PHYLUM: ASCOMYCOTA

CLASS: ASCOMYCETES

Readings: Pages 180 - 213 Alexopolous, Mimms and Blackwell 4th Edition - Chapter 7

l These fungi are all truly mycelial with some yeast stages.

l Derived from chytridiomycete lines based on cell wall data.

Subdivisions:

* Distinguished from all other fungi by the sexual meiospores, ascospores, borne in an ascus (pl. asci).

* The ascus is a sac-like structure - hence ascomycetes are sometimes known as the sac fungi.

* Usually (not always) 8 ascospores in an ascus

* Ascospores are often borne in a protective fruiting body known as an ascocarp.

* Hyphae is septate with simple septa

* All have Chitin-β1,3-glucan cell walls

* No motile spore stages

* Predominately Haploid/Dikaryotic

* Many ascomycetes produce asexual conidia (sing. conidium)

* More than one type of conidium may be produced by one species

CLASSIFICATION:

There are four major types ways asci are presented: Fig 7-18 (None, Cleistothecium, Perithecium and Apothecium.)

Note: also that there are other stromatic types eg. Fig 7-19, 7-20 (Xylaria).

The group is divided on the basis of the ascocarp and the asci.

« Nonascocarpic (naked asci) v's Ascocarpic

l Prototunicate v's Unitunicate v's Bitunicate asci

Class Ascomycetes (only class in phylum)

Archiascomycetes - Nonascocarpic; Naked, Prototunicate asci

Plectomycetes - Ascocarpic; Unitunicate asci; No hymenium; Cleistothecia

Pyrenomycetes - Ascocarpic; Unitunicate asci; Hymenium present; Perithecia

Discomycetes - Ascocarpic; Unitunicate and Bitunicate Asci, Hymenium present, Apothecia

Laboulbeniomycetes - Ascocarpic; Unitunicate asci; Hymenium present; Perithecia; Obligate arthropod parasites

Loculoascomycetes - Ascocarpic; Bitunicate asci; Hymenium present; Ascostroma, Psuedothecia

SEXUAL REPRODUCTION IN THE ASCOMYCETES

l The basis of sexual reproduction in the Ascomycetes is the same as all other organisms: To bring together two nucleii that will eventually fuse. This is achieved by four main methods:

1) Gametangial copulation

* Similar to Zygomycete copulation

* Two undifferentiated gametangia touch and fuse

* The fusion cell develops into a single ascus.

* No dikaryon is formed because karyogamy occurs right after plasmogamy.

2) Gametangial Contact (Gametangy)

* Two differentiated gametangia fuse

* Ascogonia (sing: Ascogonium) and Antheridia (sing: Antheridium)

* Fertilization of the ascogonium occurs via a trichogyne when the male nucleus passes into the ascogonium

* The antheridia may be non-functional and fertilization occurs with trichogyne nucelii, hyphal nucleii, or nucleii in the ascogonium itself (It is often multinuceate).

3) Spermatization

* Nucleii reach the Ascogonium via spermatia, microconidia or conidia

* Transfer male nucelii to trichogyne or fertile hyphae.

* Spermatia are formed on the spermophore and may be in spermogonia

4) Somatogamy

* Fusion of somatic hyphae

* Compatible nucleii migrate to ascogonial site through hyphae

l COMPATIBILITY

l ASCOMYCETES may be Heterothallic or Homothallic

* Heterothallic strains carry two complementary alleles at one locus

* The zygote is always heterozygous A1/A2 in heterothallic strains

* After meiosis in the ascus the haploid ascospores are either A1 or A2.

l GENERALIZED LIFE CYCLES OF ASCOMYCETES

* Figure 10-4: Saccharomyces

*Figure 11-1: Ascocarpic ascomycetes

l ASCOSPOROGENESIS - From plasmogamy to Ascospores in the Ascocarpic Ascomycetes (Fig 7-7)

* (Ascogonium forms on fertile hypha; coiled or globose or lobate; uninucleate or multinucleate

* Compatible male nucleii are brought to the ascogonium by one of the above methods

* Karyogamy does not occur and a dikaryon is formed

* Ascogonium forms papillae, which eventually elongate into ascogenous hyphae containing paired compatible nucleii whihc undergo synchronous mitosis

* Primary septum forms at tip of ascogenous hypha to delimit a uninucleate tip cell proximal to which are a series of dikaryotic cells.

* Dikaryotic cells elongate to from croziers or hook shaped cells

* The tip and basal cell are formed by septum formation and the middle cell be comes the dikaryotic ascus mother cell

* The nucleii in the ascus mother cell undergo karyogamy and meiosis then one single mitotic division to form 8 haploid ascospores

* Proliferation can occur by fusion of the tip and basal cells or by further mitosis of the dikaryotic ascus mother cell.

(See Fig 7-7) 

l The alignment of the ascospores in the ascus has proved to be one of the most important tools in genetic analysis.

* Neurospora crassa is a traditional genetic guinea pig for 5 reasons:

1) Meiosis occurs right after karyogamy allowing easy control of parental genotypes

2) Haploid ascospores represent all four viable products of meiosis. Always aligned in order from the meiotic tetrad.

3) Haploid mycelia express genotype. Recessives are not masked

4) Can be easily grown in culture

5) Conidia (asexual) represent simple clonal propagation

l The linear alignment of ascospores in the ascus is of considerable importance to genetics and an interesting question so we look at it in a little more detail.

l Ascopores are formed in pairs from the original meiotic tetrad

* Often binucleate (second mitotic division within the spore itself)

* May have more nucleii through further cycles of mitosis and nuclear abortion

* The ascospores are aligned in precise order from the meiotic tetrad

l Hence in uniseriate asci the order of characters represents the timing of segregation in meiosis.

l Secondary mitosis with cytokinesis leads to biseriate asci