Trunk segment numbers and sequential segmentation in myriapods

Appearance of a transparent protrusion containing two pairs of legs on the apodous ring preceding the anamorphic molt in a millipede, Niponia nodulosa

BACKGROUND

Arthropods gradually change their forms through repeated molting events during postembryonic development. Anamorphosis, i.e., segment addition during postembryonic development, is seen in some arthropod lineages. In all millipede species (Myriapoda, Diplopoda), for example, postembryonic processes go through anamorphosis. Jean-Henri Fabre proposed 168 years ago the "law of anamorphosis", that is, "new rings appear between the penultimate ring and the telson" and "all apodous rings in a given stadium become podous rings in the next stadium", but the developmental process at the anamorphic molt remains largely unknown. In this study, therefore, by observing the morphological and histological changes at the time of molting, the detailed processes of leg- and ring-addition during anamorphosis were characterized in a millipede, Niponia nodulosa (Polydesmida, Cryptodesmidae).

RESULTS

In the preparatory period, a few days before molting, scanning electron microscopy, confocal laser scanning microscopy, and histological observations revealed that two pairs of wrinkled leg primordia were present under the cuticle of each apodous ring. In the rigidation period, just prior to molt, observations of external morphology showed that a transparent protrusion was observed on the median line of the ventral surface on each apodous ring. Confocal laser scanning microscopy and histological observations revealed that the transparent protrusion covered by an arthrodial membrane contained a leg bundle consisting of two pairs of legs. On the other hand, ring primordia were observed anterior to the telson just before molts.

CONCLUSIONS

Preceding the anamorphic molt in which two pairs of legs are added on an apodous ring, a transparent protrusion containing the leg pairs (a leg bundle) appears on each apodous ring. The morphogenetic process of the rapid protrusion of leg bundles, that is enabled by thin and elastic cuticle, suggested that millipedes have acquired a resting period and unique morphogenesis to efficiently add new legs and rings.

Two-way exchanges between animal and plant biology, with focus on evo-devo

By definition, biology is the science of all living beings. However, horizons restricted to either plants or animals have characterized the development of life sciences well beyond the emergence of unified perspectives applying to all forms of life, such as the cell theory or the theory of evolution. Separation between botanical and zoological traditions is not destined to go extinct easily, or shortly. Disciplinary isolation is emphasized by institutional contexts such as scientific societies and their congresses, specialist journals, disciplines recognized as teaching subjects and legitimate and fundable research fields. By shaping the personal agendas of individual scientists, this has a strong impact on the development of biology. In some fields, botanical and zoological contributions have long being effectively intertwined, but in many others plant and animal biology have failed to progress beyond a marginal dialogue. Characteristically, the so-called “general biology” and the philosophy of biology are still zoocentric (and often vertebrato- or even anthropocentric). In this article, I discuss legitimacy and fruitfulness of some old lexical and conceptual exchanges between the two traditions (cell, tissue, and embryo). Finally, moving to recent developments, I compare the contributions of plant vs. animal biology to the establishment of evolutionary developmental biology. We cannot expect that stronger integration between the different strands of life sciences will soon emerge by self-organization, but highlighting this persisting imbalance between plant and animal biology will arguably foster progress.

Extreme multisegmentation in a giant bivalved arthropod from the Cambrian Burgess Shale

The origin of mandibulate arthropods can be traced back to the Cambrian period to several carapace-bearing arthropod groups, but their morphological diversity is still not well characterized. Here, we describe Balhuticaris voltae, a bivalved arthropod from the 506-million-year-old Burgess Shale (Marble Canyon, British Columbia, Canada). This species has an extremely elongated and multisegmented body bearing ca. 110 pairs of homonomous biramous limbs, the highest number among Cambrian arthropods, and, at 245 mm, it represents one of the largest Cambrian arthropods known. Its unusual carapace resembles an arch; it covers only the frontalmost section of the body but extends ventrally beyond the legs. Balhuticaris had a complex sensory system and was probably an active swimmer thanks to its powerful paddle-shaped exopods and a long and flexible body. Balhuticaris increases the ecological and functional diversity of bivalved arthropods and suggests that cases of gigantism occurred in more arthropod groups than previously recognized.

