Introduction to Fungi, Third Edition

150 CHYTRIDIOMYCOTA
Fig 6.17 Scanning electron
micrograph of two thalli of
Rhizophlyctisrosea on a cellophane
membrane. Pit corrosion is visible
where a thallus has been lifted
from the substratum (arrows).
wall (Fig. 6.16f ). Whether they are formed
sexually in R. rosea is not known. Couch (1939)
has, however, put forward evidence that the
fungus is heterothallic because single isolates
grown in culture failed to produce resting
sporangia whereas these structures did form
when certain cultures were paired. Stanier (1942)
has reported the occurrence of biflagellate zoospores,
but whether these represented zygotes
seemed doubtful. In the homothallic chitinophilic
fungus Rhizophlyctis oceanis, Karling (1969)
has described frequent fusions between zoospores.
These fusions are possibly sexual, but
unfortunately Karling was unable to cultivate
the resulting thalli to the stage of resting spore
development.
On germination, the resting sporangium of
R. rosea functions as a prosporangium, although
it is uncertain whether resting sporangia are
important for survival in nature. Willoughby
(2001) has shown that R. rosea could be recovered
from cellophane baits in as little as 5 6 h after
placing air-dried soil samples in water, and it
was concluded that these zoospores were derived
from sporangia instead of resting spores which
need a longer time to produce zoospores.
The nutritional requirements of R. rosea are
simple. It shows vigorous growth on cellulose
as the sole carbon source but it can utilize
a range of carbohydrates such as glucose,
cellobiose and starch. The pink colour of the
sporangia is due to the presence of carotenoid
pigments such as g-carotene, lycopene and a
xanthophyll.
6.4 Neocallimastigales
(rumen fungi)
A very interesting and unusual group of zoosporic
fungi inhabits the rumens (foreguts) of
ruminants (herbivorous mammals which regurgitate
and masticate previously ingested food)
like cows, sheep and deer. They have also been
found in some non-ruminants such as horses and
elephants and probably occur in the guts of
many large herbivores. These fungi are obligate
anaerobes which can flourish in the rumen
because oxygen is depleted there by the intense
respiratory activity of a dense population of
protozoa and bacteria, some of which are
facultative anaerobes capable of scavenging free
oxygen. Their zoospores were at first thought to
be protozoa and were not recognized as belonging
to fungi because obligately anaerobic fungi