character to Excavates and Discicristates, or a convergent
evolution (homoplasy) (Simpson 2003).
The association of the nucleus (or of each nucleus, when
there are several) with a group of undulipodiums is considered by some authors as evidence of the endosymbiotic
origin of the kinetic apparatus in eukaryotes (Margulis and
Dolan 1997).
7.13.2 Parabasalia
The Parabasalia (¼Parabasala, Parabasalidea, parabasalids)
encompass the trichomonads and a number of taxa formerly
grouped as the “hypermastigids.” The kinetosomes are
linked to parabasal fibers that attach to prominent Golgi
complexes, constituting the “parabasal apparatus” which is
distinctive to Parabasalia. The undulipodiums are grouped
by four (“kinetid”). There may be a single kinetid in
Trichomonads (Trichomonas and Tritrichomonas), but
these can be quite numerous in “hypermastigids” (up to
100,000 undulipodiums). When there are many nuclei,
each kinetid is associated with a nucleus (e.g., in
Calonympha).
Sexual reproduction exists, or has existed until recently,
in Parabasalia, but it is poorly understood. In Trichomonas
vaginalis (trichomonads), the meiosis-specific genes are
present, although meiosis was not actually observed. In
“hypermastigids,” cell divisions reminiscent of meiosis, but
with a single division (unlike most other eukaryotes, in
which there are normally two), were observed (Dacks and
Roger 1999; Malik et al. 2008).
Parabasalia are obligate heterotrophic organisms.
Trichomonads live freely or are parasites. Trichomonas
vaginalis is a parasite of the human urogenital tract and is
the main agent of sexually transmitted disease (STD) on a
global scale, with about 175 million new cases each year
(Malik et al 2008). “Hypermastigids” are mutualistic
symbionts in the gut of termites and cockroaches.
Calonympha grassii lives in the gut of the termite
Cryptotermes brevis and participates in the digestion of
cellulose and hemicellulose, due to endosymbiotic
bacteria. Hoplonympha sp., which lives in the gut of the
termites Hodothermopsis sjoestedti and of cockroaches, is
associated with ectosymbiotic bacteria (Bacteroidales)
(Noda et al. 2006).
7.13.3 Diplomonadida
In Diplomonads, there are usually two diploid nuclei per
cell, both being transcriptionally active. They have identical
copies of the genes. Each nucleus is associated with a set of
microtubules and four undulipodiums; this structure is called
karyomastigont. The Giardia lamblia genome is relatively
small (only 11.7 Mbp), with many genes acquired by horizontal gene transfer (HGT) from Bacteria and Archaea. The
Golgi apparatus is absent (Lecointre and Le Guyader 2006;
Morrison et al. 2007).
The life cycle involves alternation between cells with
undulipodium (“trophozoites”), actively swimming, and
cysts that ensure dissemination and infection. Although sexual reproduction has not been observed directly, the presence in Giardia of genes specifically involved in meiosis and
the observation of genetic recombination are evidence of its
presence (Logsdon 2007; Malik et al. 2008).
Diplomonads live in habitats poor in oxygen and rich in
organic matter, especially in the digestive tract of
vertebrates. Giardia lamblia is a parasite of the intestine of
man, in whom it causes giardiasis, a disease characterized in
particular by diarrhea (Simpson 2003; Lecointre and Le
Guyader 2006).
7.14 Kingdom Opisthokonta
7.14.1 General Remarks
The Opisthokonta
34 (¼Opisthochonta, opistokonts) are by
far, with about 1,350,000 species, the largest taxon within
the eukaryotes. They include Metazoa (¼animals in the
modern sense), choanoflagellates, Fungi (modern meaning;
only a part of what was called fungi, customary meaning;
Fig. 5.27), Microsporidia (a “protozoan” taxon that early
molecular phylogenies considered as close to ancestral
eukaryotes) and a number of poorly known taxa such as
nucleariid amoebas and Mesomycetozoa (see below).
The grouping of Metazoa and Fungi (modern meaning)
within the same kingdom, Opisthokonta, was unexpected, on
the basis of available cytological and biochemical data; this
grouping was therefore one of the surprises resulting from
the early molecular phylogenies (Cavalier-Smith 1987b).
From a genetic point of view, Opisthokonta clearly appear
as a monophyletic and robust taxon (Steenkamp et al. 2006).
For example, the gene tyrosyl-tRNA synthetase, of Archean
origin, clearly separates Opisthokonta from all other
eukaryotes (Huang et al. 2005). From a biochemical point
of view, the presence of chitin as a structural macromolecule
is an ancestral characteristic of Opisthokonta, although
it may be present in other eukaryotic taxa (e.g., in
Rhodobionta). Considering the cytological characters,
34 Opisthokonta: from the ancient Greek opistho (hind) and kontos
(pole, in reference to the undulipodium).
