Conidial Ontogeny

Speaker:Anurag KerkettaPh.D. Scholar Department of Plant PathologyIGKV, Raipur

Introduction

• Deuteromycetes or Fungi imperfecti are fungi without a teleomorph. It is believed to be the primitive condition of Ascomycetes and Basidiomycetes.

• Most food- and airborne fungi belong to this group: Penicillium, Aspergillus, Fusarium and Cladosporium etc.

• This group of fungi is artificial and is characterized by their way of sporulation, and produced conidia-asexual spore .

• Besides true conidia, some fungi, especially animal and human pathogens, produce other types of asexual spores: microconidia, blastospores, and arthrospores.

• Microconidia are very small conidia. Blastospores (Gr. Blastos- bud, shoot, + sporos- seed, spore) are asexual spores formed by budding either directly from a hypha, or from any other cell.

• Arthrospores (Gr. Arthron- joint, + sporos- seed, spore) are formed by the breaking up of the hyphae into their component cells. They are no different from oidia. All three of germinate to form mycelium and function the same as conidia.

Important terms

• Conidiogenesis = the mode of conidium formation.• Conidia = specialized non-motile asexual spore.• Conidiogenous cell = specialized cell which give rise

to the conidia.Term such as phialides and annelide are used to designate different types of conidiogenous cells.An phialide considered to be a conidiogenous cell with an open end through which conidia with only partlycontinuous with the conidiogenous cell development in basipetal succession.

An annellide is conidiogenous cell , undergoes repeated percurrent proliferation during production of chain of conidia so that the elongating conidiogenous cell becomemarked with a series of scar.

• Conidiophore = entire system of fertile hyphae. Fig: Penicillium sp.

Conidia

Conidiogenous cell

Conidiophore

Vegetative hypha

Conidia and Conidial Ontogeny

• Conidiospores, commonly known as conidia, are asexual reproductive structures. The word is derived from the Greek konis ‘dust’ + the diminutive suffix -idium (Sutton, 1986).

• Conidia are found in many different groups of fungi, but especially within Deuteromycotina. The term conidium has, unfortunately, been used in a number of different ways, so that it no longer has any precise meaning.

• In many fungi conidia represent a means of rapid spread and colonization from an initial focus of infection. Conidia may originated from conidiogenous cell in several ways.

• There is great variation in conidial ontogeny, which may be either thallic orblastic.

1. Thallic- is used to describe development where there is no enlargement of the conidium initial, i.e. the conidium arises by conversion of a pre-existing segment of the fungal thallus. An example : Galactomyces candidus, in which the conidia are formed by dissolution of septa along a hypha.

Fig: Galactomyces candidus

2. In most conidia, development is blastic, i.e. there is enlargement of the conidium initial before it is delimited by a septum.

• Two main kinds of blastic development have been distinguished:i. Holoblastic,ii. Enteroblastic,Holoblastic development- All the wall layers of the conidiogenous cell

balloon out to form a conidium initial recognizably larger than the conidiogenous cell. Conidia of Sclerotinia fructigena.

Fig: Sclerotinia fructigena

• Enteroblastic development: only the innerwall layers of the conidiogenous cell are involved in conidium formation. The inner wall layers balloon out through a narrow channel in the outer wall.

• Enteroblastic development are found in Helminthosporium velutinum and Pleospora herbarum.

• Although only a single conidium may be produced at a locus, in many cases a number of conidia are produced simultaneously or successively at newly developing loci.

• The conidia may arise apically with the conidiogenous cell growing out after each conidium is delimited apically, a process is called progressive conidial locus development.

Hennebert and Sutton (1994) Drawing by L M Barona.

• Retrogressive conidial locus development is said to occur when the conidiogenous cell shortens after each conidium is produced. Stationary, when there is no change.

• Several other conidiation have been described , sympodial; in which the conidial locus is sub apical and shifts to lateral as successive conidia develop.

Retrogressive Stationary sympodial (Hennebert and Sutton (1994) Drawing by L M Barona.)

• Conidia can develop in two ways in chain, if the oldest conidia of a chain is at the tip and youngest at the base- the conidia are said to form basipetally.

• If the youngest conidium is at the tip- conidial succession is acropetal.

• Ripe conidia may also be liberated in two basic ways, schizolytic and rhexolytic.

• In scohizlytic dehiscence, the halves ofa double septum split apart by thebreakdown of a kind of middle lamella.

• In rhexolytic dehiscence the entire septum • Separates with the conidium.

scohizlytic rhexolytic secession secession

• There are eight different kinds of conidium development: six are blastic, two thallic development.

i. Blastic-acropetal or blastic-synchronous conidiogenesis.ii. blastic-sympodial conidiogenesis.iii. blastic-annellidic or blastic-percurrent conidiogenesis.iv. Blastic-phialidic conidiogenesis.v. Blastic-retrogressive conidiogenesis.vi. Basauxic conidiogenesis.vii. Thallic-arthric conidiogenesis.viii. Thallic-solitary conidiogenesis

Type I: blastic-acropetal or blastic-synchronous conidiogenesis

• The Monilia anamorph of Monilinia fructicola (Unitunicatae Inoperculatae: Leotiales), the brown rot fungus of peach and other stone fruits, and the Cladosporium anamorph of Mycosphaerella tassiana (Bitunicatae: Dothideales), a common mould on decaying organic matter; produce conidia in chains by apical budding.

