Introduction to Fungi, Third Edition

ASEXUAL REPRODUCTION
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Fig18.12 Subapical hypha of a dikaryon of Omphalotus olearius. (a) Interference contrast image showing the segments delimited by
clamp connections (arrowheads). (b) DAPI fluorescence staining of the same hypha showing that each segment contains two paired
nuclei (bright fluorescent objects).
thick-walled cells making up the rind (Townsend,
1954; Cairney, 1991). The best-known example of
a rhizomorph is that of the honey fungus
Armillaria mellea, a serious tree pathogen whose
flat, black, bootlace-like strands often persist
for long periods beneath the bark of trees killed
by the fungus (see pp. 16 18 and Fig. 18.13b).
Many ectomycorrhizal fungi especially in the
boletoid clade form hyphal aggregates which
show intermediate features between mycelial
cords and rhizomorphs.
Sclerotia
Sclerotia, an adaptation to prolonged survival
and propagation, develop in some basidiomycetes
(see pp. 18 20). They vary in size from
50 mm to several centimetres and in weight from
10 mg to several kilogrammes. They also vary in
organization from loose aggregations of dark
hyphae to highly differentiated structures with a
rind of smaller, dark thick-walled cells and a
medulla of larger, colourless, thin-walled cells
packed with food reserves (Willetts, 1971, 1972;
Clémençon, 2004). Many of the sclerotial types
described on pp. 18 20 are produced by
Basidiomycota, e.g. the loose type (Rhizoctonia
solani) and the strand type (Sclerotium rolfsii). Some
sclerotia are massive, as in Polyporus mylittae
where they may give rise to fruit bodies
(carpogenic development; Figs. 18.13c,d).
Basidiocarps of many common agarics such as
Coprinus cinereus, Collybia tuberosa, Hygrophoropsis
aurantiaca and Paxillus involutus may develop from
smaller sclerotia, and these are also characteristic
of the clavarioid fungus Typhula (Corner,
1950). Sclerotial germination, especially of plant
pathogenic fungi such as Rhizoctonia, is more
usually by the outgrowth of mycelium (myceliogenic
germination).
Pseudosclerotia with a similar function to
sclerotia, but consisting of a compacted mass of
intermixed substratum, soil, stones, etc., support
the fruiting of certain polypores such as Polyporus
tuberaster (the stone fungus, tuckahoe), and
Meripilus giganteus.
18.8 Asexual reproduction
Conidium formation is less commonly reported
in the Basidiomycota than in the Ascomycota.
Conidia may develop on monokaryotic or dikaryotic
mycelia, sometimes on both. They may also
form on basidiocarps. Conidia may have an
asexual function in propagation and dispersal
or may also fulfil a sexual role. We can only
consider a few examples. In terms of structure
and ontogeny, basidiomycete conidia are of three
basic kinds which are summarized below (see
Kendrick & Watling, 1979; Clémençon, 2004).