Introduction to Kingdom Fungi

Pl P 421/521 General Mycology LECTURE 2

What is a fungus?

A eukaryotic, heterotrophic organism devoid of chlorophyll that obtains its nutrients by absorption, and reproduces by spores.

Modes of nutrition Fungi=absorptive heterotrophs Animals=phagotrophic heterotroph Heterotroph (chemo-organotrophs):

an organism incapable of synthesizing carbohydrates from inorganic sources; requires preformed organic compounds produced by other organisms

Plants=autotrophs

Hyphae (singular, hypha) Cylindrical, branching filaments composed

of a tubular cell wall filled with cytoplasm and organelles

Most fungal hyphae are 2-10 m diameter

http://www.uoguelph.ca/~gbarron/MISCELLANEOUS/hairpen.htm

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

Septa Septa—regular cross-walls formed in hyphae.

Hyphae with septa are septate, those lacking septa except to delimit reproductive structures and aging hyphae are called aseptate or coenocytic. primary septa are formed as a process of hyphal

extension and generally have a septal pore, which allows for cytoplasmic and organelle movement.

Secondary or adventitious septa are imperforate, formed to wall off ageing parts of the mycelium.

Septal pores--Ascomycota Woronin bodies

Septal pores--Basidiomycota

Dolipore septa, septal pore caps or parenthosomes

Septal pore cap

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

Tree of eukaryotes, showing variation in genome size. From Keeling and Slamovits (2005). Current Opinion in Genetics and Development 15: 601-608

“Unikont”—eukaryotic cell with one flagellum

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 Tatum

Using the common bread mold Neurospora crassa, in 1941 developed the classic concept of “one gene, one enzyme”

Awarded Nobel Prize in 1945

Fungal nuclei Usually haploid Nuclear membrane persists during division Nuclear associated organelles (NAOs):

Associated with the nuclear envelope; function as microtubule-organizing centers during mitosis and meiosis

Spindle pole bodies In fungi that lack a flagellated stage in lifecycle

Centrioles In fungi and other organisms possessing flagellated stage

in lifecycle

Spindle Pole Body

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

Storage Compounds

Glycogen, lipids and trehalose in fungi and animals

Starch in plants

Animal Cell Plant Cell

G. T. Cole. 1986. Microbiol. Rev. 50: 95-132

Fungal Reproduction

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

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:

Chytridiomycota Form motile spores called zoospores Meiosis occurs in resting sporangium

Glomeromycota Form spores containing hundreds of nuclei; no known sexual reproduction

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

Concept of fungal holomorph

Asexual and sexual reproduction may be separated in time and space

The holomorph is the entire fungus—including asexual and sexual stages if both are formed

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)

2n n

Fungal life cycles

Plasmogamyn n+n

Karyogamy n+n 2n

Life cycle is predominantly haploid (n)

mitosis

Meiosis

n n + n

Fungal life cycles

Karyogamyn + n 2n

Meiosis 2n n

Life cycle is predominantly dikaryotic (n + n)

mitosis

Plasmogamy

n + n 2n

Fungal life cycles

Meiosis2n n

Plasmogamy n n + n

Life cycle is predominantly diploid (2n)

mitosis

Karyogamy