Introduction to Fungi, Third Edition

MONOBLEPHARIDALES
163
Fig 6.24 Summary diagram of zoospore ultrastructure
of Monoblepharis polymorpha. Abbreviations: angular cisternae
(ac), endoplasmic reticulum (er), kinetosome (K), lipid globule
(L), mitochondria (M), microbody (mb), microtubule (mt),
nucleus (N), non-flagellated centriole (nfc), transition zone
plug (O), kinetosome prop (P), ribosomal aggregate (R),
rumposome (Ru), striated disc (sd), vacuole (V).The diameter
of the zoospore is about 6 mm. Reprinted with permission
from Mollicone and Longcore (1994), Mycologia.
ß The Mycological Society of America.
smaller in diameter than the axonemal microtubules,
running parallel to them within the
axoneme and connected at intervals to doublets
3 and 8 (Mollicone & Longcore, 1999).
6.6.2 Monoblepharis
Species of Monoblepharis occur in quiet silt-free
pools containing neutral or slightly alkaline
water (i.e. pH 6.4 7.5) on waterlogged twigs on
which the bark is still present. Twigs of birch,
ash, elm and especially oak are suitable substrata,
and although samples taken at varying
times throughout the year may yield growths
of the fungus, there are two main periods of
vegetative growth, one in spring and another
in autumn, with resting periods during the
summer and winter months. Low temperatures
appear to favour asexual development and good
growth can be obtained on twigs incubated in
dishes of distilled water at temperatures around
3°C. The mycelium is delicate and vacuolate.
The hyphae are multinucleate. During the
formation of a sporangium, a multinucleate
tip is cut off by a septum. The cytoplasm cleaves
around the nuclei to form zoospore initials
which are at first angular and then later pearshaped.
The ripe sporangium is cylindrical or
club-shaped and may not be much wider than
the hypha bearing it. A pore is formed at
the tip of the sporangium through which the
zoospores escape by amoeboid crawling. The
free zoospores swim away. On coming to rest,
a zoospore encysts and germinates by emitting
a germ tube. The single nucleus of the zoospore
cyst divides and further nuclear divisions occur
as the germ tube elongates.
Sexual reproduction can be induced by
incubating twigs at room temperature. Light
also affects reproduction in M. macrandra.
Cultures of this fungus incubated in the dark
produced only gametothalli whilst those grown
in light formed only sporothalli (Marek, 1984).
In M. polymorpha and related species, the antheridia
are epigynous, becoming cut off by a basal
septum. Beneath the antheridium the hypha
becomes swollen somewhat asymmetrically so
that the antheridium is displaced into a lateral
position. The swollen subterminal part becomes
spherical and is then cut off by a basal septum
to form the oogonium. In M. sphaerica and
some other species, the arrangement of the sex
organs is the reverse of that in M. polymorpha,
i.e. hypogynous. In M. macrandra the antheridia
and oogonia may grow as solitary organs at the