Arthropods: Developmental diversity within a (super) phylum

Abstract

The expression patterns of developmental genes provide new markers that address the homology of body parts and provide clues as to how body plans have evolved. Such markers support the idea that insect wings evolved from limbs but refute the idea that insect and crustacean jaws are fundamentally different in structure. They also confirm that arthropod tagmosis reflects underlying patterns of Hox gene regulation but they do not yet resolve to what extent Hox expression domains may serve to define segment homologies.

Full text

Perspective
Arthropods: Developmental diversity within a
(super) phylum
Michael Akam*
Laboratory for Development and Evolution, University Museum of Zoology, Department of Zoology, Downing Street, Cambridge, CB2 3EJ, United Kingdom
The expression patterns of developmental genes provide new markers that address the homology of body parts and provide clues as to
how body plans have evolved. Such markers support the idea that insect wings evolved from limbs but refute the idea that insect and
crustacean jaws are fundamentally different in structure. They also confirm that arthropod tagmosis reflects underlying patterns of Hox
gene regulation but they do not yet resolve to what extent Hox expression domains may serve to define segment homologies.
The goal of much evo-devo research is
to understand how developmental
mechanisms evolve to generate new body
plans. A prerequisite is to understand how
body plans themselves may best be com-
pared. For macroevolutionary compari-
sions, this is no trivial task—a fact evident
from century-old disputes as to the ho-
mology of parts. Molecular embryology
provides new markers to address these old
questions, markers that also provide clues
to the molecular changes that underlie
evolutionary transformations. Arthropods
provide an excellent test case for this
approach because their diversity is con-
strained by the literal straight jacket of a
modular exoskeleton. I review the first
fruits of such studies below. In some cases,
these data clearly support or refute pre-
vious hypotheses, fulfulling much of their
promise. In other cases, interpretation
remains difficult. However, given the po-
tential richness of this data set, there is
good cause to be optimistic that further
studies will resolve, rather than com-
pound, ambiguities.
The Place of Arthropods in the Tree of Life.
The context for any study that seeks to
understand the evolution of development
must be a phylogeny, albeit uncertain. Ar-
thropods are no longer considered to be the
kin of annelids, but both molecular and
morphological data support the traditional
association between arthropods proper and
the segmented lobopods, typified by Peripa-
tus. This pan-arthropod grouping would
now be placed by many within a larger
assemblage of molting animals, the Ecdyso-
zoa [refs. 1 and 2; see the article by Adoutte
in this issue of PNAS (3)]. The basal radi-
ation of the arthropods is not yet resolved,
but both molecular and new morphological
data support a close relationship between
insects and crustaceans, to the exclusion of
chelicerates (4– 8). The position of the myri-
apods remains uncertain, although molecu-
lar analyses consistently place them outside
an insect
兾
crustacean clade.
Homology of Limbs and Segmentation. Con-
served details of engrailed gene expression
support a common origin for segmenta-
tion within the arthropods. Engrailed pro-
tein marks the posterior parts of segments
and, in all arthropods tested, limb buds
arise at the boundary of engrailed ex press-
ing and non-expressing cells (9, 10). No
studies have examined segmentation gene
expression in other pan-arthropodan
phyla.
The limbs of insects and myriapods
have a single proximo-distal ax is—they
are uniramous. Crustacean limbs are fre-
quently branched, with two (biramous) or
more proximo-distal axes. These branched
structures arise by the appearance of mul-
tiple growth foci at different dorso-ventral
positions around the distal margin of the
limb bud. However, all limb branches arise
at the same interface between engrailed
expressing and non-expressing cells: i.e.,
at the same A
兾
P position (11, 12). This
pattern provides no developmental sup-
port for a model of arthropod segment
evolution that derives biramous limbs
from the fusion of two primitive un-
iramous segments (13).
Current models propose that the
proximo
兾
distal axis of the Drosophila
limb is specified by the overlap of decap-
entaplegic (dpp) and wingless signaling ter-
ritories, with distal territories defined by
expression of the distalless gene (14). Ho-
mologues of distalless and wingless are
both involved in patterning the multiple
branches of crustacean limbs, but the pat-
tern of their expression in the most com-
plex multiply branched limbs does not
suggest a simple reiteration of the insect
model in each limb branch (15, 16).
Molecular markers support the hypoth-
esis that the wings of insects may derive
from the dorsal branches of an ancestrally
branched arthropod limb, and not from an
extension of the notum, as has been pro-
posed more recently. Two genes charac-
teristically expressed in the developing
wing of Drosophila, nubbin and apterous,
are both expressed specifically in the dorsal
lobe of the multiply branched limb of the
branchiopod crustacean, Artemia (17).
