Reevaluating the Arthropod Tree of Life

Abstract

Arthropods are the most diverse group of animals and have been so since the Cambrian radiation. They belong to the protostome clade Ecdysozoa, with Onychophora (velvet worms) as their most likely sister group and tardigrades (water bears) the next closest relative. The arthropod tree of life can be interpreted as a five-taxon network, containing Pycnogonida, Euchelicerata, Myriapoda, Crustacea, and Hexapoda, the last two forming the clade Tetraconata or Pancrustacea. The unrooted relationship of Tetraconata to the three other lineages is well established, but of three possible rooting positions the Mandibulata hypothesis receives the most support. Novel approaches to studying anatomy with noninvasive three-dimensional reconstruction techniques, the application of these techniques to new and old fossils, and the so-called next-generation sequencing techniques are at the forefront of understanding arthropod relationships. Cambrian fossils assigned to the arthropod stem group inform on the origin of arthropod characters from a lobopodian ancestry. Monophyly of Pycnogonida, Euchelicerata, Myriapoda, Tetraconata, and Hexapoda is well supported, but the interrelationships of arachnid orders and the details of crustacean paraphyly with respect to Hexapoda remain the major unsolved phylogenetic problems.

Full text

EN57CH09-Giribet ARI 31 October 2011 7:32
Reevaluating the Arthropod
Tree of Life
Gonzalo Giribet1,∗and Gregory D. Edgecombe2
1Museum of Comparative Zoology, Department of Organismic and Evolutionary Biology,
Harvard University, Cambridge, Massachusetts 02138; email: ggiribet@oeb.harvard.edu
2Department of Palaeontology, The Natural History Museum, London SW7 5BD,
United Kingdom; email: g.edgecombe@nhm.ac.uk
Annu. Rev. Entomol. 2012. 57:167–86
First published online as a Review in Advance on
September 9, 2011
The Annual Review of Entomology is online at
ento.annualreviews.org
This article’s doi:
10.1146/annurev-ento-120710-100659
Copyright c
2012 by Annual Reviews.
All rights reserved
0066-4170/12/0107-0167$20.00
∗Corresponding author
Keywords
arthropod phylogeny, anatomy, fossils, molecular data, phylogenomics
Abstract
Arthropods are the most diverse group of animals and have been so since
the Cambrian radiation. They belong to the protostome clade Ecdysozoa,
with Onychophora (velvet worms) as their most likely sister group and
tardigrades (water bears) the next closest relative. The arthropod tree of
life can be interpreted as a five-taxon network, containing Pycnogonida,
Euchelicerata, Myriapoda, Crustacea, and Hexapoda, the last two forming
the clade Tetraconata or Pancrustacea. The unrooted relationship of Tetra-
conata to the three other lineages is well established, but of three possible
rooting positions the Mandibulata hypothesis receives the most support.
Novel approaches to studying anatomy with noninvasive three-dimensional
reconstruction techniques, the application of these techniques to new and
old fossils, and the so-called next-generation sequencing techniques are at
the forefront of understanding arthropod relationships. Cambrian fossils
assigned to the arthropod stem group inform on the origin of arthropod
characters from a lobopodian ancestry. Monophyly of Pycnogonida, Euche-
licerata, Myriapoda, Tetraconata, and Hexapoda is well supported, but the
interrelationships of arachnid orders and the details of crustacean paraphyly
with respect to Hexapoda remain the major unsolved phylogenetic problems.
167
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Tagmosis: type of
body organization
where batches of
segments acquire a
specific function and
delimit different body
regions
INTRODUCTION
Arthropods, with nearly 85% of the described extant animal species and the richest fossil record
of any animal group (22), are by far the most successful metazoan phylum. Mites, for example,
can be found in any ecosystem on earth, from the deepest seafloor to the highest mountain
peak, and spiders have even been collected ballooning through the stratosphere. Insects thrive
in almost every terrestrial environment and were the first true conquerors of the air. A study of
arthropod abundance in a Bornean lowland tropical rainforest shows that arthropod biomass in
the aboveground regions was 23.6 kg ha−1, that abundance was 23.9 million individuals ha−1,and
that density on leaf surfaces was 280 individuals m−2leaf area (17). Likewise, copepods and krill
constitute a sizeable fraction of the marine biomass and sustain a large part of the ocean’s food
chain. Arthropods are the most important ecosystem builders on land and are fundamental for
breaking down and recycling organic matter in the soil. They are also of tremendous importance
to humans as a food source, as pollinators, as producers of material goods (e.g., wax, honey, silk),
and for biomedical studies, but they are also pests, vectors of disease, and the direct source of stings,
bites, and envenomation, most prominently in the case of spiders, scorpions, and centipedes.
The contemporary importance of arthropods in terms of diversity and ecosystem function is
the outcome of a geological history that spans at least 525 million years, since the main burst of
the Cambrian radiation (11). Arthropod body fossils date to the early Cambrian and are preceded
by trace fossils indicative of arthropods for at least 5 million years in the earliest Cambrian.
Trilobites, which appear nearly as early as any other arthropods, are the most diverse animal
clade in the Cambrian Period, and when unmineralized diversity is considered alongside the
more typically preserved “shelly” fossil record—as in sites of exceptional preservation such as the
Burgess Shale and Chengjiang—arthropods are both the most abundant and the most species-rich
Cambrian animal group (44). The fossil record provides a chronology for the conquest of land
by arachnids and myriapods by at least the Silurian Period (94). The evolution of land plants
has been tightly connected to the evolution of insects in a series of mutualistic interactions, with
insects acting mostly as probable pollinators of gymnosperms since the mid-Mesozoic (80) and
most prominently nectar-feeding flies, butterflies, and beetles pollinating angiosperms diversified
in concert with plants during the Cretaceous (80).
