A three-phase model of arthropod segmentation

Molecular and morphological evidence (expression patterns of pair-rule genes and segmental position of the genital openings and other segmental markers) suggest that the segmental units of the arthropod body are specified, in early ontogeny, by three spatially and/or temporally distinct mechanisms and do not appear in a strict antero-posterior sequence. A first anterior set of indivisible segments (naupliar segments, possibly three in all arthropods) is followed by a set of more caudal (post-naupliar) primary units (eosegments, possibly ten in all arthropods) which then undergo a process of secondary segmentation, thus giving rise to a higher number of definitive segments (merosegments). The number of merosegments deriving from each eosegment is characteristic of the different arthropod clades and is mostly stable at the level of the traditional arthropodan classes or subclasses. All their segmentation patterns, however, including those found in the segmental organisation of highly segmented forms (such as centipedes and millipedes, notostracan, lipostracan and anostracan crustaceans, and trilobites) are reducible to the basic groundplan with three naupliar and ten postnaupliar segments. These basic units of arthropod segmentation may also have an equivalent in other Ecdysozoa, despite the lack of any segmentation (nematodes) or, at least, of an overt segmentation (kinorhynchs).     

The evolution of segmentation of centipede trunk and appendages

The segmentation of centipedes is interpreted in the light of a biphasic model of segmentation (holomeric plus meromeric). The mid-body anomaly (e.g. in the alternating short and long terga, or in the sequence of segments with and without spiracles) is regarded as due to an early patterning of the embryo, occurring before the onset of meromeric segmentation and affecting a level within the fourth eosegment of the trunk. Comparisons with the Diplopoda suggest that genital structures such as millipede gonopods did probably develop originally at this spot, whose position remained marked even after the transition from a putatively progoneate to the current opisthogoneate condition of centipedes, perhaps following gene duplication and divergence of expression patterns of the paralogues. A new lower limit for the number of leg-bearing segments [27, in a male specimen of Schendylops oligopus (Pereira, Minelli & Barbieri,1995)] is established for Geophilomorpha. Coevolutionary trends involving the segmentation of the trunk, the segmentation of the appendages (especially the antennae), the postembryonic developmental schedule and the presence or absence of regeneration ability supports a recent view of the appendages as evolutionarily divergent duplicates of the main body axis.     

Larval ontogenetic stages of Chaetopterus: developmental heterochrony in the evolution of chaetopterid polychaetes

Seven post-gastrulation larval stages are described for the sedentary polychaete Chaetopterus. Analysis of larval anatomy and morphology through ontogeny reveals significant differences in the temporal sequence of segmentation, and in the character of segments formed, from the typical embryological pattern described for other polychaete families, such as nereidids or spionids. When compared in alternative phylogenetic schemes, these differences represent significant developmental heterochrony, among other evolutionary transitions, which has arisen in the chaetopterid lineage. The heterochrony is correlated with the extreme morphological regionalization along the anterior-posterior body axis, a feature that is also characteristic of chaetopterids.     

Homology,limbs, and genitalia

SUMMARY Similarities in genetic control between the main body axis and its appendages have been generally explained in terms of genetic co-option. In particular, arthropod and vertebrate appendages have been explained to invoke a common ancestor already provided with patterned body outgrowths or independent recruitment in limb patterning of genes or genetic cassettes originally used for purposes other than axis patterning. An alternative explanation is that body appendages, including genitalia, are evolutionarily divergent duplicates (paramorphs) of the main body axis. However, are all metazoan limbs and genitalia homologous? The concept of body appendages as paramorphs of the main body axis eliminates the requirement for the last common ancestor of limb-bearing animals to have been provided with limbs. Moreover, the possibility for an animal to express complex organs ectopically demonstrates that positional and special homology may be ontogenetically and evolutionarily uncoupled. To assess the homology of animal genitalia, we need to take into account three different sets of mechanisms, all contributing to their positional and/or special homology and respectively involved (1) in the patterning of the main body axis, (2) in axis duplication, followed by limb patterning mechanisms diverging away from those still patterning the main body axis (axis paramorphism), and (3) in controlling the specification of sexual/genital features, which often, but not necessarily, come into play by modifying already developed and patterned body appendages. This analysis demonstrates that a combinatorial approach to homology helps disentangling phylogenetic and ontogenetic layers of homology.     