Damaged Dickinsonia specimens provide clues to Ediacaran vendobiont biology

Recently reported specimens of the enigmatic Ediacaran fossil Dickinsonia from Russia show damage and repair that provides evidence of how they grew, and of their biological affinities. Marginal and terminal areas of wilting deformation are necrotic zones separating regenerated growth, sometimes on two divergent axes, rather than a single axis. Necrotic zones of damage to Dickinsonia are not a thick scar or callus, like a wound or amputation. Nor are they smooth transitions to a regenerated tail or arm. The wilted necrotic zone is most like damage by freezing, salt, or sunburn of leaves and lichens, compatible with evidence of terrestrial habitat from associated frigid and gypsic paleosols. Dickinsonia did not regrow by postembryonic addition of modules from a subterminal or patterned growth zone as in earthworms, myriapods, trilobites, crustaceans, and lizards. Rather Dickinsonia postembryonic regrowth from sublethal damage was from microscopic apical and lateral meristems, as in plants and lichens. Considered as fungal, Dickinsonia, and perhaps others of Class Vendobionta, were more likely Glomeromycota or Mucoromycotina, rather than Ascomycota or Basidiomycota.

Developmental dynamics is revealed in the early Cambrian arthropod Chuandianella ovata

Segmentation and tagmatization have contributed to the preeminent success of arthropods since their first appearance in the Cambrian. However, the exact mechanism of segmentogenesis is still insufficiently known in living and extinct groups. Here, we describe the postembryonic development of a Waptiid arthropod Chuandianella ovata from the early Cambrian Chengjiang biota, South China. The new data illuminate a complex dynamic pattern of anamorphosis and epimorphosis, and a three-step process of segmentogenesis, i.e., the elongation of the terminal segment, delineation of an incipient segment, and full separation of a new segment. Compensatory growth is accomplished by rapid growth of new segments and/or generation of additional segments, which results in the trimorphism of the posterior tagma. Such complex developmental dynamics has rarely been known in the arthropod fossil record and its presence in early history helps to understand the rapid diversification of arthropods in the early Cambrian.

There and Back Again: Hox Clusters Use Both DNA Strands

Bilaterian animals operate the clusters of Hox genes through a rich repertoire of diverse mechanisms. In this review, we will summarize and analyze the accumulated data concerning long non-coding RNAs (lncRNAs) that are transcribed from sense (coding) DNA strands of Hox clusters. It was shown that antisense regulatory RNAs control the work of Hox genes in cis and trans, participate in the establishment and maintenance of the epigenetic code of Hox loci, and can even serve as a source of regulatory peptides that switch cellular energetic metabolism. Moreover, these molecules can be considered as a force that consolidates the cluster into a single whole. We will discuss the examples of antisense transcription of Hox genes in well-studied systems (cell cultures, morphogenesis of vertebrates) and bear upon some interesting examples of antisense Hox RNAs in non-model Protostomia.

Intraspecific variation in the Cambrian: new observations on the morphology of the Chengjiang euarthropod Sinoburius lunaris

Background

The Chengjiang biota from southwest China (518-million-years old, early Cambrian) has yielded nearly 300 species, of which more than 80 species represent early chelicerates, crustaceans and relatives. The application of µCT-techniques combined with 3D software (e.g., Drishti), has been shown to be a powerful tool in revealing and analyzing 3D features of the Chengjiang euarthropods. In order to address several open questions that remained from previous studies on the morphology of the xandarellid euarthropod Sinoburius lunaris, we reinvestigated the µCT data with Amira to obtain a different approach of visualization and to generate new volume-rendered models. Furthermore, we used Blender to design 3D models showing aspects of intraspecific variation.

Results

New findings are: (1) antennulae consist of additional proximal articles that have not been detected before; (2) compared to other appendages, the second post-antennular appendage has a unique shape, and its endopod is comprised of only five articles (instead of seven); (3) the pygidium bears four pairs of appendages which are observed in all specimens. On the other hand, differences between specimens also have been detected. These include the presence/absence of diplotergites resulting in different numbers of post-antennular appendages and tergites and different distances between the tip of the hypostome and the anterior margin of the head shield.

