284 M. BABADOOST
Composition of the cell wall is an important characteristic that has
been used to separate oomycetes from true fungi. The cell wall of
oomycetes is composed predominately of훽-1,3- and훽-1,6-glucans and
cellulose, while the cell wall of true fungi is primarily chitin. However,
chitin has been reported in some species of oomycetes (Campose-Takaki
et al. 1982; Ruiz-Herrera 1992; Bulone et al. 1990). Also, cell walls of
oomycetes contain the amino acid hydroxyproline, which is another
characteristic that separates these organisms from true fungi (Alexopou-
los et al. 1996). The bulk of the oomycete wall is composed of the훽-1,3-
and훽-1,6-glucans. Cellulose makes up only 4–20% of the dry weight of
the cell wall of oomycetes (Bartnicki-Garcia 1970).
Oomycetes have both sexual and asexual reproductions (Alex-
opoulos et al. 1996; Latijnhouwers et al. 2003). Sexual reproduction
in oomycetes is heterogametangic. In most species, the gametangia
typically are differentiated into small structures called antheridia
(male; singular: anthridium) and oogonia (female; singular: oogonium).
Antheridia and oogonia may develop from the same thallus or from
different thalli. Gametangia produced on the same thallus may or
may not be compatible. Developing antheridia are attracted to oogonia
under the influence of hormones. Following fertilization of oogonia by
the antheridia, oospores develop. Oospores are thick-walled, resilient,
long-lived structures capable of surviving unfavorable environmental
conditions. Upon germination, an oospore gives rise to a diploid thallus.
Asexual reproduction in oomycetes is by means of zoospores that
develop either in sporangia or in an evanescent vesicle that is pro-
duced from the sporangium (Dick 1990; Alexopoulos et al. 1996). In
the obligate plant parasitic oomycetes (e.g., downy mildew pathogens),
the sporangia resemble conidia, as they are produced either at the tips
of specialized, branched sporangiophores or in chains at the tips of
short, club-shaped sporangiophores. Zoospores are biflagellate, with a
tinsel flagellum and a whiplash flagellum. The tinsel flagellum is longer
and directed forward while the whiplash flagellum trails behind (Dick
1990). The structure and behavior of zoospores have been studied in
detail (Heath 1977; Holloway and Heath 1977; Lange and Olson 1983).
Each zoospore contains a single pear-shaped nucleus. Near the tapered
end of the nucleus lies a pair of kinetosomes from which the flagella
arise. One or two golgi bodies typically are situated near the surface
of the nucleus and numerous mitochondria, lipid bodies, ribosomes,
and vacuoles are in the cytoplasm. Oomycete zoospores also contain a
variety of microbody-like organelles. In some species, these organelles
are associated with the kinetosome and are called “K-bodies” (Powell
et al. 1985). These structures also have been reported in the zoospores