Conserved versus innovative features in animal body organization

The origin of evolutionary novelties is a central topic in evolutionary developmental biology (evo-devo) studies. In any new feature, there is a conserved component that is either structural or related to the underlying genetic control, but it is not always obvious what is really new and what is conserved. Nevertheless, disentangling this blending of old and new features is basic to understanding mechanisms of evolutionary change. The origin of arthropod appendages illustrates the complexity in tracing the origin of evolutionary novelties. At the base of the lineage, the main body axis was already segmented and antero-posteriorly patterned, and the genetic tool kit required to form lateral outgrowths was already available. The novelty was possibly the developmental decision to "read" the available axial information and to exploit it for differentiating segmentally patterned and axially segmented appendages. Some important novelties bridge the gap between what have been traditionally distinguished as systemic and local changes. For example, the origin of the body cavities evolved by several animal groups may have been initiated by simple changes in cell-to-cell adhesive properties. Any possible change in an existing ontogenetic pathway has the potential to generate novelties.     

Evolution of networks for body plan patterning; interplay of modularity, robustness and evolvability

A major goal of evolutionary developmental biology (evo-devo) is to understand how multicellular body plans of increasing complexity have evolved, and how the corresponding developmental programs are genetically encoded. It has been repeatedly argued that key to the evolution of increased body plan complexity is the modularity of the underlying developmental gene regulatory networks (GRNs). This modularity is considered essential for network robustness and evolvability. In our opinion, these ideas, appealing as they may sound, have not been sufficiently tested. Here we use computer simulations to study the evolution of GRNs' underlying body plan patterning. We select for body plan segmentation and differentiation, as these are considered to be major innovations in metazoan evolution. To allow modular networks to evolve, we independently select for segmentation and differentiation. We study both the occurrence and relation of robustness, evolvability and modularity of evolved networks. Interestingly, we observed two distinct evolutionary strategies to evolve a segmented, differentiated body plan. In the first strategy, first segments and then differentiation domains evolve (SF strategy). In the second scenario segments and domains evolve simultaneously (SS strategy). We demonstrate that under indirect selection for robustness the SF strategy becomes dominant. In addition, as a byproduct of this larger robustness, the SF strategy is also more evolvable. Finally, using a combined functional and architectural approach, we determine network modularity. We find that while SS networks generate segments and domains in an integrated manner, SF networks use largely independent modules to produce segments and domains. Surprisingly, we find that widely used, purely architectural methods for determining network modularity completely fail to establish this higher modularity of SF networks. Finally, we observe that, as a free side effect of evolving segmentation and differentiation in combination, we obtained in-silico developmental mechanisms resembling mechanisms used in vertebrate development.     

Myriapod metamerism and arthropod segmentation

Outstanding progress in understanding segmentation of tracheate arthropods (Atelocerata), i.e. Chilopoda, Diplopoda, Pauropoda, Symphyla and Insecta, has been gained through experimental studies carried out on a single, very derivative organism, i.e. Drosophila. We stress the need for a broader comparative approach. We have studied the segmental structure of the trunk in geophilomorph centipedes, where we can identify morphogenetic units of two, four, eight or 16 segments. Accordingly, we sketch an improved model for arthropod segmentation, with the following initial steps: (a) biochemical marking of a very few repetitive units (eosegments); (b) iterative duplications of this first periodicity, until the embryo acquires an array of biochemical markings matching the whole number of segments of the future larva or juvenile specimen; (c) transpatterning, stabilization and interpretation of this 'segmental' arrangement; (d) possible repatterning, to give a final repetitive pattern we define as metasegmental. Finally, we express some doubt about the homology between annelid and arthropod segmentation.     

Lamprey as an evo-devo model: lessons from comparative embryology and molecular phylogenetics

Lamprey, the living jawless vertebrate, has been regarded as one of the most primitive groups of vertebrates. The evolutionary phylogenetic position of the lamprey promises to provide hints about the origin of the vertebrate genome as well as the origin of the body plan, a part of which may be written in the genome. Since the lamprey split from the gnathostome lineage early in the history of vertebrates, the shared developmental mechanisms in lampreys and gnathostomes can be regarded as possessed by the hypothetical common ancestor of these animals, whereas the gnathostome-specific developmental mechanisms that are absent from lampreys indicate that they are relatively new, added to the developmental program only after the split of gnathostomes. Thus, the sequential establishment of the gnathostome body plan is inherently related to the history of genomic duplication events. In this review, recent molecular developmental and evolutionary molecular research on the living lampreys are summarized and discussed, taking vertebrate comparative morphology and embryology into consideration.     

