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Biology 2e

24.2 Classifications of Fungi

Biology 2e24.2 Classifications of Fungi
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  1. Preface
  2. The Chemistry of Life
    1. 1 The Study of Life
      1. Introduction
      2. 1.1 The Science of Biology
      3. 1.2 Themes and Concepts of Biology
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 2 The Chemical Foundation of Life
      1. Introduction
      2. 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks
      3. 2.2 Water
      4. 2.3 Carbon
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 3 Biological Macromolecules
      1. Introduction
      2. 3.1 Synthesis of Biological Macromolecules
      3. 3.2 Carbohydrates
      4. 3.3 Lipids
      5. 3.4 Proteins
      6. 3.5 Nucleic Acids
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  3. The Cell
    1. 4 Cell Structure
      1. Introduction
      2. 4.1 Studying Cells
      3. 4.2 Prokaryotic Cells
      4. 4.3 Eukaryotic Cells
      5. 4.4 The Endomembrane System and Proteins
      6. 4.5 The Cytoskeleton
      7. 4.6 Connections between Cells and Cellular Activities
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 5 Structure and Function of Plasma Membranes
      1. Introduction
      2. 5.1 Components and Structure
      3. 5.2 Passive Transport
      4. 5.3 Active Transport
      5. 5.4 Bulk Transport
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 6 Metabolism
      1. Introduction
      2. 6.1 Energy and Metabolism
      3. 6.2 Potential, Kinetic, Free, and Activation Energy
      4. 6.3 The Laws of Thermodynamics
      5. 6.4 ATP: Adenosine Triphosphate
      6. 6.5 Enzymes
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 7 Cellular Respiration
      1. Introduction
      2. 7.1 Energy in Living Systems
      3. 7.2 Glycolysis
      4. 7.3 Oxidation of Pyruvate and the Citric Acid Cycle
      5. 7.4 Oxidative Phosphorylation
      6. 7.5 Metabolism without Oxygen
      7. 7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
      8. 7.7 Regulation of Cellular Respiration
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    5. 8 Photosynthesis
      1. Introduction
      2. 8.1 Overview of Photosynthesis
      3. 8.2 The Light-Dependent Reactions of Photosynthesis
      4. 8.3 Using Light Energy to Make Organic Molecules
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    6. 9 Cell Communication
      1. Introduction
      2. 9.1 Signaling Molecules and Cellular Receptors
      3. 9.2 Propagation of the Signal
      4. 9.3 Response to the Signal
      5. 9.4 Signaling in Single-Celled Organisms
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 10 Cell Reproduction
      1. Introduction
      2. 10.1 Cell Division
      3. 10.2 The Cell Cycle
      4. 10.3 Control of the Cell Cycle
      5. 10.4 Cancer and the Cell Cycle
      6. 10.5 Prokaryotic Cell Division
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  4. Genetics
    1. 11 Meiosis and Sexual Reproduction
      1. Introduction
      2. 11.1 The Process of Meiosis
      3. 11.2 Sexual Reproduction
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 12 Mendel's Experiments and Heredity
      1. Introduction
      2. 12.1 Mendel’s Experiments and the Laws of Probability
      3. 12.2 Characteristics and Traits
      4. 12.3 Laws of Inheritance
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 13 Modern Understandings of Inheritance
      1. Introduction
      2. 13.1 Chromosomal Theory and Genetic Linkage
      3. 13.2 Chromosomal Basis of Inherited Disorders
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    4. 14 DNA Structure and Function
      1. Introduction
      2. 14.1 Historical Basis of Modern Understanding
      3. 14.2 DNA Structure and Sequencing
      4. 14.3 Basics of DNA Replication
      5. 14.4 DNA Replication in Prokaryotes
      6. 14.5 DNA Replication in Eukaryotes
      7. 14.6 DNA Repair
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    5. 15 Genes and Proteins
      1. Introduction
      2. 15.1 The Genetic Code
      3. 15.2 Prokaryotic Transcription
      4. 15.3 Eukaryotic Transcription
      5. 15.4 RNA Processing in Eukaryotes
      6. 15.5 Ribosomes and Protein Synthesis
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 16 Gene Expression
      1. Introduction
      2. 16.1 Regulation of Gene Expression
      3. 16.2 Prokaryotic Gene Regulation
      4. 16.3 Eukaryotic Epigenetic Gene Regulation
      5. 16.4 Eukaryotic Transcription Gene Regulation
      6. 16.5 Eukaryotic Post-transcriptional Gene Regulation
      7. 16.6 Eukaryotic Translational and Post-translational Gene Regulation
