全 文 ：Journal of Systematics and Evolution 46 (3): 263–273 (2008) doi: 10.3724/SP.J.1002.2008.08060
(formerly Acta Phytotaxonomica Sinica) http://www.plantsystematics.com
An overview of the phylogeny and diversity of eukaryotes
Sandra L. BALDAUF*
(Department of Evolution, Genomics and Systematics, Evolutionary Biology Centre, University of Uppsala, Norbyvägen 18 D, Uppsala 75236, Sweden)
Abstract Our understanding of eukaryote biology is dominated by the study of land plants, animals and fungi.
However, these are only three isolated fragments of the full diversity of extant eukaryotes. The majority of eu-
karyotes, in terms of major taxa and probably also sheer numbers of cells, consists of exclusively or predomi-
nantly unicellular lineages. A surprising number of these lineages are poorly characterized. Nonetheless, they are
fundamental to our understanding of eukaryote biology and the underlying forces that shaped it. This article
consists of an overview of the current state of our understanding of the eukaryote tree. This includes the identity
of the major groups of eukaryotes, some of their important, defining or simply interesting features and the pro-
posed relationships of these groups to each other.
Key words biodiversity, eukaryotes, molecular evolution, phylogeny, systematics, tree of life.

Eukaryotes are only one of the three domains of
life, along with Bacteria and Archaea, yet we are
particularly intrigued by eukaryotes. This is at least
partly because they include the organisms we can see.
However, the vast diversity of eukaryotes are single
celled organisms, and their importance to our under-
standing of ourselves, our world and our history are
immense. Knowledge of the morphological, functional
and ecological diversity of microbial eukaryotes is
essential for numerous practical reasons, but also
because they teach us about the must fundamental
rules of biology. For every rule we think we know,
there is some organisms some where that bends or
breaks those rules, and these breaks and bends hold
important clues to how biology works.
Eukaryotes are by definition complex-celled or-
ganisms. Even the “simplest” have nuclei with highly
structured chromatin, introns and large spliceosomal
complexes to remove them (Collins & Penny, 2005),
and complex membrane pores to control traffic in and
out (Jékely, 2005). The cytoplasm is structured by an
extensive cytoskeleton facilitating intracellular traffic,
endo- and exocytosis, amoeboid locomotion (Cava-
lier-Smith, 2002). There is a vast array of organelles
usually including, at a minimum, a mitochondrion or
its derivative (Embley & Martin, 2006) and a golgi
apparatus for synthesizing and recycling membranes
and modifying their proteins (Mironov et al., 2007).
Eukaryotic flagella are large complex intracellular
structures unrelated to the simple bacterial structures
of the same name (Pazour et al., 2005). Life histories
also tend to be complex in eukaryotes, often with
multiple highly distinct forms, sometimes including
complex multicellular ones.
On the other hand, eukaryotes are fairly metab-
olically uniform. This is in contrast to bacteria, whose
metabolic diversity is vast and quite possibly largely
unknown (Frias-Lopez et al., 2008). Eukaryotes
mostly rely on endosymbiotic former bacteria, the
mitochondrion and chloroplast, for the bulk of their
ATP production. The former are probably universal
among extant eukaryotes, although they have been
drastically functionally reduced multiple times (Em-
bley & Martin, 2006). Chloroplasts arose later and,
with one known exception (Nowack et al., 2008),
probably all trace to a single endosymbiotic event
early in the evolution of the former “Plantae” (now,
Archaeplastida). Nonetheless, photosynthesis is
widespread in eukaryotes. This most often takes the
form of temporarily acquired plastids that must be
replaced every generation from external sources
(Fehling et al., 2007). This is found in nearly every
major group of eukaryotes except amitochondriate
excavates, including animals from that are essentially
algae (Fehling et al., 2007). This has evolved into
permanent secondary endosymbioses at least three
times, and many of the most ecologically and eco-
nomically important algae photosynthesize with
permanently stolen plastids (“kleptoplasts”; Archi-
bald, 2005). Eukaryotes are also involved in a wide
variety of other endosymbioses for which there seems
to be a continuum of host-symbiont interdependency
(Moya et al., 2008).

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Received: 22 April 2008 Accepted: 3 May 2008
* Author for correspondence. E-mail: sandra.baldauf@ebc.uu.se;
Tel.: +46 (0) 184-716-452; Fax: +46 (0) 184-716-457.
