Opisthokonta have mitochondria with plate-like flattened
cristae, as in Archaeplastida and Cryptophytes. Finally,
these are the only eukaryotes to have a single undulipodium,
located at the rear of the cell and acting as a propellant
(Fig. 7.10; cf. Sect. 7.4).
In the simplified phylogenetic tree presented here
(Fig. 7.1), four higher taxa of Opisthokonta are shown:
metazoans, choanoflagellates, Fungi (modern meaning;
including Microsporidia), and Mesomycetozoa. This
simplified tree does little to account for the complexity of
the deep roots of the phylogenetic tree, still largely uncertain, and the diversity of amoeboid taxa located close to the
roots of Opisthokonta. For example, Nuclearia would fall
toward the root of the Fungi (modern meaning), Amoebidium
corallochytrium toward the root of choanoflagellates, and
Ministeria toward the root of Metazoa (Steenkamp et al.
2006). Ministeria and Nuclearia are amoebae equipped
with thin pseudopodia (filopodia), the latter superficially
resembling Centrohelida (cf. Sect. 7.5.2) and Actinophryda
(cf. Sect. 7.9.1).
7.14.2 Microsporidia
Microsporidia are considered either as a sister group of the
Fungi (Liu et al. 2006), or a taxon of Fungi (Hibbett et al.
2007), or placed at the base of the Fungi together with the
“Zygomycota” to which they could belong (Dyer 2008; Lee
et al. 2008), or the sister group of Chytridiomycota, or within
the Chytridiomycota to which they may belong. The choice
to treat them in a separate section of this chapter is therefore
more pedagogic than scientific as it allows description of
their highly derived and unique characters. About 1,500
species have been described and their true number could
exceed one million (Larsson 2009).
Microsporidia are characterized by very small genomes,
even smaller than bacterial genomes. Mitochondria are
absent, which is not an ancestral state, as hydrogenosomes
or mitosomes, considered as remnants of mitochondria, are
present (Germot et al. 1997; Williams et al. 2002). Cytoplasmic ribosomes are of the 70S type and not of the 80S as
generally observed in other eukaryotes. This could also be a
derived, and not ancestral, character as mitochondria and
their 70S ribosomes are absent (Cavalier-Smith 2002). The
cells are not mobile and undulipodiums have never been
observed. All these derived characters could be explained
by the fact that microsporidia are all obligate intracellular
parasites.
A cell wall is only present at the cyst stage. It is made of
two layers, the endospore composed of chitin and the exospore. Within the cyst, a polar tube coiled in a helix, derived
from the Golgi apparatus, is fixed to an anchoring disk at the
anterior part of the cell, next to a lamellar structure called the
polaroplast (Fig. 7.52; Dyer 2008; Larsson 2009). A vacuole
is located at the posterior part of the cell. The polar tube
serves to inject the cyst content into the host cell (Fig. 7.53).
Injection results from the sudden swelling of the vacuole and
is completed in less than 2 s.
Once the cyst content is within the host cell, it expands as
a multinucleated cytoplasm or coenocyte, which at the end
of the process occupies almost all of the host cell volume and
starts producing cysts also named “spores” in the literature
(Fig. 7.53). The life cycle of Microsporidia does not
include an obvious sexual stage. However, sexuality could
be present, or has been present in the past, as suggested from
the occurrence of genes linked to sexual processes in
Microsporidia’s genomes (Dyer 2008; Lee et al. 2008;
Larsson 2009).
Microsporidia are thus obligate intracellular parasites
mostly of teleost fishes and of arthropods. Antonospora
locustae is a grasshopper pathogen commercialized for the
biological control of locusts. Nosema bombycis is responsible for a silk worm disease. Nosema ceranae is a benign
pathogen of the Japanese bee Apis cerana which turned out
to be a severe, even deadly, pathogen for the European bee
A. melifera upon its introduction in the far East. Following
re-importation of contaminated bee colonies from Asia to
Europe and North America, this pathogen is now responsible
for disease outbreaks in bees in these geographical areas
(Chauzat et al. 2007). Finally, some Microsporidia parasite
mammals as exemplified by Enterocytozoon bieneusi which
causes diarrhoea in humans (Dyer 2008).
7.14.3 Fungi (Modern Meaning)
Fungi
35 in the current meaning of this taxonomic term form
a monophyletic group from which have been successively
excluded in the last decade’s organisms belonging to
the bacteria (actinobacteria and planctomycetes), Rhizaria
(Phytomyxea; cf. Sect. 7.6.4), Alveolata (Ellobiopsidae;
cf. Sect. 7.8.1) and Stramenopiles (Oobionta and
Labyrinthulobionta; cf. Sects. 7.9.1 and 7.9.2). With more
than 100,000 described species (Hawksworth 2001;
Lecointre and Le Guyader 2006), the Fungi include species
as diverse as lichen (generally belonging to Ascomycota)
and mushroom-forming species (generally belonging to
Basidiomycota, such as saffron milk cap, button mushroom,
cepe, and death cap). The true number of species in the
35 Fungi is the plural of the Latin word ‘Fungus’, which means ‘mushroom’. By convention, the term ‘Fungi’ is used here even when used in
the singular.