Morphologically, arthropods are characterized by a special body plan formed by numerous
segments, grouped into functional units or tagmata (Figure 1). Segmentation and tagmosis are
most certainly responsible for the diversity of the group, allowing arthropods to adapt to differ-
ent environmental conditions. Most arthropods concentrate the sensorial functions in an anterior
tagma or head, the locomotory function (walking or swimming legs and wings) in an intermediate
tagma (thorax in insects), and the reproductive functions in a posterior tagma or abdomen. These
tagmata fuse or are otherwise modified in many groups; myriapods have the body divided into
head and trunk (Figure 1d), arachnids fuse the head and thorax into a prosoma and the abdomen is
called opisthosoma (Figure 1c), and crustaceans vary their body plan enormously. Remipedes, for
example, have a basic division between head and trunk (Figure 1f), whereas most malacostracans
structure their body into a cephalon, pereion, and pleon (Figure 1e). In the arthropod groundpat-
tern, each segment bears a pair of appendages that can be modified for specific functions, and the
homology of the head appendages has been debated for a long time (see Figure 2 for current inter-
pretations). The appendages of the head are transformed into mouthparts (mandibles, maxillae),
grasping appendages (chelicerae, so-called frontal appendages of many Cambrian arthropods),
and sensorial organs (antennae, antennulae, pedipalps). The trunk appendages can deviate from a
locomotory role by acquiring a function in reproduction (e.g., gonopods of millipedes) or respira-
tion (e.g., limbs of many crustaceans). The enormous possibilities for adaptation of the appendages
168 Giribet ·Edgecombe
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EN57CH09-Giribet ARI 31 October 2011 7:32
a
b
c
b
c
d
g
e
f
Figure 1
Arthropod body plans. (a)Mesoperipatus tholloni (Onychophora, Peripatidae) from Gabon. (b)Anoplodactylus
evansi (Pycnogonida) from New South Wales, Australia, photographed by M. Harris. (c)Euscorpius sp.
(Chelicerata, Arachnida, Scorpiones) from Sicily, Italy. (d)Scolopendra laeta (Myriapoda, Chilopoda) from
Western Australia. (e)Quadrimaera sp. (Crustacea, Malacostraca, Amphipoda) from British Virgin Islands,
photographed by A.J. Baldinger and E.A. Lazo-Wasem. ( f)Speleonectes tulumensis (Crustacea, Remipedia)
from Mexico, photographed by J. Pakes. ( g) Japygoidea sp. (Hexapoda, Diplura) from New Zealand. All
photos, except where specified, by the authors.
and the regional specialization of a modular body plan have been interpreted as responsible for
the success of the arthropods (9). The developmental genetic basis for the differentiation of ap-
pendicular structures along the body axis, e.g., chelicerae, pedipalps, walking legs, book lungs, or
spinnerets, in the case of spiders, is being elucidated (71).
Taxonomically the phylum Arthropoda includes several major lineages that traditionally have
received the ranks of subphylum, class, or subclass, and their interrelationships are the crux of
ongoing debate over arthropod phylogeny. For the sake of consistency and convenience, we
use the following names in this review: Pycnogonida, Euchelicerata, Myriapoda, Crustacea, and
Hexapoda. For some authors Chelicerata includes pycnogonids, and we follow this practice and
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EN57CH09-Giribet ARI 31 October 2011 7:32
Hexapoda/
Myriapoda
Euchelicerata
Crustacea
Pycnogonida
pc
dc
tc
Chelifore
Palp
Walking leg
Ocular segment
Labrum
Chelicera
Pedipalp
Walking leg
Antenna
1st antenna
2nd antenna
Mandible
Figure 2
Alignment of head segments and homology of appendicular structures in the major arthropod lineages.
Abbreviations: dc, deutocerebrum; pc, protocerebrum; tc tritocerebrum.
use the term Euchelicerata to refer to the nonpycnogonid chelicerates (36). The pycnogonid body
plan differs markedly from that of other chelicerates, and maintaining the two groups makes
the phylogenetic alternatives easier to discuss. Crustacea and Hexapoda form a clade named
Tetraconata or Pancrustacea (81), although Crustacea is most likely paraphyletic with respect
to a monophyletic Hexapoda.
THE PHYLOGENETIC POSITION OF ARTHROPODS
Arthropods are protostome animals and as such have an apical dorsal brain with a ventral longi-
tudinal nerve chord and a mouth that typically originates from the embryonic blastopore. They
have been traditionally considered to have a primary body cavity, or coelom, that has been re-
stricted to the pericardium, gonoducts, and nephridial structures (coxal glands, antennal/maxillary
glands). The true coelomic nature of arthropods is however questionable (4). Similarly, although
many authors at one time considered arthropods to have a modified spiral cleavage—as found in
annelids, mollusks, nemerteans, and platyhelminthes—this idea is now rejected (91). Their lateral
jointed appendages have been homologized with the lobopods of onychophorans (Figure 1a),
a view strengthened by similar genetic patterning of the proximo-distal axes (51), as well as with
the limbs of tardigrades (90). Earlier they were also considered possible homologs of the an-
nelid parapodia, a homology that is generally rejected by systematists today (apart from a broad
correspondence as lateral outgrowths of the body).
The position of arthropods among animals has changed radically in the past two decades as
a result of refinements in cladistic analysis and especially by the introduction of molecular data.
