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21-2 Classification of Fungi
Photo Credit: ©D. Cavagnaro/DRK Photo
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2. The Kingdom Fungi has over 100,000 species.
Classification
The Kingdom Fungi has over 100,000 species.
Fungi are classified according to their structure and method of reproduction.
The four main groups of fungi are:
Common molds (Zygomycota)
Sac fungi (Ascomycota)
Club fungi (Basidiomycota)
Imperfect fungi (Deuteromycota)
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The Common Molds
The Common Molds
Familiar molds that grow on meat, cheese, and bread are called zygomycetes.
Zygomycetes have life cycles that include a zygospore.
A zygospore is a resting spore that contains zygotes formed during the sexual phase of the mold's life cycle.
Their hyphae generally lack cross walls, although their reproductive structure cells do have cross walls.
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4. Structure and Function of Bread Mold
The Common Molds
Structure and Function of Bread Mold 
Black bread mold, Rhizopus stolonifer, is a zygomycete.
Black bread mold has two types of hyphae:
Rhizoids are rootlike hyphae that penetrate the bread's surface- release digestive enzymes and absorb the organic material.
Stolons are stemlike hyphae that run along the surface of the bread.
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The Common Molds
Life Cycle of Molds
Black bread molds reproduce both sexually and asexually.
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The Common Molds
Hyphae from different mating types fuse and produce gamete-forming structures called gametangia.
Gametangia
+ Mating type (N)
- Mating type (N)
Stolon
Rhizoids
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The Common Molds
Haploid (N) gametes produced in the gametangia fuse with gametes of the opposite mating type to form diploid (2N) zygotes.
Zygotes develop into thick-walled zygospores, which may remain dormant for months.
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The Common Molds
In favorable conditions, the zygospore germinates, undergoes meiosis, and releases new haploid spores.
Zygospore (2N)
Sporangium
Spores (N)
MEIOSIS
Zygospore (2N)
Sexual Reproduction
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The Common Molds
The sporangium reproduces asexually by releasing haploid spores produced by meiosis.
Sporangium
Spores (N)
Sporangiophore
Rhizoids
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The Common Molds
Life Cycle of a Black Bread Mold
FERTILIZATION
MEIOSIS
Zygomycetes have life cycles that include a zygospore. During sexual reproduction in the bread mold Rhizopus stolonifer, hyphae from two different mating types form gametangia. The gametangia fuse, and zygotes form within a zygospore. The zygospore develops a thick wall and can remain dormant for long periods. The zygospore eventually germinates, and a sporangium emerges. The sporangium reproduces asexually by releasing haploid spores produced by meiosis.
Sexual Reproduction
Asexual Reproduction
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The Sac Fungi
The Sac Fungi
The phylum Ascomycota is named for the ascus, a reproductive structure that contains spores and organisms belonging to the phylum are commonly called ascomycetes.
There are more than 30,000 species of ascomycetes, making it the largest phylum of Fungi.
Life Cycle of Sac Fungi
The life cycle of an ascomycete usually includes both asexual and sexual reproduction.
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The Sac Fungi
In asexual reproduction, spores called conidia form at tips of conidiophores.
Conidiophores are specialized hyphae.
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The Sac Fungi
During sexual reproduction, haploid hyphae of two different mating types (+ and - ) grow close together.
Hyphae (N) + (N)
Hyphae (N)
Gametangia
+ Mating type (N)
- Mating type (N)
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The Sac Fungi
The N + N hyphae then produce a fruiting body in which sexual reproduction continues.
Gametangia from the two mating types fuse, but the haploid nuclei do not fuse.
This fusion produces hyphae with haploid nuclei of each mating type.
The ascus forms within the fruiting body.
Within the ascus, two nuclei of different mating types fuse to form a diploid zygote (2N).
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The Sac Fungi
Fruiting body (N + N)
Ascus (N + N)
Zygote (2N)
Asci
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The Sac Fungi
The zygote divides by meiosis, producing four haploid cells.
In most ascomycetes, meiosis is followed by mitosis, so that eight cells called ascospores are produced.
An ascospore can germinate and grow into a haploid mycelium.
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The Sac Fungi
Ascus
8 Ascospores (N)
Hypha (N)
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The Sac Fungi
Sexual Reproduction
The life cycle of ascomycetes includes both asexual and sexual reproduction. During asexual reproduction, spores called conidia are formed at the tips of specialized hyphae called conidiophores. During sexual reproduction, hyphae of two mating types fuse to form hyphae with two haploid nuclei (N + N). The N + N hyphae then form a fruiting body, which eventually releases ascospores. Ascomycetes are named for the ascus, the reproductive structure that contains ascospores.
Asexual Reproduction
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The Sac Fungi
Yeasts
Yeasts are unicellular fungi.
Yeasts reproduce asexually by budding.
Dry granules of yeast contain ascospores, which become active in a moist environment.
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The Club Fungi
The Club Fungi
The phylum Basidiomycota, or club fungi, gets its name from a specialized reproductive structure that resembles a club.
The spore-bearing structure is called the basidium.
Basidia are found on the gills that grow on the underside of mushroom caps.
Life Cycle of Club Fungi
Basidiomycetes undergo an elaborate life cycle.
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The Club Fungi
A basidiospore germinates to produce a haploid primary mycelium.
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The Club Fungi
The mycelia of different mating types fuse to produce a secondary mycelium.
The cells of the secondary mycelium contain haploid nuclei of each mating type.
Can become enormous: hundreds of meters across.
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The Club Fungi
When the right combination of moisture and nutrients occurs, spore-producing fruiting bodies push above the ground.
They have an incredible growth rate, becoming fully developed overnight, caused by cell enlargement, not cell division.
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The Club Fungi
When the mushroom cap opens, it exposes hundreds of tiny gills on its underside.
Each gill is lined with basidia.
Fruiting body (N + N)
Gills lined with basidia
Gills
Cap
Stalk
Base
Basidia (N +N)
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The Club Fungi
Nuclei in each basidium fuse, forming a diploid zygote, which undergoes meiosis, forming clusters of haploid basidiospores.
Zygote (2N)
Basidiospores (N)
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The Club Fungi
Basidiospores form at the edge of each basidium and are ready to be scattered.
Zygote (2N)
Basidiospores (N)
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The Club Fungi
Life Cycle of Club Fungi
The club fungi are named after the club shape of their reproductive structure, the basidium. The cap of a basidiomycete such as a mushroom is composed of tightly packed hyphae. The lower side of the cap is composed of gills—thin blades of tissue lined with basidia that produce basidiospores.
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The Club Fungi
Diversity of Club Fungi
Basidiomycetes include shelf fungi, puffballs, earthstars, jelly fungi, and rusts.
See page 535
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The Imperfect Fungi
The Imperfect Fungi
Imperfect fungi, or Deuteromycota, are fungi that cannot be placed in other phyla because researchers have never been able to observe a sexual phase in their life cycles.
A majority of impert fungi resemble ascomycetes.
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The Imperfect Fungi
A well-known genera of the imperfect fungi is Penicillium.
Penicillium notatum is a mold that grows on fruit and is the source of the antibiotic penicillin.
It reproduces asexually by conidia, so scientists believe it evolved from an ascomycete that lost the sexual phase of its life cycle.
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