Segmentation: mono- or polyphyletic?

Understanding the evolutionary origins of segmented body plans in the metazoa has been a long-standing fascination for scientists. Competing hypotheses explaining the presence of distinct segmented taxa range from the suggestion that all segmentation in the metazoa is homologous to the proposal that segmentation arose independently many times, even within an individual clade or species. A major new source of information regarding the extent of homology vs. homoplasy of segmentation in recent years has been an examination of the extent to which molecular mechanisms underlying the segmentation process are conserved, the rationale being that a shared history will be apparent by the presence of common molecular components of a developmental program that give rise to a segmented body plan. There has been substantial progress recently in understanding the molecular mechanisms underlying the segmentation process in many groups, specifically within the three overtly segmented phyla: Annelida, Arthropoda and Chordata. This review will discuss what we currently know about the segmentation process in each group and how our understanding of the development of segmented structures in distinct taxa have influenced the hypotheses explaining the presence of a segmented body plan in the metazoa.     

Larval neurogenesis in Sabellaria alveolata reveals plasticity in polychaete neural patterning

The investigation of neurogenesis in polychaetes not only facilitates insights into the developmental biology of this group, but also provides new data for phylogenetic analyses. This should eventually lead toward a better understanding of metazoan evolution including key issues such as the ontogenetic processes that underlie body segmentation. We here document the development of the larval nervous system in the polychaete Sabellaria alveolata using fluorescence-coupled antibodies directed against serotonin, FMRFamide, and tubulin in combination with confocal laser scanning microscopy and 3D reconstruction software. The overall pattern of neurogenesis in S. alveolata resembles the condition found in other planktonic polychaete trochophores where the larval neural body plan including a serotonergic prototroch nerve ring is directly followed by adult features of the nervous system such as circumesophageal connectives and paired ventral nerve cords. However, distinct features are also found in S. alveolata, such as the innervation of the apical organ with ring-shaped neurons, the low number of immunoreactive perikarya, and the lack of a posterior serotonergic cell. Moreover, in the larvae of S. alveolata, two distinct modes of neuronal development are expressed, viz. the simultaneous formation of the first three segmental neurons of the peripheral nervous system on the one hand versus the sequential appearance of the ventral commissures on the other. This highlights the complex mechanisms that underlie annelid body segmentation and indicates divergent developmental pathways within polychaete annelids that lead to the segmented nervous system of the adult.     

Substitution saturation and nuclear paralogs of commonly employed phylogenetic markers in the Caryophyllidea, an unusual group of non-segmented tapeworms (Platyhelminthes)

Caryophyllidean cestodes (Platyhelminthes) represent an unusual group of tapeworms lacking serially repeated body parts that potentially diverged from the common ancestor of the Eucestoda prior to the evolution of segmentation. Here we evaluate the utility of two nuclear and two mitochondrial molecular markers (ssrDNA and lsrDNA, nad3 and cox1) for use in circumscribing generic boundaries and estimating interrelationships in the group. We show that these commonly employed markers do not contain sufficient signal to infer well-supported phylogenetic estimates due to substitution saturation. Moreover, we detected multiple trnK+nad3+trnS+trnW+cox1 haplotypes within individuals, indicating a history of gene exchange between the mitochondrial and nuclear genomes. The presence of such nuclear paralogs (i.e. numts), to our knowledge described here in cestodes for the first time, together with the results of phylogenetic, saturation and split-decomposition analyses all suggest that finding informative markers for estimating caryophyllidean evolution is unusually problematic in comparison to other major lineages of tapeworms.     

A morphologist's perspective on terminal growth and segmentation

When approaching the study of terminal growth and segmentation, comparative morphology provides an important guide to formulate questions. There are often problems in unambiguously identifying the axis along which we wish to study terminal (often, actually, subterminal) growth, especially when the trunk axis is posteriorly prolonged in an appendage (as with the tail of vertebrates), or when the polarity of the "external animal" is other than the polarity of the "internal animal," as in polypoid bilaterians. We cannot ignore that the rear end of the main body axis is possibly defined very early in development in some groups, for example, arthropods, whereas in others, vertebrates, for example, it is defined much later. We cannot think of segmentation as always corresponding to the sequential posterior addition of new units, thus ignoring the widespread occurrence of double segmentation. A more subtle problem is represented by the overlapping of different processes, all of them contributing to elongating the body, such as segmentation, cell proliferation, cell rearrangement, and cell growth. Within a segmented trunk, cell proliferation and differentiation may go on in parallel from as many growth points as there are groups of regularly spaced cells. The main consequence, however, is not so much to expedite elongation as to reduce the disparity of metabolic conditions, gene expression patterns and "relative age" of different body districts, otherwise possibly troublesome within the limited space of the embryo.     

