Conidial ontogeny refers to the modes of formation and development of asexual spores known as conidia. There are eight types of conidial ontogeny: six involve blastic or enlargement development and two involve thallic or non-enlargement development. Examples are provided to illustrate the different types including phialidic, annellidic, synchronous, and arthroconidial development. Key terms are also defined such as conidiogenous cell, conidiophore, and schizolytic versus rhexolytic dehiscence.

1. Conidial Ontogeny
Speaker:
Anurag Kerketta
Ph.D. Scholar
Department of Plant Pathology
IGKV, Raipur

2. 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.

3. • 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.

4. 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 partly
continuous 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 become
marked with a series of scar.
• Conidiophore = entire system of fertile
hyphae. Fig: Penicillium sp.
Conidia
Conidiogenous
cell
Conidiophore
Vegetative
hypha

5. 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.

6. • There is great variation in conidial ontogeny, which may be either
thallic or
blastic.
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

7. 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

8. • 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.

9. • 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.

10. • 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.)

11. • 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 of
a double septum split apart by the
breakdown of a kind of middle lamella.
• In rhexolytic dehiscence the entire septum
• Separates with the conidium.
scohizlytic rhexolytic
secession secession

12. • 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

13. 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

14. • 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.

15. • 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)

16. 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.

17. • 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

18. 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 conidiogenous
cell

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

20. 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.

21. 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.

22. 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.

23. 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').

24. • 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).

25. 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.

26. Thank you