Journal of Systematics and Evolution Vol. 46 No. 3 2008 264
Our understanding of deep eukaryote phylogeny
has begun to coalesce around data from large scale
sequencing projects (Burki et al., 2007; Hackett et al.,
2007; Rodriguez-Ezpeleta et al., 2007). As a result,
most at least moderately well studied eukaryotes can
now be assigned to a small number of major groups
(Fig. 1). However, there are at least three important
caveates to this. First, there are still many groups of
eukaryotes, including whole major divisions, about
which we know very little (Adl et al., 2005) including
their true diversity (e.g., Bass & Cavalier-Smith,
2004). Second, we still have a very poor understand-
ing of the cryptic diversity of eukaryotes, which could
be vast (e.g., Slapeta et al., 2005). Finally, we are only
just beginning to uncover the vast diversity of bacte-
rial-sized (pico- and nano-) eukaryotes, first discov-
ered in clone libraries derived by PCR amplification
of pooled “environmental” DNAs (culture independ-
ent PCR or ciPCR) (Moreira & Lopéz-Garcia, 2002).
Some of these species are as small as 1 μm or less in
diameter, and they appear to include whole new
divisions of eukaryotes (Massana et al., 2006).
1 Overview of the tree
Most well studied eukaryotes can now be as-
signed to one of four to five major groups. These are
(1) Unikonts, (2) Archaeplastida, (3) Rhizaria+
Alveolates+Stramenopiles (RAS), and (4) Excavates,
which are probably at least two distinct groups re-
ferred to here as the 1.4.1) mitochondriate Excavates,
and 1.4.2) core (amitochondriate) Excavates. Unikonts
include all eukaryotes thought to be primitively
uniflagellate, that is, Opisthokonts (including animals
and fungi) and Amoebozoa (Cavalier-Smith, 2002).
The RAS group was only recently recognized and
includes most of the former “chromalveolates” plus
Rhizaria (Burki et al., 2007; Hackett et al., 2007).
Archaeplastida is the group in which eukaryotic
photosynthesis first arose (Adl et al., 2005; Archibald,
2005). Mitochondriate excavates include the former
discicristates and core Jakobids. They are probably not
directly related to the amitochondriate excavates, a
collection of highly derived taxa with simplified
internal cell structure and lacking aerobic mitochon-
dria.
1.1 Unikonts
1.1.1 Opisthokonts The close evolutionary rela-
tionship between animals and fungi is now firmly
established (Rodriguez-Ezpeleta et al., 2005; Hackett
et al., 2007; Yoon et al., 2008). Since the earliest
branches of both lineages are single celled organisms,
it is also clear and not particularly surprising that
multicellularity evolved independently and funda-
mentally quite differently in the two groups. In fact,
both lines have at least two unicellular sister taxa—
mesomycetozoa and choanoflagellates in the case of
Metazoa (Steenkamp et al., 2006; Ruiz-Trillo et al.,
2008) and nucleariid amoebas and chytrids in the case
of Fungi (Medina et al., 2003; Steenkamp et al.,
2006). Intriguingly, some data split the mesomyceto-
zoa, placing Capsaspora owczarzaki, which was once
classified as a nucleariid, as the earliest branch of
Holozoa (Ruiz-Trillo et al., 2008). If correct, this
would suggest that the last common ancestor of
animals and fungi was a nucleariid-like amoeba.
The first branches of true fungi are chytridiomy-
cetes, which appear to be paraphyletic (James et al.,
2006). These aquatic unicells with pseudohyphae are
the only fungi with flagella, which occur singly on
their zoospores. All other fungi are multicellular,
hyphal organisms with absorptive nutrition, probably
all at least capable of producing multicellular fruiting
bodies. The Glomales (arbuscular mycorrhizal fungi)
are found exclusively in symbioses with plants, in-
vading the outer root cells to develop highly branched
tree-like structures to facilitate nutrient exchange with
their hosts. This symbiosis probably dates to very
early in land plant evolution and was probably an
important factor in the successful invasion of land by
plants (Read et al., 2000; Wang & Qiu, 2006). The
overall outline of the fungal tree of life is now coming
clear (James et al., 2006), although the number of
undiscovered species is probably immense (Van-
denkoornhuyse et al., 2002) and is estimated by some
to be as much as 95% of all extant species (James et
al., 2006).
The earliest known branch(es) of Holozoa are the
mesomycetozoa, which may or may not be para-
phyletic (see above), followed by the choanoflagel-
lates, which are the closest sister group to Metazoa
(Steenkamp et al., 2006; Ruiz-Trillo et al., 2008).
Mesomycetozoa are parasites or symbionts and in-
clude pseudohyphal, flagellate and amoeboid forms.
