Introduction to Fungi, Third Edition

162 CHYTRIDIOMYCOTA
pathways. There are important differences in the
activities of certain enzymes (Cantino et al., 1968;
Lovett, 1975). In the absence of bicarbonate,
there is evidence for the operation of a tricarboxylic
acid cycle, whereas in the presence of
bicarbonate, part of this cycle is reversed, leading
to alternative pathways of primary carbon
metabolism. In addition, a polyphenol oxidase,
absent in the thin-walled thallus, replaces the
normal cytochrome oxidase. There is also
increased synthesis of melanin and of chitin in
the presence of bicarbonate. The effect of
bicarbonate can be brought about by increased
levels of CO 2 .
Another unusual feature is that B. emersonii
fixes CO 2 more rapidly in the light than in the
dark. In the presence of CO 2 , light-grown thalli
show a number of differences when compared
with dark-grown controls. Illuminated thalli take
about three hours longer to mature, and are
larger than dark-grown thalli. They also have an
increased rate of nuclear division and a higher
nucleic acid content. The most effective wavelengths
for this increased CO 2 fixation (or
lumisynthesis) lie between 400 and 500 nm, i.e.
at the blue end of the spectrum. This suggests
that the photoreceptor should be a yellowish
substance. Attempts to identify the photoreceptor
have as yet been unsuccessful, but it is known
not to be a carotenoid.
6.6 Monoblepharidales
This group includes about 20 species and is
represented by 5 genera, namely Monoblepharis,
Monoblepharella, Gonapodya, Oedogoniomyces and
Harpochytrium. Fungi belonging to this order
can be isolated from soil samples or from twigs
or fruits submerged in freshwater, sometimes
under anoxic conditions (Karling, 1977; Fuller
& Clay, 1993). Whisler (1987) has given details
of isolation techniques. Most species are saprotrophs
and several are available in culture. In all
genera the thallus is eucarpic either with
rhizoids or a holdfast, and with branched or
unbranched filaments. The walls contain
microfibrils of chitin (Bartnicki-Garcia, 1968),
but the walls of G. prolifera also contain cellulose
(Fuller & Clay, 1993). A characteristic feature is
the frothy or alveolate appearance of the
cytoplasm caused by the presence of numerous
vacuoles often arranged in a regular pattern.
Asexual reproduction is by posteriorly uniflagellate
zoospores which are borne in terminal,
cylindrical or flask-shaped sporangia. Sexual
reproduction, where known, is unique for fungi
in being oogamous with a large egg and a
smaller, posteriorly flagellate spermatozoid. The
egg may be retained within the oogonium or
may move to its mouth by amoeboid movement
in some species of Monoblepharis, or propelled by
the lashing of the flagellum of the spermatozoid
in Monoblepharella and Gonapodya.
6.6.1 The zoospore
The fine structure of zoospores is similar in
representatives of all five genera (Fig. 6.24; see
Mollicone & Longcore, 1994, 1999). In all cases
the body of the zoospore is oval, the narrow
part facing forward and with a long whiplash
flagellum trailing from the wider posterior.
Amoeboid changes of shape may occur and
swimming zoospores may develop pseudopodia
anteriorly. The body of the zoospore is differentiated
into three regions: an anterior region
which is often devoid of organelles apart from
lipid globules, a few vacuoles and tubular
cisternae; a central region which contains the
nucleus, surrounded by ribosomal aggregations
(sometimes termed the nuclear cap), microbodies
and spherical mitochondria with flattened
cristae; and a posterior ‘foamy’ region at
the base of which are the functional kinetosome,
a non-functional kinetosome and a rumposomal
complex. The functional kinetosome is surrounded
by a striated disc, apparently anchored
to annular cisternae. From an electron-dense
region of the striated disc, about 31 34 microtubules
extend outwards into the body of the
zoospore. Water expulsion vacuoles have been
identified in the anterior part of the zoospore
of G. prolifera. Another distinctive feature in this
fungus is the presence of a pair of paraxonemal
structures, solid cylindrical fibres which are