Ascomycota Fungi

• Ascomycota is a large phylum of fungi
• They possess septate assimilative hyphae with chitinous walls

Introduction to Ascomycota

• This phylum includes over 32,000 species of familiar and economically vital fungi
• Ascomycetes encompasses blue-green, red, and brown molds, which cause food spoilage
• Neurospora sp. (salmon-colored bread mold) is an ascomycete with a key role in modern genetics
• This phylum causes serious plant diseases such as powdery mildews, chestnut blight, and Dutch elm disease
• Dutch elm disease is caused by Ophiostoma ulmi, a fungus native to some European countries
• Yeasts, edible morels, and truffles are also ascomycetes
• Ascomycota is not fully known, and there are thousands of additional species awaiting scientific description
• Filamentous when growing, except for unicellular yeasts
• Hyphae are septate, divided by perforated cross walls
• Hyphal cells of vegetative mycelium can be either uninucleate or multinucleate
• Some species are homothallic (self-fertile), producing sexual structures from a single genetic strain
• Other species are heterothallic, needing a combination of + and - strains to reproduce

Asexual Reproduction

• Asexual reproduction in the majority involves specialized spores referred to as conidia
• Conidia means "fine dust" in Greek
• Conidia are produced at the tips of modified hyphae called conidiophores
• Conidiophores means "conidia bearers"
• Conidia are usually multinucleate

Sexual Reproduction

• Sexual reproduction always involves the formation of an ascus (plural: asci), a sac-like structure
• This is characteristic of this phylum and distinguishing it from all other fungi
• Ascomycetes are divided into two main groups depending on ascus morphology
• Unitunicate asci have one thin wall
• Bitunicate asci have each ascus surrounded by a double wall

Ascus Formation

• Ascus usually forms within a complex structure of tightly interwoven hyphae referred to as the ascoma, also known as an ascocarp
• Many ascomata are macroscopic and the only part that most people see
• Ascoma:
  • Apothecium is cup-shaped, open
  • Cleistothecium is closed and spherical
  • Perithecium is flask-shaped with a small pore for ascospore escape
• The layer of asci is the hymenium, or hymeneal layer
• Ascostroma fungi have asci formed inside a chamber named ascostroma, within a mass of tissue called a stroma
• Ascostroma chambers differ from perithecia because they have no wall or hymenium
• Ascostroma are called pseudothecia

Life Cycle

• Mycelium starts with ascospore germination
• Post vegetative growth, the mycelium starts to form conidiophores which bear conidia
• Many conidia crops are produced during growing season
• Conidia are responsible for asexual propagation of the fungus

Ascus Formation Details

• Ascus formation occurs on the same mycelium producing conidia
• Ascus formation is preceded by formation of multinucleate gametangia called antheridia and ascogonia
• Male nuclei of the antheridium pass into ascogonium via the trichogyne, which is an outgrowth of the ascogonium
• Plasmogamy, fusion of two protoplasts, occurs
• Male nuclei pair with genetically different female nuclei inside common cytoplasm but don't fuse
• Hyphal filaments grow out of the ascogonium and elongate into ascogenous hyphae
• As ascogenous hypha develop, pairs of nuclei migrate into them, and simultaneous mitotic divisions occur in hyphae and ascogonium
• Cell division in developing ascogenous hyphae is such that the resulting cells are dikaryotic, containing two haploid nuclei
• An ascus first forms at the tip of the developing dikaryotic, ascogenous hypha
• Throughout ascus formation, one of the binucleate cells of the dikaryotic hypha grows to form a hook known as a crozier
• In the hooked cell, the two nuclei divide so that their spindle fibres are parallel and more or less vertical in orientation
• Two daughter nuclei are closely located at the top of the hook
• One of the other nuclei is near the tip, and the other is near the basal septum of the hook
• Two septa form, dividing the hook into three cells
• The middle cell becomes the ascus
• Karyogamy occurs in this middle cell: two nuclei fuse to form a diploid nucleus (zygote), which is the only diploid nucleus in ascomycete life cycle
• Soon after karyogamy, young ascus elongates
• Diploid nucleus then undergoes meiosis, typically followed by one mitotic division, resulting in four to eight nuclei
• These haploid nuclei are then cut off in cytoplasm segments to form ascospores
• Ascus becomes turgid at maturity and bursts, sending ascospores explosively into the air

