chapter 31 Flashcards
Defining Fungi
Mycologists believe there may be as many as 1.5 million fungal species
Single-celled or multicellular
Sexual or asexual
Specialized to extract and absorb nutrients from surroundings
Animal and fungi last shared a common ancestor 460 MYA
Some debate on timing
7 monophyletic phyla
Microsporidia Blastocladiomycota Neocallismastigamycota Chytridiomycota Glomeromycota Basidiomycota Ascomycota Zygomycota is not a monophyletic phyla
General Biology of the Fungi
Multicellular fungi consist of long, slender filaments called hyphae
Some hyphae are continuous
Others are divided by septa
Cytoplasm flows throughout hyphae
Allows rapid growth under good conditions
Except—when reproductive structures form, they are cut off by complete septa that lack perforations or the perforations become blocked
Mycelium – mass of connected hyphae
Grows through and digests its substrate
Fungal cell walls include chitin
Also found in the hard shells (exoskeletons) of arthropods
Hyphae may have more than one nucleus
Monokaryotic – 1 nucleus 1n or 2n
Dikaryotic – 2 nuclei 1n + 1n
Both genomes transcribed
If the 2 nuclei fuse = karyogamy
Sometimes many nuclei intermingle in the common cytoplasm of the fungal mycelium
Heterokaryotic – nuclei from genetically distinct individuals
Homokaryotic – nuclei are genetically similar to one another
Fungi have an unusual mitosis
Cell is not relevant unit of reproduction
Nuclear envelope does not break down and re-form
Instead, the spindle apparatus is formed within it
Fungi lack centrioles
Spindle plaques regulate microtubule formation during mitosis
Reproduction
Capable of both sexual and asexual reproduction
Sexual reproduction
Fusion of two haploid hyphae of compatible mating types
In some fungi, fusion immediately results in a diploid (2n) cell
Others, have a dikaryon stage (1n + 1n) before parental nuclei form diploid nucleus
May form mushrooms or puffballs
Spores are the most common means of reproduction among fungi
May form from sexual or asexual processes
Asexually produced spores are produced by
conidiophores and the spores are called conidia
Most are dispersed by wind
Nutrition
Obtain food by secreting digestive enzymes into surroundings
Then absorb the organic molecules produced by this external digestion
Great surface area-to-volume ratio
Fungi can break down cellulose and lignin
Decompose wood
Some fungi are carnivorous
Microsporidia
Obligate, intracellular, animal parasites
Long thought to be protists
Lack mitochondria
Ancestors lost them
Encephalitozoon cuniculi
Commonly cause disease in immunosuppressed patients
Infect hosts with their spores, which contain a polar tube—go inside cells
Infects intestinal and neuronal cells, leading to diarrhea and neurodegenerative disease
Chytridiomycota
Chytridiomycetes or chytrids Aquatic, flagellated fungi Closely related to ancestral fungi Have motile zoospores Batrachochytrium dendrobatidis has been implicated in amphibian die-offs
Blastocladiomycetes
Uniflagellated zoospores Allomyces example Water mold Haplodiplontic life cycle Female gametes secrete pheromone to attract male gametes Giant mitochondria in its zoospores
Neocallimastigamycota
Digest plant biomass in mammalian herbivore rumens
Mammal depends on fungi for sufficient calories
Greatly reduced mitochondria lack cristae
Zoospores have multiple flagella
Horizontal gene transfer brought cellulase gene from bacteria into Neocallimastix genome
Zygomycota
Zygomycetes are incredibly diverse
Not monophyletic – still under research
Include the common bread molds
A few human pathogens
Sexual reproduction
Fusion of gametangia
Haploid nuclei fuse to form diploid zygote nuclei – karyogamy
Develops into zygosporangium in which zygospore develops
Meiosis occurs during germination of zygospore
Releases haploid spores
Asexual reproduction more common
Sporangiophores have sporangia that release spores
Glomeromycota
Glomeromycetes are a tiny group of fungi
Form intracellular associations with plant roots called arbuscular mycorrhizae
Cannot survive in absence of host plant
No evidence of sexual reproduction
Basidiomycota
Basidiomycetes are some of the most familiar fungi
Mushrooms, toadstools, puffballs, shelf fungi, etc.
