Introduction to Fungi, Third Edition

78 STRAMINIPILA: OOMYCOTA
Fig 5.3 Life cycle of Saprolegnia.Vegetative hyphae are diploid and coenocytic. Asexual reproduction is by means of diplanetic
(auxiliary and principal) zoospores.The principal zoospore state is polyplanetic. Saprolegnia is homothallic, and sexual reproduction
is initiated by the formation of antheridia and oogonia.For simplicity, only a single nucleus is shown in each of the oospheres and
in the antheridium. Each oogonium contains several oospheres. Karyogamy occurs soon after fertilization of an oosphere by an
antheridial nucleus.The oospore may germinate by means of a germ sporangium (not shown) or a hyphal tip.Open and closed circles
represent haploid nuclei of opposite mating type; diploid nuclei are larger and half-filled. Key events in the life cycle are meiosis (M),
plasmogamy (P) and karyogamy (K).
between the plasma membrane and the outer
spore wall (epispore). Upon germination, the
endospore is thought to coat the emerging germ
tube with wall material, and some material may
also be taken up by endocytosis. A large storage
vacuole inside the oospore protoplast is called
the ooplast. It arises by fusion of dense-body
vesicles and, like them, contains mycolaminarin
and phosphate. Dick (1995, 2001a) speculated
that the ooplast contributes membrane precursor
material during the process of oospore germination.
The third storage compartment consists
of one or several lipid droplets which provide
the endogenous energy supply required for germination.
Ultrastructural changes during oospore
germination have been described by Beakes
(1981).
5.1.4 Ecology and significance
Oomycota have a major impact on mankind as
pathogens causing plant diseases of epidemic
proportions. Two events have had particularly
far-reaching political and social consequences,
and have shaped and interlinked the young
disciplines of mycology and plant pathology in
the nineteenth century. These were the great
Irish potato famine of 1845 1848 caused by
Phytophthora infestans (Bourke, 1991), and the
occurrence of downy mildew of grapes caused
by Plasmopara viticola (Large, 1940). The former