Chapter 31 Fungi

Concept 31.1: Nutrition and Fungal Lifestyles Fungi are heterotrophs but do not ingest their food Fungi secrete into their surroundings exoenzymes that break down complex molecules And then absorb the remaining smaller compounds

Fungi exhibit diverse lifestyles Decomposers Parasites Mutualistic symbionts

The morphology of multicellular fungi Body Structure The morphology of multicellular fungi Enhances their ability to absorb nutrients from their surroundings Hyphae. The mushroom and its subterranean mycelium are a continuous network of hyphae. Reproductive structure. The mushroom produces tiny cells called spores. Spore-producing structures 20 m Mycelium Figure 31.2

Fungi consist of Most fungi Mycelia, networks of branched hyphae adapted for absorption Most fungi Have cell walls made of chitin, unlike plant cells

Some fungi Coenocytic fungi Have hyphae divided into cells by septa, with pores allowing cell-to-cell movement of materials Coenocytic fungi Lack septa Cell wall Cell wall Nuclei Pore Septum Nuclei Figure 31.3a, b (a) Septate hypha (b) Coenocytic hypha

Mycorrhizae Are mutually beneficial relationships between fungi and plant roots Fungal hyphae between plant cells (colorized SEM) 100 m mycorrhizae mycorrhizae Figure 37.12a

Fungi propagate themselves Concept 31.2: Fungi propagate themselves By producing vast numbers of spores, either sexually or asexually

The sexual life cycle involves Sexual Reproduction The sexual life cycle involves Cell fusion, plasmogamy Nuclear fusion, karyogamy An intervening heterokaryotic stage Occurs between plasmogamy and karyogamy in which cells have haploid nuclei from two parents

The generalized life cycle of fungi Key Haploid (n) Heterokaryotic (unfused nuclei from different parents) Diploid (2n) PLASMOGAMY (fusion of cytoplasm) stage KARYOGAMY (fusion of nuclei) SEXUAL REPRODUCTION Spore-producing structures Spores ASEXUAL Zygote Mycelium GERMINATION MEIOSIS Figure 31.5

Many molds and yeasts have no known sexual stage Asexual Reproduction Many molds and yeasts have no known sexual stage Mycologists have traditionally called these imperfect fungi 10 m Parent cell Bud Figure 31.7 2.5 m Figure 31.6

Concept 31.3: The Origin of Fungi Molecular evidence Supports the hypothesis that fungi and animals diverged from a common ancestor that was unicellular and bore flagella

The oldest undisputed fossils of fungi Are only about 460 million years old 50 m Figure 31.8 

Fungi were among the earliest colonizers of land The Move to Land Fungi were among the earliest colonizers of land Probably as symbionts with early land plants

Concept 31.4: Fungi have radiated into a diverse set of lineages Much of their phylogeny is still uncertain Molecular analysis has helped clarify the evolutionary relationships between fungal groups

Arbuscular mycorrhizal fungi The phylogeny of fungi Chytrids Zygote fungi Arbuscular mycorrhizal fungi Sac fungi Club fungi Chytridiomycota Zygomycota Glomeromycota Ascomycota Basidiomycota Figure 31.9

A review of fungal phyla Table 31.1

Fungi in the phylum Zygomycota, the zygomycetes Include fast-growing molds, parasites, and commensal symbionts Are named for their sexually produced zygosporangia

The life cycle of Rhizopus stolonifer Is fairly typical of zygomycetes Mycelia have various mating types (here designated +, with red nuclei, and , with blue nuclei). 1 Neighboring mycelia of different mating types form hyphal extensions called gametangia, each walled off around several haploid nuclei by a septum. 2 Rhizopus growing on bread ASEXUAL REPRODUCTION Mycelium Dispersal and germination MEIOSIS KARYOGAMY PLASMOGAMY Key Haploid (n) Heterokaryotic (n + n) Diploid Sporangium Diploid nuclei Zygosporangium (heterokaryotic) 100 m Young zygosporangium (heterokaryotic) SEXUAL REPRODUCTION Mating type (+) Mating type () Gametangia with haploid nuclei 50 m Sporangia A heterokaryotic zygosporangium forms, containing multiple haploid nuclei from the two parents. 3 The spores germinate and grow into new mycelia. 8 9 Mycelia can also reproduce asexually by forming sporangia that produce genetically identical haploid spores. The sporangium disperses genetically diverse, haploid spores. 7 4 This cell develops a rough, thick-walled coating that can resist dry environments and other harsh conditions for months. 5 When conditions are favourable, karyogamy occurs, followed by meiosis. 6 The zygosporangium then breaks dormancy, germinating into a short sporangium. Figure 31.12

Some zygomycetes, such as Pilobolus Can actually “aim” their sporangia toward conditions associated with good food sources 0.5 mm Figure 31.13

Zygosporangia, which are resistant to freezing and drying Are capable of persisting through unfavorable conditions Can undergo meiosis when conditions improve

Glomeromycetes Form a distinct type of endomycorrhizae called arbuscular mycorrhizae 90% of all plants form mycorrhizae with glomeromycetes 2.5 m Figure 31.15

Fungi in the phylum Ascomycota Ascomycetes Fungi in the phylum Ascomycota Are found in a variety of marine, freshwater, and terrestrial habitats Are defined by the production of sexual spores in saclike asci, which are usually contained in fruiting bodies called ascocarps

Ascomycetes Vary in size and complexity from unicellular yeasts to elaborate cup fungi and morels (a) The cup-shaped ascocarps (fruiting bodies) of Aleuria aurantia give this species its common name: orange peel fungus. (b) The edible ascocarp of Morchella esculenta, the succulent morel, is often found under trees in orchards. (c) 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. (d) Neurospora crassa feeds as a mold on bread and other food (SEM). 10 m Figure 31.16a–d

