282 M. BABADOOST
imbalances, low oxygen, atmospheric pollutants, pesticide damages,
and genetic abnormalities.
II. OOMYCETES
A. Taxonomic Position of Oomycetes
Oomycetes, also known as fungal-like organisms, are members of the
kingdom Chromista (=Stramenopila) (Cavalier-Smith 1986; Alexopou-
los et al. 1996; Margulis and Schwartz 2000). This group was originally
classified, among the fungi, as Oomycota (the name “oomycota” means
“egg fungus”), and later treated as Protistas, based on general morphol-
ogy and lifestyle. However, further studies showed that oomycetes are
allied with brown algae (Sogin and Silberman 1998; Baldauf et al. 2000).
Kreisel (1969) and Sheffer (1975) suggested excluding oomycetes from
the true fungi.
Oomycetes are morphologically similar to true fungi but they do
not have a phylogenic relationship (Cavalier-Smith 1987; Dick 1990,
1997; Lang et al. 1999; Baldauf et al. 2000; Heffer Link et al. 2002;
Rossman and Palm 2006; Fry and Grunwald 2010; Carris et al. 2012).
Alexopoulos et al. (1996) reported the following characteristics that
separate oomycetes from true fungi: (1) asexual reproduction by means
of biflagellate zoospore; (2) the production of diploid thallus in which
meiosis occurs in the developing gametangia; (3) oogamous reproduc-
tion by gametangial contact that results in the production of an oospore;
(4) cell wall composed primarily of훽-glucans and cellulose; and (5)
mitochondria with tubular cristae.
B. Biology of Oomycetes
There are more than 500 species of oomycetes (Anon 2010). Most of
the oomycete species are saprophytic and primarily inhabit aquatic and
soil, which play key roles in decomposition and recycling of organic
matter (Margulis and Schwartz 2000). Most of the aquatic species live
on dead plants and animals and are thought to play a major role in
the degradation and recycling of nutrients in aquatic ecosystem (Dick
1990). Some of the aquatic species are parasites and attack either algae
or animals such as rotifers, nematodes, mosquito larvae, crayfish, and
catfish (Dick 1990). Soil-inhabitant oomycetes may be saprophytic, par-
asitic on animals, and pathogenic on plants.Pythium insidiosum, a soil-
inhabitant oomycete, is known to cause pythiosis in mammals (de Cock
et al. 1987; Rippon 1988).