242
C.-F. Boudouresque
evolution (homoplasy) (Simpson 2003).
The association of the nucleus (or of each nucleus, when
there are several) with a group of undulipodiums is considered by some authors as evidence of the endosymbiotic
origin of the kinetic apparatus in eukaryotes (Margulis and
Dolan 1997).
7.13.2 Parabasalia
The Parabasalia (¼Parabasala, Parabasalidea, parabasalids)
encompass the trichomonads and a number of taxa formerly
grouped as the “hypermastigids.” The kinetosomes are
linked to parabasal fibers that attach to prominent Golgi
complexes, constituting the “parabasal apparatus” which is
distinctive to Parabasalia. The undulipodiums are grouped
by four (“kinetid”). There may be a single kinetid in
Trichomonads (Trichomonas and Tritrichomonas), but
these can be quite numerous in “hypermastigids” (up to
100,000 undulipodiums). When there are many nuclei,
each kinetid is associated with a nucleus (e.g., in
Calonympha).
Sexual reproduction exists, or has existed until recently,
in Parabasalia, but it is poorly understood. In Trichomonas
vaginalis (trichomonads), the meiosis-specific genes are
present, although meiosis was not actually observed. In
“hypermastigids,” cell divisions reminiscent of meiosis, but
with a single division (unlike most other eukaryotes, in
which there are normally two), were observed (Dacks and
Roger 1999; Malik et al. 2008).
Parabasalia are obligate heterotrophic organisms.
Trichomonads live freely or are parasites. Trichomonas
vaginalis is a parasite of the human urogenital tract and is
the main agent of sexually transmitted disease (STD) on a
global scale, with about 175 million new cases each year
(Malik et al 2008). “Hypermastigids” are mutualistic
symbionts in the gut of termites and cockroaches.
Calonympha grassii lives in the gut of the termite
Cryptotermes brevis and participates in the digestion of
cellulose and hemicellulose, due to endosymbiotic
bacteria. Hoplonympha sp., which lives in the gut of the
termites Hodothermopsis sjoestedti and of cockroaches, is
associated with ectosymbiotic bacteria (Bacteroidales)
(Noda et al. 2006).
7.13.3 Diplomonadida
In Diplomonads, there are usually two diploid nuclei per
cell, both being transcriptionally active. They have identical
copies of the genes. Each nucleus is associated with a set of
microtubules and four undulipodiums; this structure is called
karyomastigont. The Giardia lamblia genome is relatively
small (only 11.7 Mbp), with many genes acquired by horizontal gene transfer (HGT) from Bacteria and Archaea. The
Golgi apparatus is absent (Lecointre and Le Guyader 2006;
Morrison et al. 2007).
The life cycle involves alternation between cells with
undulipodium (“trophozoites”), actively swimming, and
cysts that ensure dissemination and infection. Although sexual reproduction has not been observed directly, the presence in Giardia of genes specifically involved in meiosis and
the observation of genetic recombination are evidence of its
presence (Logsdon 2007; Malik et al. 2008).
Diplomonads live in habitats poor in oxygen and rich in
organic matter, especially in the digestive tract of
vertebrates. Giardia lamblia is a parasite of the intestine of
man, in whom it causes giardiasis, a disease characterized in
particular by diarrhea (Simpson 2003; Lecointre and Le
Guyader 2006).
7.14 Kingdom Opisthokonta
7.14.1 General Remarks
The Opisthokonta
34 (¼Opisthochonta, opistokonts) are by
far, with about 1,350,000 species, the largest taxon within
the eukaryotes. They include Metazoa (¼animals in the
modern sense), choanoflagellates, Fungi (modern meaning;
only a part of what was called fungi, customary meaning;
Fig. 5.27), Microsporidia (a “protozoan” taxon that early
molecular phylogenies considered as close to ancestral
eukaryotes) and a number of poorly known taxa such as
nucleariid amoebas and Mesomycetozoa (see below).
The grouping of Metazoa and Fungi (modern meaning)
within the same kingdom, Opisthokonta, was unexpected, on
the basis of available cytological and biochemical data; this
grouping was therefore one of the surprises resulting from
the early molecular phylogenies (Cavalier-Smith 1987b).
From a genetic point of view, Opisthokonta clearly appear
as a monophyletic and robust taxon (Steenkamp et al. 2006).
For example, the gene tyrosyl-tRNA synthetase, of Archean
origin, clearly separates Opisthokonta from all other
eukaryotes (Huang et al. 2005). From a biochemical point
of view, the presence of chitin as a structural macromolecule
is an ancestral characteristic of Opisthokonta, although
it may be present in other eukaryotic taxa (e.g., in
Rhodobionta). Considering the cytological characters,
34 Opisthokonta: from the ancient Greek opistho (hind) and kontos
(pole, in reference to the undulipodium).
242
C.-F. Boudouresque