Introduction to Fungi, Third Edition

THE MECHANISM OF BASIDIOSPORE DISCHARGE
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point of abscission (Yoon & McLaughlin, 1986;
Money, 1998).
18.5 The mechanism of
basidiospore discharge
Fig18.5 Asymmetric expansion of basidiospores during
development. (a) Coprinus cinereus.Changes in shape and the
axis of growth (shown on the right) at successive stages in
basidiospore development.The numbers on the left indicate
spore stages. (b) Boletusrubinellus.Changes in shape and the
major axis of growth (long arrows) at successive
developmental stages.For further explanation see text.
(a)fromMcLaughlin(1977)and(b)fromYoonandMcLaughlin
(1984), by copyright permission of the American Journalof
Botany.
homogeneous. Corner (1948) believed that the
non-spherical shape of basidiospores was the
result of differential setting of the wall material.
This may well be true, but more recent studies
have shown that the basidiospore wall also
varies in thickness. Many basidiospores have
smooth outer walls, but others have characteristic
ornamentations, e.g. spines, folds or ridges.
The ornamentations generally develop by extension
of the outer wall layer of the basidiospore
(Pegler & Young, 1971; Clémençon, 2004). In such
spores, e.g. those of Lactarius and Russula, only
the main body of the spore is ornamented,
whereas the region of the adaxial face immediately
above the hilar appendix is smooth. This
region is termed the suprahilar plage, disc or
depression (Pegler & Young, 1971, 1979). It plays
an important part in the spore discharge
mechanism (see below).
Abscission of the basidiospore from its sterigma
is preceded by the formation of a plug of
material, the hilar plug which blocks the spore
hilum, and a sterigmal plug of wall material
immediately below the hilum. Between the two
plugs a septum appears which represents the
With the exception of the gasteromycetes (see
Chapter 20), in most terrestrial basidiomycetes
basidiospores are ballistospores, i.e. they are
actively projected from basidia. Various suggestions
have been made as to the mechanism of
ballistospore discharge (see Webster & Chien,
1990), but the one which we now accept is the
surface tension catapult, originally suggested by
Buller (1922) and Ingold (1939). Discussions of
the various theories of basidiospore discharge
have been written by Webster et al. (1988) and
Money (1998). Shortly before discharge, dissolution
of the abscission layer occurs, indicated by
a slight wobble in the position of the spore. Then
a spherical drop of liquid, Buller’s drop, forms at
the hilar appendix and a shallower liquid
deposit, the adaxial drop (adaxial blob), appears
on the face of the spore above the hilar appendix
(Fig. 18.7). Cinephotography has been used to
illustrate these events (Webster & Hard, 1998b;
Webster, 2006b). Both drops increase in size until
they eventually coalesce, and spore discharge
then immediately occurs (Pringle et al., 2005).
Experimental investigations on the phenomenon
of ballistospore discharge have focused
on Itersonilia perplexans, an unusual heterobasidiomycete
with large ballistospores. This fungus
is a weak plant pathogen and is commonly
associated with lesions caused by other pathogens
such as rust and smut fungi. It also grows in
basidiocarps of certain jelly fungi and can be
readily isolated by allowing it to shoot off its
ballistospores from the basidiocarps of Dacrymyces
stillatus or Auricularia auricula-judae (Ingold, 1983a,
1984a). In culture it forms a clamped dikaryotic
mycelium, the tips of whose branches swell to
form clamped sporogenous cells, each with a
single ballistospore (Fig. 18.6a). The sporogenous
cells do not fully match the definition of basidia
because nuclear fusion and meiosis do not occur
in them; instead the dikaryotic cell forms a