Introductory Mycology – Cell Structures

What are fungi?

• Eukaryotic, spore-bearing, heterotrophic organisms that produce extracellular enzymes and absorb their nutrition.

Body Plan• unicellular (yeast), filamentous, or both (=dimorphic)

• Hypha (pl. hyphae) is the basic “cellular” unit in filamentous fungi; they may be septate or coenocytic (aseptate); collectively a mycelium

• limited tissue differentiation and division of labor

• somatic & reproductive structures

• plectenchyma: all organized fungal tissue, somatic & reproductive

Fungal nuclei

•1--3 m diam•3--40 chromosomes•Up to 13--40 Mb (million base pairs) DNA coding for 6,000 to 13,000 genes•Intranuclear division--nuclear envelope remains intact during mitosis (unlike plants and animals)

Organism # bp   # genes

Escherichia coli  4,600,000 4288

Saccharomyces cerevisiae 13,000,000 5885

Caenorhabditis elegans ~100,000,000 ~14,000

Arabidopsis thaliana ~120,000,000 ~10,000

Drosophila melanogaster ~170,000,000 ~12,000

humans ~3,400,000,000 ~80,000

Fungi as model organisms

•Small genome relative to other eukaryotes•Many fungal genes are homologous to those in other eukaryotes•Easy to grow, short life cycles•Haploid genomes amenable to mutation•Sexual stage for analysis of segregation and recombination of genes; all products of meiosis can be retrieved in haploid spores•Asexual (clonal) reproduction

Beadle and TatumUsing the common bread mold Neurospora crassa, in 1941 developed the classic concept of “one gene, one enzyme”Awarded Nobel Prize in 1945

Nuclear Status• Eukaryotic; uni, bi- or multinucleate• Haploid, diploid (less frequent)• Monokaryon (1 nucleus per hyphal compartment)

• Dikaryon (2 nuclei per hyphal compartment) • Homokaryotic

• Heterokaryotic

• Mitosis– intranuclear: nuclear membrane doesn't

breakdown during mitosis– centric in flagellated forms; typical centrioles

of eukaryotes– noncentric in nonflagellated forms; possess

spindle pole bodies (SPBs); differ from centrioles in lacking microtubular component

Organelles• typical eukaryote assemblage of organelles

+ fungal specific ones• mitochondria • endoplasmic reticulum • Golgi equivalents

single cisternal elements • vacuoles • microbodies

funx in fatty acid degradation, N metabolism

Other organelles• Mitochondria—flattened or plate-like

mitochondrial cristae in Fungi (similar to animals)

• Golgi bodies—consist of a single, tubular cisternal element (stacked, plate-like cisternae in animals and plants)

• Other types: – ribosomes, endoplasmic reticulum, vacuoles, lipid

bodies, glycogen storage particles, microbodies, microtubules, vesicles

Cell Wall• well defined

• chitin

1-4 n-acetyal glucosamine -glucans

polymers of glucose

1-3 glucose

• cellulose in some

1-4 glucose

chitin -glucans

Chitin

Cellulose

1-3 glucan

Fungal cell wall composition

Structural components:chitin microfibrils [ß(1-4)-linked polymer of N-acetylglucosamine]chitosan in Zygomycota [ß(1-4)-linked polymer of glucosamine] ß-linked glucans

Gel-like components:Mannoproteins (form matrix throughout wall

Other cell wall components

Antigenic glycoproteins, agglutinans, adhesions—on cell wall surface Melanins—dark brown to black pigments (confer resistance to enzyme lysis, confer mechanical strength and protect cells from UV light, solar radiation and desiccation)Plasma membrane—semi-permeable

•fungal specific organelles involved in cell wall growth

Spitzenkorper or vacuole

associated with growing hyphal tips in septate fungi

chitosome

microvesicles transporting chitin synthases to growing cell wall

Nutrition• Heterotrophic• Secrete extracellular enzymes • Absorptive nutrition• Saprobes: decay dead organic matter• pathogens: biotroph, necrotroph • symbionts: parasites - commensals - mutualists

Spores - a minute propagative unit functioning as a seed, but differing from it in that a spore does not contain a preformed embryo

Fruiting body - any complex fungal structure that contains or bears spores; a sporocarp

Reproduction

• Sexual reproduction: spores meiotically derived nuclei

• Homothallic (selfing)• Heterothallic (outcrossing)• Genetic mating system

– MAT loci – 1 to hundreds of “sexes”

• Asexual reproduction– Spores with mitotically derived nuclei

Fungal Reproduction

• Many fungi have the ability to reproduce by asexual and sexual means

Asexual Reproduction in Fungi• Fragmentation – hyphae simply break off.

