Introduction to Fungi, Third Edition

ASEXUAL REPRODUCTION
503
Fig18.14 Arthroconidia in two
Homobasidiomycetes. (a) Arthroconidia of
Flammulina velutipes formed on a monokaryotic
mycelium.When the chains of conidia
disarticulate, the conidia remain dry and are
dispersed by wind currents. (b) Arthroconidia
(oidia) of Coprinus cinereus formed on a
monokaryon.The sticky oidia accumulate in
mucilage in globose heads and are dispersed by
insects visiting the dung on which this fungus
grows. (c) The homing reaction in C. cinereus as
seen in an agar culture.The tips of lateral
branches of monokaryotic hyphae have been
stimulated chemotropically to grow or curve
towards oidia of compatible mating type
placed near them a few hours earlier.
Plasmogamy between a hyphal tip and a
compatible oidium is followed by transfer of a
nucleus from the oidium into the monokaryon,
converting it into a dikaryon.
segments (Heinz & Niederpruem, 1970; Polak
et al., 1997). The oidia collect in mucilaginous
globules from which they are dispersed by
insects.
Oidia, whether wet or dry, can function as
spermatizing agents. If an oidium is placed a
little distance ahead of a monokaryotic hypha,
the growing hypha changes direction, being
attracted chemotropically towards the oidium.
This phenomenon is termed the ‘homing reaction’
(Fig. 18.14c). The response has been detected
over distances up to 75 mm. This is remarkable in
view of the fact that the width of the approaching
monokaryon hypha is about 2.5 mm and the
growing zone of the hyphal tip is about 0.5 mm
(Kemp, 1975a). The homing reaction is elicited
not only between compatible oidium hypha
combinations but also between incompatible
associations. It may even be triggered by oidia
of different species. Where the oidium and
approaching hypha are compatible, plasmogamy,
i.e. fusion of the hyphal tip and the
oidium, takes place, followed by nuclear
migration and the eventual establishment of a
dikaryon. Plasmogamy may also occur between
an oidium and an unrelated approaching hypha,
i.e. one belonging to a different species. In this
case the introduction of a nucleus from the
oidium into a cell of the unrelated hypha results
in a lethal response, involving the death of the
receptor cell and possibly some adjacent cells.
Kemp (1975a,b) has argued that the lethal
response is important in maintaining interspecific
barriers.
18.8.2 Blastic conidia
Blastic development involves the marked
enlargement of a recognizable conidium initial
before the conidium is delimited by a septum.
This is usually achieved by a blowing-out of part
of the conidiophore wall. There are many
different ways in which blastic conidia can
develop (Kendrick & Watling, 1979). For example,
they may develop singly, synchronously in
clusters, or in succession. The growth form