1. CHAPTER 31 Fungi

2. Introduction  Mycologist = trained fungi biologist.  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  About 100,000 species of fungi have been described  It is estimated there are actually 1.5 million species of fungi

5. Concept 31.1 Nutrition and Structure  Despite their diversity, fungi share key traits, most importantly the way in which they derive nutrition  Fungi are heterotrophs that acquire their nutrients by absorption.  They absorb small organic molecules from the surrounding medium (outside their body).  Exoenzymes, powerful hydrolytic enzymes secreted by the fungus, digest food outside its body to simpler compounds that the fungus can absorb and use.  Exoenzymes are to fungi what _____ are to us.  Saprobic and parasitic fungi occur in nature.

6.  Fungi exhibit diverse lifestyles  Decomposers  Parasites  Mutualists

7. Body Structure  The most common body structures are multicellular filaments and single cells (yeasts)  Some species grow as either filaments or yeasts; others grow as both

8.  The morphology of multicellular fungi enhances their ability to absorb nutrients  The vegetative bodies of most fungi are constructed of tiny filaments called hyphae that form an interwoven mat called a mycelium.  Most fungi have cell walls made of chitin

9.  Most fungi are multicellular with hyphae divided into cells by cross walls, or septa.  These generally have pores large enough for ribosomes, mitochondria, and even nuclei to flow from cell to cell.  Fungi that lack septa, coenocytic fungi, consist of a continuous cytoplasmic mass with hundreds or thousands of nuclei.  This results from repeated nuclear division without cytoplasmic division.

10.  Parasitic fungi usually have some hyphae modified as haustoria, nutrient-absorbing hyphal tips that penetrate the tissues of their host.  Some fungi even have hyphae adapted for preying on animals.

11.  Mycorrhizae are mutually beneficial relationships between fungi and plant roots  Mycorrhizal fungi deliver phosphate ions and minerals to plants  Most vascular plants have mycorrhizae

12. 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  Fungi can produce spores from different types of life cycles

13. Figure 31.5-1 Key Haploid (n) Heterokaryotic Diploid (2n) Spores Spore-producing structures ASEXUAL REPRODUCTION GERMINATION Mycelium

14. Figure 31.5-2 PLASMOGAMY Key Haploid (n) Heterokaryotic Diploid (2n) Spores Spore-producing structures ASEXUAL REPRODUCTION SEXUAL REPRODUCTION GERMINATION Zygote Heterokaryotic stage KARYOGAMY Mycelium

15. Figure 31.5-3 PLASMOGAMY Key Haploid (n) Heterokaryotic Diploid (2n) Spores Spore-producing structures ASEXUAL REPRODUCTION SEXUAL REPRODUCTION GERMINATION MEIOSIS Spores Zygote Heterokaryotic stage KARYOGAMY Mycelium

16. Sexual Reproduction  Fungal nuclei are normally haploid, with the exception of transient diploid stages formed during the sexual life cycles  Sexual reproduction requires the fusion of hyphae from different mating types  Fungi use sexual signaling molecules called pheromones to communicate their mating type

17.  Plasmogamy is the union of cytoplasm from two parent mycelia  In most 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

18.  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  The paired processes of karyogamy and meiosis produce genetic variation

19. Asexual Reproduction  In addition to sexual reproduction, many fungi can reproduce asexually  Molds produce haploid spores by mitosis and form visible mycelia

20. Figure 31.6 1.5  m

21.  Other fungi that can reproduce asexually are yeasts, which are single cells  Instead of producing spores, yeasts reproduce asexually by simple cell division and the pinching of “bud cells” from a parent cell  Some fungi can grow as yeasts and as filamentous mycelia

22. Figure 31.7 10  m Parent cell Bud

23.  Many molds and yeasts have no known sexual stage  Mycologists have traditionally called these deuteromycetes, or imperfect fungi  This is not a sound taxonomic group; fungi are reclassified once their sexual stage is discovered

24. Concept 31.3: The ancestor of fungi was an aquatic, single-celled, flagellated protist  Fungi and animals are more closely related to each other than they are to plants or other eukaryotes

25. Concept 31.4 Fungi Diversity  Roughly 100,000 species of fungi are known and mycologists estimate that there are actually about 1.5 million species worldwide.  Molecular analyses supports the division of the fungi into five phyla, but some uncertainty still remains.

27.  The chytrids are mainly aquatic.  Some are saprobes, while others parasitize protists, plants, and animals.  The presence of flagellated zoospores had been used as evidence for excluding chytrids from kingdom Fungi which lack flagellated cells.  However, recent molecular evidence supports the hypothesis that chytrids are the most primitive fungi. Chytrids

28.  Most of the 1000 zygomycete, or zygote fungi, are terrestrial, living in soil or on decaying plant and animal material.  They include fast-growing molds, parasites, and commensal symbionts  The zygomycetes are named for their sexually produced zygosporangia  One zygomycete group form mycorrhizae, mutualistic associations with the roots of plants.  Zygomycete hyphae are coenocytic, with septa found only in reproductive structures. Zygomycetes

29.  The life cycle and biology of Rhizopus, black bread mold, is typical of zygomycetes.  Horizontal hyphae spread out over food, penetrate it, and digest nutrients.  In the asexual phase, hundreds of haploid spores develop in sporangia at the tips of upright hyphae.  If environmental conditions deteriorate, this species of Rhizopus reproduces sexually.