The Mandible. In insects, myriapods, and
crustaceans, the first mouthpart segment
is modified to form a biting jaw, the man-
dible. The structure of the mandible has
been a key character supporting a phylog-
eny that groups the insects with the myri-
apods to the exclusion of the crustaceans.
From the evidence of functional morphol-
ogy, Sidnie Manton argued that the myri-
apod and insect mandibles were con-
structed from a whole limb whereas the
crustacean mandible was derived from
only the basal segment of the appendage
(a so-called gnathobasic mandible). De-
velopmental data do not agree with this
interpretation. The insect mandible,
uniquely among insect appendages, does
not express distalless at any stage of its
development, strongly suggesting that it
does not correspond to a whole limb.
However, distalless expression is also lost
from the developing mandible of myri-
apods, and of those crustaceans that lack
a mandibular palp (18, 19). By this crite-
rion, the biting structures of all arthropod
mandibles are gnathobasic in the adult.
The nature of the mandible is therefore
not useful for defining relationships be-
tween these three groups. It is, however, a
character that unites myriapods, insects,
and crustaceans (traditionally termed the
mandibulate arthropods) to the exclusion
of the chelicerates, where all of the limbs
retain distal elements, and distalless
expression.
Ancestral Patterns of Arthropod Segmenta-
tion. The ancestral arthropod has tradi-
tionally been envisaged as an animal with
a large and somewhat ill defined number
*E-mail: m.akam@zoo.cam.ac.uk.
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of similar trunk segments. However, cur-
rent arthropod phylogenies suggest that
we should look again at animals showing
characteristics that may be interpreted as
intermediate between those of arthropods
and onychophorans. Several have been
described from the Cambrian—most re-
cently, Kerygmachela, from the Sirius Pas-
sat fauna of Greenland (20). This animal
has a lobopod-like body, but spiny and
兾
or
segmented appendages at the anterior and
posterior (Fig. 1). Perhaps the first jointed
appendages of arthropods were the anten-
nae and cerci, with trunk appendages de-
rived by the transfer of developmental
programs that first evolved to build these
sensory structures.
These putative intermediate Cambrian
forms have relatively few trunk segments,
often 11 (20). If these lie close to the
arthropod stem lineage, then the ancestral
arthropod may itself have had a relatively
short trunk with a well defined segment
number. This implies a mechanism gener-
ating a specific number of segments, not
an indefinite budding process akin to that
of annelids. The large and variable num-
ber of segments seen in many trilobites,
some crustaceans, and some myriapods
would then be a derived character, not the
ancestral state. Indeed, among centipedes,
the orders with large and variable segment
numbers are derived, not basal (21).
The Ancestral Complement of Arthropod Hox
Genes. Comparison of the Hox gene com-
plements of different phyla, and of
different classes within the arthropods,
suggests that the ancestral Hox cluster of
the arthropods contained 10 linked
genes, corresponding to the 8 canonical
Hox genes of Drosophila and two more
genes—one orthologous to the Hox3
gene of vertebrates, which in insects gave
rise to the zen and bicoid genes, and one
additional central gene that gave rise to
the segmentation gene ftz of Drosophila
and its relatives in other insects (refs. 2
and 22; C. Cook and M.A., unpublished
work).
In chelicerate arthropods, the ftz and
zen related genes, as well as all of the
canonical Hox genes that have been ana-
lyzed, are expressed in restricted domains
along the body axis (10, 23, 24). These
expression patterns presumably reflect a
conserved ancestral role for all of the Hox
genes in the specification of axial position.
It is not yet clear when in arthropod
evolution the Hox3 and ftz-related genes
acquired the new functions in embryonic
patterning seen in higher insects, or lost
their old functions.
Hox Genes, Tagmosis, and Segment Morphol-
ogy. Arthropod bodies are subdivided into
distinct regions comprising arrays of func-
tionally integrated and, to a greater or
lesser extent, morphologically similar seg-
ments, termed tagmata (from the Greek
regiment). Available data support the hy-
pothesis that the abrupt and extensive
changes in segment morphology that char-
acterize the boundaries between tagmata
reflect discontinuities in Hox gene expres-
sion (10, 25–32).
