Introduction to Fungi, Third Edition

76 STRAMINIPILA: OOMYCOTA
Fig 5.1 Asexual reproductive stages in Saprolegnia.
(a) Auxiliary (primary) zoospore. (b) Principal
(secondary) zoospore. Schematic drawings, based
partly on Dick (2001a).
rather than lamellate cristae (see Fig. 4.1). The
vacuolar system of Oomycota is also unusual in
containing dense-body vesicles or ‘fingerprint
vacuoles’ (see Fig. 5.24b) which consist of deposits
of a phosphorylated b-(1,3)-glucan polymer, mycolaminarin.
Mycolaminarin may serve as a storage
compound for carbohydrates as well as phosphate
(Hemmes, 1983), and the polyphosphate
storage deposits which are typically found within
vacuoles of true Fungi are absent from vacuoles
of Oomycota (Chilvers et al., 1985). Apart from
that, however, vacuoles of Oomycota share many
features with those of true Fungi, including the
membranous continuities which often link adjacent
vacuoles and provide a means of transport
by peristalsis (Rees et al., 1994; see Fig. 1.9).
Cytoplasmic glycogen granules, which are one
of the major carbohydrate storage sites in
Eumycota, are absent from hyphae of Oomycota
(Bartnicki-Garcia & Wang, 1983).
5.1.2 The zoospore
The Oomycota are characterized by motile asexual
spores (zoospores) which are produced in
spherical or elongated zoosporangia. They are
heterokont, possessing one straminipilous and
one whiplash-type flagellum. Two types of zoospore
may be produced and, if so, the auxiliary
zoospore is the first formed. It is grapeseedshaped,
with both flagella inserted apically
(Fig. 5.1a), and it encysts soon after its formation.
Encystment is by withdrawal of the flagella,
so that a tuft of tripartite tubular hairs (TTHs;
see p. 68) is left behind on the surface of the
developing cyst (Dick, 2001b). The cyst germinates
to give rise to the principal zoospore, which is
by far the more common type and also the more
vigorous swimmer. This typical and readily recognized
oomycete zoospore is uniform in appearance
across the phylum (Lange & Olson, 1983;
Dick, 2001a). In species lacking auxiliary zoospores,
the principal zoospore is usually produced
directly from a sporangium. It is kidney-shaped,
with the flagella inserted laterally in a kinetosome
boss which in turn is located within the
lateral groove (Fig. 5.1b). Encystment is initiated
by the shedding, rather than withdrawal, of the
flagella; no tufts of TTHs are left on the cyst
surface (Dick, 2001a). Fascinating insights into
the cytology of zoospore encystment have been