      8. 16.7 Cancer and Gene Regulation
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    7. 17 Biotechnology and Genomics
      1. Introduction
      2. 17.1 Biotechnology
      3. 17.2 Mapping Genomes
      4. 17.3 Whole-Genome Sequencing
      5. 17.4 Applying Genomics
      6. 17.5 Genomics and Proteomics
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  5. Evolutionary Processes
    1. 18 Evolution and the Origin of Species
      1. Introduction
      2. 18.1 Understanding Evolution
      3. 18.2 Formation of New Species
      4. 18.3 Reconnection and Speciation Rates
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 19 The Evolution of Populations
      1. Introduction
      2. 19.1 Population Evolution
      3. 19.2 Population Genetics
      4. 19.3 Adaptive Evolution
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 20 Phylogenies and the History of Life
      1. Introduction
      2. 20.1 Organizing Life on Earth
      3. 20.2 Determining Evolutionary Relationships
      4. 20.3 Perspectives on the Phylogenetic Tree
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  6. Biological Diversity
    1. 21 Viruses
      1. Introduction
      2. 21.1 Viral Evolution, Morphology, and Classification
      3. 21.2 Virus Infections and Hosts
      4. 21.3 Prevention and Treatment of Viral Infections
      5. 21.4 Other Acellular Entities: Prions and Viroids
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 22 Prokaryotes: Bacteria and Archaea
      1. Introduction
      2. 22.1 Prokaryotic Diversity
      3. 22.2 Structure of Prokaryotes: Bacteria and Archaea
      4. 22.3 Prokaryotic Metabolism
      5. 22.4 Bacterial Diseases in Humans
      6. 22.5 Beneficial Prokaryotes
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    3. 23 Protists
      1. Introduction
      2. 23.1 Eukaryotic Origins
      3. 23.2 Characteristics of Protists
      4. 23.3 Groups of Protists
      5. 23.4 Ecology of Protists
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    4. 24 Fungi
      1. Introduction
      2. 24.1 Characteristics of Fungi
      3. 24.2 Classifications of Fungi
      4. 24.3 Ecology of Fungi
      5. 24.4 Fungal Parasites and Pathogens
      6. 24.5 Importance of Fungi in Human Life
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 25 Seedless Plants
      1. Introduction
      2. 25.1 Early Plant Life
      3. 25.2 Green Algae: Precursors of Land Plants
      4. 25.3 Bryophytes
      5. 25.4 Seedless Vascular Plants
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    6. 26 Seed Plants
      1. Introduction
      2. 26.1 Evolution of Seed Plants
      3. 26.2 Gymnosperms
      4. 26.3 Angiosperms
      5. 26.4 The Role of Seed Plants
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 27 Introduction to Animal Diversity
      1. Introduction
      2. 27.1 Features of the Animal Kingdom
      3. 27.2 Features Used to Classify Animals
      4. 27.3 Animal Phylogeny
      5. 27.4 The Evolutionary History of the Animal Kingdom
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    8. 28 Invertebrates
      1. Introduction
      2. 28.1 Phylum Porifera
      3. 28.2 Phylum Cnidaria
      4. 28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans
      5. 28.4 Superphylum Lophotrochozoa: Molluscs and Annelids
      6. 28.5 Superphylum Ecdysozoa: Nematodes and Tardigrades
      7. 28.6 Superphylum Ecdysozoa: Arthropods
      8. 28.7 Superphylum Deuterostomia
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    9. 29 Vertebrates
      1. Introduction
      2. 29.1 Chordates
      3. 29.2 Fishes
      4. 29.3 Amphibians
      5. 29.4 Reptiles
      6. 29.5 Birds
      7. 29.6 Mammals
      8. 29.7 The Evolution of Primates
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
  7. Plant Structure and Function
    1. 30 Plant Form and Physiology
      1. Introduction
      2. 30.1 The Plant Body
      3. 30.2 Stems
      4. 30.3 Roots
      5. 30.4 Leaves
      6. 30.5 Transport of Water and Solutes in Plants
      7. 30.6 Plant Sensory Systems and Responses
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 31 Soil and Plant Nutrition
      1. Introduction
      2. 31.1 Nutritional Requirements of Plants
      3. 31.2 The Soil
      4. 31.3 Nutritional Adaptations of Plants
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 32 Plant Reproduction
      1. Introduction
      2. 32.1 Reproductive Development and Structure
      3. 32.2 Pollination and Fertilization
      4. 32.3 Asexual Reproduction
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  8. Animal Structure and Function
    1. 33 The Animal Body: Basic Form and Function
      1. Introduction
      2. 33.1 Animal Form and Function