Unicellular Ascomycota

• Some ascomycetes lack an ascomata
• Order Endomycetales contains yeasts that can be unicellular or mycelial
• Order Taphrinales include members that have mycelial hyphae

ENDOMYCETALES (Yeasts)

• Yeasts are mainly unicellular and reproduce asexually by fission, or by pinching off small buds, rather than by spore formation
• Sexual reproduction in yeasts occurs when either two cells or two ascospores unite to form a diploid zygote
• Zygote may produce asexual buds, or may undergo meiosis to produce four haploid nuclei
• In some species, there may also be a subsequent mitotic division that produces eight haploid nuclei
• Single cell in these unicellular yeasts become an ascus
• Species are holocarpic, meaning the entire organism becomes the reproductive structure
• Walls are laid down around the nuclei inside the ascus/zygote wall so eight ascospores form
• These ascospores are freed when the ascus wall breaks down
• Ascospores either bud asexually or fuse with another cell to repeat the sexual process
• Saccharomyces spp. and Schizosaccharomyces spp. are two yeast genera commonly used in the baking, and brewing industries
• In both genera, asci are formed by fusion of two haploid cells
• In unicellular, holocarpic yeasts, there is no ascocarp

TAPHRINALES

• Primarily parasitic fungi attaching to higher plants, causing deformities and galls (tumorous growths) on leaves, stems, and the fruits
• Individual thalli are mycelial
• Taphrina deformans (leaf curl of peaches and almonds):
• Frequently causes loss in peach and almond crops and also parasitizes other members of the Rosaceae flowering plant family
• Hyphae penetrate between cells of the hosts' leaves and stems, creating abnormal growth
• This excess growth makes the leaves curl up
• Hyphae produce naked asci (not surrounded by an ascoma) on the surface of the infected tissue
• Asci develop from dikaryotic hyphae which are formed by the sexual fusing of two compatible hyphal strains deep within host tissue

Ascomatal Fungi

• Remaining ascomycota orders are separated based on the types of asci, the form of ascospore release, and the type of ascoma surrounding asci
• All these fungi have ascomata arising from ascogenous hyphae
• Ascomatal fungi are divided into two groups based on ascoma characteristics
• The first group has unitunicate asci, which are asci with only one thin wall surrounding the developing ascospores
• Releases spores through a small pore (an operculum)
• Spores possess a structure called an apical ring apparatus
• Spores are forcibly expelled into the air
• The second group has bitunicate asci, which are asci with a double wall
• Spores are expelled through an apical pore

EUROTIALES

• All fungi in the EUROTIALES order have short-lived asci scattered within a cleistothecial ascoma
• Ascoma is a completely enclosed sphere composed of dikaryotic hyphae
• Ascospore release needs cleistothecium wall to degenerate or break
• Ascomycetes in this order have apparently lost ability to forcibly discharge ascospores from the ascus
• Asci are not arranged in any ordered layer (hymenium) in the ascoma
• Asci are scattered randomly throughout the cleistothecium
• These fungi reproduce primarily by formation of conidia which are pinched off from conidiophores

Penicillium sp. (blue and green molds)

• Penicillium have is known for its connection with antibiotics
• Penicillin is a by-product of Penicillium notatum
• Penicillium notatum is released into the culture medium and inhibits growth of gram-positive bacteria
• Penicillin was discovered by Sir Alexander Fleming in 1929, but was not exploited until World War II
• Penicillin represses bacterial growth without being toxic to animal tissues
• Molds became of significant economic interest
• Penicillium sp. gives flavour, odour and character to some types of cheese
• P. roquefortii was first found in caves near the French village of Roquefort
• P. camembertii gives camembert cheese its special qualities

Aspergillus spp.