Choanoflagellates, known for their resemblance to the
collared cells of sponges (Porifera), encode metazoan
developmental proteins (King et al., 2008). Some data
indicate that the enigmatic Ministeriids are the closest
sister taxa to Metazoa (Cavalier-Smith & Chao, 2002;
Steenkamp et al., 2006). However, only one species
(Ministeria vibrans) has been examined, and it always
forms a long branch in phylogenetic trees. Although a
second species has been described, it has been lost
from culture and not seen since (Simpson & Patterson,
BALDAUF: Phylogeny and diversity of eukaryotes

265



Fig. 1. A consensus phylogeny of the major groups of eukaryotes based on published molecular phylogenetic and ultrastructural data (adapted from
Baldauf, 2003). Dotted lines indicate positions of major lineages of Stramenopiles known primarily from ciPCR (Massana et al., 2006). The two
currently proposed positions for the eukaryote root are also indicated.


2006). Dire warnings to the contrary (Rokas et al.,
2005), large molecular data sets and careful analysis
has led to tremendous recent progress in resolving the
deepest branches of Metazoa (Dunn et al., 2008).
1.1.2 Amoebozoa The Amoebozoa include several
divisions of free-living amoebas as well as amito-
chondriate amoeboflagellates (Archamoebae) and
social (Mycetozoa) amoebas, which may be each
other’s closest relatives (Smirnov et al., 2005; Ni-
kolaev et al., 2006). There are also many species of
uncertain affinity, and the higher-order phylogeny of
the group is very uncertain. This is due partly to the
difficulty in isolating and identify these species and
because of a general lack of molecular data of any
kind for most of them. Amoebozoan amoebae tend to
have lobose or tube-like pseudopods, a single nucleus,
and tubular branched mitochondrial cristae (Adl et al.,
2005). They range in size from a few microns to
several millimeters, and many smaller forms probably
remain to be discovered. Cyst formation to survive
desiccation or to invade hosts is common. Most taxa
are free-living in soil where they are important as
bacterial predators. The common soil amoebae of the
Arcellinidae are the only amoebozoans to form tests,
which they construct from organic material.
Medically the most important amoebozoans are
the Entamoebae, which are tentatively grouped to-
gether with “pelobionts” as the Archamoebae (Bapt-
este et al., 2002; Cavalier-Smith et al., 2004). Ar-
chaemobids tend to live in low-oxygen environments
and have mitochondria that are reduced to genome-
free mitosomes (Tovar et al., 1999), which continue to
function in the production of heme and other com-
pounds (Embley & Martin, 2006). Pelobionts are
amoeboflagellates that can be as large as 3 mm long
(Pelomyxa) and have 1-many non-motile flagella (Adl
Journal of Systematics and Evolution Vol. 46 No. 3 2008 266
et al., 2005). Entamoebae are small, non-flagellate and
mostly commensal or parasitic, living in the mouth
and intestinal tracts of various Metazoa. Entamoeba
histolytica, causative agent of amoebic dysentery,
appears to have developed this life style very recently.
It is morphologically and molecularly nearly indistin-
guishable from the harmless commensal, E. dispar,
and many of the enzymes required for its anaerobic
life style were acquired relatively recently by hori-
zontal gene transfer from bacteria (Clark et al., 2007).
The most dramatic amoebozoans are the Macro-
mycetozoa, the myxogastrid (plasmodial) and dic-
tyostelid (cellular) slime molds. These have very
different trophic (feeding) stages but similar fruiting
bodies, albeit formed in very different ways. Since
their discovery ~150 years ago, Dictyostelia and
Myxogastria have been variously classified, together
or separately, as plants, animals, or fungi. However,
abundant molecular data confirm that they are amoe-
bozoans (Bapteste et al., 2002; Yoon et al., 2008).
Their closest relatives are protostelid slime molds,
although it is not clear if these are monophyletic.