Conclusions

Those new observations reveal intraspecific variation among Chengjiang euarthropods not observed before and encourage considerations about possible sexual dimorphic pairs or ontogenetic stages. Sinoburius lunaris is a variable species with respect to its morphological characters, cautioning that taxon-specific variabilities need to be considered when exploring new species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01854-1.

The Development of Arthropod Segmentation Across the Embryonic/Post-embryonic Divide – An Evolutionary Perspective

In many arthropods, the appearance of new segments and their differentiation are not completed by the end of embryogenesis but continue, in different form and degree, well after hatching, in some cases up to the last post-embryonic molt. Focusing on the segmentation process currently described as post-embryonic segment addition (or, anamorphosis), we revise here the current knowledge and discuss it in an evolutionary framework which involves data from fossils, comparative morphology of extant taxa and gene expression. We advise that for a better understanding of the developmental changes underlying the evolution of arthropod segmentation, some key concepts should be applied in a critical way. These include the notion of the segment as a body block and the idea that hatching represents a well-defined divide, shared by all arthropods, between two contrasting developmental phases, embryonic vs. post-embryonic. This eventually reveals the complexity of the developmental processes occurring across hatching, which can evolve in different directions and with a different pace, creating the observed vagueness of the embryonic/post-embryonic divide.

Life after the mother's hug: Late post-embryonic development of Cryptops parisi (Chilopoda: Scolopendromorpha: Cryptopidae)

Here we describe in detail the late post-embryonic development of the common European scolopendromorph centipede Cryptops parisi. Canonical variate analyses of two groups of external morphological characters, viz., cephalic capsule characters (head length, length of the anterior and posterior paramedian cephalic sutures) and coxopleuron surface characters (number of pores in the coxal pore-field, number of setae on the posterior coxopleuron edge, their number on the coxal pore-field, and their number posterior to the coxal pore-field) were conducted on a large sample of specimens collected from two localities in Serbia. Ten free-living stages are recognized: three pre-adult stages (adolescens I, II, and III) and seven adult stages (one maturus junior stage, four maturus, and two maturus senior stages). The fourth late post-embryonic stage is the first mature stage in both sexes. Sexual dimorphism in the aforementioned characters was not observed. Morphological variation of coxopleuron characters was more informative for the discrimination of developmental stages in Cryptops than the morphological variation of cephalic capsule characters.

Development and evolution of segmentation assessed by geometric morphometrics: The centipede Strigamia maritima as a case study

Using the centipede model species Strigamia maritima as a subject of study, we illustrate the potential of geometric morphometrics for investigating the development and evolution of segmentation, with a specific focus on post-embryonic segmental patterning. We show how these techniques can contribute detailed descriptive data for comparative purposes, but also precious information on some features of the developmental system that are considered relevant for the evolvability of a segmented body architecture, such as developmental stability and canalization. Morphometric analyses allow to separately investigate several sources of phenotypic variation along a segmented body axis, like constitutive and random segment heteronomy, both within and among individuals. Specifically, in S. maritima, the segmental pattern of ventral sclerite shapes mirrors that of their bilateral fluctuating asymmetry and among-individual variation in associating the most anterior and most posterior segments in diverging from the central ones. Also, among segments, there seems to be a correlation between fluctuating asymmetry and shape variation among individuals, suggesting that canalization and developmental stability are somehow associated. Overall, these associations might stem from a joint influence of the segmental position on the two processes of developmental buffering.

Linking gene regulation to cell behaviors in the posterior growth zone of sequentially segmenting arthropods

Virtually all arthropods all arthropods add their body segments sequentially, one by one in an anterior to posterior progression. That process requires not only segment specification but typically growth and elongation. Here we review the functions of some of the key genes that regulate segmentation: Wnt, caudal, Notch pathway, and pair-rule genes, and discuss what can be inferred about their evolution. We focus on how these regulatory factors are integrated with growth and elongation and discuss the importance and challenges of baseline measures of growth and elongation. We emphasize a perspective that integrates the genetic regulation of segment patterning with the cellular mechanisms of growth and elongation.