• The youngest conidium is at the tip of the chain. The chain branches when two buds, rather than one, develop on a terminal conidium (which may then be called a ramo conidium).

fig: Monilia

• The hyphomycetous anamorphs Botrytis and Gonatobotryum produce many conidia synchronously on a swollen cell: Gonatobotryum goes on to form acropetal chains of secondary conidia, while Botrytis does not.

• Botryosporium also produces conidia synchronously on swollen cells. In this genus, the branches bearing these vesicles are arranged along an extremely tall, graceful, white conidiophore up to 2 mm long in a sequence from youngest at the tip to oldest near the base. This fungus, which often turns up in greenhouses growing on dead leaves, is sometimes called 'the beautiful hyphomycete.

fig: Botryosporium (Pictures courtesy of Dr. Roland Weber)

Type II :blastic-sympodial conidiogenesis

• In species of Beauveria, hyphomycetous insect pathogens which are now being used in biological control of potato beetle, the narrow apex of the conidiogenous cell extends sympodially: each new apex becomes converted into a blastic conidium, then the next apex grows out from behind and to one side of it. The more conidia are produced, the longer the conidiogenous cell becomes.

• Although Leptographium anamorphs of Ophiostoma (Prototunicatae: Ophiostomatales) have single conidiophores, these have complex heads with several tiers of supporting cells (metulae), the ultimate ones bearing many sympodially (or percurrently) extending conidiogenous cells, and innumerable conidia accumulate in a slimy head; these spores are insect dispersed. Basifimbria (teleomorph unknown), which is common on horse dung, has simple conidiophores that elongate sympodially during conidiation.

Fig: Leptographium Basifimbria

Type III: blastic-annellidic or blastic-percurrent conidiogenesis

• In the Spilocaea anamorph of Venturia inaequalis, the apple scab fungus, each seceding conidium leaves a ring like scar, an annellation, around the conidiogenous cell, which then grows on through the scar ('percurrently') to produce the next conidium.

• Conidiogenous cells that have produced x spores bear x annular scars hence the name annellidic. flame shaped conidia of Spilocaea with truncate bases, and several annellations on the central conidiogenous cell, which is just developing a new conidium.

Annellated conidiogenouscell

• Annellophora africana shows many, widely spaced annellations, each of which was the level at which a conidium was formed and released.

Fig: Annellophora africana

Type IV : Blastic-phialidic conidiogenesis

• Many common moulds produce conidia in rapid basipetal succession from the open end of special conidiogenous cells called phialides. Important genera such as Penicillium, Aspergillus, Fusarium, Stachybotrys, Trichoderma and Chalara are all phialidic.

Type V: Blastic-retrogressive conidiogenesis

• In Basipetospora (thermotolerant fungus used in Indonesia in the preparation of a red food colouring), a conidium forms at the tip of the relatively undifferentiated conidiogenous hypha and is delimited by a crosswall; then a short zone of the hypha just below the conidium balloons out to produce the second conidium.

• After this has been delimited by a septum, the next segment of the hypha plasticizes and blows out, and so on. As the chain of conidia elongates, the conidiogenous hypha becomes shorter.

Type VI: basauxic conidiogenesis

• the Oidium anamorph of Blumeria graminis, whitish chains of conidia (the 'powdery mildew') cover the host leaves. Each chain consists of a graded series of gradually maturing conidia, the oldest at the tip, the youngest barely differentiated from the hyphal cell just below it. New material is added at the base of the chain in a form of intercalary growth, arising from a sometimes swollen mother cell which appears to be a highly modified phialide.

Type VII: Thallic-arthric conidiogenesis

• In the Geotrichum anamorphs of Dipodascus spp. (Saccharomycetes), an assimilative hypha stops growing, then becomes divided up into short lengths by irregularly arising septa. These are double septa which split apart schizolytically to give a 'chain' of short cylindrical 'fission arthroconidia' that disarticulates and appears jointed (hence 'arthric').

• In Coremiella, some hyphal cells degenerate to release the intervening cells as 'alternate arthroconidia.‘

• In Oidiodendron, a common soil mould, the branches of an often tree like conidiophore disarticulate into conidia, ultimately leaving only the denuded 'trunk,' (the stipe).

Type VIII: Thallic-solitary conidiogenesis

• The Microsporum anamorphs of Nannizzia (Prototunicatae: Onygenales), which can digest keratin, and cause skin diseases in humans, develop large thallic phragmospores at the ends of hyphae. These conidia are liberated rhexolytically.

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