Introduction to Fungi, Third Edition

LABYRINTHULOMYCOTA
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phylogenetically closely related to the Oomycota
(van der Auwera et al., 1995; Hausner et al., 2000;
see Fig. 4.2). Treatments of the group have been
given by Karling (1977), Fuller (1990, 2001) and
Dick (2001a). The diagnostic feature is the
zoospore with its single anterior straminipilous
flagellum (Fig. 4.5). This kind of zoospore is not
found in any other known life form. The
zoospore of hyphochytrids contains one prominent
Golgi stack, one nucleus, and lipid droplets
and microbodies (Barr & Allan, 1985; Cooney
et al., 1985). The latter are not arranged in a
microbody lipid complex like they are in
chytrids (cf. Fig. 6.3). The TTHs are localized
within Golgi-derived vesicles. The flagellum
arises from a kinetosome, with microtubules
rooting deeply within the spore and probably
maintaining its shape. A second (dormant)
kinetosome lies adjacent but at an angle, at the
same position as that which gives rise to the
backward-directed smooth flagellum in zoospores
of Oomycota. This whiplash flagellum is
missing in Hyphochytriomycota, and Barr and
Allan (1985) have speculated that it could have
been lost during evolution of the latter from the
former. Like the Oomycota, hyphochytrids
synthesize lysine by the a,e-diaminopimelic acid
(DAP) pathway (Vogel, 1964).
Hyphochytrids occur in the soil and in
aquatic environments (both freshwater and
marine) as saprotrophs or parasites of algae,
oospores of Oomycota or azygospores of
Glomales. Hyphochytrium peniliae was reported
once as the cause of a devastating epidemic of
marine crayfish (Artemchuk & Zelezinkaya,
1969), but no further cases have been observed
since. Some species can be isolated into pure
culture relatively easily (Fuller, 1990).
Zoospores encyst by withdrawing their flagellum
and secreting a wall, leaving the TTHs
dispersed on the surface of the cyst wall
(Beakes, 1987). The cyst germinates by enlargement
or by putting out rhizoids. Because of the
similarity of their vegetative thalli with those of
Chytridiomycota (see Chapter 6), hyphochytrids
have been studied primarily by comparison with
chytrids, and the same terminology has been
used (see Fig. 6.1). Depending on the species,
cysts germinate to develop in three different
ways, which have been used to subdivide
the Hyphochytriomycota into families:
(1) Holocarpic thalli are produced by simple
enlargement of the cyst. The entire content of
the sac-like thallus ultimately becomes converted
into zoospores (Anisolpidiae, e.g. Anisolpidium
which parasitizes marine algae; Canter, 1950).
(2) In eucarpic monocentric thalli, the cyst
produces a bunch of rhizoids at one end, which
anchor the enlarging thallus to the substratum
and/or absorb nutrients (Rhizidiomycetidae, e.g.
Rhizidiomyces; Wynn & Epton, 1979). (3) In
eucarpic polycentric thalli, a broad hypha-like
germ tube emerges, branches and produces
several zoosporangia (Hyphochytriaceae, e.g.
Hyphochytrium; Ayers & Lumsden, 1977). The
asexual life cycle is completed when a fresh
crop of zoospores is released. Sexual reproduction
has not yet been reliably described for the
hyphochytrids.
4.4 Labyrinthulomycota
Whereas the Hyphochytriomycota described in
the previous section have a strong resemblance
to true fungi (especially Chytridiomycota),
the Labyrinthulomycota do not, and the
only justification for mentioning them here is
the fact that they have traditionally been
studied by mycologists. They have been the
subject of numerous taxonomic rearrangements,
and are known under many different names such
as Labyrinthomorpha, Labyrinthista and
Labyrinthulea. Some 48 species are currently
recognized (Kirk et al., 2001). DNA sequence
comparisons have placed them within the
Straminipila (Fig. 4.2; Hausner et al., 2000;
Leander & Porter, 2001), and they are characterized
by having heterokont flagellation, i.e.
possessing a straminipilous and a whiplash
flagellum with a pointed tip (Fig. 4.7). In addition,
they have mitochondria with tubular cristae.
Recent treatments of this group can be found in
Moss (1986), Porter (1990) and Dick (2001a).
Labyrinthulomycota occur in freshwater and
marine environments where they are attached to
solid substrata by means of networks of slime