31. Glomeromycetes  The glomeromycetes (phylum Glomeromycota) were once considered zygomycetes  They are now classified in a separate clade  Glomeromycetes form arbuscular mycorrhizae, fungi that extend hyphae through the cell walls of root cells.

32.  Mycologists have described over 65,000 species of ascomycetes, or sac fungi.  They range in size and complexity from unicellular yeasts to elaborate cup fungi and morels. Ascomycetes

33.  Ascomycetes live in a variety of aquatic, and terrestrial habitats.  Some are plant pathogens. Many are saprobes, particularly of plant material.  The phylum is defined by production of sexual spores in saclike asci, usually contained in fruiting bodies called ascocarps  Ascomycetes reproduce asexually by enormous numbers of asexual spores called conidia  Neurospora crassa, a bread mold, is a model organism with a well-studied genome

34.  Approximately 30,000 fungi, including mushrooms, shelf fungi, puffballs, and rusts, are classified in the phylum Basidiomycota.  The phylum is defined by a clublike structure called a basidium, used in sexual reproduction. Basidiomycetes

35.  Basidiomycetes are important decomposers of wood and other plant materials.  Of all fungi, these are the best at decomposing the complex polymer lignin, abundant in wood.  Two groups of basidiomycetes, the rusts and smuts, include particularly destructive plant parasites.

36.  By concentration growth in the hyphae of mushrooms, a basidiomycete mycelium can erect basidiocarps (containing basidia) in just a few hours.  Some species may produce “fairy rings” overnight.  At the center of the ring are areas where the mycelium has already consumed all the available nutrients.  As the mycelium radiates out, it decomposes the organic matter in the soil and mushrooms form just behind this advancing edge.

37.  The four of the five phyla can be distinguished by their reproductive features.

38.  A mold is a rapidly growing, asexually reproducing fungus.  The mycelia of these fungi grow as saprobes or parasites on a variety of substrates.  Early in life, a mold, a term that applies properly only to the asexual stage, produces asexual spores.  Later, the same fungus may reproduce sexually, producing zygosporangia, ascocarps, or basidiocarps. Molds

39.  Some molds cannot be classified as zygomycetes, ascomycetes, or basidiomycetes because they have no known sexual stages.  Collectively called deuteromycetes, or imperfect fungi, these fungi reproduce asexually by producing haploid spores.  This is an informal grouping without phylogenetic basis.

40.  Yeasts are unicellular fungi that inhabit liquid or moist habitats, including plant sap and animal tissues.  Humans have used yeasts to raise bread or ferment alcoholic beverages for thousands of years.  Various strains of the yeast Saccharomyces, an ascomycete, have been developed as baker’s yeast and brewer’s yeast. Yeasts

41. Concept 31.5 Fungi Ecology  Fungi interact with other organisms in many ways  Fungi are efficient decomposers  Fungi form mutualistic relationships with plants, algae, cyanobacteria, and animals  Mycorrhizae are enormously important in natural ecosystems and agriculture

42. Fungus-Animal Symbioses  Some fungi share their digestive services with animals  These fungi help break down plant material in the guts of cows and other grazing mammals  Many species of ants use the digestive power of fungi by raising them in “farms”

43. Figure 31.22

44.  While often mistaken for mosses or other simple plants when viewed at a distance, lichens are actually a symbiotic association of millions of photosynthetic microorganisms held in a mesh of fungal hyphae. Lichens

45.  The fungal hyphae provides most of the lichen’s mass and gives it its overall shape and structure.  The algal component usually occupies an inner layer below the lichen surface.

46.  Fungi and bacteria are the principle decomposers that keep ecosystems stocked with nutrients.  Without decomposers, elements would be consumed.  However, aggressive decomposition can be a problem.  Between 10% and 50% of the world’s fruit harvest is lost each year to fungal attack.  Fungi do not distinguish between wood debris and human structures built of wood. Fungi as principle decomposers

47.  About 30% of the 100,000 known species of fungi are parasites, mostly on or in plants.  Invasive ascomycetes have had drastic effects on forest trees, such as American elms and American chestnut, in the northeastern United States.  Other fungi, such as rusts and ergots, infect grain crops, causing tremendous economic losses each year. Fungi as Pathogens

48.  Some fungi that attack food crops produce compounds that are harmful to humans.  For example, the mold Aspergillus can contaminate improperly stored grains and peanuts with aflatoxins, which are carcinogenic.  Poisons produced by the ascomycete Claviceps purpurea can cause gangrene, nervous spasms, burning sensations, hallucinations, and temporary insanity when infected rye is milled into flour and consumed.  On the other hand, some toxin extracted from fungi have medicinal uses when administered at weak doses.

49.  The general term for a fungal infection is mycosis.  Infections of ascomycetes produce the disease ringworm, known as athlete's foot when they grow on the feet.  Inhaled infections of other species can cause tuberculosis-like symptoms.  Candida albicans is a normal inhabitant of the human body, but it can become an opportunistic pathogen causing yeast infections.

50.  Yeast are even more important in food production.  Yeasts are used in baking, brewing, and winemaking.  Contributing to medicine, some fungi produce antibiotics used to treat bacterial diseases.  The first antibiotic discovered was penicillin, made by the common mold ascomycete Penicillium. Practical Uses of Fungi