No Hox gene is known to be expressed
in or anterior to the first appendage pair
of any arthropod: i.e., the antennae of
insects (corresponding to the first antenna
of crustacea), or the eponymous cheliceral
segment of chelicerates. Insect antennae
require the absence of Hox gene expres-
sion for normal development, and it is to
this state that appendage development
defaults when Hox genes are deleted (33).
Thus, we may surmise that the character-
istic differences between the first append-
age bearing segment of chelicerates and
mandibulates are independent of Hox
genes, and reflect other differences in the
segment patterning machinery of these
two arthropod groups.
In the prosoma of chelicerates, anterior
Hox genes are expressed in extensively
overlapping patterns (10, 30, 32), resem-
bling more the patterns seen in verte-
brates, and in the thorax and abdomen of
insects, than the well resolved segment
specific patterns observed for anterior
Hox genes in the head of insects and
crustaceans (29, 34). Comparison of Hox
gene expression in the heads of several
insect and crustacean species reveals con-
siderable variation in the precise domains
of Hox gene expression. Abzhanov and
Kaufman (29) suggest that these restricted
patterns have been derived independently
from an ancestral pattern more similar to
that seen in chelicerates, presumably as
the morphology of anterior segments has
become more diversified.
In comparisons between mandibulates
and chelicerates, Hox gene expression is in
general no guide to the form or function of
trunk appendages. For example, ‘‘walking
legs’’ express a quite different suite of Hox
genes in the two groups. This contrasts
markedly with the conser ved relationship
between the expression of some regula-
tory genes and the development of specific
organs [e.g., the pax 6 gene and eyes (35)].
Perhaps this is because what distinguishes
different appendages is not, in general, the
possession of unique cell types, but more
subtle aspects of tissue patterning and
relative growth, the regulation of which
may become linked to new transcription
factors on relatively short evolutionary
time scales. There is one possible excep-
tion to this rule: The most posterior Hox
gene, Abdominal-B, is expressed in the
genitalia in at least some insects, crusta-
ceans, and chelicerates (25, 31).
Although Hox genes cannot in general
be tied to particular morphologies, there
are striking analogies in the way that Hox
genes are used to pattern segments within
the leg bearing tagmata of insects and
spiders. In both cases, a regiment of fun-
damentally similar appendages are more
or less subtly differentiated one from one
another. In insects, all of the thoracic
segments initially express Antennapedia,
which in combination with other region-
specific transcription factors (e.g., teashirt)
appears to specify a thoracic ground state.
The legs are differentiated one from one
Fig. 1. Proposed reconstruction of the Cambrian lobopod, Kerygmachela kierkegaardi, from ref. 20. This
animal had typical onychophoran trunk appendages, but remarkably arthropod-like sensory appendages
at front and back. [Reproduced with permission from the Royal Society of Edinburgh from Transactions
of the Royal Society of Edinburgh: Earth Sciences, volume 89 (1999 for 1998), pp. 249–290.]
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another by the locally modulated expres-
sion of other Hox genes [in this case, Sex
combs reduced (Scr) and Ultrabithorax
(Ubx) (36, 37)]. In the spiders, all of the
leg buds express Deformed, and other an-
terior Hox genes, but in later develop-
ment, the appendages are distinguished by
distinct patterns of Scr expression, which is
expressed only in the more posterior legs
(30, 32). Perhaps we see here convergent
evolution of the role of Hox genes, but
using different members of the gene fam-
ily in the two groups.
Segment Homologies Between Mandibulate
and Chelicerate Arthropods. The serial or-
dering of Hox gene expression along the
body axis is largely conser ved in arthro-
pods, as it is in many other phyla. It is
perhaps more remarkable that, if seg-
ments of insects and chelicerates are sim-
ilarly numbered by counting from the first
appendage-bearing segment backwards,
then the anterior boundaries of expression
for several of the Hox genes lie between
the same pairs of segments—labial be-
tween segments 1 and 2, deformed be-
tween segments 2 and 3, etc. (10, 30, 32,
38) This pattern has led two groups to
propose that the anterior limits of Hox
gene expression are conserved ancestral
characteristics that reflect segment ho-
mologies and, on this basis, to resolve
between two long-standing models for
segment organization in insect and cheli-
cerate heads (10, 32). However, not all are
convinced that Hox gene expression
boundaries can be used as markers for
segment homology (30). Data from the
myriapods, and from other chelicerate
and crustacean groups, are needed to
resolve this question.