There are over 6,000 described species of
Myxogastria, also known as “giant amoebas”. Most of
what is known about their life cycle comes from
studies of Physarum polycephalum. Following mating,
these amoeboflagellates transform into plasmodia that
can grow to 100+ decimeters in diameter (Olive &
Stoianovich, 1975). Plasmodia are motile, can contain
10,000s of synchronously dividing nuclei and may
have thickened branching channels throughout their
cytoplasm. However, there are never any internal cell
membranes and the nuclei remain undifferentiated,
even when the plasmodia break down to form fruiting
bodies (sporophores) (Marwan et al., 2005; Gloeckner
et al., 2008). Nonetheless, these macroscopic, often
colorful structures can appear highly complex with
multiple distinct tissue-like layers, internal structures
and ornamentation. This is in striking contrast to
Dictyostelia or social amoebas (~100 described spe-
cies; Schaap et al., 2006), which spend most of their
life cycle as solitary amoebas. Under appropriate
conditions 104–105 amoebae aggregate, generally to
form 2–5 mm long motile “slugs”. The “head” of the
slug senses environmental stimuli, directs slug migra-
tion, and forms the inert cellulosic stalk of the rela-
tively inconspicuous sporophores. The tail cells then
rise to the top of the stalk and form the live spores
(Strmecki et al., 2005; Romeralo et al., in press). Less
is known about the protostelids, which form simple
largely microscopic sporophores (Olive & Stoiano-
vich, 1975; Spiegel et al., 1995).
1.2 Archaeplastida
This is the group in which eukaryotic photosyn-
thesis first arose (Adl et al., 2005), and all species in
the group are photosynthetic with the exception of a
few minor parasitic lines. There are three highly
distinct lineages—Rhodophyta, Glaucophyta and
Chloroplastida (green algae and land plants)—and, so
far, no apparent intermediate branches between them.
All eukaryotic photosynthesis originates from this
group, with one very recent exception (Nowack et al.,
2008), and many different lines of evidence support
monophyly of archaeplastid chloroplasts (Reyes-
Prieto et al., 2007). Molecular phylogenetic support
for the monophyly of their nuclear genomes has been
more elusive (Rodriguez-Ezpeleta et al., 2005), possi-
bly due to the antiquity of the group and/or extremely
sparse molecular sampling of rhodophyte and glauco-
phyte taxa.
Glaucophytes are unicells that vary from biflag-
ellates to coccoid non-flagellates to palmelloid forms
(non-motile cells in a mucilaginous matrix). Their
plastids (cyanelles) resemble those of red algae in that
they have phycobiliproteins and unstacked thylakoids
but lack chlorophyll b. However cyanelles are also
unique in that they have a bacterial-like peptidoglycan
cell wall sandwiched between their inner and outer
membranes (Steiner et al., 2005). Rhodophytes vary
from large seaweeds to crustose mats that, to the
naked eye look more like rocks than living plants.
Two major subgroups are recognized, Bangiophyta
and Florideophyta, both of which probably invented
multicellularity independently. Chloroplastida include
the Chlorophyta, Ulvophyta, Prasinophyta and Strep-
systera (Charaphyta and land plants), although prasi-
nophytes maybe para- or even polyphyletic. There are
probably multiple inventions of multicellularity in
Chloroplastida, as Chlorophytes, Ulvophytes, and
Strepsystera are all mixtures of uni- and multicellular
forms.
1.3 RAS Group
RAS (or SAR) unites three of the largest, most
diverse divisions of eukaryotes, the Rhizaria, Alveo-
lates and Stramenopiles (formerly, Heterokonts)
(Burki et al., 2007; Hackett et al., 2007). There was
very little evidence to suggest the existence of this
supergroup until the very recent acquisition of sub-
stantial EST (expressed sequence tag) data from
Rhizaria (Burki et al., 2007; Hackett et al., 2007). The
group may also include the remaining chlorophyll c
algae, the Haptophytes and Cryptophytes, but mo-
lecular phylogenetic support for this is still very weak
(Burki et al., 2007; Hackett et al., 2007).
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267
1.3.1 Rhizaria These are largely, but not entirely,
amoeboid forms. The amoebas tend to have fine
pointed (filose) pseudopodia and to build shells (tests)
from various materials. The most famous divisions are
Radiolaria, Foraminifera, Plasmodiophora, Heliozoa
and Cercozoa (Nikolaev et al., 2004). The Radiolaria
were popular subjects of Haeckel, who designed
striking lithographs based on their almost snow-
flake-like tests. These amoebae are exclusively marine
and have internal mineralized “skeletons” from which
radiate long arm-like microtubular rays (axopodia).
They include the Acantharia, with skeletons composed
of strontium sulfate, and the Polycistinae, whose
skeletons vary from simple spicules to complex
helmet-shaped structures. The third traditional group
of “Radiolaria”, the Phaeodaria, evolved independ-
ently and have siliceous skeletons usually made of
hollow radial spines (Nikolaev et al., 2004).