• Budding – small outgrowth of hyphae pinches off

• Formation of spores – Sporangiospores: produced in sporangia (sac-like

structures) located on a sporangiophore.– Conidiospores: produced at the tips of specialized

hyphae

Fungal reproduction

• Anamorph= asexual stage – Mitospore=spore formed via asexual

reproduction (mitosis), commonly called a conidium or sporangiospore

• Teleomorph= sexual stage– Meiospore=spore formed via sexual

reproduction (e.g., resulting from meiosis), type of spore varies by phylum

Kingdom Fungi• Phyla:

– Zygomycota• Form asexual spores called sporangiospores• Meiosis occurs in zygospore

– Ascomycota (including Deuteromycetes)• Form asexual spores called conidia• Meiosis occurs in ascus

– Basidiomycota• Meiosis occurs in basidium

Fungal life cycles

• The vegetative thallus predominates in the life cycle of a fungus

• The thallus may be haploid (1n), dikaryotic (n+n) or diploid (2n) in different groups of fungi

• Ploidy of thallus is determined by the timing of these events in the life cycle:– Plasmogamy (cell fusion)– Karyogamy (nuclear fusion)– Meiosis (reduction division)

Misc.

• Life cycle: simple to complex; wide variety

• Sporocarps: microscopic or macroscopic, limited tissue differientiation

• Habitat: ubiquitous

• Studied by mycologists!!!!!!

2n n

Fungal life cycles

Plasmogamy

n n+n

Karyogamy

n+n 2n

Life cycle is predominantly haploid (n)

mitosis

Meiosis

n n + n

Fungal life cycles

Karyogamy

n + n 2n

Meiosis

2n n

Life cycle is predominantly dikaryotic (n + n)

mitosis

Plasmogamy

n + n 2n

Fungal life cycles

Meiosis

2n n

Plasmogamy

n n + n

Life cycle is predominantly diploid (2n)

mitosis

Karyogamy

What are fungi?

• Eukaryotic, spore-bearing, heterotrophic organisms that produce extracelluar enzymes and absorb their nutrition.

Fungi vs. "fungi"

• Based on the phenotypic definitions or traits attributed to fungi, fungi do not comprise a single monophyletic group of organisms

• more than one evolutionary origin

• not all "fungi" are members of the Kingdom Fungi

Some characters that separate the the Kingdom Fungi from “protistan fungi”

Kingdom Fungi Protistan fungi

mitochondria: cristae flattened cristae tubular

motile cells: no motile cells or motile cells with anteriorposterior flagellum or lateral

heterokont flagella cell wall carbohydrate: glucans, chitin glucans, cellulose lysine biosynthesis: alpha-aminoadepic acid (AAA) diaminopimelic (DAP)

storage compound glycogen mycolaminarins

sterols ergosterol fucosterol

Phyla: Chytridiomycota Oomycota(emphasized Zygomycota Myxomycotain class) Glomeromycota Dictyosteliomycota

Basidiomycota Ascomycota

General characteristics of the Phyla of the Kingdom Fungibasal lineages

Zygomycota: (1000)•generally coenocytic mycelium •production of zygosporangia & zygospores•no sporocarp production

Basidiomycota: (22500)•septate mycelium•clamp connections•complex dolipore septa •dikaryotic, haploid mycelium •production of exospores (basidiospores) on a basidium •production of complex sporocarps

Ascomycota: (35000)•septate mycelium•simple septa •monokaryotic, haploid mycelium •production of endospores (ascospores)

in an ascus •production of complex sporocarps•often dominant asexual reproduction

Kingdom - Fungi Phylum - Basidiomycota

Class - HymenomycetesOrder - Agaricales

Family - AmanitaceaeGenus - Amanita

Species - A. muscaria

Amanita muscaria

Nomenclature

Medically important fungi are in four phyla:

1. Ascomycota - Sexual reproduction in a sack called an ascus with the production of ascopspores.( Aspergillus, Blastomyces dermatidis, Histoplasma capsulatus)

2. Basidiomycota -Sexual reproduction in a sack called a basidium with the production of basidiospores.( Cryptococcus neoformans)

3. Zygomycota - sexual reproduction by gametes and asexual reproduction with the formation of zygospores.( Rhizopus, Mucor)

4. Deuteromycota(Fungi Imperfecti \Mitosporic Fungi) - no recognizable form of sexual reproduction. Includes most pathogenic fungi ( Sporothrix, Coccidioides immitis, Candida, Pneumocystis).

Introductory Mycology- Cell Structure