Introduction to Fungi, Third Edition

496 BASIDIOMYCOTA
mushroom Agaricus campestris produced 1.8 10 9
spores over 2 days at an average rate of
40 million h 1 . Estimates for some other basidiomycetes
are given in Table 18.1. The mycelia of
all these fungi are perennial and an individual
mycelium may produce numerous basidiocarps
over a period of many years. It is therefore clear
that an individual basidiospore has an infinitesimal
chance of successfully establishing a
fruiting mycelium.
Fig18.8 Representation of events associated
with ballistospore discharge. (a) Ballistospore
attached to its sterigma before drop
formation.The closed circle within the spore
indicates the centre of mass of the spore.
(b) Buller’s drop appears at the hilar appendix.
The adaxial drop emerges on the spore wall
above it and extends downwards as it
increases in size.The centre of mass of spore
plus drop moves to a position nearer the hilar
appendix. (c) Contact between the two drops
is followed by immediate coalescence and the
combined mass of liquid moves rapidly up the
adaxial face of the spore away from the hilar
appendix.The centre of mass moves very
rapidly in the direction of the thin arrow and
the spore drop system gains kinetic energy
and momentum in the same direction,
simultaneously exerting an opposite force F
on the sterigma at the hilum (thick arrow).
Some angular momentum is also exerted on
the spore, related to the distance a between
the hilum and the hilar appendix. Reprinted
from Webster et al. (1988), with permission
from Elsevier.
18.7 Basidiospore germination and
hyphal growth
18.7.1 Germination
Basidiospores may remain dormant and retain
viability for several months or even for a few
years if conditions are unsuitable for germination.
Dormancy is frequently exogenous, i.e. the
spores require some external chemical or physical
stimulus before germination can occur.
Germination may be direct by production of a
germ tube, by repetition (i.e. the formation of a
secondary ballistospore), or by the formation of
conidia. Repetitious germination is common in
certain jelly fungi (Tremellales) (see Figs. 18.6b,
21.7b and 21.13d). Germination by the formation
of conidia is illustrated for Dacrymyces stillatus
(Fig. 21.4a), Auricularia auricula-judae (Fig. 21.6c)
and by yeast-like budding in Tremella frondosa (Fig.
21.13c). During direct germination, germ tubes
usually emerge through a special germ pore at
the hilum or, as in Coprinus, through a pore at
the opposite end of the spore.
18.7.2 Monokaryotic and dikaryotic
hyphae
Because the nuclear divisions involved in basidiospore
formation are meiotic, basidiospores
are haploid. Since the post-meiotic nuclear
divisions are mitotic, if there are several nuclei
in a single basidiospore these are usually
genetically identical. They are said to be homokaryotic
(Gr. homos ¼ equal, alike; karyon ¼ a nut,
here meaning nucleus). At germination, repeated
mitotic nuclear divisions occur and the early
germ tubes may consequently be multinucleate
and coenocytic. Transverse septa are laid down
behind the growing hyphal tip and eventually
divide the hypha into segments which contain
only a single nucleus. The uninucleate segments
and the hyphae which contain them are said to
be monokaryotic. The terms homokaryon and
monokaryon or primary mycelium have also
been applied to such haploid mycelia. As part of
the sexual cycle, monokaryotic hyphae of genetically
distinct mating type undergo plasmogamy
(somatogamy), i.e. they fuse together and initiate
the formation of a mycelium made up of