Phylogenetic position of the Tardigrada based on the 18S ribosomal RNA gene sequences

The phylogenetic position of the Tardigrada remains uncertain. This is due to the limited information available, and the uncertainty of whether some characters are homologous or analogous with other taxa. Based on some morphological characters, current discussion centres on whether the taxon branches from the annelid-arthropod lineage, or lies within the arthropod complex. The molecular data presented here from an analysis of the 18S rRNA gene sequences are used to test the validity of these two hypotheses. Phylogenetic inference by the maximum parsimony and distance (neighbour-joining) methods suggests that the Tardigrada is a sister group of the major protostome eucoelomate assemblage that emerged before the arthropods, annelids, molluscs, and sipunculids evolved. The tardigrade clade also appears as an independent lineage separate from the nematode clade, thus supporting the current idea that tardigrades do not have a close aschelminth relationship. The molecular data also imply that several morphological features, considered significant in determining the phylogenetic relationships of tardigrades, are not synapomorphic characters.     

Arthropod-like expression patterns of engrailed and wingless in the annelid Platynereis dumerilii suggest a role in segment formation

The origin of animal segmentation, the periodic repetition of anatomical structures along the anteroposterior axis, is a long-standing issue that has been recently revived by comparative developmental genetics. In particular, a similar extensive morphological segmentation (or metamerism) is commonly recognized in annelids and arthropods. Mostly based on this supposedly homologous segmentation, these phyla have been united for a long time into the clade Articulata. However, recent phylogenetic analysis dismissed the Articulata and thus challenged the segmentation homology hypothesis. Here, we report the expression patterns of genes orthologous to the arthropod segmentation genes engrailed and wingless in the annelid Platynereis dumerilii. In Platynereis, engrailed and wingless are expressed in continuous ectodermal stripes on either side of the segmental boundary before, during, and after its formation; this expression pattern suggests that these genes are involved in segment formation. The striking similarities of engrailed and wingless expressions in Platynereis and arthropods may be due to evolutionary convergence or common heritage. In agreement with similarities in segment ontogeny and morphological organization in arthropods and annelids, we interpret our results as molecular evidence of a segmented ancestor of protostomes.     

Notch/Delta signalling is not required for segment generation in the basally branching insect Gryllus bimaculatus

Arthropods and vertebrates display a segmental body organisation along all or part of the anterior-posterior axis. Whether this reflects a shared, ancestral developmental genetic mechanism for segmentation is uncertain. In vertebrates, segments are formed sequentially by a segmentation 'clock' of oscillating gene expression involving Notch pathway components. Recent studies in spiders and basal insects have suggested that segmentation in these arthropods also involves Notch-based signalling. These observations have been interpreted as evidence for a shared, ancestral gene network for insect, arthropod and bilaterian segmentation. However, because this pathway can play multiple roles in development, elucidating the specific requirements for Notch signalling is important for understanding the ancestry of segmentation. Here we show that Delta, a ligand of the Notch pathway, is not required for segment formation in the cricket Gryllus bimaculatus, which retains ancestral characteristics of arthropod embryogenesis. Segment patterning genes are expressed before Delta in abdominal segments, and Delta expression does not oscillate in the pre-segmental region or in formed segments. Instead, Delta is required for neuroectoderm and mesectoderm formation; embryos missing these tissues are developmentally delayed and show defects in segment morphology but normal segment number. Thus, what initially appear to be 'segmentation phenotypes' can in fact be due to developmental delays and cell specification errors. Our data do not support an essential or ancestral role of Notch signalling in segment generation across the arthropods, and show that the pleiotropy of the Notch pathway can confound speculation on possible segmentation mechanisms in the last common bilaterian ancestor.     

