The vegetative form of Phytomyxea is a coenocyte, i.e. a
giant multinucleate cell (Fig. 7.23), located within the host
organism. Plasmodiophorida possess a complex “extrusome”
used for penetrating food cells (Braselton 1995; Bulman et al.
2001). The plasmodiophorid nuclei divide by an unusual and
characteristic type of nuclear division, the cruciform nuclear
division: an elongated nucleolus is arranged perpendicularly
to the chromosomes at the equatorial plate (Fig. 7.24;
Braselton 1995, 2009). The cell wall, where present, is composed of chitin.
The life cycle is digenetic diphasic, with alternation
between haploid and diploid phases (Fig. 7.23), or monogenetic, with only a diploid phase (Chadefaud 1960). Spores
and gametes have two undulipodiums usually naked, i.e.
without mastigonemes, and of unequal length. Resistant
spores (cysts) are present. The reproductive cells are spread
when the host cell bursts.
The Plasmodiophorida are intracellular parasites of
Embryophyta
(Viridiplantae,
Archaeplastida)
and
Chromobionta (Stramenopiles), in which they determine the
formation of kinds of galls. For example, Plasmodiophora
brassicae causes swellings or distortions of the roots of
cabbages and related plants (the “clubroot disease”),
Spongospora subterranea causes the “powdery scrab,” a disease of potato tubers, and Tetramyxa parasitica is a parasite of
Potamogeton, a genus of mostly freshwater Embryophyta
(Chadefaud 1960, 1978). Polymyxa betae is a root parasite of
the sugar beet and vector of the beet necrotic yellow vein virus.
Polymyxa betae, P. graminis, and Spongospora subterranea
are obligate parasites of Magnoliophyta (Viridiplantae); they
are responsible for the transmission of several very important
Gametogen (n)
Formation of gametes (n)
Fertilization
Zygote (2n)
Sporogen (2n)
Meiosis
Spores
(n)
Fertilization
Fig. 7.23 The life cycle of a Plasmodophora species (Phytomyxea). Open circles ¼ haploid nuclei, solid circles ¼ diploid nuclei (Modified and
redrawn from Chadefaud (1960))
Fig. 7.24 The cruciform nuclear division. This type of division is one
of the characteristic features of Plasmodiophorida (Photo: courtesy of
Braselton (2009))
218
C.-F. Boudouresque
giant multinucleate cell (Fig. 7.23), located within the host
organism. Plasmodiophorida possess a complex “extrusome”
used for penetrating food cells (Braselton 1995; Bulman et al.
2001). The plasmodiophorid nuclei divide by an unusual and
characteristic type of nuclear division, the cruciform nuclear
division: an elongated nucleolus is arranged perpendicularly
to the chromosomes at the equatorial plate (Fig. 7.24;
Braselton 1995, 2009). The cell wall, where present, is composed of chitin.
The life cycle is digenetic diphasic, with alternation
between haploid and diploid phases (Fig. 7.23), or monogenetic, with only a diploid phase (Chadefaud 1960). Spores
and gametes have two undulipodiums usually naked, i.e.
without mastigonemes, and of unequal length. Resistant
spores (cysts) are present. The reproductive cells are spread
when the host cell bursts.
The Plasmodiophorida are intracellular parasites of
Embryophyta
(Viridiplantae,
Archaeplastida)
and
Chromobionta (Stramenopiles), in which they determine the
formation of kinds of galls. For example, Plasmodiophora
brassicae causes swellings or distortions of the roots of
cabbages and related plants (the “clubroot disease”),
Spongospora subterranea causes the “powdery scrab,” a disease of potato tubers, and Tetramyxa parasitica is a parasite of
Potamogeton, a genus of mostly freshwater Embryophyta
(Chadefaud 1960, 1978). Polymyxa betae is a root parasite of
the sugar beet and vector of the beet necrotic yellow vein virus.
Polymyxa betae, P. graminis, and Spongospora subterranea
are obligate parasites of Magnoliophyta (Viridiplantae); they
are responsible for the transmission of several very important
Gametogen (n)
Formation of gametes (n)
Fertilization
Zygote (2n)
Sporogen (2n)
Meiosis
Spores
(n)
Fertilization
Fig. 7.23 The life cycle of a Plasmodophora species (Phytomyxea). Open circles ¼ haploid nuclei, solid circles ¼ diploid nuclei (Modified and
redrawn from Chadefaud (1960))
Fig. 7.24 The cruciform nuclear division. This type of division is one
of the characteristic features of Plasmodiophorida (Photo: courtesy of
Braselton (2009))
218
C.-F. Boudouresque