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14.3: Introduction to Fungi

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Lesson Objectives

  • Identify what fungi are.
  • Describe habitats of fungi.
  • Outline the structure of fungi.
  • Describe fungi reproduction.
  • Summarize the evolution of fungi.
  • Give an overview of fungi classification.

Vocabulary

budding
type of asexual reproduction in yeasts in which an offspring cell pinches off from the parent cell
chitin
tough carbohydrate that makes up the cell walls of fungi and the exoskeletons of insects and other arthropods
fungi (singular, fungus)
kingdom in the domain Eukarya that includes molds, mushrooms, and yeasts
hyphae (singular, hypha)
thread-like filaments that make up the body of a fungus and consist of one or more cells surrounded by a tubular cell wall
mycelium
body of a fungus that consists of a mass of threadlike filaments called hyphae
zygospore
diploid spore in fungi that is produced by the fusion of two haploid parent cells

Introduction

Do you see the organisms growing on the bread in Figure below? They belong to the Kingdom Fungi. Molds growing on foods are some of the most common fungi in our everyday lives. These organisms may seem useless, gross, and costly. But fungi play very important roles in almost every terrestrial ecosystem on Earth.

The mold growing on this bread is a common fungus.

What Are Fungi?

Fungi (singular, fungus) are a kingdom in the domain Eukarya. The fungi kingdom may contain more than a million species, but fewer than 100,000 have been identified. As shown in Figure below, fungi include mushrooms and yeasts in addition to molds.

Several examples of fungi are pictured here.

Most fungi are multicellular, but some exist as single cells. Fungi spend most of their life cycle in the haploid state. They form diploid cells only during sexual reproduction. Like the cells of protists and plants, the cells of fungi have cell walls. But fungi are unique in having cell walls made of chitin instead of cellulose. Chitin is a tough carbohydrate that also makes up the exoskeleton (outer skeleton) of insects and related organisms.

Habitats of Fungi

You probably already know where some species of fungi live. No doubt, you’ve seen them growing on rotting logs and moist soil. In fact, most fungi live on dead matter or soil. However, some fungi are aquatic. Others live in or on other organisms in symbiotic relationships.

Structure of Fungi

Except for yeasts, which grow as single cells, most fungi grow as thread-like filaments, like those shown in Figure below. The filaments are called hyphae (singular, hypha). Each hypha consists of one or more cells surrounded by a tubular cell wall. A mass of hyphae make up the body of a fungus, which is called a mycelium (plural, mycelia).

These branches are hyphae, or filaments, of a mold called Penicillium.

A mycelium may range in size from microscopic to very large. In fact, one of the largest living organisms on Earth is the mycelium of a single fungus. A small part of a similar fungus is pictured in Figure below. The giant fungus covers 8.9 square kilometers (3.4 square miles) in an Oregon forest. That’s about the size of a small city. The fungus didn’t grow that large over night. It’s estimated to be 2,400 years old—and it’s still growing!

The fungus shown here has been dubbed the “humongous fungus” because it covers such a large area.

Reproduction of Fungi

The majority of fungi can reproduce both asexually and sexually. This allows them to adjust to conditions in the environment. They can spread quickly through asexual reproduction when conditions are stable. They can increase their genetic variation through sexual reproduction when conditions are changing and variation may help them survive.

Asexual Reproduction

Almost all fungi reproduce asexually by producing spores. A fungi spore is a haploid cell produced by mitosis from a haploid parent cell. It is genetically identical to the parent cell. Fungi spores can develop into new haploid individuals without being fertilized.

Spores may be dispersed by moving water, wind, or other organisms. Some fungi even have “cannons” that “shoot” the spores far from the parent organism. This helps to ensure that the offspring will not have to compete with the parents for space or other resources. You are probably familiar with puffballs, like the one in Figure below. They release a cloud of spores when knocked or stepped on. Wherever the spores happen to land, they do not germinate until conditions are favorable for growth. Then they develop into new hyphae.

Puffballs release spores when disturbed.

Yeasts do not produce spores. Instead, they reproduce asexually by budding. Budding is the pinching off of an offspring from the parent cell. The offspring cell is genetically identical to the parent. Budding in yeast is pictured in Figure below.