On the larval development of<Emphasis Type="Italic"> Eubranchipus grubii</Emphasis> (Crustacea,Branchiopoda, Anostraca), with notes on the basal phylogeny of the Branchiopoda

Selected larval stages of Eubranchipus grubii (Anostraca) from Danish temporary waters are examined by scanning electron microscopy in a phylogenetic context. The study focuses on limb development and body segmentation. It is shown that the large, proximal endite of the trunk limbs in the adult Anostraca is actually a fusion product of two smaller endites which make their appearance in the early larval development. This gives a total of six endites along the inner margin of the trunk limbs. An unsegmented endopod follows more distally. A small additional, seventh endite makes a short appearance in late larvae, but has disappeared in the adults. The naupliar feeding apparatus is of the same type as found in other branchiopods, and has previously been suggested as an autapomorphy for the Branchiopoda. The similarities between the naupliar feeding apparatus of E. grubii and other branchiopods include the presence of a long protopod with a characteristic morphology of the coxal and basipodal masticatory spines/setae, and a three-segmented mandibular palp (basipod and two endopod segments) with a largely similar setation in all taxa. The mode of trunk limb development is also the same as seen in most other recent branchiopods. The phylogenetic significance for the basal phylogeny of the Branchiopoda of these and other morphological features is discussed in relation to the phylogenetic position of two branchiopod fossils, Lepidocaris rhyniensis and Rehbachiella kinnekullensis. While R. kinnekullensis has previously been suggested to be a stem lineage branchiopod, the position of L. rhyniensis is more uncertain. Three different possible phylogenetic positions of L. rhyniensis are discussed: (a) L. rhyniensis as a stem lineage anostracan, (b) L. rhyniensis as a stem lineage branchiopod or (c) L. rhyniensis as a stem lineage phyllopod. It seems most plausible to consider L. rhyniensis a stem lineage anostracan.     

Origins of anteroposterior patterning and Hox gene regulation during chordate evolution

All chordates share a basic body plan and many common features of early development. Anteroposterior (AP) regions of the vertebrate neural tube are specified by a combinatorial pattern of Hox gene expression that is conserved in urochordates and cephalochordates. Another primitive feature of Hox gene regulation in all chordates is a sensitivity to retinoic acid during embryogenesis, and recent developmental genetic studies have demonstrated the essential role for retinoid signalling in vertebrates. Two AP regions develop within the chordate neural tube during gastrulation: an anterior 'forebrain-midbrain' region specified by Otx genes and a posterior 'hindbrain-spinal cord' region specified by Hox genes. A third, intermediate region corresponding to the midbrain or midbrain-hindbrain boundary develops at around the same time in vertebrates, and comparative data suggest that this was also present in the chordate ancestor. Within the anterior part of the Hox-expressing domain, however, vertebrates appear to have evolved unique roles for segmentation genes, such as Krox-20, in patterning the hindbrain. Genetic approaches in mammals and zebrafish, coupled with molecular phylogenetic studies in ascidians, amphioxus and lampreys, promise to reveal how the complex mechanisms that specify the vertebrate body plan may have arisen from a relatively simple set of ancestral developmental components.     

An interspecific test of Bergmann's rule reveals inconsistent body size patterns across several lineages of water beetles (Coleoptera: Dytiscidae)

1. Bergmann's rule sensu lato, the ecogeographic pattern relating animals' body size with environmental temperature (or latitude), has been shown to be inconsistent among insect taxa. Body size clines remain largely unexplored in aquatic insects, which may show contrasting patterns to those found in terrestrial groups because of the physiological or mechanical constraints of the aquatic environment. 2. Bergmann's rule was tested using data on body size, phylogeny and distribution for 93 species belonging to four lineages of dytiscid water beetles. The relationship between size and latitude was explored at two taxonomic resolutions – within each independent lineage, and for the whole dataset – employing phylogenetic generalised least‐squares to control for phylogenetic inertia. The potential influence of habitat preference (lotic versus lentic) on body size clines was also considered. 3. Within‐lineage analyses showed negative relationships (i.e. converse Bergmann's rule), but only in two lineages (specifically in those that included both lotic and lentic species). By contrast, no relationship was found between body size and latitude for the whole dataset. 4. These results suggest that there may be no universal interspecific trends in latitudinal variation of body size in aquatic insects, even among closely related groups, and show the need to account for phylogenetic inertia. Furthermore, habitat preferences should be considered when exploring latitudinal clines in body size in aquatic taxa at the interspecific level.