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
243
cristae, as in Archaeplastida and Cryptophytes. Finally,
these are the only eukaryotes to have a single undulipodium,
located at the rear of the cell and acting as a propellant
(Fig. 7.10; cf. Sect. 7.4).
In the simplified phylogenetic tree presented here
(Fig. 7.1), four higher taxa of Opisthokonta are shown:
metazoans, choanoflagellates, Fungi (modern meaning;
including Microsporidia), and Mesomycetozoa. This
simplified tree does little to account for the complexity of
the deep roots of the phylogenetic tree, still largely uncertain, and the diversity of amoeboid taxa located close to the
roots of Opisthokonta. For example, Nuclearia would fall
toward the root of the Fungi (modern meaning), Amoebidium
corallochytrium toward the root of choanoflagellates, and
Ministeria toward the root of Metazoa (Steenkamp et al.
2006). Ministeria and Nuclearia are amoebae equipped
with thin pseudopodia (filopodia), the latter superficially
resembling Centrohelida (cf. Sect. 7.5.2) and Actinophryda
(cf. Sect. 7.9.1).
7.14.2 Microsporidia
Microsporidia are considered either as a sister group of the
Fungi (Liu et al. 2006), or a taxon of Fungi (Hibbett et al.
2007), or placed at the base of the Fungi together with the
“Zygomycota” to which they could belong (Dyer 2008; Lee
et al. 2008), or the sister group of Chytridiomycota, or within
the Chytridiomycota to which they may belong. The choice
to treat them in a separate section of this chapter is therefore
more pedagogic than scientific as it allows description of
their highly derived and unique characters. About 1,500
species have been described and their true number could
exceed one million (Larsson 2009).
Microsporidia are characterized by very small genomes,
even smaller than bacterial genomes. Mitochondria are
absent, which is not an ancestral state, as hydrogenosomes
or mitosomes, considered as remnants of mitochondria, are
present (Germot et al. 1997; Williams et al. 2002). Cytoplasmic ribosomes are of the 70S type and not of the 80S as
generally observed in other eukaryotes. This could also be a
derived, and not ancestral, character as mitochondria and
their 70S ribosomes are absent (Cavalier-Smith 2002). The
cells are not mobile and undulipodiums have never been
observed. All these derived characters could be explained
by the fact that microsporidia are all obligate intracellular
parasites.
A cell wall is only present at the cyst stage. It is made of
two layers, the endospore composed of chitin and the exospore. Within the cyst, a polar tube coiled in a helix, derived
from the Golgi apparatus, is fixed to an anchoring disk at the
anterior part of the cell, next to a lamellar structure called the
polaroplast (Fig. 7.52; Dyer 2008; Larsson 2009). A vacuole
is located at the posterior part of the cell. The polar tube
serves to inject the cyst content into the host cell (Fig. 7.53).
Injection results from the sudden swelling of the vacuole and
is completed in less than 2 s.
Once the cyst content is within the host cell, it expands as
a multinucleated cytoplasm or coenocyte, which at the end
of the process occupies almost all of the host cell volume and
starts producing cysts also named “spores” in the literature
(Fig. 7.53). The life cycle of Microsporidia does not
include an obvious sexual stage. However, sexuality could
be present, or has been present in the past, as suggested from
the occurrence of genes linked to sexual processes in
Microsporidia’s genomes (Dyer 2008; Lee et al. 2008;
Larsson 2009).
Microsporidia are thus obligate intracellular parasites
mostly of teleost fishes and of arthropods. Antonospora
locustae is a grasshopper pathogen commercialized for the
biological control of locusts. Nosema bombycis is responsible for a silk worm disease. Nosema ceranae is a benign
pathogen of the Japanese bee Apis cerana which turned out
to be a severe, even deadly, pathogen for the European bee
A. melifera upon its introduction in the far East. Following
re-importation of contaminated bee colonies from Asia to
Europe and North America, this pathogen is now responsible
for disease outbreaks in bees in these geographical areas
(Chauzat et al. 2007). Finally, some Microsporidia parasite
mammals as exemplified by Enterocytozoon bieneusi which
causes diarrhoea in humans (Dyer 2008).
7.14.3 Fungi (Modern Meaning)
Fungi
35 in the current meaning of this taxonomic term form
a monophyletic group from which have been successively
excluded in the last decade’s organisms belonging to
the bacteria (actinobacteria and planctomycetes), Rhizaria
(Phytomyxea; cf. Sect. 7.6.4), Alveolata (Ellobiopsidae;
cf. Sect. 7.8.1) and Stramenopiles (Oobionta and
Labyrinthulobionta; cf. Sects. 7.9.1 and 7.9.2). With more
than 100,000 described species (Hawksworth 2001;
Lecointre and Le Guyader 2006), the Fungi include species
as diverse as lichen (generally belonging to Ascomycota)
and mushroom-forming species (generally belonging to
Basidiomycota, such as saffron milk cap, button mushroom,
cepe, and death cap). The true number of species in the
35 Fungi is the plural of the Latin word ‘Fungus’, which means ‘mushroom’. By convention, the term ‘Fungi’ is used here even when used in
the singular.
7 Taxonomy and Phylogeny of Unicellular Eukaryotes
243