      3. 33.2 Animal Primary Tissues
      4. 33.3 Homeostasis
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 34 Animal Nutrition and the Digestive System
      1. Introduction
      2. 34.1 Digestive Systems
      3. 34.2 Nutrition and Energy Production
      4. 34.3 Digestive System Processes
      5. 34.4 Digestive System Regulation
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 35 The Nervous System
      1. Introduction
      2. 35.1 Neurons and Glial Cells
      3. 35.2 How Neurons Communicate
      4. 35.3 The Central Nervous System
      5. 35.4 The Peripheral Nervous System
      6. 35.5 Nervous System Disorders
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 36 Sensory Systems
      1. Introduction
      2. 36.1 Sensory Processes
      3. 36.2 Somatosensation
      4. 36.3 Taste and Smell
      5. 36.4 Hearing and Vestibular Sensation
      6. 36.5 Vision
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 37 The Endocrine System
      1. Introduction
      2. 37.1 Types of Hormones
      3. 37.2 How Hormones Work
      4. 37.3 Regulation of Body Processes
      5. 37.4 Regulation of Hormone Production
      6. 37.5 Endocrine Glands
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 38 The Musculoskeletal System
      1. Introduction
      2. 38.1 Types of Skeletal Systems
      3. 38.2 Bone
      4. 38.3 Joints and Skeletal Movement
      5. 38.4 Muscle Contraction and Locomotion
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 39 The Respiratory System
      1. Introduction
      2. 39.1 Systems of Gas Exchange
      3. 39.2 Gas Exchange across Respiratory Surfaces
      4. 39.3 Breathing
      5. 39.4 Transport of Gases in Human Bodily Fluids
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    8. 40 The Circulatory System
      1. Introduction
      2. 40.1 Overview of the Circulatory System
      3. 40.2 Components of the Blood
      4. 40.3 Mammalian Heart and Blood Vessels
      5. 40.4 Blood Flow and Blood Pressure Regulation
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    9. 41 Osmotic Regulation and Excretion
      1. Introduction
      2. 41.1 Osmoregulation and Osmotic Balance
      3. 41.2 The Kidneys and Osmoregulatory Organs
      4. 41.3 Excretion Systems
      5. 41.4 Nitrogenous Wastes
      6. 41.5 Hormonal Control of Osmoregulatory Functions
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    10. 42 The Immune System
      1. Introduction
      2. 42.1 Innate Immune Response
      3. 42.2 Adaptive Immune Response
      4. 42.3 Antibodies
      5. 42.4 Disruptions in the Immune System
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    11. 43 Animal Reproduction and Development
      1. Introduction
      2. 43.1 Reproduction Methods
      3. 43.2 Fertilization
      4. 43.3 Human Reproductive Anatomy and Gametogenesis
      5. 43.4 Hormonal Control of Human Reproduction
      6. 43.5 Human Pregnancy and Birth
      7. 43.6 Fertilization and Early Embryonic Development
      8. 43.7 Organogenesis and Vertebrate Formation
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
  9. Ecology
    1. 44 Ecology and the Biosphere
      1. Introduction
      2. 44.1 The Scope of Ecology
      3. 44.2 Biogeography
      4. 44.3 Terrestrial Biomes
      5. 44.4 Aquatic Biomes
      6. 44.5 Climate and the Effects of Global Climate Change
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 45 Population and Community Ecology
      1. Introduction
      2. 45.1 Population Demography
      3. 45.2 Life Histories and Natural Selection
      4. 45.3 Environmental Limits to Population Growth
      5. 45.4 Population Dynamics and Regulation
      6. 45.5 Human Population Growth
      7. 45.6 Community Ecology
      8. 45.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    3. 46 Ecosystems
      1. Introduction
      2. 46.1 Ecology of Ecosystems
      3. 46.2 Energy Flow through Ecosystems
      4. 46.3 Biogeochemical Cycles
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    4. 47 Conservation Biology and Biodiversity
      1. Introduction
      2. 47.1 The Biodiversity Crisis
      3. 47.2 The Importance of Biodiversity to Human Life
      4. 47.3 Threats to Biodiversity
      5. 47.4 Preserving Biodiversity
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
  10. A | The Periodic Table of Elements
  11. B | Geological Time
  12. C | Measurements and the Metric System
  13. Index
By the end of this section, you will be able to do the following:
  • Identify fungi and place them into the five major phyla according to current classification
  • Describe each phylum in terms of major representative species and patterns of reproduction