Traditionally, arthropods (and their allies, onychophorans and tardigrades) were grouped with
annelids in a clade named Articulata by Cuvier in the early nineteenth century, in reference
to the segmental body plan in both phyla (92). The competing Ecdysozoa hypothesis (32, 89),
170 Giribet ·Edgecombe
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EN57CH09-Giribet ARI 31 October 2011 7:32
EST: expressed
sequence tag
Arthrodial
membrane: the
unsclerotized part of
the arthropod
exoskeleton, for
example, between
joints
allying arthropods, onychophorans, and tardigrades with a group of mostly pseudocoelomate an-
imal phyla (Figure 3) that share a cuticle that is molted at least once during their life cycle, was
proposed originally on the basis of 18S rRNA sequence data (1) but is now broadly accepted because
of support from diverse kinds of molecular information (22, 104). The exact sister group relation-
ship of arthropods is, however, still debated. Alternative hypotheses suggest either Onychophora,
Tardigrada, or a clade composed of them both as the candidate sister group of arthropods, with
phylogenomic data decanting toward the first option (2, 21, 42, 85). Whether tardigrades are
related to Onychophora +Arthropoda or to Nematoda remains more contentious, as both alter-
natives are resolved for the same EST (expressed sequence tag) (21) or mitogenomic (87) datasets
under different analytical conditions. In the latter case, conditions intended to counter certain
kinds of systematic error strengthen the support for tardigrades grouping with arthropods and
onychophorans rather than with nematodes. The alliance of Tardigrada with Onychophora and
Arthropoda is consistent with a single origin of paired, segmental ventrolateral appendages in a
unique common ancestor, and the name Panarthropoda is usually applied to this group.
Arthropod monophyly [including the parasitic Pentastomida (68) as ingroup crustaceans] is
now nearly universally accepted based on morphological, developmental, and molecular evidence
(see review in Reference 23). Advocacy for arthropod polyphyly in the 1960s and 1970s (62) was
not based on characters that supported alternative sister group hypotheses and was abandoned
on unsound logical grounds. Evidence for arthropod monophyly comes from the shared presence
of a sclerotized exoskeleton, legs composed of sclerotized podomeres separated by arthrodial
membranes, muscles that attach at intersegmental tendons, compound eyes in which new eye
elements are added in a proliferation zone at the sides of the developing eye field (40), and the
presence of two optic neuropils. Segmentation gene characters (30) and a stereotypical pattern
of how neural precursors segregate (25) can also be identified as autapomorphies for Arthropoda
compared with Onychophora and Tardigrada.
THE RELATIONSHIPS AMONG THE MAJOR
ARTHROPOD LINEAGES
Relationships among major arthropod lineages have been debated for centuries, and for a
long time the only nearly universally accepted result was the monophyly of Atelocerata—a
group that included hexapods and myriapods. However, the addition of molecular and novel
anatomical and developmental data has helped us reinterpret arthropod relationships, such that
hexapods are associated with crustaceans instead of with myriapods in a clade named Tetraconata
(=Pancrustacea) in reference to the shared presence of four crystalline cone cells in the compound
eye ommatidia in both groups (81). We are still far from having a perfectly resolved arthropod
tree of life, but several patterns, including a basic unrooted topology, are congruent among all new
sources of data. Today, nearly all authors interpret the arthropod phylogeny problem as a rooting
problem of five taxa (13, 37). Three alternative roots (of seven possible positions) are consistently
recovered in different analyses, with support falling mostly on one hypothesis—the monophyly
of all arthropods with a mandible, or Mandibulata—as the sister clade to Chelicerata. Alternative
rootings support pycnogonids as sister to all other arthropods ( =Cormogonida) (34, 117), or a
clade named Paradoxopoda ( =Myriochelata) that joins myriapods with the chelicerate groups
(61, 75).
In this section we focus on developments in three key areas, comparative anatomy, the fossil
record, and novel molecular approaches, each of which has advanced greatly since the publication
of the first arthropod phylogenies combining morphology and multiple molecular markers (34,
109). Since then, the amount of molecular data devoted to this problem has increased exponentially
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EN57CH09-Giribet ARI 31 October 2011 7:32
with recent genomic approaches. The techniques used to analyze fossil information as well as
developmental and anatomical data have improved considerably, especially with the increased
usage of confocal laser scanning microscopy, but also with the appearance of new techniques such
as X-ray microtomography (31), serial grinding with computer reconstruction of virtual fossils
(97), and synchrotron X-ray tomographic microscopy (29, 56, 98).
Contributions from Anatomy
Nervous system characters, including the ultrastructure of the eyes and configurations of the optic
neuropils, played an important role in arthropod phylogenetics in the early twentieth century,
with major contributions by N. Holmgren and B. Hanstr ¨
om in particular. One of the major
insights of this early neuroanatomical research, the putative ancestry of hexapods from crustaceans
rather than from myriapods, was revitalized in the past 20 years by neuroanatomists using new
staining/immunoreactivity and imaging techniques and cladistic methods, an approach called
neurophylogeny (82).
Current datasets based on neural characters (39, 101, 102) reinforce a closer relationship be-
tween Malacostraca and Hexapoda than either shares with Branchiopoda or Maxillopoda, as ev-
idenced by such shared features as optic neuropils with a nesting of the lamina, medulla, lobula,
and lobula plates and their connections by chiasmata. Branchiopod brains could be secondarily
simplified from a more malacostracan or remipede-like ancestor (102), although character po-
larities are dependent on the exact pattern of relationships between these crustacean groups and
Hexapoda.
For centuries the internal anatomy of arthropods has been studied by dissection and/or serial
sectioning of small species and subsequent examination by light or electron microscopy. Tra-
ditional comparative morphological analyses and subsequent three-dimensional reconstructions
suffer from a number of drawbacks. This is evident particularly in the case of soft tissue studies that
are technically demanding, time consuming, and often prone to producing artifacts (116). Some
of these problems have been overcome by employing noninvasive, nondestructive imaging tech-
niques, initially confocal laser microscopy and then more recently microcomputed tomography
or magnetic resonance imaging (29, 43). Micro-computer tomography techniques and three-
dimensional reconstruction have also been applied to the study of the circulatory system of several
arthropods using corrosion casting (111), and these have clarified important phylogenetic ques-
tions, e.g., within crustaceans (112). Rapid and relatively inexpensive imaging techniques will be
required if morphology is to continue playing a role in formulating phylogenetic hypotheses in a
world ever more inundated by molecular data.
