particularity of being claimed by traditional botanists, under
the name Bicosoecophyceae,
21 while they are predators basically devoid of photosynthesis (Fig. 7.1). The term “stramenopiles” was proposed by Patterson (1989) to account for all
taxa, unicellular or not, with tripartite tubular mastigonemes
on their undulipodiums. Tripartite mastigonemes, which may
be taken as the defining characteristic of stramenopiles, are
composed of a short basal segment, a long tubular shaft, and
two or three thin terminal filaments.
Stramenopiles are a major lineage of eukaryotes currently
containing more than 100,000 species (van den Hoek et al.
1998; Lecointre and Le Guyader 2006). By the number of
species, stramenopiles (mainly Chromobionta) are one of the
“big three” taxa of eukaryotes, after opisthokonts (mainly
Metazoa and Fungi, modern meaning) and Archaeplastida
(mainly embryophyta).
Actinophryda
22
(¼actinophryida,
actinophryids,
actinophrids) were for a long time placed within Heliozoa,
“sun animacules,” with e.g. Radiolaria (today: Rhizaria) and
Centrohelida (today: insertae sedis; maybe Archaepastida),
due to their radiating long pseudopodia (axopods). Subsequently, genetic analyses showed that Heliozoa were actually a polyphyletic group and that actinophryids were
emerging within the stramenopiles (Fig. 7.1; Nicolaev
et al. 2004). Actinophryda are unicellular and spherical in
shape, 50–1,000 μm in diameter, without any cell wall or test
and with long raylike pseudopodia, supported internally by a
microtubule (axopods), radiating from the cell body, as in
Centrohelida (cf. Sect. 7.5.2). Asexual reproduction takes
place by binary fission. Under unfavorable conditions, false
sexual reproduction (autogamy or self-fertilization) occurs;
the organism forms a multi-walled cyst; within the cyst, two
gametes are produced, which then fuse together (Fig. 7.39).
The Actinophryda thrive mainly in the freshwater plankton,
especially lakes and rivers; a few species can be found in soil
and marine habitats. They passively feed on some unicellular species (e.g., small Ciliophora and Dinobionta) which
adhere to the axopods.
Bicosoecida
(¼Bicosoecea,
Bicosoecophyceae,
Bicosoecids) are unicellular heterotrophic organisms. The
cell bears two unequal undulipodiums, is generally less
than 10 μm in diameter and may or may not have a lorica,
a covering that does not completely enclose the cell. It may
be attached to the substrate by the “posterior” undulipodium
or swim freely. There is a groove on the side of the cell,
through which small prey (e.g., bacteria) may be ingested.
Cafeteria roenbergensis is a voracious predator very
a Vegetative
cell (2n)
b
Encystment
c Mitosis
d Meiosis
spores (n)
e 3 out of 4 nuclei
degenerate
f The remaining two cells
become amoeboid gametes
(n)
g Autogamy
encysted
zygote (2n)
Fig. 7.39 False sexual reproduction (autogamy) in an Actinophryda. Open circles ¼ haploid nuclei, solid circles ¼ diploid nuclei. The encysted
zygote will germinate when environmental conditions become favorable
21 The suffix ‘phyceae’ is used to designate taxa (taxonomic rank: class)
belonging to ‘algae’ (traditional sense). ‘Phyceae’ comes from the
ancient Greek ‘phykos’ which means ‘seaweed’.
22 Actinophryda: from the ancient Greek aktina (ray) and ophrys
(eyebrow).
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
229
the name Bicosoecophyceae,
21 while they are predators basically devoid of photosynthesis (Fig. 7.1). The term “stramenopiles” was proposed by Patterson (1989) to account for all
taxa, unicellular or not, with tripartite tubular mastigonemes
on their undulipodiums. Tripartite mastigonemes, which may
be taken as the defining characteristic of stramenopiles, are
composed of a short basal segment, a long tubular shaft, and
two or three thin terminal filaments.
Stramenopiles are a major lineage of eukaryotes currently
containing more than 100,000 species (van den Hoek et al.
1998; Lecointre and Le Guyader 2006). By the number of
species, stramenopiles (mainly Chromobionta) are one of the
“big three” taxa of eukaryotes, after opisthokonts (mainly
Metazoa and Fungi, modern meaning) and Archaeplastida
(mainly embryophyta).
Actinophryda
22
(¼actinophryida,
actinophryids,
actinophrids) were for a long time placed within Heliozoa,
“sun animacules,” with e.g. Radiolaria (today: Rhizaria) and
Centrohelida (today: insertae sedis; maybe Archaepastida),
due to their radiating long pseudopodia (axopods). Subsequently, genetic analyses showed that Heliozoa were actually a polyphyletic group and that actinophryids were
emerging within the stramenopiles (Fig. 7.1; Nicolaev
et al. 2004). Actinophryda are unicellular and spherical in
shape, 50–1,000 μm in diameter, without any cell wall or test
and with long raylike pseudopodia, supported internally by a
microtubule (axopods), radiating from the cell body, as in
Centrohelida (cf. Sect. 7.5.2). Asexual reproduction takes
place by binary fission. Under unfavorable conditions, false
sexual reproduction (autogamy or self-fertilization) occurs;
the organism forms a multi-walled cyst; within the cyst, two
gametes are produced, which then fuse together (Fig. 7.39).
The Actinophryda thrive mainly in the freshwater plankton,
especially lakes and rivers; a few species can be found in soil
and marine habitats. They passively feed on some unicellular species (e.g., small Ciliophora and Dinobionta) which
adhere to the axopods.
Bicosoecida
(¼Bicosoecea,
Bicosoecophyceae,
Bicosoecids) are unicellular heterotrophic organisms. The
cell bears two unequal undulipodiums, is generally less
than 10 μm in diameter and may or may not have a lorica,
a covering that does not completely enclose the cell. It may
be attached to the substrate by the “posterior” undulipodium
or swim freely. There is a groove on the side of the cell,
through which small prey (e.g., bacteria) may be ingested.
Cafeteria roenbergensis is a voracious predator very
a Vegetative
cell (2n)
b
Encystment
c Mitosis
d Meiosis
spores (n)
e 3 out of 4 nuclei
degenerate
f The remaining two cells
become amoeboid gametes
(n)
g Autogamy
encysted
zygote (2n)
Fig. 7.39 False sexual reproduction (autogamy) in an Actinophryda. Open circles ¼ haploid nuclei, solid circles ¼ diploid nuclei. The encysted
zygote will germinate when environmental conditions become favorable
21 The suffix ‘phyceae’ is used to designate taxa (taxonomic rank: class)
belonging to ‘algae’ (traditional sense). ‘Phyceae’ comes from the
ancient Greek ‘phykos’ which means ‘seaweed’.
22 Actinophryda: from the ancient Greek aktina (ray) and ophrys
(eyebrow).
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
229