Introduction to Fungi, Third Edition

84 STRAMINIPILA: OOMYCOTA
Fig 5.5 Surface features of Saprolegnia. (a) Detail of
an auxiliary zoospore cyst of S. parasitica showing
the tuft of TTHs (mt) at the point where the
straminipilous flagellum was withdrawn. (b) Surface of
a principal zoospore cyst of S. parasitica; the long boat
hook spines are arranged in fascicles. (c) Surface of a
principal zoospore cyst of S. hypogyna with discrete
boat hooks of intermediate length. All bars ¼ 2 mm.
All images kindly provided by M.W. Dick and I.C.
Hallett; (b) reprinted from Hallett and Dick (1986),
with permission from Elsevier.
rather than withdraw their flagella. The first step
in encystment is the fusion of vesicles called
K-bodies with the plasma membrane. These are
so called because they are located near the
kinetosome. The material they secrete is involved
in attachment of the zoospore to a substratum,
which occurs in the region of the groove near the
flagellar bases, designated the ventral region
(Lehnen & Powell, 1989). The cyst wall and preformed
boat hook spines are secreted by fusion of
encystment vesicles with the plasma membrane
(Beakes, 1987; Burr & Beakes, 1994). The length
and arrangement of spines on the surface of a
mature principal cyst are characteristic features
of individual species (Figs. 5.5b,c). They probably
mediate attachment of the cyst to the host, and
pathogenic isolates of Saprolegnia have much
longer spines than saprotrophic ones (Burr &
Beakes, 1994). Alternatively, the boat hooks may
mediate attachment to the water meniscus.