Introduction to Fungi, Third Edition

SAPROLEGNIALES
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cell wall-softening enzymes and the role of the
cytoskeleton in pushing forward the growing tip
(see pp. 6 9; Money & Harold, 1992, 1993; Money,
1997; Money & Hill, 1997).
The Saprolegniales are the only order within
the Oomycota to produce both auxiliary and
principal zoospores, although both forms are not
produced in all genera. The production of two
distinct motile stages is termed diplanetism.
It has also been called dimorphism, but this
term has several different meanings and is best
avoided in the current context. Depending on
environmental conditions, the cysts of principal
zoospores may germinate either by means of a
germ tube developing into a hypha or by the
emergence of a new principal zoospore. The
repetition of the same type of motile spore is
called polyplanetism.
Sexual reproduction in the Saprolegniales is
oogamous, with a large, usually spherical oogonium
containing one or several oospheres.
Antheridial branches apply themselves to the
wall of the oogonium and penetrate the wall
by fertilization tubes through which a single
nucleus is introduced into each oosphere. A
feature of many Saprolegniales, especially when
grown in culture, is the formation of thick-walled
enlarged terminal or intercalary portions of
hyphae which become packed with dense cytoplasm
and are cut off from the rest of the
mycelium by septa. These structures, which may
occur singly or in chains (see Fig. 5.6g), are termed
gemmae or chlamydospores, and their formation
can be induced by manipulating the culture conditions.
Morphologically less distinct but otherwise
similar structures are frequently found in
old cultures. Although it is known that chlamydospores
cannot survive desiccation or prolonged
freezing, they remain viable for long periods in
less extreme conditions. They may function as
female gametangia or as zoosporangia, but more
frequently they germinate by means of a germ
tube. Another feature of old cultures is the
fragmentation of cylindrical pieces of mycelium
cut off at each end by a septum.
Members of the Saprolegniales can be isolated
readily from water, mud and soil by floating split
boiled hemp seeds or dead house flies in dishes
containing pond water, or by covering soil
samples or waterlogged twigs with water
(Stevens, 1974; Dick, 1990a). Within about 4 days
the fungi can be recognized by their stiff,
radiating, coarse hyphae bearing terminal sporangia,
and cultures can be prepared by transferring
hyphal tips or zoospores to cornmeal agar
or other suitable media. The most commonly
encountered genera are Achlya, Dictyuchus,
Saprolegnia, Thraustotheca and Aphanomyces. With
the exception of a few obligately parasitic species,
most of the Saprolegniales will grow readily in
pure culture even on chemically defined media,
and extensive studies of their nutritional physiology
have been undertaken (summarized by
Cantino, 1955; Gleason, 1976; Jennings, 1995).
Most species examined have no requirement for
vitamins. Organic forms of sulphur such as
cysteine, cystine, glutathione and methionine
are preferred, and most species are unable to
reduce sulphate. Organic nitrogen sources such
as amino acids, peptone and casein are preferred
to inorganic sources. Ammonium is widely
utilized, but nitrate is not. Glucose is the most
widely utilized carbon source, but many species
also degrade maltose, starch and glycogen. In
liquid culture, Saprolegnia can be maintained in
the vegetative state indefinitely if supplied with
organic nutrients in the form of broth. When the
nutrients are replaced by water, the hyphal tips
quickly develop into zoosporangia. The formation
of sexual organs can similarly be affected by
manipulating the external conditions in some
species, and the concentration of salts in the
medium may play a decisive role (Barksdale,
1962; Davey & Papavizas, 1962).
5.2.1 Saprolegnia (Saprolegniaceae)
Species of Saprolegnia are common in soil and in
freshwater as saprotrophs on plant and animal
remains. A few species such as S. parasitica and
S. polymorpha cause disease in fish and their eggs
(Plate 2a). Salmonid fish are particularly affected,
and the disease can cause significant damage
in fish farms around the world (Willoughby,
1994, 1998a). Control by fungicides is difficult
but possible (Willoughby & Roberts, 1992). The
disease is also seen in wild salmon and other fish
(Söderhäll et al., 1991; Bly et al., 1992). Pathogenic