelia).
This phase is known as the germination phase.
Plasmogamy: Fusion Of Mating Types
Eventually the different strands of the mycelium interweave and fuse in a process known as plasmogamy. The newly fused cells contain the haploid, n nuclei of both the + and – mating types. Since these fused cells have two nuclei, they are called dikaryotic cells, or dikarya.
When these fused mycelia strands combine, they undergo mitosis, which duplicates the chromosomes from one set, haploid n, to two sets, diploid 2n. The 2n hyphae eventually form a small mushroom, which grows into the mature mushroom, or basidiocarp, complete with the gills that can produce more basidiospores.
From there, the process can start all over again.
Mushrooms spring up from the ground and form the fruiting bodies of underground fungal masses. These masses of hyphae filaments spread along underneath the soil, forming a mycelium.The mycelium started out as an individual spore, called a basidiospore, which formed from the gills of a mushroom.
These basidiospores were formed by the process of karyogamy, where a single cell holds four separate nuclei. The spores form the underground masses of mycelia, which have two mating types: Mating Type + and Mating Type -. These two types recombine during germination to form dikaryotic mycelia, in the process known as plasmogamy.
Eventually the dikaryotic mycelium forms a bud that sprouts from the ground and becomes – voila! – a fruiting body, or a mushroom, once more.A big difference between humans and the Basidiomycota, is that their adult form is haploid, with the mycelia containing just one copy of the chromosomes. Our adult cells, on the other hand, are diploid, with two copies of the chromosomes. The reproductive part of a Basidiomycota fungus is actually the diploid organ.The next time you enjoy a mushroom meal, or spot some basidiocarps sprouting up in the forest after it rains, remember – you’re looking at a very small part of a larger life cycle of the Basidiomycota, just beneath your feet.