The heterokaryotic phase of a fungal life cycle is a

Objectives

• Understand fungal body plans, including mycelia, hyphae, and yeasts.
• Understand the general characteristics of fungal life cycles, including the heterokaryotic stage.
• Compare and contrast the fungi with plants and animals in terms of body plan and life cycle.

Specimens: microscope slides

• Rhizopus mycelium and conjugation.
• Penicillium conidiophores
• Aspergillus conidiophores. 
• Lichen cross section.
• Coprinus entire pileus.
• Peziza cup with asci.
• Mycorrhizae. These are fungi that are attached to plant roots. We may have more than one kind in lab.

Other specimens

• Molds in rotting wood or leaves. You should be able to recognize the fungal hyphae as distinct from the plant tissues on which they are feeding.
• Fresh mushrooms. Compare these to the prepared slides of Coprinus.
• Live Saccharomyces yeast. This is bread yeast; compare these to the prepared slides of Saccharomyces.
• Whole lichens. Compare these to the microscope slides of lichens.
• Plant roots with mycorrhizal fungi.

Introduction

Fungi are among our closest relatives outside the animal kingdom. Like animals, they are heterotrophs. Their bodies are specialized for absorbing their food, and, like animals, they do much of their digestion outside their cells. They secrete enzymes to break down food so it can be absorbed into the cells. However, unlike animals, they don’t swim, walk, or fly to find their food. Fungi move into new food sources by growing or by the passive dispersal of nonmotile spores. The basic body design of fungi is simple, but highly functional: it’s all about absorbing food molecules from the environment.

Like plants, fungi have cell walls. However, the cell wall material of fungi is completely different from that of plants. Fungi have cell walls made of chitin, the same tough polysaccharide that makes up insect exoskeletons. Plants have cell walls made of cellulose, a different polysaccharide.

Multicellular fungi are typically composed of long, thin filaments called hyphae; the whole body of many hyphae is called a mycelium. Multicellular fungi are always composed of hyphae; even mushrooms, which are the reproductive structures of underground fungi, are composed of these threadlike filaments of cells. Fungal mycelia have enormous surface area for absorption. Also, the hyphae can rapidly grow through soil as they move toward new food sources. Some fungi are unicellular; these are called yeasts. Yeasts are described in more detail below.

Saccharomyces

Saccharomyces is the common yeast often used in making wine, beer, and bread.

The term "yeast" simply refers to fungi that grow as single, roundish cells and don't form hyphae. Yeast is not a taxonomic group; it's a description of a body type.

These eukaryotic cells contain a nucleus (stained dark in this image) and a central vacuole (light staining), as well as other internal membrane-bound structures.

Aspergillus

This is a typical mold, showing asexual spore formation.

Various species of Aspergillus are used commercially, for example in the production of citric acid or digesting the starch in rice as a step in making sake. Some species of Aspergillus occasionally act as human pathogens.

Aspergillus is a typical mold, a mold is a fungus with a body composed of thin, stringy hyphae. The whole body of connected hyphae is called a mycelium. The mycelium is haploid, except for reproductive structures.

Aspergillus is an ascomycete (phylum Ascomycota). The sporangia (called conidiophores) produce  asexual spores called conidia. Conidia germinate to form new haploid mycelia for growth or specialized hyphae that perform plasmogamy.

Compare this slide to the life cycle of Neurospora (fig. 31.16 in Campbell), but our slides may contain only the asexual sporangia.

Molds can spread rapidly because their thin hyphae penetrate into new food sources (rotting fruit, for example) and can grow very rapidly. Molds can also produce huge numbers of asexual spores via mitosis.

Molds also reproduce sexually, but this is much less common than asexual spore formation.

Rhizopus

Rhizopus is another common mold. Like Aspergillus, it typically makes a huge number of asexual spores and undergoes sexual reproduction less frequently.

This image shows sporangia, which produce spores. These sporangia could be produced either sexually or asexually.

During sexual reproduction, two haploid hyphae from different parents perform plasmogamy, joining together, producing a heterokaryotic cell. This multinucleate heterokaryotic cell forms a zygosporangium.

Pairs of nuclei (one from each parent) perform karyogamy in the zygosporangium, fusing to form a diploid nucleus. These diploid nuclei are zygotes; they immediately undergo meiosis to begin producing haploid sexual spores. A sporangium sprouts out of the zygosporangium to release these spores.

