Introduction to Fungi, Third Edition

PYTHIALES
95
are keratinophilic, occurring in the soil or in
water on insect remains (Dick, 1970; Seymour &
Johnson, 1973).
5.3 Pythiales
The order Pythiales includes two families, the
Pythiaceae and Pythiogetonaceae (Dick, 2001a;
Kirk et al., 2001). The Pythiogetonaceae are a small
group of aquatic saprotrophs presently comprising
one genus and six species. They occur in
anoxic sediments at the bottom of freshwater
lakes and are facultatively anaerobic as well as
obligately fermentative, i.e. they break down
sugars incompletely to give organic acids irrespective
of the presence or absence of oxygen
(Emerson & Natvig, 1981; Natvig & Gleason, 1983).
Another member of the Pythiogetonaceae,
Pythiogeton zeae, causes root and stalk rot in
maize (Jee et al., 2000). The Pythiogetonaceae are
clearly related to the Pythiaceae by DNA sequence
homology (Voglmayr et al., 1999).
Only the Pythiaceae will be considered
further in this book. This is a large family of
over 200 species in approximately 10 genera, of
which 2 are of outstanding significance: Pythium
and Phytophthora. Phytophthora species are primarily
pathogenic to plants from which they can be
isolated and grown in pure culture. The genus
Pythium is best known for its saprotrophic soilinhabiting
members, many of which are opportunistic
pathogens especially in young plants.
There are also obligately pathogenic Pythium
spp. Generally, Pythium spp. parasitize a wider
diversity of hosts than Phytophthora, including
mammals, fungi and algae.
5.3.1 Life cycle of Pythiaceae
The life cycle of Phytophthora infestans is summarized
in Fig. 5.19. Asexual reproduction in Pythium
and Phytophthora is by means of sporangia
which vary in shape from swollen hyphae or
globose structures (Pythium) to lemon-shaped
(Phytophthora). Sporangia are borne on more or
less undifferentiated hyphae. In most cases,
sporangia germinate to produce zoospores
which are of the principal (kidney-shaped) type.
In many Pythium spp., the final stages of zoospore
differentiation take place outside the sporangium
in a walled vesicle, followed by breakdown
of the soft wall and release of the zoospores. In
Phytophthora, in contrast, zoospores differentiate
within the sporangium and are released directly
or via a very short-lived vesicle which is surrounded
only by a membrane. About 20% of
the total respiratory activity within a released
zoospore is used up to fuel propulsion (Hölker
et al., 1993). The forward-directed straminipilous
flagellum generates about 10 times more thrust
than the posterior whiplash flagellum which acts
mainly as a rudder (Erwin & Ribeiro, 1996).
Zoospores can swim for several hours before they
encyst. The process of encystment has been
examined in great detail for Phytophthora (see
p. 102). Cysts usually germinate by means of
a germ tube, only rarely producing a further
zoospore stage. In many species, sporangia can
germinate either indirectly by releasing zoospores
or directly by means of a germ tube,
depending on environmental conditions and
age of the sporangium.
Sexual reproduction is oogamous. Each oogonium
contains a single oosphere (except for
Pythium multisporum in which there are several).
The antheridial and oogonial initials are
commonly multinucleate at their inception and
further nuclear divisions may occur during
development. Meiosis eventually takes place in
the gametangia so that karyogamy occurs
between haploid antheridial and oogonial
nuclei. In many forms, there is only one functional
male and female nucleus, but in others
multiple fusions occur. Oospores germinate
either by producing a single germ sporangium,
or by sending out vegetative hyphae.
Most members of the Pythiaceae are homothallic,
although heterothallism and relative
sexuality have been reported, e.g. for
Phytophthora infestans (Fig. 5.19) and Pythium
sylvaticum. Heterothallic species are thought to
be derived from homothallic ones (Kroon et al.,
2004). The situation of mating in heterothallic
strains is rather complex and still only incompletely
understood. A system of two mating types
(A1 and A2) seems to be superimposed on a
hormonal control mechanism of mating akin to