Introduction to Fungi, Third Edition

SAPROLEGNIALES
85
Either way, attachment must be very effective
because trout or char, placed in a water bath with
principal zoospores of S. parasitica for 10 min and
followed by 1 h in clean water, had an extremely
high concentration of cysts attached to the skin
(Willoughby & Pickering, 1977).
Principal zoospore cysts can germinate either
by means of a germ tube (Fig. 5.4p) or by
releasing a further principal zoospore which in
turn may germinate directly or by releasing yet
another motile stage. Saprolegnia is therefore
polyplanetic. The auxiliary and principal zoospores,
as well as the cysts they form, differ
morphologically from each other, i.e. they are
diplanetic.
Sexual reproduction in Saprolegnia
All members of the genus Saprolegnia characterized
to date are homothallic, i.e. a culture
derived from a single zoospore will give rise to
a mycelium forming both oogonia and antheridia.
In contrast, Achlya also contains heterothallic
species in which sexual reproduction occurs
only when two different strains are juxtaposed,
one forming oogonia, the other antheridia (see
Fig. 5.10).
Sexual reproduction follows a similar course
in all members of the Saprolegniales. Oogonia
containing one or several eggs are fertilized by
antheridial branches. Fertilization is accomplished
by the penetration of fertilization tubes
into the oogonium. In some species, ripe oogonia
are found without antheridia associated with
them (Fig. 5.6f); this could be due either to the
fusion of two haploid nuclei from adjacent
meiotic events in a single oogonium (apomixis)
or the formation of an oospore around a diploid
nucleus that never underwent meiosis (parthenogenesis).
Both processes are impossible to
distinguish without detailed cytological evidence
(Dick, 2001a). The typical arrangement of oogonia
and antheridia in Saprolegnia is shown in
Fig. 5.6. Antheridial branches arising from the
stalk of the oogonium or the same hypha as the
oogonium are said to be monoclinous whereas
they are diclinous if they originate from different
hyphae.
Fig 5.6 Saprolegnia litoralis.(a d) Stages in the development of oogonia. (c) Oogonium showing furrowed cytoplasm indicative
of centrifugal cleavage. (d) Outlines of two oospheres become visible. (e) Oogonium with two mature oospores. (f) Intercalary
oogonium lacking antheridia.The oospores have developed by apomixis or parthogenesis. (g) Chain of chlamydospores.