Contributions from the Fossil Record
A contribution of fossils to understanding arthropod evolution is in providing snapshots of ex-
tinct diversity, morphology, and inferred ecology. Fossils are our only record of gigantism in
lineages such as stem-group arachnids (i.e., eurypterids), an extinct clade of millipedes with pos-
sible affinities to the minute extant order Penicillata (arthropleurids in Carboniferous coal swamp
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→
Figure 3
Hypothesis of the protostome tree of life, placing Arthropoda within the ecdysozoan phyla. This tree is a summary of diverse sources,
with emphasis on groups recognized in phylogenomic analyses.
172 Giribet ·Edgecombe
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Chaetognatha
Entoprocta
Cycliophora
Priapulida
Kinorhyncha
Loricifera
Nematoda
Onychophora
Tardigrada
Arthropoda
Bryozoa
Brachiopoda
Nemertea
Annelida
Mollusca
Nematozoa
Ecdysozoa
Scalidophora
Polyzoa
Trochozoa
Platyzoa
Gnathifera
Gastrotricha
Gnathostomulida
Micrognathozoa
Rotifera
Phoronida
Platyhelminthes
Nematomorpha
Nem
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Stem group:
a paraphyletic
assemblage of fossil
taxa that diverged basal
to a crown group but is
more closely related to
it than are its closest
extant relatives
Crown group:
a clade composed of
the most recent
common ancestor of
the extant members of
a taxon and all of its
descendants
forests), and insects with wingspans on the order of 71 cm (Permian griffinflies of the extinct order
Protodonata). Inclusion of fossils may influence estimates of the interrelationships of extant taxa,
as exemplified by cladograms for the basal lineages of beetles (5) and the interordinal relationships
of arachnids (35, 96).
A particularly significant forum for fossils in arthropod phylogenetics concerns Cambrian
taxa recognized as constituting stem-group Arthropoda (23). Most information about these fossils
comes from sites with soft-part preservation, so-called Burgess Shale-type localities, approximately
40 of which are known from the Cambrian worldwide. The most important of these localities are
the Burgess Shale of western Canada (Stage 5 in the contemporary 10-stage Cambrian timescale),
the Chengjiang biota of south China (Cambrian Stage 3), and Sirius Passet in north Greenland
(Cambrian Stage 3).
Broad consensus has been reached that anomalocaridids (including such animals as Anomalocaris
and Hurdia)andOpabinia are stem-group arthropods that branched from the stem lineage after
the acquisition of stalked, compound eyes but before the evolution of a sclerotized tergal exo-
skeleton (11, 23). Whether anomalocaridids and other large-bodied, lobopod-bearing Cambrian
animals with which they share spinose frontal appendages, such as Kerygmachela and Pambdelurion,
unite as a clade named Dinocaridida (59) or comprise a paraphyletic series in the arthropod stem
group (16) is debated. A Devonian taxon with a radial mouthpart and anomalocaridid-like frontal
appendages, Schinderhannes (55), may be positioned even more crownward than anomalocaridids
in the arthropod stem group because it appears to share additional derived characters with the
arthropod crown group (notably an articulated tergal exoskeleton).
Another emerging point of agreement is that a growing sample of taxa (mostly from Chengjiang)
with lobopodial trunk limbs collectively known as Cambrian lobopodians represent a grade of
panarthropods that includes stem-group Onychophora and stem-group Arthropoda and possibly
stem-group Tardigrada or stem-group Panarthropoda. The most recent phylogenetic analyses of
these taxa resolve “armored” lobopodians with paired, segmentally arranged dorsal spines such as
Hallucigenia and Luolishania either on the arthropod stem lineage (59), though branching stemward
of the dinocaridids, or on the onychophoran stem lineage (58).
Another style of fossil preservation has figured prominently in research on the early history of
some major crown-group euarthropod clades, especially the crustaceans. Orsten refers to secon-
darily phosphatized fossils, known from numerous localities that span the Early Cambrian to Early
Ordovician window. This phosphate replacement permits exquisitely preserved, uncompacted
fossils smaller than 2 mm to be extracted from limestones and examined by scanning electron
microscopy, providing unique insights into larval development and highly detailed information on
appendage morphology. Orsten arthropods have recently been revealed from the early Cambrian,
from rocks as old as the Chengjiang fauna, demonstrating that crown-group Arthropoda and,
more precisely, crown-group “crustaceans” have a fossil record as early as Cambrian Stage 3.
These fossils include Yicaris (114), an entomostracan crustacean, and a metanauplius named Wu-
jicaris (115), which is convincingly identified as a maxillopodan crustacean. Late Cambrian Orsten
of Sweden has contributed a series of species that have been resolved as stem-lineage crustaceans
(41). Character analysis to date has interpreted these fossils in the context of crustacean mono-
phyly, but alternative placements with Crustacea as a grade within Tetraconata remain an open
question.
Although molecular techniques now allow essentially precise dating of old arthropod lineages
(70, 88), paleontology contributes most of the data on the age of modern lineages, and minimum
ages from fossils calibrate molecular estimates for divergencies. Most molecular estimates of the
splits between the deep arthropod clades such as Chelicerata versus Mandibulata (or Myriochelata
versus Tetraconata) date these events to the Ediacaran Period (635–542 My) or even earlier, to
174 Giribet ·Edgecombe
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PCR: polymerase
chain reaction
cDNA:
complementary DNA
mRNA:
messenger RNA
Transcriptome: the
fraction of the genome
that encompasses all
transcribed genes
the Cryogenian (74). These “long fuse” time trees suggest a considerable duration that lacks any
credible fossils. Body plan conservatism is exceptional in some groups of arthropods, notably in
Silurian pycnogonids (97) and scorpions, and in Devonian Opiliones (19).
