Introduction to Fungi, Third Edition

SAPROLEGNIALES
87
studies on the nature of mating hormones
(pheromones); additionally, more recent work
has focused on zoospore release. Both aspects are
described below.
Asexual reproduction in Achlya
The development of zoosporangia in Achlya is
similar in all aspects to that in Saprolegnia but
has been better researched. The central vacuole
in the developing cylindrical sporangium is
typical of the Saprolegniales and originates
from the fusion of dense body vesicles containing
mycolaminarin. The centrifugal cleavage of
cytoplasm from the vacuole towards the plasma
membrane, and the partitioning of individual
spores, are controlled mainly by the actin
cytoskeleton (Heath & Harold, 1992). In the
Pythiales, vital roles of microtubules in the
organization of differentiating cytoplasm have
been described (see p. 102), and microtubules
may have similar but as yet undescribed functions
in the Saprolegniales. As the plasma
membrane of the Achlya zoosporangium is
breached, the zoosporangial volume decreases
by about 10% due to the loss of turgor pressure.
Since the membranes of the vacuole contribute
to the zoospore plasma membrane, the vacuolar
contents of water-soluble mycolaminarins (b-1,3-
glucans) are released into the sporangium. These
molecules are osmotically active but are too
large to diffuse through the sporangial wall, thus
causing the osmotic inward movement of water
into the sporangium, which in turn pressurizes
the sporangium and drives the rapid discharge of
the auxiliary zoospores (Money & Webster, 1985,
1988; Money et al., 1988).
On discharge, the zoospores do not swim
away but cluster in a hollow ball at the mouth of
the zoosporangium and encyst there (Fig. 5.8a).
In fact, it is doubtful whether the term ‘zoospore’
is altogether appropriate as functional flagella
are probably not formed. Partial fragmentation
of the cyst ball frequently occurs and may have
ecological significance in the dispersal of cysts
prior to the release of principal zoospores. Unlike
certain species of Saprolegnia, Achlya cysts are
normally found at the bottom of culture dishes,
and presumably also at the water/bottom sediment
interface in natural environments. Cysts of
Fig 5.8 Achlya colorata. (a) Zoosporangium showing a clump
of primary cysts at the mouth.Note the lateral proliferation of
the hypha from beneath the old sporangium. (b) Full and empty
auxiliary cysts. (c) Stages in the release of principal zoospores
from an auxiliary cyst. (d) Principal zoospores. (e) Principal
cyst. (f) Principal cyst germinating by means of a germ tube.
A. klebsiana may remain viable for at least two
months when stored aseptically at 5°C (Reischer,
1951). However, most auxiliary cysts remain at
the mouth of the sporangium for a few hours
and then each cyst releases a principal zoospore
through a small pore (Figs. 5.8b,c). After a period
of swimming, principal zoospores encyst, and
principal cysts germinate either by a germ tube
or by releasing another principal zoospore. When
the zoosporangium of Achlya has released its