Foraminifera are widely distributed in all types of
marine environments but also occur in freshwater and
on land. Their amoebae have reticulated pseudopods
with bidirectional cytoplasmic flow. Most also build
tests, which are organic, agglutinated or calcareous
and with one or more chambers (Lee et al., 2000).
Both foraminiferan (Polycistinae) and radiolarian
(Phaeodaria) skeletons contribute substantially to the
microfossil record extending back to the Cambrian.
These fossilized tests are used in micropaleontology
as biostratigraphic markers and in paleoceanography
as indicators of ancient water temperatures, ocean
depths, circulation patterns, and the age of water
masses.
The remainder of the Rhizaria form a large het-
erogeneous assemblage, the Cercozoa, which includes
various amoebae, some formerly classified as helio-
zoans (desmothoracids) or radiolarians (Phaeodarea),
as well as flagellates, amoeboflagellates and plasmo-
dial parasites (Nikolaev et al., 2004). They also in-
clude chlorarachniophytes, which are closely related
to heterotrophic amoeboflagellates (cercomonads,
Archibald, 2005) but have acquired photosynthesis.
This they have done by secondary endosymbiosis of a
green alga, and they retain a highly reduced version
(nucleomorph) of the original alga’s nucleus to help
service their plastids (Archibald, 2007). Cercozoa also
include common freshwater and/or terrestrial species,
such as euglyphids, which have silica tests, and the
plasmodial parasites Haplosporidians (endoparasites
of freshwater and marine invertebrates) and Plas-
modiophorids. The latter are important endoparasites
of plants or stramenopile algae (Nikolaev et al., 2004).
1.3.2 Stramenopiles Stramenopiles are character-
ized by flagella with rows of stiff, tripartite hairs
(stramenopiles), which reverse the flow around the
flagellum so that the cell is dragged forward rather
than pushed along. Most also possess a second,
shorter, smooth flagellum (hence the alternative name
“heterokont”). This is an extraordinarily diverse group
including numerous lineages of single-celled hetero-
trophs (bicosoecid, pseudociliates), plasmodial para-
sites (oomycetes), cosmopolitan and often highly
abundant single-celled algae (diatoms, ochromonads)
and large to giant multicellular algae (xanthophytes,
phaeophytes). Environmental sampling (ciPCR)
suggests that there may be additional major divisions
of the group, consisting largely, if not entirely, of
ultra-small species (Moreira & Lopéz-Garcia, 2002).
There are at least five known lineages of
non-photosynthetic stramenopiles. Oomycetes (water
molds and downy mildews) were previously classified
as fungi and include numerous extremely destructive
plant parasites such as Phytophthora infestans, the
cause of potato blight. The bicosoecids are small
heterotrophic biflagellates, such as Cafeteria, possibly
the world’s most abundant predator (Moreira &
Lopéz-Garcia, 2002). Labyrinthulids (slime nets) form
filamentous “railway-like” networks patrolled by
amoeboid-like cells. Opalinids look almost like cili-
ates, except that their numerous flagella have stra-
menopile hairs. The taxonomically enigmatic Blasto-
cystis spp. are commensals in the guts of cold-blooded
animals and appear to be in the process of converting
their mitochondrion into a hydrogenosome (Stech-
mann et al., 2008).
Photosynthetic stramenopiles constitute at least
eleven distinct lineages, including some of the most
important and abundant algae. Diatoms have intri-
cately patterned bipartite silica tests that fit together
like lidded boxes. They are ubiquitous and often
abundant in marine and fresh water, with ~11,000
described and possibly as much as 107+ undescribed
species (Fehling et al., 2007). Phaeophytes (brown
algae) are particularly widespread in temperate inter-
tidal and subtidal zones. They have true parenchyma
and build “forests” in near-shore waters, supporting
complex ecosystems including fish and marine mam-
mals. Xanthophytes (giant sea kelps) are the keystone
species of deep sea kelp forests, another set of com-
plex marine ecosystems. Environmental sampling also
suggests that there are at least eight additional major
divisions of extremely small (pico- and nano-sized)
stramenopiles, which are widespread in occurrence
but so far known only from ciPCR libraries (Massana
et al., 2006).