The kingdom Fungi contains five major phyla that were established according to their mode of sexual reproduction or using molecular data. Polyphyletic, unrelated fungi that reproduce without a sexual cycle, were once placed for convenience in a sixth group, the Deuteromycota, called a “form phylum,” because superficially they appeared to be similar. However, most mycologists have discontinued this practice. Rapid advances in molecular biology and the sequencing of 18S rRNA (ribosomal RNA) continue to show new and different relationships among the various categories of fungi.

The five true phyla of fungi are the Chytridiomycota (Chytrids), the Zygomycota (conjugated fungi), the Ascomycota (sac fungi), the Basidiomycota (club fungi) and the recently described Phylum Glomeromycota (Figure 24.10).

The image displays the phyologenetic tree for fungi. From a central source, branches include basidiomycota and ascomycota (which exist at the end of the same branch), glomeromycota, zygomycota, and chytridiomycota. Animalia are diplayed coming from a separate branch from the same source ancestor, but are not a part of the fungi.
Figure 24.10 Fungal phyla. Note: “-mycota” is used to designate a phylum while “-mycetes” formally denotes a class or is used informally to refer to all members of the phylum.

Chytridiomycota: The Chytrids

The only class in the Phylum Chytridiomycota is the Chytridiomycetes. The chytrids are the simplest and most primitive Eumycota, or true fungi. The evolutionary record shows that the first recognizable chytrids appeared during the late pre-Cambrian period, more than 500 million years ago. Like all fungi, chytrids have chitin in their cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. Most chytrids are unicellular; however, a few form multicellular organisms and hyphae, which have no septa between cells (coenocytic). The Chytrids are the only fungi that have retained flagella. They produce both gametes and diploid zoospores that swim with the help of a single flagellum. An unusual feature of the chytrids is that both male and female gametes are flagellated.

The ecological habitat and cell structure of chytrids have much in common with protists. Chytrids usually live in aquatic environments, although some species live on land. Some species thrive as parasites on plants, insects, or amphibians (Figure 24.11), while others are saprobes. The chytrid species Allomyces is well characterized as an experimental organism. Its reproductive cycle includes both asexual and sexual phases. Allomyces produces diploid or haploid flagellated zoospores in a sporangium.

