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Section E

Sub- division :- Ascomycotina(sac fungi)

Members of the Ascomycotina are known as the Sac Fungi. The majority of fungi that lack morphological evidence of sexual reproduction are placed here. Examples of sac fungi are yeasts, morels, truffles, and Penicillium. This includes the salmon-coloured bread mold Neurospora sp., which has played an important role in the development of modern genetics.  Important features of Ascomycotina:-

  • These fungi posses well-developed, profusely branched mycelium except the unicellular yeasts.
  • Hyphae with regular cross-walls called septa and haploid.Which are centrally perforated to allow movement of cytoplasm, and sometimes nuclei, between compartments.
  • The hyphal cells of the vegetative mycelium may be either uninucleate or multinucleate.
  • Cell walls are composed mostly of chitin.
  • All produce an ascus (sac-like structure) that contains haploid (n) ascospores after meiosis .
  • Plasmogamy is separated from karyogamy in time so that a dikaryotic phase is produced – the ascogenous hyphae represent the dikaryotic hyphae(or at least in those which produce a teleomorph).
  • Sexual life cycle is basically similar – haploid-dikaryotic life cycle.
  • Some species of Ascomycota can self-fertilize and produce sexual structures from a single genetic strain; others require a combination of + and - strains.

Nutrition and Symbioses :-Ascomycotina are heterotrophs and obtain nutrients from dead or living organisms :-

  • As saprotrophs they can consume almost any carbonaceous substrate such as cellulose (found in plant cell walls), lignin (found in wood) and recycled dead plant material .

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  • As parasites, ascomycetes account for most of plant pathogens including powdery mildews that attack fruits, chestnut blight, and Dutch Elm disease (caused by Ceratocytis ulmi).
  • Lichens- Symbiotic association between a sac fungus and a photosynthetic green algae or cyanobacteria
  • Mycorrhizae :- Symbiotic association of a sac fungus living on plant roots.
  • Endophytes:- Symbiotic association of a sac fungus with the leaves and stems of plants.

Reproduction and life cycle Fungi are Holomorphs.

  • Holomorph a fungus that is characterized both by sexual and asexual reproductive states.
  • Teleomorph :-the fungus when reproducing sexually.
  • Anamorph :- the fungus when reproducing asexually.
  • Asexual reproduction :-
    • Yeasts reproduces by budding or fission.
    • Asexual reproduction in the majority of the Ascomycetes occurs by the formation of specialized spores, known as conidia.
    • Which are formed on tips of modified hyphae called conidiophores.
    • Conidia are formed in longitudinal chains on the conidiophores.
    • Each conidium contains one or more nuclei.
    • Conidia form on the surface of conidiophores in contrast to spores that form within sporangia in Rhizopus.
    •   When mature, conidia are released in large numbers and germinate to produce new organisms.

  • Sexual reproduction :-In these fungi sexual reproduction occurs by the formation of multinucleate gametangia called :-
    • Male gametangia may be an antheridium or conidium-like structure – spermatium.
    • Female gametangium - ascogonium, may have a long projection, the trichogyne
  • Asci formation occur on the same mycelia that produce conidia.
  • The male nuclei of the antheridium pass into the ascogonium or an ascogonium and a spermatium ,through the trichogyne.
  • Plasmogamy or the fusion of the two cytoplasms, has now taken place.
  • The male nuclei then pair with the genetically different female nuclei within the common cytoplasm but do not fuse.
  • Hyphae now begin to grow out of the ascogonium.
  • As the hyphae develop, pairs of nuclei migrate into them and simultaneous mitotic divisions occur in the hyphae and ascogonium -dikaryotic cells.
  • The pairing of two genetically (+ and -) different types of nuclei followed by septation results in the formation of a number of dikaryotic cells (i.e. containing two haploid nuclei, one from each strain).
  • Sterile haploid hyphae known as peridium envelope the ascogonium to form the multicellular ascoma or ascocarp(fruiting body).   Many ascogenous hyphae produced inside the developing ascoma.
  • Tips of ascogenous hyphae form croziers (hooks) before developing into an ascus.
  • Ascoma contains two types of hyphae :-
    • Ascogenous hyphae – dikaryotic, form asci through crozier formation.
    • Sterile hyphae – haploid, form bulk of ascoma.
  • The two nuclei in the terminal cell (ascus) of the dikaryotic hyphae then fuse into a single diploid nucleus ("karyogamy"). The zygote is only diploid phase in life cycle.
  • The ascus then elongates and the diploid nucleus divides by meiosis, forming 4 haploid nuclei.
  • Each haploid nucleus usually divides again by mitosis, generally resulting in a total of 8 haploid nuclei but this may vary. 
  • These haploid nuclei are then cut off in segments of the cytoplasm to form ascospores.
  • In most Ascomycotina, the ascus becomes turgid at maturity and finally bursts, sending its ascospores explosively into the air.
  • Within the ascocarp, there is usually a layer containing the asci, and sterile cells known as paraphyses. This layer is the hymenial layer.

The form of the ascocarp may be one of four types :-

There are four basic types of ascus:-

  • Unitunicate-operculate ascus has single wall, operculum (lid/) at the top. The operculum pops open when the ascus becomes mature to release ascospores. - found in apothecia
  • Unitunicate-inoperculate ascus has no operculum but instead contains a special elastic ring at the top. On ripening it briefly expands and so lets the spores shoot out - found in perithecia and some apothecia.
  • Prototunicate asci are mostly spherical in shape and they have no active dispersal mechanism at all. The ripe ascus wall simply dissolves so that the spores can escape, or it is broken open by other influences such as animals. Found in cleistothecia, some perithecia, and hypogeous ascomata.
  • Bitunicate ascus is enclosed in a double wall. This consists of a thin brittle outer shell and a thick elastic inner wall. At maturity the thin outer wall splits, and the thick inner wall absorbs water and expands upward, carrying the ascospores with it . This 'Jack-in-a-box' design allows the ascus to stretch up into the neck of the ascoma to expel its spores. Bitunicate asci occur only in pseudothecia.