Introduction to Fungi, Third Edition

490 BASIDIOMYCOTA
wall is indeed thinner and differs in structure
from the other parts of the wall (Yoon &
McLaughlin, 1984). Beneath these areas there is
some evidence of cytoplasmic differentiation,
such as the presence of microtubules and vesicles.
The growth of sterigmata can be compared to
hyphal tip growth, except that a Spitzenkörper
is absent (McLaughlin, 1973). At the tips of the
sterigmata of Coprinus cinereus, vesicles apparently
fusing with the plasma membrane have been
observed by transmission electron microscopy
studies, and vesicles of similar size were also
found in the basidium (McLaughlin, 1973). Corner
(1948) suggested that the force for the development
of basidia comes from the expansion of the
basal vacuole which acts as a piston, ramming
the cytoplasm into the spores. Ripe basidia thus
contain a large vacuole but very little cytoplasm
(Fig. 18.1e). The vacuole, which is filled with
liquid, keeps the basidium turgid until spore
discharge has occurred.
The tip of the sterigma expands to form a
small spherical knob, the apophysis (Gr. apo- ¼
away from, separate; physis ¼ growth). Further
development of basidiospores is asymmetric,
expansion being more rapid towards the outside
of the long axis of the basidium. The narrow
point of attachment of the spore at the tip of the
sterigma is the eventual point of spore separation
and is termed the hilum. The non-expanded
part of the apophysis persists as the hilar
appendix (Fig. 18.3a).
18.3.2 Nuclear events
Typically, a basidium is at first binucleate; it is
formed on a dikaryotic mycelium, i.e. a mycelium
with segments containing two haploid nuclei
which are usually genetically different (see
Section 18.9). In this cell nuclear fusion (karyogamy)
occurs (see Fig. 18.4) and is followed immediately
by meiosis, giving rise to four haploid
nuclei. As in most fungi, division is intranuclear;
the nuclear membrane remains intact. Meiosis
occurs in the upper part of the basidium. In
narrow basidia the plane of the second meiotic
nuclear division lies parallel to the long axis of
the basidium. This type of nuclear division is
termed chiastic (Gr. chiastos ¼ crossed, arranged
diagonally) and basidia with nuclear division of
this sort are termed chiastobasidia. In contrast,
in broader basidia, the plane of both meiotic
nuclear divisions is transverse to the long
axis. Nuclear divisions of this type are stichic
(Gr. stichos ¼ a line or row of things) and the
corresponding term for basidia with such division
is stichobasidium. The plane of nuclear
division has relevance to taxonomy; chiastobasidia
are found in mushrooms and toadstools
whilst stichobasidia are more characteristic of
certain genera of bracket fungi in the polyporoid
clade. Terms used to define different parts of
a basidium are probasidium, the part within
which karyogamy occurs, and metabasidium,
the part within which meiosis occurs (see Kirk
et al., 2001). The four haploid nuclei formed
during meiosis move into the basidiospores
which are therefore usually four in number. As
they pass through the sterigmata, the nuclei
are often elongated and tapered apically. They
may be led through the sterigmata by microtubules
attached to the nuclear spindle pole
bodies (Thielke, 1982; Lingle et al., 1992).
In many basidiomycetes, meiosis is followed
by a post-meiotic mitosis (Duncan & Galbraith,
1972; Clémençon, 2004) which may happen in
different places: (1) In the upper part of the
basidium. Four of the eight nuclei enter the
spores, and those remaining in the basidium
abort. The ripe spores are thus uninucleate, e.g.
in Cantharellus cibarius. (2) At the base of or inside
the sterigmata. One nucleus enters each spore
and the other four remain in the basidium
and degenerate, e.g. in Collybia butyracea. (3) In
the young spore. Four of the daughter nuclei
migrate back into the basidium where they
degenerate, e.g. in Paxillus involutus. (4) In the
young spore, but all eight nuclei remain in the
four spores and none abort; the basidium is left
devoid of nuclei.
18.4 Basidiospore development
On the adaxial side of the apophysis, an electrondense
cytoplasmic region appears at the moment