 Micrograph A shows an arthropod, which is a teardrop-shaped transparent organism about 90 microns across and 120 microns long. Tentacle-like appendages jut out from the front, wide end of the organism, and clusters of cilia-like appendages just out from either side and the back. Transparent oval organisms about 20 microns across cling to the arthropod. Micrograph B shows a similar oval transparent organisms clinging to rod-shaped algae about 5 microns across and 200 microns long.
Figure 24.11 Chytrids. The chytrid Batrachochytrium dendrobatidis is seen in these light micrographs as transparent spheres growing on (a) a freshwater arthropod (water mite) and (b) algae. This chytrid causes skin diseases in many species of amphibians, resulting in species decline and extinction. (credit: modification of work by Johnson ML, Speare R., CDC)

Zygomycota: The Conjugated Fungi

The zygomycetes are a relatively small group of fungi belonging to the Phylum Zygomycota. They include the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and vegetables. Most species are saprobes, living off decaying organic material; a few are parasites, particularly of insects. Zygomycetes play a considerable commercial role. For example, the metabolic products of some species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones.

Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the vegetative stage. The fungi usually reproduce asexually by producing sporangiospores (Figure 24.12). The black tips of bread mold are the swollen sporangia packed with black spores (Figure 24.13). When spores land on a suitable substrate, they germinate and produce a new mycelium. Sexual reproduction starts when environmental conditions become unfavorable. Two opposing mating strains (type + and type –) must be in close proximity for gametangia from the hyphae to be produced and fuse, leading to karyogamy. Each zygospore can contain several diploid nuclei. The developing diploid zygospores have thick coats that protect them from desiccation and other hazards. They may remain dormant until environmental conditions are favorable. When the zygospore germinates, it undergoes meiosis and produces haploid spores, which will, in turn, grow into a new organism. This form of sexual reproduction in fungi is called conjugation (although it differs markedly from conjugation in bacteria and protists), giving rise to the name “conjugated fungi”.

The asexual and sexual life cycles of zygomycetes are shown. In the asexual life cycle, 1n spores undergo mitosis to form long chains of cells called mycelia. Germination results in the formation of more spores. In the sexual life cycle, spores germinate to form mycelia with two different mating types: plus and minus. If the plus and minus mating types are in close proximity, extensions called gametangia form between them. In a process called plasmogamy, the gametangia fuse to form a zygosporangium with multiple haploid nuclei. A thick, protective coat forms around the zygosporangium. In a process called karyogamy, the nuclei fuse to form a zygote with multiple diploid (2n) nuclei. The zygote undergoes meiosis and germination. A sporangium grows on a short stalk. Haploid spores are formed inside. The spores germinate, ending the cycle.
Figure 24.12 Zygomycete life cycle. Zygomycetes have asexual and sexual phases in their life cycles. In the asexual phase, spores are produced from haploid sporangia by mitosis (not shown). In the sexual phase, plus and minus haploid mating types conjugate to form a heterokaryotic zygosporangium. Karyogamy then produces a diploid zygote. Diploid cells in the zygote undergo meiosis and germinate to form a haploid sporangium, which releases the next generation of haploid spores.
 The photo shows a thick layer of green mold growing on bread. Fuzzy white projections grow from the mold.
Figure 24.13 Rhizopus spores. Asexual sporangia grow at the end of stalks, which appear as (a) white fuzz seen on this bread mold, Rhizopus stolonifer. The black tips (b) of bread mold are the spore-containing sporangia. (credit b: modification of work by "polandeze"/Flickr)

Ascomycota: The Sac Fungi

The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an ascus (plural, asci), a sac-like structure that contains haploid ascospores. Filamentous ascomycetes produce hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to another. Conidia and asci, which are used respectively for asexual and sexual reproduction, are usually separated from the vegetative hyphae by blocked (non-perforated) septa. Many ascomycetes are of commercial importance. Some play a beneficial role for humanity, such as the yeasts used in baking, brewing, and wine fermentation, and directly as food delicacies such as truffles and morels. Aspergillus oryzae is used in the fermentation of rice to produce sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia poses a significant threat to AIDS patients who have a compromised immune system. Ascomycetes not only infest and destroy crops directly; they also produce poisonous secondary metabolites that make crops unfit for consumption.

