Environmental Engineering Reference
In-Depth Information
Kingdom: Chromalveolata
Fungi
Animals
Plants
Chromista
Straminipila
Oomycetes
Phylum: Heterokontophyta
Class: Oomycota
Orders (& families)
Lagenidiales
Lagenidiaceae
Olpidiosidaceae
Sir olpidiaceae
Leptomitales
Leptomitaceae
Peronosporales
Albuginaceae
Peronosporaceae
Pythiaceae
Rhipidiales
Rhipidaceae
Saprolegniales
Ectrogellaceae
Haliphthoraceae
Leptolegniellaceae
Saprolegniaceae
Thraustochytriales
Myxomycota
Archaea
Plasmodiophoromycota
?
Bacteria
Fig. 12.3
Common ancestry of Oömycota
(Dick
1990
,
1997
; Kwon-Chung and Bennett
1992
; Rossman and Palm
2006
).
This algal affi nity was postulated as early as
1858 by Pringsheim (Barr
1992
) and is supported
by analysis of small subunit ribosomal RNA
sequences (Bruns et al.
1991
; Ariztia et al.
1991
;
Wainwright et al.
1993
). The oömycetes may be
related to the progenitors of the heterokont
algae, representing an evolutionary sideline that
never acquired plastids (Christensen
1990
; and
Bhattacharya et al.
1992
). Alternatively, they may
have evolved from algae, having lost their plas-
tids and adopted saprobic and parasitic strategies
in the shadows of freshwater pools, within soils
and inside plants (Dawkins
1989
). With a few
important exceptions, the organisms studied by
mycologists satisfy the following criteria: (i)
absorptive mode of nutrition, (ii) growth by polar-
ized hyphal extension and (iii) reproduction
involving the formation of spores. While some
fungi do not seem to exhibit hyphal growth, the
fact that even
Saccharomyces cerevisiae
produces
invasive pseudo-hyphae under certain nutritional
conditions (Gimeno et al.
1992
) suggests that
polarized growth is an almost universal hallmark of
the fungi. A marked propensity towards parasitism
is a fourth characteristic that seems to be
increasingly applicable to fungi; while this does
not apply to all species, most classes of fungi con-
tain parasitic members, and there are a growing
number of reports of fungi regarded as saprobes
establishing parasitic symbioses with plants and
animals (Sternberg
1994
; Plattner and Hall
1995
).
The oömycetes conform to this working defi nition
on all counts, but this does not of course unite
Fig. 12.2
Systematic position of Oömycota
because of their fi lamentous growth and
because they feed on decaying matter like fungi
(Alexopoulos et al.
1996
). The cell wall of oömy-
cetes, however, is not composed of chitin, as in
the fungi, but is made up of a mix of cellulosic
compounds and glycan. The nuclei within the
fi laments are diploid, with two sets of genetic
information, not haploid as in the fungi. The
ultrastructure, biochemistry and molecular
sequences of these organisms indicate that they
belong with the Chromista (Fig.
12.3
). The free-
swimming spores, which are produced, bear two
dissimilar fl agella, one with mastigonemes; this
feature is common in the chromists, as is the
presence of the chemical mycolaminarin, an
energy storage molecule similar to those found in
kelps and diatoms. Thus, although oömycetes are
in the minority as heterotrophic chromists, they
quite defi nitely belong with these other chromist
groups. Molecular sequences show their phylo-
genetic roots with the Chromista, the chromophyte
algae and other Protista, rather than the true fungi
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