Introduction to Fungi, Third Edition

ALEURIA (PYRONEMATACEAE)
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simultaneously divide by mitosis (conjugate
mitosis). Two septa cut off a uninucleate terminal
cell, a binucleate penultimate cell and
a uninucleate antepenultimate cell (the stalk
cell). The binucleate penultimate cell is the ascus
mother cell and the two nuclei within it fuse,
i.e. karyogamy now occurs. The diploid fusion
nucleus undergoes meiosis and the four resulting
haploid nuclei then divide mitotically so
that eight haploid nuclei result around which
the eight ascospores are subsequently cleaved
(Reeves, 1967). No further nuclear divisions occur
so that each ascospore contains one haploid
nucleus. No special inclusions are seen in the
septa of the crozier, but electron-dense plugs are
formed at the base of the ascus (Hung & Wells,
1971; Kimbrough, 1994). When the uninucleate
terminal cell grows backwards and makes
contact with the stalk cell, their walls break
down and a new binucleate cell is formed which
grows on to form a further crozier and another
ascus, a process which is repeated so that a single
ascogenous hypha may produce several asci
(Fig. 14.2b). The ascus mother cell elongates and
acquires a cylindrical shape. It is surrounded by
filamentous paraphyses. These develop from the
stalks of the ascogonia (I. M. Wilson, 1952) and
also appear to arise from ascogenous hyphae
(Fig. 14.2c). As the ascus matures it extends above
the layer of paraphyses and explodes, throwing
out its ascospores. The operculum may persist
as a hinged lid (Fig. 14.2c) or may be blown
off. During the development of asci, before
the ascospores are cleaved out, the operculum
becomes apparent as a thickened rim of wall
material at the upper end of the ascus.
Mature ascospores have three wall layers,
a thicker, electron-transparent inner layer (the
endospore), a thinner electron-opaque epispore,
and an outer fibrous perispore of variable thickness.
The perispore lies immediately within
the ascospore-investing membrane, and, as ascospores
mature, this membrane continues to
produce vesicles, leading to degradation of the
perispore. At discharge, the ascospores do not
stick together but remain separate from each
other (Hung, 1977). Merkus (1976) has described
the development and structure of the ascospores
of P. omphalodes in similar terms.
Much is known about the conditions under
which P. domesticum forms apothecia and sclerotia
(Moore-Landecker, 1975, 1992). Light is
required for apothecium development, with
white, blue and far-red light being particularly
effective. Sclerotium formation is inhibited by
intense blue light.
14.2.2 Ecology of Pyronema
In nature, the apothecia of both species of
Pyronema are among the first to appear on
burnt ground following volcanic eruptions, wildfires,
controlled burns and bonfires. Pyronema
forms part of a characteristic group of ‘phoenicoid
fungi’, i.e. fungi arising from ashes. Many
other operculate discomycetes are also phoenicoid
(Carpenter & Trappe, 1985; Dix & Webster,
1995). The ascospores of P. domesticum germinate
readily at 20°C, although a short exposure to
50°C enhances germination. Apothecium formation
is inhibited by the presence of other
soil-inhabiting organisms and it is possible that
the preference for burnt ground and steamsterilized
soil is associated with its rapid growth
and inability to compete with other soil biota
(El-Abyad & Webster, 1968a,b).
Unnatural (i.e. man-made) situations in which
P. domesticum fruits are steam-sterilized soils
and composts used in horticulture, and plaster
prepared by slaking lime, a process which generates
heat. Supposedly sterile surgical gauzes
manufactured from Chinese cotton have been
found to be contaminated with P. domesticum
due to insufficient radiation treatment during
manufacture. The source is likely to be raw
cotton materials possibly already contaminated
in the field (Yan, 1998). Laboratory experiments
have shown that the g-irradiation resistance
of P. domesticum ascospores is higher even than
that of Bacillus endospores (Richter & Barnard,
2002).
14.3 Aleuria (Pyronemataceae)
Aleuria seems to be closely related to Pyronema
(Landvik et al., 1997). There are about 10 species
of Aleuria, growing especially on forest soil.