How reversible is development? Contrast between developmentally plastic gain and loss of segments in barnacle feeding legs

Segmented organisms and structures have fascinated biologists since William Bateson first described homeotic transformation and recognized the fundamental evolutionary significance of segmental organization. On evolutionary time scales, segments may be lost or gained during major morphological transitions. But how segment loss compares to gain on developmental time scales remains mysterious. Here, we examine the ease of reverse development (opposite to normal growth) by comparing developmentally plastic leg segment loss versus gain in individual barnacles transplanted between different water flow conditions. Plastic segment addition occurred rapidly (one to two molts) and exclusively near the limb base. In contrast, developmentally plastic segment loss-the first observation in any arthropod-took much longer (>10 molts) and, remarkably, occurred throughout the leg (23% of losses occurred mid-limb). Segment loss was not a simple reversal of segment addition. Intersegmental membranes fused first, followed by elimination of duplicate tendons and gradual shortening (but not loss) of duplicate setae. Setal loss, in particular, may impose a severe developmental constraint on arthropod segment fusion. This asymmetric developmental potential (time lag of phenotypic response)-plastic segment addition (amplified normal development) is faster and more orderly than segment loss (reverse development)-adds a new dimension to models of developmental plasticity because the cost of making a developmental mistake in one direction will be greater than in the other.

Population genetic structure of a centipede species with high levels of developmental instability

European populations of the geophilomorph centipede Haplophilus subterraneus show a high proportion of individuals with morphological anomalies, suggesting high levels of developmental instability. The broad geographic distribution of this phenomenon seems to exclude local environmental causes, but the source of instability is still to be identified. The goal of the present study was to collect quantitative data on the occurrence of phenodeviants in different populations, along with data on the patterns of genetic variation within and between populations, in order to investigate possible association between developmental instability and genetic features. In a sample of 11 populations of H. subterraneus, distributed in western and central Europe, we looked for phenodeviants, in particular with respect to trunk morphology, and studied genetic variation through the genotyping of microsatellite loci. Overall, no support was found to the idea that developmental instability in H. subterraneus is related to a specific patterns of genetic variation, including inbreeding estimates. We identified a major genetic partition that subdivides French populations from the others, and a low divergence among northwestern areas, which are possibly related to the post-glacial recolonization from southern refugia and/or to recent anthropogenic soil displacements. A weak correlation between individual number of leg bearing segments and the occurrence of trunk anomalies seems to support a trade-off between these two developmental traits. These results, complemented by preliminary data on developmental stability in two related species, suggest that the phenomenon has not a simple taxonomic distribution, while it exhibits an apparent localization in central and eastern Europe.

Cell size versus body size in geophilomorph centipedes

Variation in animal body size is the result of a complex interplay between variation in cell number and cell size, but the latter has seldom been considered in wide-ranging comparative studies, although distinct patterns of variation have been described in the evolution of different lineages. We investigated the correlation between epidermal cell size and body size in a sample of 29 geophilomorph centipede species, representative of a wide range of body sizes, from 6 mm dwarf species to gigantic species more than 200 mm long, exploiting the marks of epidermal cells on the overlying cuticle in the form of micro-sculptures called scutes. We found conspicuous and significant variation in average scute area, both between suprageneric taxa and between genera, while the within-species range of variation is comparatively small. This supports the view that the average epidermal cell size is to some extent taxon specific. However, regression analyses show that neither body size nor the number of leg-bearing segments explain this variation, which suggests that cell size is not an usual target of change for body size evolution in this group of arthropods, although there is evidence of its correlation with other morphological variables, like cuticle thickness. Scute sizes of miniaturized geophilomorph species are well within the range of the lineage to which the species belong, suggesting recent evolutionary transitions to smaller body size.