Journal of Systematics and Evolution Vol. 46 No. 3 2008 268
1.3.3 Alveolates These include ciliates, dinoflag-
ellates, and apicomplexans, which are united by
molecular phylogeny and by the presence of cortical
alveoli underlying their plasma membranes (Haus-
mann et al., 2003), and two divisions of pico-
eukaryotes (<10 μm diameter) largely known only
from ciPCR (Lopéz-Garcia et al., 2001). Ciliates are
highly speciose aquatic unicells characterized by an
abundance of flagella and dimorphic nuclei, that is,
micro- and macronuclei. The micronucleus is a tran-
scriptionally inactive germ nucleus, with genes that
can consist of numerous fragments, sometimes ar-
ranged in scrambled order and even distributed over
multiple loci. The situation is not lethal because
transcription occurs in the macronuclei, which are, in
the most extreme cases, essentially cDNA libraries
consisting of multiple, correctly processed copies of
the micronuclear genes (Prescott, 2000; Dalby &
Prescott, 2004).
Dinoflagellates are a diverse, predominantly uni-
cellular group with characteristic often-elaborate
plates or armor and two unequal flagella that give rise
to a unique rotatory swimming motion. Although the
group was probably primitively photosynthetic only
about half of the extant species still are. They also
have extremely reduced plastid genomes, with most
genes relocated to the cell nucleus (Bachvaroff et al.,
2004). This may explain why they are particularly
adept at acquiring and retaining exogenous plastids
(Yoon et al., 2002; Archibald, 2005). Dinoflagellates
are important symbionts of coral and other hydrozoans
and the main source of harmful algal blooms (HABs;
e.g., red tides), where they produce some of the most
potent neurotoxins known. They also have some of the
largest known nuclear genomes, with large amounts of
repetitive DNA, which makes the prospect of a full
dinoflagellate genome sequence unlikely for some
time, if ever. This repetitive DNA may play a struc-
tural role in compensating for the nearly complete lack
of histones in dinoflagellate nuclei (Hackett et al.,
2005).
Apicomplexa are the sister group to the dinoflag-
ellates and include some of the most important proto-
zoan disease agents of both invertebrates and verte-
brates. Nearly all are obligate intracellular parasites,
including the causative agents of malaria (Plasmodium
spp.) and toxoplasmosis. Their name derives from
their characteristic apical complex, which functions in
the attachment and initial penetration of the host cell.
All species retain a vestigial plastid (apicoplast), most
likely of red algal origin (Fast et al., 2001) and re-
quired for heme, lipid and isoprenoid biosynthesis
(Waller & McFadden, 2005).
One of the most striking features of Alveolates
and Stramenopiles is the widespread occurrence of
secondary chloroplasts (kleptoplasts). Stealing plastids
is a complex process, since ~95% of chloroplast
proteins are encoded in the host nucleus (Reyes-Prieto
et al., 2007). Thus, the secondary host must acquire
not just a plastid, but the ~3600 nuclear genes needed
to maintain it. This presumably takes quite a while,
during which time a working remnant of the primary
host nucleus must be maintained in the new host cell
(Archibald, 2005). Such a remnant has now been
observed in chlorarachniophytes (see above) and in
cryptophytes and haptophytes (see below).
1.3.4 Haptophytes and cryptophytes These are
both primarily chlorophyll c algae. However, although
their plastids clearly share a common origin with the
chlorophyll c plastids of stramenopiles, there is little
evidence that their nuclear genomes do. The group of
Cryptophytes + Haptophytes, if it indeed it is a group,
is potentially quite large. Two major new lineages
have been discovered recently—the Telonemids
(Shalchian-Tabrizi et al., 2007) and Klephablepharids
(Not et al., 2007).
Haptophytes are named for their haptoneme, an
anterior appendage used for adhesion and prey cap-
ture. They include coccolithophorids, which are
unicells covered in overlapping calcium carbonate
scales (coccoliths). Blooms of the coccolithophore
Emiliana huxleyi can be 1,000+ miles across and
visible from space. These massive blooms substan-
tially affect the temperature and optical qualities of
ocean waters, and when they die, they release enough
dimethyl sulfoxide to seed clouds (Buitenhuis et al.,
1996). Blooms end in massive die-offs caused by a
marine virus, and the resulting limestone deposits are
the largest inorganic reservoirs of carbon on Earth.
The cryptophytes are relatively small (mostly
2–10 μm diameter) unicells primarily found in cold or
deep waters. Similar to the chlorarachniophytes, their
plastids are accompanied by a remnant of the primary
host nucleus. Both the cryptophyte and chlorarach-
niophyte nucleomorphs encode some of the proteins
required for plastid function. However, mostly they
encode proteins needed to main the nucleomorph itself
(Lane et al., 2007). Cryptophytes are abundant and
ubiquitous and are commonly involved in temporary
endosymbioses (Fehling et al., 2007).