Yeast reproduce asexually by budding.

Sexual Reproduction

Sexual reproduction also occurs in virtually all fungi. This involves mating between two haploid hyphae. During mating, two haploid parent cells fuse, forming a diploid spore called a zygospore. The zygospore is genetically different from the parents. After the zygospore germinates, it can undergo meiosis, forming haploid cells that develop into new hyphae.

Evolution of Fungi

DNA evidence suggests that almost all fungi have a single common ancestor. The earliest fungi may have evolved about 600 million years ago or even earlier. They were probably aquatic organisms with a flagellum. Fungi first colonized the land at least 460 million years ago, around the same time as plants. Fossils of terrestrial fungi date back almost 400 million years (see Figure below). Starting about 250 million years ago, the fossil record shows fungi were abundant in many places. They may have been the dominant life forms on Earth at that time.

This rock contains fossilized fungi. The fungi lived 396 million years ago in what is now Scotland. They were preserved when they were covered with lava from a volcano. The lava cooled and hardened into rock.

Classification of Fungi

For a long time, scientists considered fungi to be members of the plant kingdom because they have obvious similarities with plants. Both fungi and plants are immobile, have cell walls, and grow in soil. Some fungi, such as lichens, even look like plants (see Figure below).

Moss (Plant) and Lichen Growing on Tree Bark. Both fungi and moss are growing on this tree. Can you tell them apart?

The Kingdom Fungi

Today, fungi are no longer classified as plants. We now know that they have unique physical, chemical, and genetic traits that set them apart from plants (and other eukaryotes). For example, the cell walls of fungi are made of chitin, not cellulose. Also, fungi absorb nutrients from other organisms, whereas plants make their own food. These are just a few of the reasons fungi are now placed in their own kingdom.

Fungal Phyla

Classification of fungi below the level of the kingdom is controversial. There is no single, widely-accepted system of fungal classification. Most classifications include several phyla (the next major taxon below the kingdom). Three of the most common phyla are compared in Table below.

Phylum Description Example
Zygomycota mainly terrestrial, live in soil and compost and on foods such as bread

black bread mold

Basidiomycota have many different shapes, considerable variation exists even within species

button mushrooms

Ascomycota found in all terrestrial ecosystems world-wide, even in Antarctica, often involved in symbiotic relationships

baker’s yeast

Lesson Summary

  • Fungi are a kingdom in the domain Eukarya that includes molds, mushrooms, and yeasts. Most fungi are multicellular. They are unique in having cell walls made of chitin.
  • Most fungi live on dead matter or soil. Some live in aquatic habitats. Many are involved in symbiotic relationships.
  • Most fungi grow as thread-like filaments called hyphae. A mass of hyphae make up the body of a fungus, called a mycelium.
  • The majority of fungi can reproduce both asexually and sexually. This allows them to adjust to conditions in the environment. Yeast reproduce asexually by budding. Other fungi reproduce asexually by producing spores. Sexual reproduction occurs when spores from two parents fuse and form a zygospore.
  • Almost all fungi have a single common ancestor. The earliest fungi may have evolved about 600 million years ago. Fungi colonized land at least 460 million years ago. By 250 million years ago, they may have been the dominant life forms on Earth.
  • Fungi used to be classified as plants. Now, they are known to have unique traits that set them apart from plants. For example, their cell walls contain chitin, not cellulose, and fungi absorb food rather than make their own. Below the level of the kingdom, fungi classification is controversial.

Lesson Review Questions

Recall

1. What are fungi?

2. List several habitats where fungi live.

3. Describe the general structure of multicellular fungi.

4. Identify ways that fungi spores may be dispersed.

5. Summarize the evolution of fungi.

6. State why fungi were once classified as plants.

Apply Concepts

7. Create a diagram to show the life cycle of a multicellular fungus.

Think Critically

8. Explain the significance of the chitin cell wall of fungi.

9. Compare and contrast a fungi spore and zygospore.

Points to Consider

In this lesson, you read that fungi differ from plants in major ways. For example, unlike plants, fungi do not make their own food by photosynthesis.

  • How do you think fungi obtain food? What organisms might they consume?
  • What roles do you think fungi might play in food chains and webs?

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