Asexual reproduction is frequent and involves the production of conidiophores that release haploid conidiospores (Figure 24.14). Sexual reproduction starts with the development of special hyphae from either one of two types of mating strains (Figure 24.14). The “male” strain produces an antheridium and the “female” strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy, without nuclear fusion. Special dikaryotic ascogenous (ascus-producing) hyphae arise from this dikaryon, in which each cell has pairs of nuclei: one from the “male” strain and one from the “female” strain. In each ascus, two haploid nuclei fuse in karyogamy. Thousands of asci fill a fruiting body called the ascocarp. The diploid nucleus in each ascus gives rise to haploid nuclei by meiosis, and spore walls form around each nucleus. The spores in each ascus contain the meiotic products of a single diploid nucleus. The ascospores are then released, germinate, and form hyphae that are disseminated in the environment and start new mycelia (Figure 24.15).

Visual Connection

 Ascomycetes have both sexual and asexual life cycles. In the asexual life cycle, the haploid (1n) mycelium branches into a chain of cells called the conidiophore. Spores bud from the end of the conidiophore and germinate to form more mycelia. In the sexual life cycle, a round structure called an antheridium buds from the male strain, and a similar structure called the ascogonium buds from the female strain. In a process called plasmogamy, the ascogonium and antheridium fuse to form a cell with multiple haploid nuclei. Mitosis and cell division result in the growth of many hyphae, which form a fruiting body called the ascocarp. The hyphae are dikaryotic, meaning they have two haploid nuclei. Asci form at the tips of these hyphae. In a process called karyogamy, the nuclei in the asci fuse to form a diploid (2n) zygote. The zygote undergoes meiosis without cell division, resulting in an ascus with four 1n nuclei arranged in a row. Each nucleus undergoes mitosis, resulting in eight ascospores, which are also arranged in a row at the tip of the hyphae. Dispersal and germination results in the growth of new mycelia.
Figure 24.14 Ascomycete life cycle. The lifecycle of an ascomycete is characterized by the production of asci during the sexual phase. In each ascus, the four nuclei produced by meiosis divide once mitotically for a total of eight haploid ascospores. The haploid phase is the predominant phase of the life cycle in Ascomycetes.

Which of the following statements is true?

  1. A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  2. A diploid ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  3. A haploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight ascospores.
  4. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form eight ascospores.
Micrograph shows asci, which appear as multiple, sphere-like shapes fused together into a structure about 7 microns across, and ascospores, which are small, light blue ovals about two microns wide by three microns long released from the asci.
Figure 24.15 Ascospores. The bright field light micrograph shows ascospores being released from asci in the fungus Talaromyces flavus var. flavus. (credit: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell)

Basidiomycota: The Club Fungi

The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cells of hyphae. The basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom, commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn (Figure 24.16). These mushroom-producing basidiomycetes are sometimes referred to as “gill fungi” because of the presence of gill-like structures on the underside of the cap. The gills are actually compacted hyphae on which the basidia are borne. This group also includes shelf fungi, which cling to the bark of trees like small shelves. In addition, the basidiomycota include smuts and rusts, which are important plant pathogens. Most edible fungi belong to the Phylum Basidiomycota; however, some basidiomycota are inedible and produce deadly toxins. For example, Cryptococcus neoformans causes severe respiratory illness. The infamous death cap mushroom (Amanita phalloides) is related to the fly agaric seen at the beginning of the previous section.

Photo shows toadstool muschrooms growing on a lawn, and forming a large circle.
Figure 24.16 Fairy ring. The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring.” The best-known fairy ring fungus has the scientific name Marasmius oreades. The body of this fungus, its mycelium, is underground and grows outward in a circle. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia to grow away from the center and leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen. (Credit: "Cropcircles"/Wikipedia Commons)]

The lifecycle of basidiomycetes includes alternation of generations (Figure 24.17). Most fungi are haploid through most of their life cycles, but the basidiomycetes produce both haploid and dikaryotic mycelia, with the dikaryotic phase being dominant. (Note: The dikaryotic phase is technically not diploid, since the nuclei remain unfused until shortly before spore production.) In the basidiomycetes, sexual spores are more common than asexual spores. The sexual spores form in the club-shaped basidium and are called basidiospores. In the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then undergoes meiosis. The haploid nuclei migrate into four different chambers appended to the basidium, and then become basidiospores.

