Identification Of Ascomycetes: The Sac Fungi

There are two basic methods of the identification of Ascomycetes.

  1. By observing their unique structure.
  2. From complex test.

By Observing Unique Structure

(This type of identification doesn’t claim complicated test or comparative studies. Only the pure and ensured observance is required.)

The primary morphological character that distinguishes the members of Ascomycetes from all other fungi is ‘Ascus’.

Ascus is a sac like structure containing the ascopores (that’s why these group of fungi are called ‘Ascomycetes’ as well as known as ‘Sac Fungi’ for their production of sac like structure called ‘Ascus’ ). The sac-like structure contains ascopores.

The ascus is cleaved from within by free cell formation after karyogamy and meiosis. There are 8 ascospores formed within each ascus but the number may vary from species to species.

The process of the development of asci (pl. of ascus) is being described here in brief.

Development of Asci

Ascomycetes have two distinct reproductive phases:

  1. Sexual phase: Involving in the formation of the asci and ascospore.
  2. Asexual phase: Involving in spore production.

In sexual phase, two compatible nuclei are brought together in the same cell and fuse to produce ascus later.

This process shows many variable ways mentioned below:

  1. Two morphologically similar gametangia touch at their tips and fuse. The fusion cell develops into ascus.
  2. Two morphologically different gametangia  (antheridia and ascogonia) are formed in this case. The male nucleus from the antheridium passes to the fusion point between antheridium and ascogonium and produce ascus (Fusion cell is not developed here).
  3. In this case, a single detached male cell works alike sperm (that’s why this process is called ‘spermatization’). The male cell becomes attached to the female receptacle. Through septal pores, the nuclei of male cell migrate to the ascogonium.

Asci may be spherical to elongated, cylindrical, ovoid or globose shape. They may be arranged in scattered fashion or in a stalk or hymenium.

From Complex Test

It seems to be too much complex to determine whether a yeast or mycelial fungus is an Ascomycete or others fungi.

In that case few method are applied to identify or determine Ascomycetes. Some of such methods are following :

Diazonium Blue B (DBB) staining method

Others fungi (basidiomycetes, yeast etc.) except Ascomycetes shows positive result in DBB staining. Where the Ascomycetes are primarily nonstaining.

So under microscope, it’s a simple task to determine whether it is Ascomycete or not.

Transmission electron microscopy (TEM)

The cell wall of Ascomycetes are of two layers- a thick, transparent inner and a thin, dense outer layer ( in TEM) . This scenereo is different from basidiomycetes or others. So it’s help us in determining Ascomycetes.

Observing Various Formation

In yeasts the manner in which new walls form in budding usually can be used to distinguish the Ascomycetes. Buds of ascomycetous yeasts new wall layers that that are continuous with the parent wall which is different than basidiomycetous buds.

Much more practice and basic knowledge about Ascomycetes is required for the exact identification of Ascomycetes.

About Tarek Siddiki Taki


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Abulais Shomrat

The article is short but precise in information. Overall, it is nice.

Please, do consider…
1. adding real photo of ascus.
2. adding photos of the three types of sexual ascus development.

N.B: Do mention the source of the image in the caption box.