Contributions from Novel Molecular Approaches
Molecular data have revolutionized our understanding of arthropod relationships since the early
1990s. For nearly two decades, molecular phylogenies relied on direct sequencing of a few selected
genes amplified with specific primers—called a target-gene approach. Systematists constructing
arthropod phylogenies often used nuclear ribosomal genes (36, 60, 108), nuclear protein-encoding
genes (78), or a combination of these with mitochondrial genes (37), or they focused on mitoge-
nomics (87)—the analysis of complete mitochondrial genomes, either their sequences, gene order
information (57), or both. However, mitogenomic data seem to present strong biases and partly
conflict with other sources of information, either from morphology or from the nuclear genome
(64, 87).
Some of the earliest papers from the 1990s presented contradictory and sometimes morpho-
logically anomalous results, but many of these problems were a result of deficient taxon sampling,
too few molecular data, systematic error, or combinations of these defects. Initially controversial
issues, such as the monophyly of Hexapoda (contradicted in several mitogenomic studies) and
Myriapoda, have stabilized in the most recent and more taxonomically complete studies. The
phylogenetic signal in support for Euchelicerata, Tetraconata, and paraphyly of Crustacea with
respect to hexapods has been strong since the beginning. However, the monophyly and internal
relationships of Arachnida, and the crustacean sister group of Hexapoda, remain the most pressing
unresolved issues in arthropod phylogenetics.
Modern target-gene approaches using large numbers of markers, as many as 62 nuclear protein-
encoding genes (77), and as many as 75 taxa (79), add support to Mandibulata and suggest a
sister group relationship of hexapods to remipedes +cephalocarids but do not resolve the exact
position of pycnogonids (a sister group relationship to Euchelicerata is recovered but without
strong support). Although the use of large numbers of markers obtained through standard PCR
(polymerase chain reaction) approaches has been an important advance, this method is time-
consuming and it is difficult to consistently amplify large numbers of genes for many taxa.
Developments in sequencing technology and shotgun approaches following the sequencing
of the first complete eukaryotic genomes changed our views on how to produce DNA sequence
data. For a fraction of the effort and cost required to amplify multiple markers, random sequenc-
ing strategies allow automated processes to be applied to collecting hundreds or thousands of
genes from complementary DNA (cDNA) libraries obtained from messenger RNA (mRNA).
Although this requires specimens specially preserved for RNA extraction (live or frozen speci-
mens, or animals preserved in special solutions such as RNAlater R
), thus limiting the usability of
recent collections for molecular work of specimens preserved in high-degree ethanol, it opened
the doors to true phylogenomic analyses based on a sizeable fraction of the transcriptome of an
organism (69). The random sequencing of clones from a cDNA library generates large numbers
of ESTs, and soon studies combined the data from full genomes with ESTs for a diverse sampling
of protostomes (21, 42) or arthropods in particular (2, 67, 84, 86). Whereas some EST-based
studies supported the Myriochelata hypothesis (21, 42, 67, 84), more recent studies support the
monophyly of Chelicerata as the sister group of Mandibulata (86), in line with the anatomical
evidence for jawed arthropods as a natural group. New characters from rare genomic changes
add more support to Mandibulata; myriapods share two putatively novel microRNAs (noncoding
regulatory genes) with crustaceans and hexapods that are not shared with chelicerates (86).
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PHYLOGENOMICS: TARGET-GENE APPROACHES VERSUS RANDOM
SEQUENCING
Phylogenomics often refers to the use of genome-level data in phylogenetic studies. Such data can be obtained
from the comparison of complete genomes or from approaches based on random sequencing of genes (ESTs if the
sequenced genes are expressed genes; the transcriptome). Although some authors apply the term phylogenomics to
the use of multiple genes obtained from direct sequencing of candidate genes (normally via PCR sequencing), we
restrict it to those studies based on genome-level data. The use of whole genomes or fractions of the transcriptome
poses analytical challenges that do not apply to target-gene approaches, in particular the problem of homology
assessment. Some authors identify sets of preselected genes for analysis, but approaches that use automated and
explicit methods for assigning homology based on reproducible criteria (21) should be preferred to approaches
based on manual curation of genomes, which do not scale well. To date, the largest analysis of animal phylogeny
includes 1,487 genes selected using these methods (42).
Most of the earliest EST libraries were obtained using standard Sanger capillary sequencers.
High-throughput sequencing with next-generation sequence technologies such as Roche 454 (63)
and Solexa illumina (45) can produce hundreds of thousands or millions of sequences per sample,
respectively, at a fraction of the cost of the earlier Sanger technology sequencing. Currently the cost
for library construction and 150-bp paired-end Illumina sequencing is approximately US$2,500,
producing up to 50 million reads. Transcriptomes for arthropods are now being produced in these
ways (28, 84), and dozens or hundreds of such libraries will become available in the next few years
(e.g., the authors have already generated illumina data for several arachnids and myriapods). The
first analyses of complete genomes of multiple species of insects are already available (15).
STANDING ISSUES WITHIN THE MAJOR
ARTHROPOD LINEAGES
The technological breakthroughs discussed above have already contributed toward resolving and
stabilizing many relationships among the arthropod taxa, but, still, several areas need improve-
ment. The exact position of the root (Figure 4), now best supported between Mandibulata and
Chelicerata, requires further testing with more genomic data on pycnogonids, arachnids, and
myriapods, because taxon sampling in those groups is sparse and the EST libraries are shallow
when compared with those of other arthropod groups. A solution to this problem is foreseeable
in the near future because several investigators have already generated the data.