Ascomycetes can reproduce Asexually by producing enormous numbers of asexual spores called conidia

A dikaryotic ascus develops. Ascomycetes The life cycle of Neurospora crassa, an ascomycete Ascomycete mycelia can also reproduce asexually by producing haploid conidia. 7 Dispersal ASEXUAL REPRODUCTION Germination Mycelium Conidiophore Mycelia Asci Eight ascospores Ascocarp Four haploid nuclei MEIOSIS KARYOGAMY PLASMOGAMY SEXUAL REPRODUCTION Diploid nucleus (zygote) Ascogonium Ascus (dikaryotic) Dikaryotic hyphae Mating type () Conidia; mating type () Key Haploid (n) Dikaryotic (n  n) Diploid (2n) Neurospora can reproduce sexually by producing specialized hyphae. Conidia of the opposite mating type fuse to these hyphae. 1 A dikaryotic ascus develops. 2 Karyogamy occurs within the ascus, producing a diploid nucleus. 3 The developing asci are contained in an ascocarp. The ascospores are discharged forcibly from the asci through an opening in the ascocarp. Germinating ascospores give rise to new mycelia. 6 5 Each haploid nucleus divides once by mitosis, yielding eight nuclei. Cell walls develop around the nuclei, forming ascospores (LM). The diploid nucleus divides by meiosis, yielding four haploid nuclei. 4 Figure 31.17

Fungi in the phylum Basidiomycota Basidiomycetes Fungi in the phylum Basidiomycota Include mushrooms and shelf fungi Are defined by a clublike structure called a basidium, with a transient diploid stage in the life cycle

Basidiomycetes Figure 31.18a–d (a) Fly agaric (Amanita muscaria), a common species in conifer forests in the northern hemisphere (b) Maiden veil fungus (Dictyphora), a fungus with an odor like rotting meat (c) Shelf fungi, important decomposers of wood (d) Puffballs emitting spores Figure 31.18a–d

The life cycle of a basidiomycete Usually includes a long-lived dikaryotic mycelium, which can erect its fruiting structure, a mushroom, in just a few hours Figure 31.19 Fairy Ring

The life cycle of a mushroom-forming basidiomycete A dikaryotic mycelium forms, growing faster then, and ultimately crowding out, the haploid parental mycelia. 2 Two haploid mycelia of different mating types undergo plasmogamy. 1 PLASMOGAMY Dikaryotic mycelium Basidiocarp (dikaryotic) KARYOGAMY Key MEIOSIS Gills lined with basidia SEXUAL REPRODUCTION Mating type () Mating type () Haploid mycelia Dispersal and germination Basidiospores Basidium with four appendages Basidium containing four haploid nuclei Basidia (dikaryotic) Diploid nuclei Basidiospore 1 m Basidium Haploid (n) Dikaryotic (n  n) Diploid (2n) 3 Environmental cues such as rain or temperature changes induce the dikaryotic mycelium to form compact masses that develop into basidiocarps (mushrooms, in this case). In a suitable environment, the basidiospores germinate and grow into short-lived haploid mycelia. 8 When mature, the basidiospores are ejected, fall from the cap, and are dispersed by the wind. 7 The basidiocarp gills are lined with terminal dikaryotic cells called basidia. 4 Each diploid nucleus yields four haploid nuclei. Each basidium grows four appendages, and one haploid nucleus enters each appendage and develops into a basidiospore (SEM). 6 Karyogamy in the basidia produces diploid nuclei, which then undergo meiosis. 5 Figure 31.20

Concept 31.5: Fungi have a powerful impact on ecosystems and human welfare

Fungi are decomposers of organic material Performing essential recycling of chemical elements between the living and nonliving world

Fungi form symbiotic relationships with Symbionts Fungi form symbiotic relationships with Plants, algae, and animals

Mycorrhizae Mycorrhizae Are enormously important in natural ecosystems and agriculture Increase plant productivity Researchers grew soybean plants in soil treated with fungicide (poison that kills fungi) to prevent the formation of mycorrhizae in the experimental group. A control group was exposed to fungi that formed mycorrhizae in the soybean plants’ roots. EXPERIMENT The soybean plant on the left is typical of the experimental group. Its stunted growth is probably due to a phosphorus deficiency. The taller, healthier plant on the right is typical of the control group and has mycorrhizae. CONCLUSION These results indicate that the presence of mycorrhizae benefits a soybean plant and support the hypothesis that mycorrhizae enhance the plant’s ability to take up phosphate and other needed minerals. Figure 31.21 RESULTS RESULTS

Fungus-Animal Symbiosis Some fungi share their digestive services with animals Helping break down plant material in the guts of cows and other grazing mammals

Many species of ants and termites take advantage of the digestive power of fungi by raising them in “farms” Figure 31.22

Lichens Are a symbiotic association of fungal hyphae with algae or cyanobacteria Ascocarp of fungus Fungal hyphae Algal layer Soredia Algal cell Fungal hyphae 10 m Figure 31.24

Lichens Are classified in three broad groups Figure 31.23a–c (a) A fruticose (shrub-like) lichen (b) A foliose (leaf-like) lichen (c) Crustose (crust-like) lichens Figure 31.23a–c

(b) Tar spot fungus on maple leaves Pathogens About 30% of known fungal species Are parasites, mostly on or in plants (a) Corn smut on corn (b) Tar spot fungus on maple leaves (c) Ergots on rye Figure 31.25a–c

Practical Uses of Fungi Humans eat many fungi And use others to make cheeses, alcoholic beverages, and bread

Antibiotics produced by fungi treat bacterial infections (i. e Antibiotics produced by fungi treat bacterial infections (i.e. penicillin) Staphylococcus Penicillium Zone of inhibited growth Figure 31.26