Each basidiospore germinates and generates monokaryotic haploid hyphae. The mycelium that results is called a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that contains haploid nuclei of two different mating strains. This is the dominant dikaryotic stage of the basidiomycete life cycle. Thus, each cell in this mycelium has two haploid nuclei, which will not fuse until formation of the basidium. Eventually, the secondary mycelium generates a basidiocarp, a fruiting body that protrudes from the ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under its cap.

Visual Connection

 The life cycle of basidiomycetes, better known as mushrooms, is shown. Basidiomycetes have a sexual life cycle that begins with the germination of 1n basidiospores into mycelia with plus and minus mating types. In a process called plasmogamy, the plus and minus mycelia form a dikaryotic mycelium. Under the right conditions, the dikaryotic mycelium grows into a basdiocarp, or mushroom. Gills on the underside of the mushroom cap contain cells called basidia. The basidia undergo karyogamy to form a 2n zygote. The zygote undergoes meiosis to form cells with four haploid (1n) nuclei. Cell division results in four basidiospores. Dispersal and germination of basidiospores ends the cycle.
Figure 24.17 Basidiomycete life cycle. The lifecycle of a basidiomycete has alternate generations with haploid and dikaryotic mycelia. Haploid primary mycelia fuse to form a dikaryotic secondary mycelium, which is the dominant stage of the life cycle, and produces the basidiocarp.

Which of the following statements is true?

  1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
  2. The result of the plasmogamy step is four basidiospores.
  3. Karyogamy results directly in the formation of mycelia.
  4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

Asexual Ascomycota and Basidiomycota

Imperfect fungi—those that do not display a sexual phase—were formerly classified in the form phylum Deuteromycota, an invalid taxon no longer used in the present, ever-developing classification of organisms. While Deuteromycota was once a classification taxon, recent molecular analysis has shown that some of the members classified in this group belong to the Ascomycota (Figure 24.18) or the Basidiomycota. Because some members of this group have not yet been appropriately classified, they are less well described in comparison to members of other fungal taxa. Most imperfect fungi live on land, with a few aquatic exceptions. They form visible mycelia with a fuzzy appearance and are commonly known as mold.

Micrograph shows Aspergillus mycelia, which look like long threads, and a spherical conidiophore about 40 microns across.
Figure 24.18 Aspergillus. Aspergillus niger is an asexually reproducing fungus (phylum Ascomycota) commonly found as a food contaminant. The spherical structure in this light micrograph is an asexual conidiophore. Molecular studies have placed Aspergillus with the ascomycetes and sexual cycles have been identified in some species. (credit: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell)

The fungi in this group have a large impact on everyday human life. The food industry relies on them for ripening some cheeses. The blue veins in Roquefort cheese and the white crust on Camembert are the result of fungal growth. The antibiotic penicillin was originally discovered on an overgrown Petri plate, on which a colony of Penicillium fungi had killed the bacterial growth surrounding it. Other fungi in this group cause serious diseases, either directly as parasites (which infect both plants and humans), or as producers of potent toxic compounds, as seen in the aflatoxins released by fungi of the genus Aspergillus.

Glomeromycota

The Glomeromycota is a newly established phylum that comprises about 230 species, all of which are involved in close associations with the roots of trees. Fossil records indicate that trees and their root symbionts share a long evolutionary history. It appears that nearly all members of this family form arbuscular mycorrhizae: the hyphae interact with the root cells forming a mutually beneficial association in which the plants supply the carbon source and energy in the form of carbohydrates to the fungus, and the fungus supplies essential minerals from the soil to the plant. The exception is Geosiphon pyriformis, which hosts the cyanobacterium Nostoc as an endosymbiont.

The glomeromycetes do not reproduce sexually and do not survive without the presence of plant roots. Although they have coenocytic hyphae like the zygomycetes, they do not form zygospores. DNA analysis shows that all glomeromycetes probably descended from a common ancestor, making them a monophyletic lineage.

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