1.4 Excavates
Taxa classified as “excavates” are unicells with
an often-large excavated groove at their anterior end
into which they trap food particles with the aid of a
BALDAUF: Phylogeny and diversity of eukaryotes

269
flagellum (suspension feeding; Simpson, 2003). While
molecular evidence strongly suggests that this is a
paraphyletic, if not a polyphyletic assemblage, the
name and attendant hypotheses are at least convenient
and will probably persist for some time. The organ-
isms grouped into this category fall into two or three
very distinct and quite possibly unrelated groups
(Simpson et al., 2006). However, their phylogeny is
challenging, as they also tend to have extremely fast
rates of molecular evolution. For convenience, they
are divided here into the “mitochondriate excavates”,
which is well supported and includes Euglenozoa,
Heterolobosea and core Jakobids (Simpson et al.,
2006), and “core excavates”, which includes two
possibly-unrelated divisions (Simpson et al., 2006).
1.4.1 Mitochondriate excavates
1.4.1.1 Euglenozoa The Euglenozoa include
Kinetoplastids, Diplonemids and Euglenids (von der
Heyden et al., 2004). Kinetoplastid genera include
Trypanosoma, Bodo, Leishmania. These are small uni-
or biflagellated cells, many of which are parasites
including the causative agents of sleeping sickness,
Chagas disease and leishmaniases. Euglenids are also
uni- or biflagellate cells, but are mostly free-living and
have a characteristic thickened pellicle made of
proteinaceous strips. Phagotrophic euglenids can
ingest whole eukaryotic cells and a subset, E. gracilis
and its relatives, have acquired a green algal chloro-
plast. All examined euglenozoans have striking mo-
lecular biology in their nuclear, mitochondrial and,
where present, plastid genomes. Self-splicing twin-
trons, where an inner intron must be spliced out before
the outer intron can assume its own catalytically
competent secondary structure, were discovered in the
E. gracilis plastid genome (Hallick et al., 1993). RNA
editing was discovered in the Trypanosoma mito-
chondrial genome. Here the genes are essentially
encoded in shorthand, and the initial transcripts re-
quire extensive nucleotide modification and oligonu-
cleotide insertion to encode a functional protein
(Lukes et al., 2005). There are also many unusual
molecular features of euglenozoan nuclear genomes
(von der Heyden et al., 2004).
1.4.1.2 Heterolobosea The Heterolobosea are
mostly amoebae, although many have flagellate
phases in their life cycles (Simpson & Patterson,
2006). They are abundant, ubiquitous and their eco-
logical importance is poorly understood but probably
very substantial. They are naked amoebae, and they
differ from lobosan amoebae in that their pseudopods
develop and move in a sporadic, “eruptive” manner.
Most are soil or freshwater bacterivores, although one,
Naegleria fowleri, is a rare but often fatal facultative
human pathogen. They also include the acrasid “slime
molds”, which were reclassified from amoebozoa to
heterolobosea based on molecular trees (Baldauf et al.,
2000). This is consistent with much earlier observa-
tions of their very unamoebozoan-like pseudopodia
(Olive & Stoianovich, 1975).
1.4.1.3 Jakobids These small free-living bac-
terivores are particularly noted for their variable
mitochondrial morphology (O’Kelly, 1993) and, more
recently, their bacterial-like mitochondrial genomes
(Lang et al., 1997). While most eukaryotes have fewer
than 20 protein-coding genes remaining in their
mitochondrial genomes, Jakobids retain more than
100. Also unlike other eukaryotes, these genes are
arranged in bacterial like operons (Lang et al., 1997),
consistent with an alpha-proteobacterial ancestry for
mitochondria.
1.4.2 Amitochondriate (core) excavates
The core excavates consist of two distinct line-
ages of uncertain affinity. The Fornicata includes
Diplomonads, Retortamonads, Carpediemonas and
possibly also Parabasalids (Simpson, 2003). Axostyla
consists of Oxymonads and Trimastix (Simpson,
2003). This huge and possibly ancient group is known
only as unicells living in anaerobic or micro-aerobic
habitats, often as commensal or parasites. Their
simplified internal cell structure and apparent lack of
mitochondria or mitochondrially-derived organelles
gave rise to the Archaezoa hypothesis, which main-
tained that these were remnants of early, pre-
mitochondriate eukaryote lineages (Cavalier-Smith &
Chao, 1996). However, genes of mitochondrial ances-
try have been found in their nuclear genomes, and
highly reduced mitochondrial relicts have recently
been discovered in many of them (Embley & Martin,
2006). Thus the Archaezoa hypothesis is now defunct.
