Introduction to Fungi, Third Edition

86 STRAMINIPILA: OOMYCOTA
The oogonial initial is multinucleate, and
nuclear divisions continue as it enlarges.
Eventually some of the nuclei degenerate, leaving
only those nuclei which are included in the
oospheres. From the central vacuole within the
oogonium, cleavage furrows radiate outwards to
divide the cytoplasm into uninucleate portions
which round off to form oospheres. Oogonium
differentiation is thus centrifugal, which is
typical of the Saprolegniales. Cleavage of the
oospheres from the cytoplasm is brought about
by the coalescence of dense body vesicles which
finally fuse with the plasma membrane of the
oogonium so that the oospheres tumble into the
centre of the oogonium (Dick, 2001a). The entire
mass of cytoplasm within the oogonium is used
up in the formation of oospheres and there is
no residual cytoplasm (periplasm) as in the
Peronosporales. The wall of the oogonium is
often uniformly thick, but in some species it
shows thin areas or pits through which fertilization
tubes may enter (Fig. 5.6e). A septum at
the base of the oogonium cuts it off from the
subtending hypha.
The antheridia are also multinucleate. The
antheridial branch grows towards the oogonium
and attaches itself to the oogonial wall. The tip of
the antheridial branch is cut off by a septum,
and the resulting antheridium puts out a fertilization
tube which penetrates the oogonial wall
and may branch within the oogonium. After the
tube has penetrated an oosphere wall, a male
nucleus eventually fuses with the single oosphere
nucleus. The fertilized oosphere (oospore) undergoes
a series of changes described by Beakes
and Gay (1978a,b). The wall of the oospore
thickens and oil globules become obvious.
Mature oospores contain a membrane-bound
vacuole-like body, the ooplast, surrounded by
cytoplasm containing various organelles, with
lipid droplets particularly prominently visible.
In Saprolegnia, the ooplast contains particles in
Brownian motion. The position of the ooplast in
the oospore is used for species identification, and
four types of oospore have been distinguished
(Fig. 5.7; Seymour, 1970; Howard, 1971). Centric
oospores have a central ooplast surrounded by
one or two peripheral layers of small lipid
droplets (e.g. S. hypogyna, S. ferax). Subcentric
oospores have several layers of small lipid
droplets on one side of the ooplast and only one
layer or none at all on the other (e.g. S. unispora,
S. terrestris). In subeccentric oospores, the small
lipid droplets have fused into several large ones
all grouped to one side, with the ooplast contacting
the plasma membrane on the opposite side
(e.g. S. eccentrica). The eccentric type (found, for
example, in S. anisospora) is similar to the
subeccentric type except that there is only one
very large lipid drop. These descriptive terms are
also used for many other species of Oomycota.
5.2.2 Achlya (Saprolegniaceae)
Phylogenetic analyses have shown that the
genera Achlya and Saprolegnia as well as minor
genera of the Saprolegniales are closely related
to each other, with possible overlaps which may
necessitate the re-assignment of some species in
future (Riethmüller et al., 1999; Leclerc et al.,
2000; Dick, 2001a). Morphologically and ecologically,
Achlya and Saprolegnia also share several key
features. Both are common in soil and in waterlogged
plant debris such as twigs, and certain
species are pathogens of fish (Willoughby, 1994;
Kitancharoen et al., 1995). Unlike Saprolegnia,
some species of Achlya are heterothallic, but
their life cycle is otherwise similar to that of
Saprolegnia given in Fig. 5.3. Heterothallic strains
of Achlya have been the subject of classical
Fig 5.7 Possible arrangements of the
ooplast (shaded organelle) and lipid
droplets (empty circles or ellipses) in
oospores of Saprolegnia. (a) Centric.
(b) Subcentric. (c) Subeccentric.
(d) Eccentric.