Introduction to Fungi, Third Edition

SAPROLEGNIALES
83
cleavage furrows separate the cytoplasm into
uninucleate pieces, each of which differentiates
into an auxiliary zoospore. As the zoospores are
cleaved, the central vacuole disappears. The tip
of the cylindrical sporangium contains clearer
cytoplasm and a flattened protuberance, the
papilla, develops at the apex. As the sporangium
ripens and the zoospores become fully
differentiated, they show limited movement
and change of shape (Figs. 5.4b d). Shortly
before discharge, there is evidence of a buildup
of turgor pressure within the sporangium
because the basal septum becomes concave,
i.e. it is bent towards the lumen of the hypha
beneath the sporangium. After cleavage, the
positive turgor pressure is lost concomitantly
with the loss of the sporangial plasma membrane
which becomes part of the zoospore
membranes, and the septum again bulges into
the sporangium while the zoospores become fully
differentiated. The sporangium undergoes a
slight change of shape at this time and the
sporangium wall breaks down at the papilla. The
spores are released quickly, many zoospores
escaping in a few seconds and moving as a
column through the opening. Osmotic phenomena
have been invoked to explain the rapidity
of discharge, and the osmotically active substances
must be large enough to be contained
by the sporangial wall. Mycolaminarin, released
from the central vacuole during zoospore differentiation,
is the likely solute (Money &
Webster, 1989). The whole process of sporangium
differentiation takes about 90 min. The zoospores
leave the sporangium backwards, with the blunt
posterior end emerging first. The size of the
zoospore is sometimes greater than the diameter
of the sporangial opening so that the zoospores
are squeezed through it. An occasional zoospore
may be left behind, swimming about in the
empty zoosporangium for a while before making
its exit. Zoospores in partially empty sporangia
orientate themselves in a linear fashion along the
central axis of the sporangium.
A characteristic feature of Saprolegnia is that,
following the discharge of a zoosporangium,
growth is renewed from the septum at its base so
that a new apex develops inside the old sporangial
wall by internal proliferation. This in
turn may develop into a zoosporangium, discharging
its spores through the old pore (Fig. 5.4f).
The process may be repeated so that several
empty zoosporangial walls may be found inside,
or partially inside, each other.
Upon release, the auxiliary zoospores slowly
revolve and eventually swim somewhat sluggishly
with the pointed end directed forwards.
They are grapeseed- or pear-shaped (‘Conference’
pear; Dick, 2001a) and bear two apically attached
flagella (see Figs. 5.1a, 5.4g). Each zoospore
also contains a diploid nucleus, mitochondria,
a contractile vacuole and numerous vesicles
(Holloway & Heath, 1977a,b). The zoospores
from a single sporangium show variation in
their period of motility, the majority encysting
within about a minute, but some remaining
motile for over an hour. The zoospore then
withdraws its flagella and encysts, i.e. the
cytoplasm becomes surrounded by a distinct
wall which is produced from pre-formed material
stored in the cytoplasmic vesicles. Only the
axonemes of the flagella are withdrawn, leaving
the TTHs of the straminipilous flagellum at
the surface of the cyst (see Fig. 5.5a). Following
a period of rest (2 3h in S. dioica), the cyst
germinates to release a further zoospore, the
principal zoospore (Figs. 5.4i,j). This differs in
shape from the auxiliary zoospore in being beanshaped,
with the two flagella inserted laterally
in a shallow groove running down one side of
the zoospore (Fig. 5.1b). The principal zoospore
may swim vigorously for several hours before
encysting. Salvin (1941) compared the rates of
movement of auxiliary and principal zoospores
in Saprolegnia and found that the latter swam
about three times more rapidly. The probable
reason for this is that the lateral insertion of
both flagella allows the straminipilous flagellum
to point forward and the whiplash one to point
backward, thereby improving the propulsion
relative to the apical insertion in which both
flagella point forward.
Movement of principal zoospores is chemotactic
and zoospores can be stimulated to
aggregate on parts of animal bodies such as the
leg of a fly, or the surface of a fish (Fischer &
Werner, 1958; Willoughby & Pickering, 1977).
When principal zoospores encyst, they shed