Also important plant pathogens like rusts and smuts
Named for basidium – club-shaped sexual reproductive structure
Karyogamy occurs within basidia
Only diploid cell in life cycle
Meiosis follows
The four haploid products are incorporated into basidiospores
Spore germination leads to the production of monokaryotic hyphae
Results in a monokaryotic mycelium, or primary mycelium
Different mating types of monokaryotic hyphae may fuse
Results in a dikaryotic mycelium, or secondary mycelium
Heterokaryotic mycelium
Basidiocarps (mushrooms) are formed entirely of secondary mycelium
Ascomycota
Contain about 75% of the known fungi
Includes bread yeasts, common molds, cup fungi, truffles, and morels—sac fungi
Serious plant pathogens – cause of chestnut blight and Dutch elm disease
Penicillin-producing fungi are in the genus Penicillium
Named for ascus – microscopic, saclike reproductive structure
Karyogamy occurs within asci
Only diploid nucleus of life cycle
Asci differentiate in ascocarp
Meiosis and mitosis follow, producing 8 haploid nuclei that become walled ascospores
Asexual reproduction is very common
Conidia formed at the ends of modified hyphae called conidiophores
Allow for the rapid colonization of a new food source
Many conidia are multinucleate
Yeast
Unicellular ascomycetes Most reproduce asexually by budding Yeasts can ferment carbohydrates Break down glucose into ethanol and CO2 Used to make bread, beer, and wine Saccharomyces cerevisiae
Yeast is a long-standing model system for genetic research
First eukaryotes to be manipulated extensively
Saccharomyces cerevisiae first eukaryote to have genome sequenced
More are being sequenced—one that causes
coccidioidomycosis—in soil in SW US—can be fatal
hey
Ecology of Fungi
Fungi, together with bacteria, are the principal decomposers in the biosphere
Break down cellulose and lignin from wood
Release carbon, nitrogen, and phosphorus
Fungi symbioses
Obligate symbiosis – essential for fungus survival
Facultative symbiosis – nonessential
Interactions
Pathogen – pathogens harm host by causing disease
Parasites cause harm to host (do not cause disease)
Commensal relationships benefit one partner but does not harm the other
Mutualistic relationships benefit both partners
Endophytic fungi
Live in the intercellular spaces inside plants
Some parasitic, some commensalistic
Some fungi protect their hosts from herbivores by producing toxins
Lichens
Symbiotic associations between a fungus and a photosynthetic partner
Cyanobacteria, green algae, or sometimes both
Most are mutualistic
Ascomycetes are found in all but about 20 of the 15,000 lichen species
Fungi in lichens are unable to grow normally without their photosynthetic partners
Fungi protect their partners from strong light and desiccation
Lichens have invaded the harshest habitats
Striking colors play a role in protecting photosynthetic partner
Sensitive to pollutants
jack
Mycorrhizae
Mutualistic relationships between fungi and plants
Found on the roots of about 90% of all known vascular plant species
Function as extensions of root system
Increase soil contact and absorption
Two principal types
Arbuscular mycorrhizae
Ectomycorrhizae
Arbuscular mycorrhizae -By far the most common Fungal partners are glomeromycetes No aboveground fruiting structures Potentially capable of increasing crop yields with lower phosphate and energy inputs
Ectomycorrhizae
-Most hosts are forest trees (pines, oaks)
Fungal partners are mostly basidiomycetes
At least 5000 species of fungi are involved in ectomycorrhizal relationships
Animal mutual symbioses
Ruminant animals host neocallimastigamycete fungi in their gut
Leaf-cutter ants have domesticated fungi which they keep in underground gardens
Ants provide fungi with leaves
Fungi are food for the ants
Fungal Parasites and Pathogens
Fungal species cause many diseases in plants
Among most harmful pests of living plants
Can also spoil harvested or stored food products
Fungi also cause human and animal diseases
Candida – thrush; vaginal infections
Pneumocystis jiroveci – pneumonia in AIDS
Athlete’s foot, ringworm, and nail fungus
Fungal diseases are difficult to treat because of the close phylogenetic relationship between fungi and animals
Batrachochytrium dendrobatidis causes chytridiomycosis
– Responsible for the worldwide decline in amphibian populations