Chelicerata
Although Euchelicerata is nearly always identified as monophyletic (but see mitogenomic analyses
in Reference 64), molecular datasets to date (35, 64, 72, 79), with a few exceptions, are at odds with
morphology (96), often not recovering the dichotomy between Xiphosura (horseshoe crabs) and
Arachnida (Figure 5). Possible causes for the difficulty in recovering these relationships are the
long history of the group, the extinction of key lineages, or intrinsic problems of the molecular
data, but identifying the cause requires more densely sampled phylogenomic analyses. Other
recurring controversies are the monophyly and phylogenetic affinities of Acari (18, 72) and the
precise position of Palpigradi and Ricinulei. Similarly, resolving the exact relationships among
the “basal” arachnid orders (Scorpiones, Opiliones, Pseudoscorpiones, and Solifugae) remains
challenging. The currently favored morphological hypothesis in which scorpions and harvestmen
176 Giribet ·Edgecombe
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Pycnogonida
Euchelicerata
Myriapoda
Hexapoda
Crustacea *
* = Paraphyletic
Cormogonida
Mandibulata
Myriochelata
Figure 4
The arthropod five-taxon rooting problem. The left rooting position recognizes the taxon Cormogonida.
The mid rooting position is the best supported and divides arthropods into Chelicerata and
Mandibulata. The right rooting position is compatible with the Myriochelata hypothesis.
are united as Stomothecata, named for a unique formation of the preoral chamber (96), conflicts
with the largest available molecular datasets for arachnids (79).
The sister group relationship between Pycnogonida and Euchelicerata is a long-standing mor-
phological argument (Figure 4), though the homology of chelifores and chelicerae remains one of
the only clearly documented autapomorphies (20). The segmental alignment of these appendages
and their identity as deutocerebral (Figure 2) have been corroborated by Hox gene expression
domains (48) and neuroanatomy (8).
Myriapoda
The long tradition of postulating that Myriapoda is nonmonophyletic stemmed from the Atelo-
cerata hypothesis. In that framework, myriapods were identified as a grade from which hexapods
evolved. Although some morphologists continue to advocate Atelocerata as a clade (3, 6), and
its members share a unique pattern of expression of the collier gene in the limbless intercalary
segment of the head (50), others have cautioned that the putative apomorphies of the group are
likely convergences due to terrestrial habits (39). The very strong molecular and neuroanatomical
support for a hexapod-crustacean clade that excludes Myriapoda means that myriapod paraphyly
is untenable (95). Analyses that used large sampling of genes (79) have resolved Myriapoda as
monophyletic, with strong support, a finding consistent with the unique structure of the tentorial
endoskeleton throughout Myriapoda. Additional molecular evidence for myriapod monophyly
comes from a novel microRNA (86) and antisense Ultrabithorax expression (49) shared by cen-
tipedes and millipedes, although the presence of these characters remains to be confirmed in
symphylans and pauropods.
The standard morphological tree for myriapod relationships (Chilopoda as sister group to
Progoneata) is retrieved in a 62-gene sampling (79). Within Progoneata, the union of diplopods
and pauropods as a clade named Dignatha is regarded as a strong anatomical and developmental
argument (95), but sequence-based analyses have instead retrieved a grouping of Pauropoda with
Symphyla rather than with Diplopoda. Pauropods and symphylans are observed to attract in
anomalous positions (sometimes even outside Arthropoda) in well-sampled analyses of nuclear
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Pycnogonida Pycnogonida
Xiphosura
Opiliones
Scorpiones
Solifugae
Pseudoscorpiones
Palpigradi
Ricinulei
Acari
Tetrapulmonata
Chilopoda
Symphyla
Pauropoda
Diplopoda
Ostracoda
Branchiura/Pentastomida
Branchiopoda
Thecostraca
Malacostraca
Copepoda
Remipedia
Cephalocarida
Collembola
Protura
Diplura
Archaeognatha
Zygentoma
Ephemeroptera
Odonata
Hemimetabola *
Holometabola
Euchelicerata
Myriapoda
Crustacea *
Hexapoda
Tetraconata
Mandibulata
Chelicerata
Arachnida
Ectognatha
(= Insecta)
Entognatha
* = Paraphyletic
Figure 5
Arthropod tree following the Mandibulata hypothesis. Not all arthropod orders are listed. For Euchelicerata, Tetrapulmonata includes
Araneae, Amblypygi, Uropygi, and Schizomida. Crustacean relationships are based mostly on Reference 79.
ribosomal genes (108), so the possibility that their grouping with nuclear coding genes may be a
long-branch artifact needs careful investigation.
Tetraconata
Tetraconata has long been recognized as a clade based on molecular data and reinforced by im-
portant morphological characters of eye ultrastructure (81), brain and optic lobe anatomy (40,
101, 102), and similarities in neurogenesis (107). The issue of hexapod monophyly, which was
disputed in some mitogenomic analyses (12), has been resolved in favor of a single origin using
178 Giribet ·Edgecombe
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larger molecular datasets (67, 79, 106). At the base of Hexapoda, the status of Entognatha as a
clade or a grade remains sensitive to taxon sampling and methods of molecular data analysis. The
Nonoculata hypothesis (sister group relationship between Protura and Diplura to the exclusion
of Collembola, the traditional sister group of Protura) was originally proposed on the basis of
nuclear ribosomal genes (33, 108), but it finds further support in phylogenomic analyses (67) and
is consistent with some morphological data (53). The internal phylogeny of insects (Ectognatha)
continues to be refined (47), including the discovery and placement of the order Mantophasma-
todea (105), the suggested paraphyly of the order Mecoptera (110), and the proposal that Isoptera
be dismissed as an order and reclassified as a family of Blattodea (46, 105). However, whether
crustaceans are monophyletic or paraphyletic with respect to hexapods (37), and, if the latter,
precisely which crustacean lineage constitutes the sister group of hexapods, remains labile (38).
