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Chapter 31

Chapter 31. Fungi. Overview: Mighty Mushrooms. Fungi are diverse and widespread They are essential for the well-being of most terrestrial ecosystems because they break down organic material and recycle vital nutrients. Concept 31.1: Fungi are heterotrophs that feed by absorption.

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Chapter 31

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  1. Chapter 31 Fungi

  2. Overview: Mighty Mushrooms • Fungi are diverse and widespread • They are essential for the well-being of most terrestrial ecosystems because they break down organic material and recycle vital nutrients

  3. Concept 31.1: Fungi are heterotrophs that feed by absorption • Despite their diversity, fungi share key traits, most importantly the way in which they derive nutrition

  4. Nutrition and Fungal Lifestyles • Fungi are heterotrophs but do not ingest their food • They secrete exoenzymes that break down complex molecules, and then they absorb the smaller compounds

  5. Fungi exhibit diverse lifestyles: • Decomposers • Parasites • Mutualistic symbionts

  6. Body Structure • The morphology of multicellular fungi enhances their ability to absorb nutrients • Fungi consist of mycelia, networks of branched hyphae adapted for absorption • The study of fungi is mycology • Most fungi have cell walls made of chitin Animation: Fungal Reproduction and Nutrition

  7. LE 31-2 Reproductive structure Hyphae Spore-producing structures 20 µm Mycelium

  8. Some fungi have hyphae divided into cells by septa, with pores allowing cell-to-cell movement • Coenocytic fungi lack septa

  9. LE 31-3 Cell wall Cell wall Nuclei Pore Septum Nuclei Septate hypha Coenocytic hypha

  10. Some unique fungi have specialized hyphae that allow them to penetrate the tissues of their host

  11. LE 31-4 Hyphae 25 µm Nematode Hyphae adapted for trapping and killing prey Plant cell wall Fungal hypha Plant cell Plant cell plasma membrane Haustorium Haustoria

  12. Mycorrhizae are mutually beneficial relationships between fungi and plant roots • Ectomycorrhizal fungi form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex • Endomycorrhizal fungi extend hyphae through the cell walls of root cells and into tubes formed by invagination of the root cell membrane

  13. Concept 31.2: Fungi produce spores through sexual or asexual life cycles • Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually

  14. LE 31-5–3 Key Heterokaryotic stage Haploid (n) Heterokaryotic (unfused nuclei from different parents) PLASMOGAMY (fusion of cytoplasm) Diploid (2n) KARYOGAMY (fusion of nuclei) Spore-producing structures Zygote SEXUAL REPRODUCTION Spores ASEXUAL REPRODUCTION Mycelium MEIOSIS GERMINATION GERMINATION Spore-producing structures Spores

  15. Sexual Reproduction • Plasomogamy is the union of two parent mycelia • In many fungi, the haploid nuclei from each parent do not fuse right away; they coexist in the mycelium, called a heterokaryon • In some fungi, the haploid nuclei pair off two to a cell; such a mycelium is said to be dikaryotic

  16. Hours, days, or even centuries may pass before the occurrence of karyogamy, nuclear fusion • During karyogamy, the haploid nuclei fuse, producing diploid cells • The diploid phase is short-lived and undergoes meiosis, producing haploid spores

  17. Asexual Reproduction • In addition to sexual reproduction, many fungi can reproduce asexually • Many of these species grow as mold, sometimes on fruit, bread, and other foods

  18. LE 31-6 2.5 µm

  19. Other fungi that can reproduce asexually are yeasts, which inhabit moist environments • Instead of producing spores, yeasts reproduce asexually by simple cell division

  20. LE 31-7 10 m Parent cell Bud

  21. Many molds and yeasts have no known sexual stage • Mycologists have traditionally called these deuteromycetes, or imperfect fungi

  22. Concept 31.3: Fungi descended from an aquatic, single-celled, flagellated protist • Systematists now recognize Fungi and Animalia as sister kingdoms • In other words, fungi and animals are more closely related to each other than they are to plants or other eukaryotes

  23. The Origin of Fungi • Molecular evidence supports the hypothesis that fungi and animals diverged from a common unicellular, flagellated ancestor • Fungi probably evolved before the colonization of land by multicellular organisms • The oldest undisputed fossils of fungi are only about 460 million years old

  24. LE 31-8 50 µm

  25. The Move to Land • Fungi were among the earliest colonizers of land, probably as symbionts with early land plants

  26. Concept 31.4: Fungi have radiated into a diverse set of lineages • Fungi phylogeny is the subject of much research • Molecular analysis has helped clarify evolutionary relationships between fungal groups, although areas of uncertainty remain

  27. LE 31-9 Arbuscular mycorrhizal fungi Sac fungi Club fungi Zygote fungi Chytrids Chytridiomycota Glomeromycota Basidiomycota Zygomycota Ascomycota

