Biology Reference
In-Depth Information
D
A
B
c
d
a
b
c
1
C
Fig. 3
Sexual reproduction in the Ascomycetes. A asci
borne singly in locules in stroma (Myriangiales). B peri-
thecia with long necks or beaks immersed in stroma
(Sphaeriales). C papillate perithecium in host
opening with a mouth or ostioles (Spaheriales).
D Discomycetes (Heliotiales), ascus (
a
) and paraphyses
(
a
) formed in a hymenial layer in a cuplike apothecium (
c
)
and (
c
1
); ascospore (
d
) germinates by germ tube
tissue,
and the ascus, a sac, typically club-shaped or
cylindrical, bearing the sexual spores, asco-
spores, usually eight in number. Asci may be
formed on or in hyphae or cells but are usually
grouped in structures, ascocarps, either in locules
in a stroma or lining a cup-shaped fruiting body
called an apothecium or the walls of an enclosed
round or flaska-shaped perithecium. The young
ascus has two nuclei, which fuse and then
undergo generally three divisions to give the
eight spores. In many genera paraphyses, thin
sterile clubs, are formed between the asci (see
Fig. 3
).
Many ascomycetes have both a parasitic and
a saprophytic stage. In their parasitic stage they
usually produce conidia or anamorph spores,
sometimes on groups of conidiophores growing
out of the mycelium, sometimes in a special pyc-
nidium. Similar structures sometimes found are
spermagonia containing spermatia, small sex cells.
Protomycetaceae Chlamydospores thick-
walled, germinating after a rest period, the exo-
spore splitting and the endospore emerging to
form a large multispored spore-sac. Parasitic on
vascular plants.
Taphrinales Chlamydospores thin-walled; asci
eight-spored but may become multispored by
budding. Genera
Exoascus
and
Taphrina
cause
leaf curl and leaf blisters and now
Exoascus
is
usually considered a synonym of
Taphrina
.
Subclass Euascomycetidae. Asci borne in
ascocarps.
Eurotiales
The order is characterized by (1) asci free on
mycelium or within sessile or stipitate ascocarps;
(2) sexual fusion, usually by trichogyne and
undifferentiated hypha; (3) spherical-to-ovoid
evanescent asci; (4) single-celled ascospores
that are oblate, spherical, globosa, or smooth or
with reticulations, spines, or thickened rings;
sometimes with appendages; (5) dry usually
phialidic conidia; and (6) being known from
a variety of often starchy, oily, or cellulosic
substrates.
Trichocomaceae Diversity of coverings are
found over the asci: pseudoparenchymatous
cleistothecia; stromata in which cleistothecia
develop; stromata without cleistothecia; or
wefty hyphal coverings over the asci.
Taphrinales
This order is now placed in the class
Archiascomycetes. Hyphae bearing terminal
chlamydospores or ascogenous cells, each of
which produces a single ascus, usually forming
a continuous hymenium-like layer on often
modified tissues of hosts. Spore sac compound
(a synascus) regarded as equivalent of numerous
asci. Parasitic on vascular plants.
