Introduction to Fungi, Third Edition

694 ANAMORPHIC FUNGI
Bandoni (1974) has advanced the idea that
tetraradiate spores may be adapted to movement
in surface films of water.
Although sigmoid spores are less efficiently
trapped than tetraradiate spores, they, too, can
develop in different ways and in unrelated
groups of fungi which have adopted an aquatic
habit, suggesting that their shape has selective
value. Observations on sigmoid spores moving
with the current flow in flat capillary tubes show
that as they approach a surface they tumble end
over end and come to rest with one spore tip in
contact with the surface. Immediately after
arrest, the spore swings parallel to the current,
thus minimizing the shear forces acting to
detach the spore (Webster & Davey, 1984). As in
tetraradiate spores, the tips of the arms of
sigmoid spores secrete mucilage, possibly in
response to a thigmotropic stimulus associated
with their tumbling movements. Mucilage is also
present on the outside of the sigmoid spore of
Mycocentrospora filiformis prior to it making
contact with a surface (Au et al., 1996). Current
flow forces the spore into contact with the
surface at a second point along its length.
Germination occurs by the development of
a germ tube from that end of the spore which
is in contact with the surface. Thus, in contrast
to the three-point contact associated with tetraradiate
spore shape, sigmoid spores make two
points of contact with surfaces to which they
adhere.
25.2.5 Spores in stream foam
Foam is an effective trap for both tetraradiate
and sigmoid spores (see Fig. 25.9). Spores
suspended in water or caught in stream foam
rarely germinate and many studies on the
distribution and seasonal abundance of aquatic
hyphomycetes using preserved foam samples
have been made. In experiments in which air
bubbles were passed through concentrated
suspensions of conidia, the concentration of
suspended spores fell very rapidly. Tetraradiate
conidia were removed more readily than conidia
of sigmoid or other shape (Iqbal & Webster,
1973a). Comparisons of spores collected in foam
or by Millipore filtration from the same stream
indicates that the spore content of foam overrepresents
the tetraradiate type of conidium
in relation to other spore types known to be
present. It cannot be assumed that all propagules
found in stream foam originate from within
the stream. Some come from fungi growing on
the living leaves of riparian trees and the spores
are brought into the stream in raindrops or
from rainwater draining down the tree trunks.
Such fungi have been distinguished as terrestrial
aquatic hyphomycetes (Ando & Tubaki,
1984a,b).
25.2.6 Adaptations of Ingoldian fungi to
the aquatic habitat
Ingoldian fungi represent an ecological group of
fungi sharing a common habitat, typically leaves
and twigs in rapidly flowing streams. It is
believed that they have been derived from
terrestrial ancestors and are able to colonize
their habitat by virtue of a number of adaptations.
These include effective spore attachment
mechanisms associated with tetraradiate and
sigmoid spore shape, rapid germination, and
rapid growth and sporulation enhanced by
turbulence and rapid water flow (Webster &
Towfik, 1972; Sanders & Webster, 1980).
Physiological adaptations include the possession
of a range of enzymes enabling them to degrade
their substrata (see below), and an ability to
grow at low temperatures, sometimes approaching
0°C, so that they can continue growth
and sporulation on submerged deciduous tree
leaves after the autumn pulse of leaf fall
(Koske & Duncan, 1974). Despite their typical
environment, many aquatic hyphomycetes can
survive for several weeks on dried leaves
previously colonized in streams and brought
out by flooding or by falling water levels (Sanders
& Webster, 1978). It is possible that overland
dispersal is achieved if colonized dried leaves
are blown about by wind and re-deposited in
streams.
25.2.7 Ecophysiological studies
There have been extensive studies on the ecology
and physiology of Ingoldian hyphomycetes
stimulated by the discovery that they play an