Kingdom Fungi: the body, the food and the reproduction

Bread, mushrooms and yeast

Bread kept in a damp place turns green or black. A mushroom pushes out of the soil after rain. A cake rises because of yeast. All of these are fungi. In the five kingdoms they are a separate kingdom, because they cannot make their own food, and their cells differ from those of plants.

The body

• Except for yeast, which is single-celled, a fungus is filamentous. Its body is made of long, thin threads called hyphae. A network of hyphae is a mycelium.
• Some hyphae have cross walls: they are septate. Others have no cross walls, with many nuclei in one tube: they are coenocytic (multinucleate and aseptate).
• The cell wall is made of chitin and polysaccharides. Chitin is the same hard material as in the shell of an insect.

Food

Fungi are heterotrophs. They take food in three ways:

• Saprophytes absorb soluble organic matter from dead things, such as dead wood and leaves.
• Parasites live on living plants or animals and take food from them.
• Symbionts live with other organisms, to the benefit of both. In lichens, a fungus lives with an alga. In mycorrhiza, a fungus lives with the roots of a plant.

Reproduction

• Vegetative: by fragmentation (pieces of mycelium), fission and budding.
• Asexual: by spores. They are conidia when made on the outside of a special hypha, sporangiospores when made inside a sporangium, and zoospores when they can swim.
• Sexual: by oospores, ascospores or basidiospores, made after two nuclei have come together.

The sexual cycle has three steps.

1. Plasmogamy: the cytoplasm of two cells fuses. The new cell has two nuclei (it is dikaryotic, written $n + n$).
2. Karyogamy: the two nuclei fuse into one diploid nucleus.
3. Meiosis: the diploid nucleus divides and gives haploid spores.

In some fungi the two cells fuse into one diploid cell at once. In the Ascomycetes and the Basidiomycetes there is a stage with two nuclei in each cell, the dikaryon, which can last a long time. Meiosis then takes place in the fruiting body.

You should find that with $8$ chromosomes in a haploid nucleus, the cell after plasmogamy holds $16$ chromosomes in two separate nuclei. After karyogamy, the one diploid nucleus also holds $16$, and each spore after meiosis holds $8$.

Try it in Python

This program sorts six fungi by the way they get their food.

Practise with new numbers

Common mistakes

• Calling all fungi many-celled. Yeast is a single cell.
• Giving fungi a cellulose wall. Their wall is of chitin and polysaccharides.
• Putting karyogamy first. First the cytoplasm fuses (plasmogamy), then the nuclei (karyogamy).
• Thinking all fungi feed on the dead. Some are parasites, and some are symbionts.

Remember these

• A fungus has a mycelium of hyphae, septate or coenocytic. Yeast is a single cell.
• The cell wall is of chitin and polysaccharides.
• Fungi are heterotrophic: saprophytes, parasites or symbionts (lichens, mycorrhiza).
• They reproduce by fragmentation, fission and budding, by asexual spores and by sexual spores.
• The sexual steps: plasmogamy, karyogamy and meiosis.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is a mycelium?

2. What is the cell wall of a fungus made of?

3. Which step comes first in the sexual cycle: karyogamy or plasmogamy?

4. What is a coenocytic hypha?