In addition to the amoebae forms sensu stricto, i.e. with
pseudopodia and without a cell wall, Chlorarachniobionta
can be present in many different forms: amoebae with a cell
wall, coccoid forms, with or without a cell wall and resistant
cysts (Fig. 7.22). The cell wall, when present, does not
contain any cellulose; it seems rather of pectic nature
(Calderon-Saenz and Schnetter 1989).
Chlorarachniobionta are photosynthetic organisms. The
cell has one or more chloroplasts, with thylakoids stacked
into three-thylakoid lamellae. The chloroplasts are derived
from a secondary endosymbiosis with a Chlorobionta
close to the current genus Tetraselmis (Prasinophyceae)
(Takahashi et al. 2007). In addition to its classical double
membrane, the chloroplast is surrounded by a second double
membrane, a remnant of the secondary endosymbiosis. A
nucleomorph,
13 remnants of the nucleus of the Chlorobionta,
is present in the periplatidial space, i.e. between the two
inner and the two outer plastid membranes. Photosynthetic
pigments are chlorophylls a and b and xanthophylls, neither
of which appear to be specific. The main storage polysaccharide is the paramylon, a carbohydrate polymer of β-1,3
glucose with numerous β-1,6 branches. Paramylon is stored
within cytoplasmic vesicles (de Reviers 2003). Outside
Chlorarachniobionta, paramylon is only known in
euglenoids (Excavates). The mitochondria have tubular cristae (Ota et al. 2007).
Chlorarachniobionta reproduce asexually by bipartition
and conidia. Some species, such as Cryptochlora perforans,
exhibit a kind of alternation of generations between a
coccoid stage and several amoeboid stages, though sexuality
is absent (Calderon-Saenz and Schnetter 1989). Cells
equipped with undulipodiums (“zoospores”), which are present in all species (Fig. 7.22), function as conidia: they produce genetically identical individuals (clones) to the one
which produced them. The fact that they are produced by
four suggests that they came from meiosis, but in this case, a
fertilization (never seen) would be needed to “close” the
cycle. Either these “zoospores” are the remnants of a missing
sex or the life cycle is still only partially known. The
undulipodium has a unique character: it is wound helically
around the cell; at its base, there are two kinetosomes, one of
them being the remnant of a second undulipodium which has
disappeared. The undulipodium is bordered by a row of tiny
unilateral hairs.
While photosynthetic, Chlorarachniobionta capture and
engulf, through their pseudopodia, small prey: bacteria,
diatoms, etc. Cryptochlora perforans pierces and enters the
dead filaments of Bryopsidophyceae (Chlorobionta) and
ingests content (Calderon-Saenz and Schnetter 1989). The
Chlorarachniobionta live in temperate and tropical marine
environment in the pelagos (Bigelowiella longifila) or the
benthos.
7.6.4 Phytomyxea
Phytomyxea is one of the taxa included within the customary
concept of “mushrooms” (cf. Sect. 5.5.3). Phytomyxea
comprises a dozen genera and about 50 species, divided into
two sets, the Phagomyxida (with one genus: Phagomyxa)
and Plasmodiophorida (Plasmodiophora, Polymyxa,
Spongospora, etc.) (Braselton 2009).
Resistant
cyst
Coccoid
vegetative
stage
Conidiocyst
Conidia
(‘autospores’)
Sporocyst?
Spores? Gametes? Conidia?
(‘zoospores’)
Fig. 7.22 The life cycle without
sexuality of Norrisiella sphaerica
(Chlorarachniobionta). The
“zoospores” that function as
conidia could match former
gametes or spores. The nuclei
(white circles) are shown; it is not
known if they are haploid (n) or
diploid (2n)
13 The nucleomorph of Chlorarachniobionta has lost most of its genes:
it has only 380 kb.
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
217
pseudopodia and without a cell wall, Chlorarachniobionta
can be present in many different forms: amoebae with a cell
wall, coccoid forms, with or without a cell wall and resistant
cysts (Fig. 7.22). The cell wall, when present, does not
contain any cellulose; it seems rather of pectic nature
(Calderon-Saenz and Schnetter 1989).
Chlorarachniobionta are photosynthetic organisms. The
cell has one or more chloroplasts, with thylakoids stacked
into three-thylakoid lamellae. The chloroplasts are derived
from a secondary endosymbiosis with a Chlorobionta
close to the current genus Tetraselmis (Prasinophyceae)
(Takahashi et al. 2007). In addition to its classical double
membrane, the chloroplast is surrounded by a second double
membrane, a remnant of the secondary endosymbiosis. A
nucleomorph,
13 remnants of the nucleus of the Chlorobionta,
is present in the periplatidial space, i.e. between the two
inner and the two outer plastid membranes. Photosynthetic
pigments are chlorophylls a and b and xanthophylls, neither
of which appear to be specific. The main storage polysaccharide is the paramylon, a carbohydrate polymer of β-1,3
glucose with numerous β-1,6 branches. Paramylon is stored
within cytoplasmic vesicles (de Reviers 2003). Outside
Chlorarachniobionta, paramylon is only known in
euglenoids (Excavates). The mitochondria have tubular cristae (Ota et al. 2007).
Chlorarachniobionta reproduce asexually by bipartition
and conidia. Some species, such as Cryptochlora perforans,
exhibit a kind of alternation of generations between a
coccoid stage and several amoeboid stages, though sexuality
is absent (Calderon-Saenz and Schnetter 1989). Cells
equipped with undulipodiums (“zoospores”), which are present in all species (Fig. 7.22), function as conidia: they produce genetically identical individuals (clones) to the one
which produced them. The fact that they are produced by
four suggests that they came from meiosis, but in this case, a
fertilization (never seen) would be needed to “close” the
cycle. Either these “zoospores” are the remnants of a missing
sex or the life cycle is still only partially known. The
undulipodium has a unique character: it is wound helically
around the cell; at its base, there are two kinetosomes, one of
them being the remnant of a second undulipodium which has
disappeared. The undulipodium is bordered by a row of tiny
unilateral hairs.
While photosynthetic, Chlorarachniobionta capture and
engulf, through their pseudopodia, small prey: bacteria,
diatoms, etc. Cryptochlora perforans pierces and enters the
dead filaments of Bryopsidophyceae (Chlorobionta) and
ingests content (Calderon-Saenz and Schnetter 1989). The
Chlorarachniobionta live in temperate and tropical marine
environment in the pelagos (Bigelowiella longifila) or the
benthos.
7.6.4 Phytomyxea
Phytomyxea is one of the taxa included within the customary
concept of “mushrooms” (cf. Sect. 5.5.3). Phytomyxea
comprises a dozen genera and about 50 species, divided into
two sets, the Phagomyxida (with one genus: Phagomyxa)
and Plasmodiophorida (Plasmodiophora, Polymyxa,
Spongospora, etc.) (Braselton 2009).
Resistant
cyst
Coccoid
vegetative
stage
Conidiocyst
Conidia
(‘autospores’)
Sporocyst?
Spores? Gametes? Conidia?
(‘zoospores’)
Fig. 7.22 The life cycle without
sexuality of Norrisiella sphaerica
(Chlorarachniobionta). The
“zoospores” that function as
conidia could match former
gametes or spores. The nuclei
(white circles) are shown; it is not
known if they are haploid (n) or
diploid (2n)
13 The nucleomorph of Chlorarachniobionta has lost most of its genes:
it has only 380 kb.
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
217