Introduction to Fungi, Third Edition

492 BASIDIOMYCOTA
Fig18.4 Life cycle of the basidiomycete Coprinus (diagrammatic and not to scale).The basidiocarp develops from a dikaryotic
secondary mycelium and produces numerous basidia on the surface of its gills beneath the cap. Progressive stages of basidium
maturation involving karyogamy (K) and meiosis (M) are indicated. Eventually, each basidium forms four basidiospores, each
containing a single haploid nucleus. In many basidiomycetes there is a post-meiotic mitosis, giving rise to two identical haploid nuclei
in each basidiospore (not shown here). Basidiospores are discharged by the surface-tension catapult mechanism involving Buller’s
drop. Discharged basidiospores germinate to form haploid (monokaryotic) mycelia with simple transverse septa. In Coprinus,these
often produce upright conidiophores which form numerous sticky haploid oidia.The apex of a monokaryotic hypha in the vicinity of
an oidium of compatible mating type will respond chemotropically by growing towards the compatible oidium (homing reaction).
Fusion (plasmogamy, P) between the hypha and the oidium initiates the formation of a dikaryotic mycelium bearing clamp
connections. Nuclear fusion does not occur at this stage.The dikaryotic mycelium can develop basidiocarps under appropriate
environmental conditions.Open and closed circles represent haploid nuclei of opposite mating type; the diploid nucleus is drawn
larger and half-filled.
Stage 3, equal enlargement. This is characterized
by spherical enlargement of the basidiospore.
Growth is at an angle of about 45° to the
sterigma apex (see Fig. 18.5a). The hilar appendix
body projects further into the spore wall. It is
conical or hemispherical, with the apex of the
cone or base of the hemisphere projecting
towards the hilar appendix. The outermost
layer of the basidiospore wall is sticky.
Stage 4, elongation. Basidiospores grow in
length and a pore cap is formed at the upper end
of the spore. The spore wall becomes darkly
pigmented, starting at the upper end.
Similar asymmetric changes in spore expansion
have been reported by Yoon and McLaughlin
(1984) for basidiospores of Boletus rubinellus
(Fig. 18.5b). Elongation of the spore occurs
mainly at later stages of development.
In certain basidiomycetes, e.g. Amanita
vaginata, the basidiospore is spherical. This
involves even wall expansion during development
and implies that the wall structure is