Nonetheless, these taxa still appear as the earliest
branches in rooted molecular trees (Bapteste et al.,
2002; Arisue et al., 2005), although this is widely,
albeit not universally, interpreted as a long-branch
attraction artefact (Philippe & Germot, 2000).
The Diplomonads typically have a striking “mir-
rored morphology”, looking like an incompletely
divided cell with two sets of nuclei, flagella and
cytoskeletons arranged back to back. Giardia intesti-
nalis is a common human gut parasite sometimes
found in remote seemingly pristine habitats. Spironu-
cleus is a parasite and plague of fish farms (Bernard et
al., 2000). Retortamonads are intestinal commensals,
roughly resembling half of a diplomonad cell (Silber-
man et al., 2002). Oxymonads are flagellated gut
Journal of Systematics and Evolution Vol. 46 No. 3 2008 270
symbionts of animals, including termites (see below).
Parabasalids are mostly parasites and symbionts,
characterized by a complex parabasal apparatus
involved in host cell attachment. They include Hy-
permastigids and Trichomonads (Dacks et al., 2001).
Hypermastigids are large (100+ μm) cells that appear
multiflagellate due to the presence of a dense covering
of elongate ectosymbiotic bacteria. They are found
almost exclusively in the hindguts of termites and
wood-eating cockroaches where they form part of a
complex microfauna responsible for the breakdown of
cellulose. Trichomonads are small teardrop-shaped
cells with four to six flagella and cause trichomoni-
asis, the most common human sexually transmitted
disease. It is quite likely that major lineages of these
groups remain to be discovered (e.g., Yubuki et al.,
2007).
1.5 Incertae sedis
In 1999, Patterson identified 230 cultured protists
of uncertain affinity (Patterson, 1999). In 2005 that
number had only decreased to 204 (Adl et al., 2005),
so much remains to be done. Most of these are small
free-living heterotrophic flagellates or amoebae, or
they are parasites of various kinds. Many will un-
doubtedly turn out to fall within one or more of the
groups described above. However, ciPCR surveys
suggest the existence of major undiscovered eu-
karyotic lineages as well (Moreira & Lopéz-Garcia,
2002), much of which probably consists of nano- and
picoeukaryotes. These are cells as small as 1 μm in
diameter that have previously escaped detection by
light microscopy because, at this level they are all but
indistinguishable from bacteria. Even supposedly
known species may be the sole representatives of what
are in fact major unsuspected lineages. For example
recent ciPCR surveys show that Apusomonads, which
also include Ancyromonads and Mastigamoeba are a
large and diverse group (Cavalier-Smith et al., 2004).
Multigene phylogenies also suggest that they may be
the sister group to Opisthokonts (Kim et al., 2006).
2 The root of the eukaryote tree
Probably the single most outstanding question in
eukaryote evolution is the location of the root of the
tree. For a long time, the predominant theory was that
species lumped together in the now-defunct Archae-
zoa lay near the root of the eukaryote tree, as they
tended to form the deepest branches in molecular trees
(Arisue et al., 2005; Baldauf et al., 1996; Bapteste et
al., 2002; Stiller & Harrell, 2005; Ciccarelli et al.,
2006). However, there has been growing distrust in
the ability of molecular phylogeny to resolve the
deepest branches in the tree of life, because of the
problem of long branch attraction (Philippe & Ger-
mot, 2000).
A radically different placement of the eukaryote
root is suggested by the fusion of the genes for dihy-
drofolate reductase and thymidylate synthase. These
genes are adjacent and co-transcribed in bacteria,
separate in Opisthokonts, and fused in representatives
of all other major eukaryote groups except core Exca-
vates and Amoebozoa, although the latter mostly lack
the genes entirely (Stechmann & Cavalier-Smith,
2003). Since gene fusions are rare, and gene fissions
undoubtedly rarer, this suggests that Archaeplastida,
RAS and Excavates, if amitochondriate and core
excavates are assumed to be a group (Simpson et al.,
2006), share a unique common ancestor excluding
Opisthokonts and possibly Amoebozoa. Thus, this
root divides all eukaryotes into two supergroups—
Unikonts (Opisthokonts + Amoebozoa) and bikonts
(everything else).
However, the near complete absence of these
genes in Amoebozoa is disconcerting, and lateral
transfer among eukaryotes and between eukaryotes
and bacteria appears to be an on-going and not too
infrequent process. Therefore, additional data are
needed to confirm this new rooting, of which little
seems to be forthcoming. Thus it is possible that this
critical question may remain outstanding for some-
time.
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