Conidium production in ascomycetes

There are several steps in the production and release of conidia, namely
(1) conidiogenesis, i.e. conidial initiation;
(2) maturation;
(3) delimitation;
(4) secession, i.e. separation from the conidiogenous cell;
(5) proliferation of the conidiogenous cell or conidiophore to form further conidia.
Conidiogenesis occurs in two ways which appear to be distinct at first glance: blastic and thallic

Fig: Two basic modes of development during conidiogenesis of a hyphal apex(a): blastic(b) and thallic (c).

Fig: Two basic modes of development during conidiogenesis of a hyphal apex(a): blastic(b) and thallic (c).

A. Blastic conidiogenesis:

The conidium develops by the blowing-out of the wall of a cell, usually from the tip of a hypha, sometimes laterally as in Aureobasidium.
Two kinds of blastic development have been distinguished.

Fig: Conidiogenesis

Fig: Conidiogenesis (a).In holoblastic conidiogenesis (b),the entire wall becomes inflated to form the conidium initial. In enteroblastic conidiogenesis (c),the conidium initial develops through a hole in the rigid outerwall.

1. Holoblastic.

All the wall layers of the conidiogenous cell contribute to the wall of the newly formed conidium.

2. Enteroblastic.

The wall of the conidiogenous cell is rigid and breaks open. The initial of the conidium is pushed through the opening and is surrounded by a newly formed wall.
Two types of enteroblastic development have been distinguished, phialidic and annellidic.

a) Phialidic

In phialidic development a basipetal succession of conidia (phialospores, phialoconidia) develops from a specialized conidiogenous cell, the phialide (Gr. diminutive of phialis = flask), usually shaped like a bottle with a narrow neck. Phialides are formed singly or in clusters at the tip of a conidiophore or, more rarely, laterally. There may be one or several nuclei in a phialide.

b) Annellidic conidiogenesis

Annellidic conidiogenesis  in many ways resembles phialidic, and indeed the term annellidic phialide is sometimes used for this type of conidiogenous cell. These are also termed annellides (Lat. annulus=little ring) or annellophores, and the spores which develop from them are annelloconidia.

As in phialidic development, the first-formed annelloconidium is holoblastic.

The difference between the two modes of development is that new wall material which is secreted within the annellide protrudes beyond its neck and the septum which cuts off the newly formed conidium also forms beyond the neck.

As each new conidium develops in basipetal fashion, a small ring of wall material (annellation) is left at the neck of the annellide, which thus grows in length as successive conidia develop. This accumulation of short collars of wall material is the annellated zone.

Examples of fungi reproducing by annelloconidia are Scopulariopsis brevicaulis  and Cephalotrichum (Doratomyces) stemonitis. Both genera contain species which are conidial forms of Microascus.

Secession of conidia

Secession of conidia, irrespective of their mode of development, is in most cases by dissolution of the septum or septa which separate them from the conidiogenous cell or from adjacent spores. This process is termed schizolytic secession (Gr. schizo= to split, divide; lyticos = able to loosen).

In some other cases secession is brought about by the collapse of a special separating cell beneath the terminal conidium. This is termed rhexolytic secession (Gr. rhexis = a rupture, breaking).

phialide-and

Phialidic conidiogenesis Annellidic conidiogenesis
1. Apex of conidiophore expands to form aphialide with a blown-out holoblastic conidium initial(o). 1. Apex of conidiophore differentiates to form a conidiogenous cell (annellide) and the initial of a holoblastic first conidium(1).
 2. The first-formed conidium(1), surrounded by a new wall secreted inside the phialide, is pushed out and breaks the outer wall of the phialide whose tip persists as a cap.  2. The first conidium is cut off by a septum.
3 . The first conidium is cut off by a septum.  3.A second conidium (2) develops beneath the first
4.  A second conidium develops below the first, also surrounded by new wall secreted inside the phialide.  4. The septum cutting off the second conidium is formed beyond the point at which the original annellide wall was ruptured and persists as an annellation.
 5. A third conidium develops in basipetal succession adding to the length of the conidial chain.The lower part of the broken original wall of the phialide has persisted as a collarette, the extent of which is shown by vertical dashed lines. Successive layers of wall material may accrete in the neck of the phialide to form a periclinal thickening.  5. The development of further conidia results in the addition of more annellations so that an annellated zone, marked by vertical dashed lines, increases in length.

B. Thallic conidiogenesis

Thallic conidiogenesis (Gr. thallos = a branch) occurs by conversion of a pre-existing hyphal element in which terminal or intercalary cells of a hypha become cut off by septa . Two kinds of thallic development have been distinguished: holothallic (Gr. holos= whole, entire) and thallic-arthric (Gr. arthron =a joint).

1. Holothallic

In holothallic development a hyphal element, e.g. a terminal segment of a hypha, is converted as a whole into a single conidium. Secession of such conidia may be schizolytic or rhexolytic. Microsporum spp. (anamorphic Arthroderma), which are skin pathogens (dermatophytes) of mammals, provide examples of this holothallic development.

2.Thallic-arthric

During thallic-arthric conidiogenesis, septa develop in a hypha and divide it up into segments which separate into individual cells by dissolution of the septa. Geotrichum candidum (anamorphic Galactomyces), a common soil fungus and frequent contaminant of milk and milk products, develops conidia in this way.

holothallic-and-thallic-arthric conidiogenesis

Fig: holothallic-and-thallic-arthric conidiogenesis

The proliferation of the conidiogenous cell or the conidiophore may occur in various ways, for example by the formation of a new growing point in the region of the conidiophore beneath the point at which the first conidium was formed. The new apex extends beyond the point of origin of the first conidium and develops a new conidiogenous cell. These methods of conidiophore regeneration are discussed more fully in relation to some of the different genera.

Holothallic conidiogenesis with rhexolytic secession Thallicarthric conidiogenesis with schizolytic secession
a. The terminal portion of a hypha is cut off by a septum a. A terminal segment of a hypha.
b. A second septum laid down near the first cuts off a subterminal segment, the separating cell. b. Septa develop, dividing the segment into several cells.
c. The terminal cell enlarges to form the conidium. c. The septa divide, each separating into two layers
d. Collapse of the separating cell causes conidium secession. d. The daughter cells separate

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