If you're looking at this slide, how would you know whether you are looking at an asexual sporangium or a sexual sporangium (zygosporangium)?  Which do you think is produced more often?

Coprinus

Coprinus is in the phylum Basidiomycota, the phylum that makes mushrooms. Like many mushroom-forming fungi, Coprinus may have very large haploid mycelia underground, while occasionally forming heterokaryotic hyphae that grow into mushrooms above ground. The main body of the mushroom is heterokaryotic. In this case, each cell has two different nuclei in each cell; this kind of heterokaryotic cell is called dikaryotic.

The main umbrella-shaped part is sometimes called the cap. Under the cap there are gills. (The gills have nothing to do with gas exchange; they're called gills because they look like fish gills.) Some of the dikaryotic cells on the gills undergo karyogamy, fusing their two different haploid nuclei to make a diploid zygote. As with all fungi, the zygote then performs meiosis, making four haploid cells. These haploid cells turn into spores.

These images show the gills in a cross-section of the mushroom at different magnifications.

This magnified view gives you a better view of the spores. Compare this to the life cycle diagram in Campbell. Are the cells you see here haploid, diploid or heterokaryotic?

Peziza

Peziza is a cup fungus in the phylum Ascomycotes. Like a mushroom (phylum Basidiomycota), Peziza produces above-ground heterokarkyotic reproductive structures that come from an underground mycelium.

One way that Peziza differs from mushrooms is that it produces spores on top of its cup, not underneath like a mushroom.

In cross-section, Peziza shows a mycelium constructed of loose hyphae underneath, with tightly packed spore-producing structures (called asci) on top.

In this image, each ascus is a long, narrow cell with eight spores inside. The ascus begins as a single dikarkyotic cell.

The two nuclei fuse, forming a single diploid nucleus; this is called karyogamy. The diploid nucleus is called a zygote.

The zygote undergoes meiosis, producing four haploid nuclei.

Each of these haploid nuclei divides once (via mitosis). The end result is eight haploid nuclei in one ascus; these nuclei form new walls and turn into spores.

In this image, you can see spores at different stages of maturation.

Lichens

A lichen is two different organisms living in a close symbiotic association.

One partner is a fungus, which forms a tough, leathery coating that provides a protected space inside. The fungal cells can tolerate harsh, dry conditions, but they cannot produce food on their own.

The other partner is an alga -- a unicellular photosynthetic organism. The algae thrive in the protected environment created by the fungal mycelium. The algae perform photosynthesis, making the sugars that can be used as energy by both the algae and the fungus.

Review

Study Questions

You don’t need to turn in answers to these questions. However, you may want to think about them to help you prepare for the next lab exam.

1. The terms plasmogamy and karyogamy are introduced in this lab. What do they mean? Why are these terms used only for the fungi? Do other groups of organisms have similar events?
2. This lab includes two examples of symbiotic interactions involving fungi. What are they? In what way are the fungi particularly suited to these kinds of symbioses?
3. What part of a plant life cycle does a mushroom correspond to? Explain in terms of both similarities and differences.
4. Compare and contrast the life cycle of a fungus with the life cycle of an animal.
5. Compare and contrast the life cycle of a fungus with the life cycle of a moss.
6. How is the fungal body plan, with hyphae, suited to heterotrophy?
7. Compare & contrast the role of spores in fungi and in plants.
8. Suppose your instructor wanted to have examples of haploid, heterokaryotic, and diploid structures of fungi on the lab exam. What specimens could he use?
9. Do fungi have meristems? Why or why not?

Terms & Concepts to Remember

• Absorption & digestion
• Cell wall
• Chitin
• Heterotroph
• Heterokaryotic
• Hyphae
• Karyogamy
• Lichen
• Life cycle: Haploid, Diploid, Heterokaryotic. Where & when meiosis happens.
• Mold
• Mushroom
• Mycorrhizae
• Mycelium
• Plasmogamy
• Rhizopus
• Saccharomyces

References & further reading

Campbell Biology, Chapter 31: Fungi. In particular, study figure 31.12 (The life cycle of Rhizopus) and figure 31.18 (The life cycle of a mushroom-forming basidiomycete).

What is the life cycle of a fungus?

What does it mean if fungal hyphae is Heterokaryotic?

What stage is a Basidiomycete life cycle?

Which the following types of fungi have no Heterokaryotic phase in their life cycle?