  28. Chytrids • Chytrids (phylum Chytridiomycota) are found in freshwater and terrestrial habitats • They can be saprobic or parasitic • Molecular evidence supports the hypothesis that chytrids diverged earliest in fungal evolution • Chytrids are unique among fungi in having flagellated spores, called zoospores

  29. LE 31-10 25 µm Hyphae Flagellum 4 µm

  30. Video: Allomyces Zoospore Release Video: Phlyctochytrium Zoospore Release

  31. Until recently, systematists thought that fungi lost flagella only once in their evolutionary history • Molecular data indicate that some “chytrids” are actually more closely related to another fungal group, the zygomycetes

  32. LE 31-11 Glomeromycetes, ascomycetes, and basidiomycetes Zygomycetes and other chytrids Some chytrids Key Loss of flagella Common ancestor

  33. Zygomycetes • The zygomycetes (phylum Zygomycota) exhibit great diversity of life histories • They include fast-growing molds, parasites, and commensal symbionts • The zygomycetes are named for their sexually produced zygosporangia • The life cycle of black bread mold (Rhizopus stolonifer) is fairly typical of the phylum

  34. LE 31-12 Key Haploid (n) Heterokaryotic (n + n) Diploid (2n) PLASMOGAMY Rhizopus growing on bread Mating type (+) Gametangia with haploid nuclei Mating type (–) 100 µm Young zygosporangium (heterokaryotic) SEXUAL REPRODUCTION Dispersal and germination Zygosporangium (heterokaryotic) KARYOGAMY Sporangia Diploid nuclei Sporangium ASEXUAL REPRODUCTION MEIOSIS Dispersal and germination Mycelium 50 µm

  35. Zygosporangia, which are resistant to freezing and drying, can survive unfavorable conditions • Some zygomycetes, such as Pilobolus, can actually “aim” their sporangia toward conditions associated with good food sources

  36. LE 31-13 0.5 mm

  37. Microsporidia • Microsporidia are unicellular parasites of animals and protists • They are now classified as zygomycetes

  38. LE 31-14 10 µm Host cell nucleus Developing microsporidian Spore

  39. Glomeromycetes • The glomeromycetes (phylum Glomeromycota) were once considered zygomycetes • They are now classified in a separate clade • Glomeromycetes form a distinct type of endomycorrhizae called arbuscular mycorrhizae

  40. LE 31-15 2.5 µm

  41. Ascomycetes • Ascomycetes (phylum Ascomycota) live in marine, freshwater, and terrestrial habitats • The phylum is defined by production of sexual spores in saclike asci, usually contained in fruiting bodies called ascocarps • Ascomycetes vary in size and complexity, from unicellular yeasts to elaborate cup fungi and morels

  42. LE 31-16 The cup-shaped ascocarps (fruiting bodies) of Aleuria aurantia give this species its common name: orange peel fungus. The edible ascocarp of Morchella esculenta, the succulent morel is often found under trees in orchards. 10 µm Neurospora crassa feeds as a mold on bread and other food (SEM). Tuber melanosporum is a truffle, an ascocarp that grows underground and emits strong odors. These ascocarps have been dug up and the middle one sliced open.

  43. Ascomycetes reproduce asexually by enormous numbers of asexual spores called conidia • Conidia are not formed inside sporangia; they are produced asexually at the tips of specialized hyphae called conidiophores

  44. LE 31-17 Key Haploid (n) Conidia; mating type (–) Dikaryotic (n + n) Diploid (2n) Dispersal Germination Mating type (+) ASEXUAL REPRODUCTION Mycelium PLASMOGAMY Ascus (dikaryotic) Mycelia Conidiophore Dikaryotic hyphae extended from ascogonium SEXUAL REPRODUCTION KARYOGAMY Germination Dispersal Diploid nucleus (zygote) Eight ascospores Asci Four haploid nuclei MEIOSIS Ascocarp

  45. Basidiomycetes • Basidomycetes (phylum Basidiomycota) include mushrooms and shelf fungi • The phylum is defined by a clublike structure called a basidium, a transient diploid stage in the life cycle

  46. LE 31-18 Maiden veil fungus (Dictyphora), a fungus with an odor like rotting meat Fly agaric (Amanita muscoria), a common species in conifer forests in the northern hemisphere Puffballs emitting spores Shelf fungi, important decomposers of wood

  47. The life cycle of a basidiomycete usually includes a long-lived dikaryotic mycelium • In response to environmental stimuli, the mycelium reproduces sexually by producing elaborate fruiting bodies call basidiocarps • Mushrooms are examples of basidiocarps

  48. The numerous basidia in a basidiocarp are sources of sexual spores called basidiospores • Asexual reproduction is much less common in basidiomycetes than in ascomycetes

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