Ascomycetes: Phylum Ascomycota

Ascomycetes

• Group of fungi characterized by their production of sexual spores in a sac-like structure called an ascus.

“Cup fungus”

Types of Ascomycete

• yeast• powdery mildew

• cup fungi–many of these produce spores suited

for airborne dispersion.

The Ascomycota

• Non motile

•Form ascospores Internally in Asci

•Most are blue-green, red and brown molds that cause food spoilage

•Penicillin

Ascomycetes Eyelash cup, Scutellinia scutellata

Ascomycetes can cause plant disease

• Brown rot of stone fruits (Monilinia fructicola)

•chestnut blight (Cryphonectria parasitica)

Dutch elm disease ( Ophiostoma ulmi)

Beneficial Ascomycete• Many Yeast are also ascomycetes

•Edible Morels and truffles

Edible black truffle, Tuber melanosporum spore-bearing structure is produced below ground mainly on oaks and hazelnuts

Edible Ascomycete

Ascomycetes growth

• Most have either unicellular or filamentous growth forms

• Hyphae have perforated septa

• Hyphal cells of Vegetative mycelim may be either uninucleated or multinucleated

• Some are homothallic others heterothallic

Life cycle of an Ascomycete

• Most species undergo asexual reproduction by the formation of multinucleated conidia

• Conidia formed from the conidiogenous cells

• Conidiogeneous cells are borne at the tips of modified hyphae called conidiophores “conidia bearers”

Conidia- the characteristic asexual spores of ascomycetes shows the stages in the formation of conidia which infects the velvetbean caterpillar

TEM of Various stages of developments of conidida

Ascomycetes spore development vs. Zygomycetes spore development

• Unlike zygomycetes which produce spores internally within a sporangium, ascomycetes produce their asexual spores externally as conidia.

Sexual reproduction in Ascomycetes

• Always involves the formation of an ascus (pl. asci)- saclike structure within which a haploid ascospores are formed following meiosis.

• Because the ascus resembles as sac, commonly referred to as “sac fungi”

• Both the asci and ascospores are unique structures that distinguish the ascomycetes from all other fungi

• Ascus formation usually occurs within a complex structure composed of tightly interwoven hyphae- the ascoma (pl. ascomata) or ascocarp.

Maturing ascospores in Asci of Ascodesmis nigricans

Enclosed asci and ascospores of Ascoma called a cleistrothecium

An ascoma of Coniochaeta showing the enclosed asci and ascospores

Perithecium- fruiting body contains ascospores

Xylaria fungus

“Dead mans fingers”

Perithecium with ascospores

Finger Xsec

Hymenial layer

• Asci usually develop on an inner surface of the asoma, a layer called the hymenium or hymenial layer

Hymenium of an ascomycete showing asci with ascospores section thru the hymenial layer of Morela

Life cycle of Ascomycete

• The mycelium grows out from a germinating ascospore

• Mycelium begins to reproduce asexually by forming conidia

• Many conidia are produced• Conidia are responsible for propagating and

disseminating the fungus

Ascomycete life cycle

Sexual Reproduction• Occurs on the same mycelium that produces conidia• The formation of multinucleate gametangia called

antheridia (male) and ascogonia (female) precedes sexual reproduction

• Male nuclei pass into the ascogonium via the trichogyne which is an outgrowth of the ascogonium

• Genetically different nuclei pair but do not fuse• Ascogenous hyphae now begin to grow• Compatible pairs of nuclei migrate and cell division occurs

and creates dikaryotic cells- two compatible haploid nuclei

• Crozier- the apical cell of the ascogenous hypha which allows the paired nuclei to divide simultaneously

• Compatible pair of nuclei fuse (karyogamy) to form a zygote

• Zygote undergoes meiosis producing ascus with 8 nuclei

• Haploid nuclei cut off to form ascospores• Ascus as it matures becomes turgid, and finally burst

to release its ascospores