Crustacean relationships have been recently reviewed (83), including summaries of the alter-
native sister group hypotheses for each major crustacean clade (52). Molecular analyses using
large numbers of genes have introduced some new, unanticipated hypotheses based on other
data sources. For example, an analysis of 62 markers suggests that a putative clade composed of
Cephalocarida +Remipedia (newly named as Xenocarida) is sister to Hexapoda, and that Bran-
chiopoda forms a clade with Malacostraca, Thecostraca, and Copepoda (79). Cladistic analyses
based on nervous system characters instead identify Malacostraca as the likely sister group of
hexapods (101, 102). In contrast, larger gene samples in phylogenomic analyses repeatedly resolve
Branchiopoda as sister to Hexapoda (although Cephalocarida and Remipedia were not sampled
in those studies) (67, 84, 86). Denser taxon sampling of key crustacean lineages is still needed
in phylogenomic analyses before a definitive solution can be proposed with strong support. The
attraction of remipedes and cephalocarids, a union not anticipated by morphology (99) but long
detected in molecular datasets (34), requires further evaluation as a potential long-branch artifact.
Among the potential crustacean sister groups of Hexapoda, Remipedia currently receives the
most focus (108). Recent studies have documented the larvae and postembryonic development of
remipedes, and some similarities to Malacostraca have been singled out (54). Brain anatomy of a
remipede provided evidence for affinities to Malacostraca and Hexapoda (39), and hexapod-type
hemocyanins have been discovered in remipedes (27). The largest available molecular datasets
for these inhabitants of anchialine caves are, as noted (79), similarly in favor of a close affinity to
Hexapoda.
ARTHROPODS AS MODELS IN DEVELOPMENTAL BIOLOGY
Arthropods in general, and the fruit fly, Drosophila melanogaster, in particular, have traditionally
served as models in developmental biology for understanding morphology or for biomedical rea-
sons. However, more recently, researchers have been studying development in other arthropods by
using modern molecular techniques such as immunoreactivity and cell labeling, among others, of-
ten with the aim of testing specific phylogenetic hypotheses. The number of these studies has grown
substantially in the past decade, and we focus on a few examples of special relevance to some of the
hypotheses addressed here. In addition, the reliable sequencing of transcriptomes is opening new
doors to studying many arthropods at levels comparable to those of previous model organisms (28).
Although the mandible and eye ultrastructure have been foci of morphological studies (76, 81)
that have supported the Mandibulata hypothesis, neurogenesis has also played an important role in
the Myriochelata versus Mandibulata debate. The neurogenesis pattern observed in selected myr-
iapods and chelicerates, in which neural precursors migrate as postmitotic clusters of cells rather
than as single cells as in the neuroblasts of hexapods and crustaceans, is considered to be homol-
ogous (14, 73, 100). In addition, myriapods and chelicerates share segmental invaginations of the
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neuroectoderm of each hemisegment from which the ventral organs are derived. The absence
of these specific patterns in onychophoran outgroups suggests that they may be autapomorphies
for Myriochelata (65). However, these putative apomorphies conflict with molecular phyloge-
netic analyses using dense arthropod sampling (79, 86), which instead defend Mandibulata rather
than Myriochelata, and neural gene expression has alternatively been viewed as compatible with
Mandibulata (25).
The composition of the arthropod brain is one of the most contentious issues in animal evo-
lution (10, 93). In particular, controversy surrounds the innervation of segmental cephalic ap-
pendages by the brain and therefore the homology of such appendages (see Figure 2). For the
major arthropod groups, Hox expression data have aided in aligning head segments (48, 93); these
data are also available for onychophorans (24). In the case of onychophorans, the major brain neu-
ropils arise from only the anterior-most body segment and only two pairs of segmental appendages
(the antenna and jaw) are innervated by the brain (66). This set of traits is taken as an indication
that the region of the central nervous system corresponding to the arthropod tritocerebrum is not
differentiated as part of the onychophoran brain (contradicting other recent investigations; 26,
103) but instead belongs to the ventral nerve cords. If the last common ancestor of Onychophora
and Arthropoda possessed a brain consisting of a protocerebrum and deutocerebrum but lacked
the tritocerebrum, the latter would be a novel character of arthropods (66).
Whether the primitive arthropod appendage is uniramous or biramous and the specific ho-
mologies between different rami (e.g., exopods, epipods, exites) in branched appendages have been
debated for centuries (7). A study (113) using a comparative cell lineage analysis of uniramous and
biramous limbs in an amphipod crustacean via single-cell labeling suggested that “biramy” in
crustaceans results from the splitting of a single limb axis and may not correspond to the state
described as biramy in many fossil arthropods, such as trilobites, in which the putative exopod
more closely resembles another axis. If correct, biramy as observed in crown-group Tetraconata
may be a relatively novel character rather than a plesiomorphy retained from the arthropod stem
group as conventionally hypothesized.
The possibilities for testing these and other evolutionary hypotheses with comparative devel-
opmental biology studies have no limits. As whole genomes of more arthropods become available
and more functional assays are applied to these questions, we should be able to provide more
explicit hypotheses of homology that will continue to be tested phylogenetically.
CONCLUSIONS
Arthropods have dominated animal diversity throughout their evolutionary history. Here we have
discussed key issues for evaluating the arthropod tree of life, focusing on novel aspects contributed
by anatomy, the early Paleozoic fossil record, and molecular approaches, the last increasingly being
phylogenomic in scope. We conclude that this knowledge and the field of developmental biology,
which is now incorporating data from nonmodel organisms, will contribute toward resolving
standing issues on homology and phylogenetic relationships.
SUMMARY POINTS
1. Arthropods are the most diverse group of animals in the extant biota and have been so
since the early Cambrian.
2. The position of arthropods among the protostome animals has been elucidated by the
Ecdysozoa hypothesis. Onychophora (velvet worms) is the most likely sister group of
arthropods.