• As in Penicillium spp., conidial stage is much more prominent than the ascus stage
• Asci are rarely seen
• In Aspergillus, conidiophore is erect and unbranched, and apex swells into a globular head
• From this, numbers of projections bud out; each projection produces a growing chain of conidia, with youngest at the bottom
• Aspergillus niger produces black conidia
• Live Aspergillus spp. are not permitted in the lab for safety reasons
• If inhaled, it may colonize lungs and cause aspergillosis, which is difficult to treat and can be fatal
• Aspergillus niger is used to produce citric acid commercially
• This organic acid is also produced by A. niger when grown under appropriate conditions
• In Asia, soy paste (miso) is produced by fermenting soybeans with Aspergillus oryzae
• Soy sauce (shoyu) is produced by fermenting soybeans with a mixture of A. oryzae and A. soyae
• Lactic acid bacteria and yeasts are also actively involved in producing the final product
• Aspergillus oryzae is important for the initial steps in brewing sake
• Saccharomyces cervisiae yeast is important later in the process

ERYSIPHALES (powdery mildews)

• Powdery mildew is caused by obligate plant parasites
• These fungi attack leaves and succulent parts of a variety of flowering plants
• Infection starts with sexual ascospores or asexual conidia germinating on surface of host plant leaf/stem
• Results in branched, septate mycelium of uninucleate cells on leaf surface; it feeds by putting haustoria into epidermal cells
• Only epidermal cells are attacked
• External mycelium releases short, erect conidiophores that each cut off a single line of barrel-shaped spores, with the youngest being at that base
• Diseased parts get covered with a "forest" of conidiophores with chains of spores
• This gives powdery appearance characteristic of disease
• Ripe spores become detached and are dispersed easily by the wind, causing new infections
• Cleistothecia are produced by late August or September
• They are minute, typically 0.1 mm in diameter
• Specialized hyphae (appendages) grow from external wall of cleistothecium
• Cleistothecia look like minute black dots under hand lens
• Cleistothecia are pathogens’ resting, hibernating stage
• Ascospores remain dormant all winter
• When asci expand in late spring, cleistothecial wall ruptures and protrudes beyond it
• Asci burst and send ascospores, causing initial infections of the season
• Cleistothecia uniquely have asci arranged in a hymenial layer
• They could be thought of as perithecia lacking ostioles

Erysiphe sp.

• Powdery mildew that infects grains and grasses

Uncinula necator

• Powdery mildew species infecting grapes

SORDARIALES

• Members occur as saprotrophs on dung, rotting wood, + soil
• Typical ascoma is a dark perithecium occurring individually without a stroma
• Order's name comes from Sordaria spp.
• Subject of research
• Neurospora spp. is one of most studied, and well characterized fungi
• Species of neurospora are used for genetic and physiological investigations

CLAVICIPITALES

• Distinguished by production of schlerotia
• Claviceps purpurea causes ergot of rye and other grasses; it is a common fungus of temperate regions
• Spores infect ovaries of grasses, and the mycelium produces minute conidia
• Minute conidia are exuded from the infected plant ovaries in a sticky fluid dubbed Honeydew
• Honeydew attracts insects
• Conidia are dispersed by insects or splashing rain and can infect additional flowers
• Hyphae grow in plant ovary and produce a hard, purplish "sclerotium" that assumes form of the grain. Sclerotia (or ergots) are resistant, overwintering structures requiring reduced temperatures
• Can retain their viability in soil many years
• Upon germination
• Each sclerotium produces one/more stalked stromata within which numerous perithecia are embedded
• Asci are in flask-shaped perithecia and arranged in a hymenial layer
• Ascospores are released through the small ostiole area