Introduction to Fungi, Third Edition

82 STRAMINIPILA: OOMYCOTA
strains or species may be closely related to nonpathogenic
ones but can be distinguished by
physiological characteristics, DNA sequencing
(Yuasa & Hatai, 1996) and the length of the
‘boat hook’ appendages on the cysts of principal
zoospores (Figs. 5.5b,c; Beakes, 1983; Burr &
Beakes, 1994).
The life cycle of Saprolegnia is summarized in
Fig. 5.3. A monographic treatment of the genus
has been published by Seymour (1970).
Asexual reproduction in Saprolegnia
Sporangia of Saprolegnia develop when a hyphal
tip, which is pointed in the vegetative condition,
swells, rounds off and becomes club-shaped. It
accumulates denser cytoplasm around the
vacuole which remains clearly visible. A septum
develops at the sporangial base and it is at first
straight or convex with respect to the sporangium,
i.e. it bulges into it (Figs. 5.4c,d). The
sporangium contains numerous nuclei, and
Fig 5.4 Saprolegnia. (a) Apex of vegetative hypha. (b d) Stages in the development of zoosporangia. (e) Release of zoospores.
(f) Proliferation of zoosporangium. A second zoosporangium is developing within the empty one. (g) Auxiliary zoospore (first
motile stage). (h) Cyst formed at the end of the first motile stage (auxiliary cyst). (i,j) Germination of auxiliary cyst to release a
second motile stage (principal zoospores).These have the typical reniform shape. (k m) Principal zoospores. (n) Principal zoospore
at the moment of encystment. Note the shed flagellum. (o) Principal cyst. (p) Principal cyst germinating by means of a germ tube.
(a f) to same scale; (g p) to same scale. Note that the straminipilous flagellum cannot be distinguished from the whiplash flagellum
at the magnification chosen.