It does not seem implausible that the
anterior segments of the common arthro-
pod ancestor already possessed unique
molecular identities, defined by Hox
genes, and that these may have become
fixed, even if they were not reflected in
overt specialization of appendages. They
may have controlled patterns of cell spe-
cialization in the mesoderm or nervous
system, and only subsequently acquired
more extensive roles in the control of
external segment morphology. The acqui-
sition of such new roles has been well
documented for subsequent evolutionary
steps in the insect lineage [e.g., appendage
suppression (39)]. However, I find it hard
to maintain the argument (10) that seg-
ments can be homologized throughout the
trunk by conserved patterns of Hox gene
expression. It is clear that domains of
Ubx
兾
abdA Hox gene expression vary
with respect to ordinal segment number,
even in quite closely related crustacean
groups (26).
Hox Genes and Segment Modification in Crus-
tacea. The crustacea in particular exhibit a
wonderful diversity of segment specializa-
tion and tagmosis. This diversity has three
aspects. One is the diversity of segments in
the adult of a single species. This is at its
most extreme in the Malacostraca, with as
many as 14 clearly distinct segment types.
We do not know in detail how this diver-
sity is controlled, but all of the evidence
suggests that it does not require the pro-
liferation of Hox genes. It is likely that the
required diversity of Hox codes is pro-
vided by increased complexity in the
regulation of a constant set of Hox genes
(40). One case in which it seems that the
number of Hox genes may have changed
is in the cirripedes—but this is a case of
gene loss, not gain. Barnacles appear to
have lost the Hox gene abdominal-A,
concomitant with loss of abdominal seg-
ments (41).
A second aspect of segment diversity is
that which has arisen between species. The
diverse forms that any one segment exhib-
its in different species probably reflect, in
large part, changes downstream of the
Hox genes. However, when it is the orga-
nization of segment types along the body
axis that varies between species, then it
seems more likely that the Hox genes will
be directly involved. Averof and Patel (26)
have examined one such case of segment
diversification—the recruitment of
anterior thoracic segments to generate
auxiliary feeding appendages called max-
illipeds. This has occurred repeatedly in
several crustacean lineages. In each case
tested, this transformation has been ac-
companied by a shift in the limits of
expression of Ubx
兾
abd-A related Hox
genes—from an inferred primitive bound-
ary at the anterior of the first thoracic
segment, to a more posterior segment.
A third aspect of segment diversity, all
too easily forgotten by Drosophila genet-
icists, is the diversity of segment morphol-
ogy during ontogeny. (The appendage
morphology of maggots is not rich!) Indi-
rect developing crustaceans are famous
for their range of larval forms. In these
larvae, the morphology of a single seg-
ment may change dramatically at specific
stages in the life cycle, often associated
with changes in locomotory or feeding
behavior. Perhaps even more remarkably,
tagmosis itself may be altered, with the
pattern of segment similarity shifting be-
tween molts (42).
These striking changes may be achieved
in two ways. The same Hox proteins may
exert differential effects at different
stages in development, perhaps because
hormonal changes modify the combinato-
rial input that controls segment morphol-
ogy. Alternatively, the axial extent of Hox
gene expression may itself change at dif-
ferent stages of development. An exam-
ple of this second mode has recently
been demonstrated in Porcellio, an isopod
crustacean.
In this pillbug (woodlouse) the series of
larval forms has been compressed into a
series of embryonic stages, but some of the
morphological transitions characteristic of
the indirect developing ancestor are still
evident. For example, the first thoracic ap-
pendage develops as a walking appendage,
identical to those of the more posterior
segments until mid embryogenesis, where-
upon it diverges from the pathway of its
thoracic homologues, coming to form a
maxilliped. Abzhanov and Kaufman (43)
show that this transition is associated with a
transition in the pattern of Hox gene expres-
sion—Scr protein is initially repressed in the
first thoracic appendage, but later ex-
pressed. Intriguingly, and exceptionally for
the Hox genes, the early regulation (repres-
sion) of Scr is at the level of translational
control, not transcription.
Conclusions. Evolutionary developmental
studies are mapping the relationships be-
tween gene expression and the diversity of
form within arthropods. We can begin to
propose models for the underlying
changes in developmental mechanisms.
Techniques to manipulate gene expres-
sion in arthropods are developing fast,
promising that the role of individual genes
may soon be tested directly. However, we
should beware of trying to explain too
much, with too little. No one gene fami-
ly—not even the Hox genes—will provide
a sufficient tool to explain the whole of
any major step in evolution.
My thanks to Michalis Averof, Max Telford,
and Chris Klingenberg for comments on the
manuscript. Work in this laboratory has been
supported principally by the Wellcome Trust
and the Biotechnology and Biological Sciences
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