180 Giribet ·Edgecombe
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3. The arthropod tree of life can be divided into five major branches: Pycnogonida,
Euchelicerata, Myriapoda, Crustacea, and Hexapoda. The monophyly of each branch
is well supported apart from Crustacea, which is likely paraphyletic with respect to
Hexapoda in a clade named Tetraconata or Pancrustacea.
4. Three competing hypotheses describe the relationships of these major lineages, but
Chelicerata is most probably sister group to Mandibulata, which includes the three groups
of arthropods with mandibles as mouthparts: myriapods, crustaceans, and hexapods.
5. Noninvasive three-dimensional reconstruction techniques for studying anatomy, the ap-
plication of such techniques to fossils, and next-generation sequencing techniques are
promising sources of new character data for arthropod phylogenetics.
6. The arthropod stem group includes lobopodians and anomalocaridids, the anatomy
of which is becoming increasingly understood from exceptionally preserved Cambrian
fossils.
7. Remaining standing issues are the internal relationships of Arachnida and the relation-
ships of the major lineages of Crustacea, including the identity of the sister group of
hexapods. Various lines of evidence point to remipedes as a strong candidate for the
hexapod sister group.
DISCLOSURE STATEMENT
The authors are not aware of any affiliations, memberships, funding, or financial holdings that
might be perceived as affecting the objectivity of this review.
ACKNOWLEDGMENTS
We are indebted to the Editorial Committee of the Annual Review of Entomology for the invita-
tion to write this review. M. Harris, C. Arango, and J. Pakes kindly assisted with photographs.
A. Schmidt-Rhaesa kindly provided the original illustrations modified for Figure 2.
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EN57-Frontmatter ARI 29 October 2011 7:8
Annual Review of
Entomology
Volume 57, 2012
Contents
Insect Responses to Major Landscape-Level Disturbance
T.D. Schowalter ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp1
Sound Strategies: The 65-Million-Year-Old Battle Between Bats
and Insects
William E. Conner and Aaron J. Corcoran ppppppppppppppppppppppppppppppppppppppppppppppppp21
Approaches and Incentives to Implement Integrated Pest Management
that Addresses Regional and Environmental Issues
Michael J. Brewer and Peter B. Goodell ppppppppppppppppppppppppppppppppppppppppppppppppppppp41
Transmission of Flea-Borne Zoonotic Agents
Rebecca J. Eisen and Kenneth L. Gage pppppppppppppppppppppppppppppppppppppppppppppppppppppp61
Insect Nuclear Receptors
Susan E. Fahrbach, Guy Smagghe, and Rodrigo A. Velarde ppppppppppppppppppppppppppppppp83
Plasmodium knowlesi: A Malaria Parasite of Monkeys and Humans
William E. Collins pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp107
Group Size and Its Effects on Collective Organization
Anna Dornhaus, Scott Powell, and Sarah Bengston pppppppppppppppppppppppppppppppppppppp123
Mosquito Genomics: Progress and Challenges
David W. Severson and Susanta K. Behura pppppppppppppppppppppppppppppppppppppppppppppp143
Reevaluating the Arthropod Tree of Life
Gonzalo Giribet and Gregory D. Edgecombe pppppppppppppppppppppppppppppppppppppppppppppp167
Morphology and Diversity of Exocrine Glands in Lepidopteran Larvae
Francesca Vegliante and Ivar Hasenfuss ppppppppppppppppppppppppppppppppppppppppppppppppppp187
Insects as Weapons of War, Terror, and Torture
Jeffrey A. Lockwood ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp205
Mites (Acari) as a Factor in Greenhouse Management
Uri Gerson and Phyllis G. Weintraub ppppppppppppppppppppppppppppppppppppppppppppppppppppp229
Evolutionary Ecology of Odonata: A Complex Life Cycle Perspective
Robby Stoks and Alex C´ordoba-Aguilar ppppppppppppppppppppppppppppppppppppppppppppppppppp249
vii
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EN57-Frontmatter ARI 29 October 2011 7:8
Insect Transgenesis: Current Applications and Future Prospects
Malcolm J. Fraser Jr. pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp267
The Ecology of Nest Movement in Social Insects
Terrence P. McGlynn ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp291
Molecular Bases of Plant Resistance to Arthropods
C. Michael Smith and Stephen L. Clement ppppppppppppppppppppppppppppppppppppppppppppppp309
Prospects for Managing Turfgrass Pests with Reduced Chemical Inputs
David W. Held and Daniel A. Potter ppppppppppppppppppppppppppppppppppppppppppppppppppppp329
Managing Social Insects of Urban Importance
Michael K. Rust and Nan-Yao Su ppppppppppppppppppppppppppppppppppppppppppppppppppppppppp355
Systematics, Biodiversity, Biogeography, and Host Associations of the
Miridae (Insecta: Hemiptera: Heteroptera: Cimicomorpha)
G. Cassis and R.T. Schuh pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp377
Essential Oils in Insect Control: Low-Risk Products in a High-Stakes
World
Catherine Regnault-Roger, Charles Vincent, and John Thor Arnason pppppppppppppppppp405
Key Aspects of the Biology of Snail-Killing Sciomyzidae Flies
William L. Murphy, Lloyd V. Knutson, Eric G. Chapman, Rory J. Mc Donnell,
Christopher D. Williams, Benjamin A. Foote, and Jean-Claude Vala pppppppppppppppp425
Advances in Insect Phylogeny at the Dawn of the Postgenomic Era
Michelle D. Trautwein, Brian M. Wiegmann, Rolf Beutel, Karl M. Kjer,
and David K. Yeates pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp449
Indexes
Cumulative Index of Contributing Authors, Volumes 48–57 ppppppppppppppppppppppppppp469
Cumulative Index of Chapter Titles, Volumes 48–57 ppppppppppppppppppppppppppppppppppp474
Errata
An online log of corrections to Annual Review of Entomology articles may be found at
http://ento.annualreviews.org/errata.shtml
viii Contents
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