Molecular phylogeny of Myriapoda provides insights into evolutionary patterns of the mode in post-embryonic development

Myriapoda, a subphylum of Arthropoda, comprises four classes, Chilopoda, Diplopoda, Pauropoda, and Symphyla. While recent molecular evidence has shown that Myriapoda is monophyletic, the internal phylogeny, which is pivotal for understanding the evolutionary history of myriapods, remains unresolved. Here we report the results of phylogenetic analyses and estimations of divergence time and ancestral state of myriapods. Phylogenetic analyses were performed based on three nuclear protein-coding genes determined from 19 myriapods representing the four classes (17 orders) and 11 outgroup species. The results revealed that Symphyla whose phylogenetic position has long been debated is the sister lineage to all other myriapods, and that the interordinal relationships within classes were consistent with traditional classifications. Ancestral state estimation based on the tree topology suggests that myriapods evolved from an ancestral state that was characterized by a hemianamorphic mode of post-embryonic development and had a relatively low number of body segments and legs.

Cloning and expression pattern of the ecdysone receptor and retinoid X receptor from the centipede Lithobius peregrinus (Chilopoda, Lithobiomorpha)

In arthropods, molting events are mediated by the binding of the ecdysone hormone to a heterodimer of two nuclear receptors: the ecdysone receptor (EcR) and the retinoid X receptor (RXR), a homolog of ultraspiracle (USP). We have cloned partial sequences of several isoforms for EcR and RXR genes from the centipede Lithobius peregrinus, and studied their expression profile during the second post-embryonic stage. LpEcR and LpRXR inferred amino acid sequences are very similar to other arthropod orthologs, especially to those of chelicerates and hemimetabolous insects, and their expression levels are significantly higher during the 48 h that precede the molt. Results obtained in this study represent the first data on the genetic basis of the ecdysone signal pathway for a myriapod, and in particular for an animal that, through a stereotyped developmental schedule paced by the molt cycle, completes trunk segmentation during post-embryonic life.

Structural aspects of leg-to-gonopod metamorphosis in male helminthomorph millipedes (Diplopoda)

Background

In the adult males of helminthomorph millipedes, one or two pairs of legs in the anterior part of the trunk are strongly modified into sexual appendages (gonopods) used for sperm transfer during the copula. Gonopods differentiate in an advanced phase of post-embryonic development, in most cases as replacement for the walking legs of the seventh trunk ring, as these first regress to tiny primordia, to eventually develop into gonopods at a subsequent stadium. These extremely localized but dramatic changes have been described as a non-systemic metamorphosis. In the present study we describe morphological and anatomical changes of trunk ring VII associated with non-systemic metamorphosis in four helminthomorph species.

Results

As documented here for the first time by means of traditional histology methods and new techniques based on confocal laser scanning microscopy, the external modifications caused by non-systemic metamorphosis are associated to a huge rearrangement of internal anatomy, mostly due to the development of gonopod apodemes and extrinsic muscles.

Conclusions

Internal changes in the seventh trunk ring, locally leading to the dorsal displacement of the ventral nerve cord and the digestive tract, are modulated in a taxon-specific manner, and are very conspicuous in the blaniulids Nopoiulus kochii and Blaniulus guttulatus, with likely major functional consequences.

Gene expression during postembryonic segmentation in the centipede Lithobius peregrinus (Chilopoda, Lithobiomorpha)

Postembryonic segmentation (anamorphosis) is widespread among arthropods, but only partially known as for its developmental mechanics and control. Studies on developmental genetics of segmentation in anamorphic arthropods are mostly limited to the germ band stage, during early phases of embryonic development. This work presents the first data on the postembryonic expression of a segmentation gene in a myriapod. Using real-time PCR, we analyzed engrailed expression patterns during the anamorphic stages of the centipede Lithobius peregrinus. A variation pattern in en RNA level during anamorphosis suggests that gene expression is precisely modulated during this period of development and that engrailed is mainly expressed in the posterior part of the body, in the newly differentiating segments of each stage. As anamorphosis is possibly the primitive segmentation mode in arthropods, the postembryonic en expression pattern documented here provides evidence for a conservation of en role in ontogeny, across the embryonic/postembryonic boundary, as well as in phylogeny, across the same boundary, but in the opposite direction, from primitive postembryonic expression to the more derived expression in clades with exclusively embryonic segmentation.

Multi-scale relationships between numbers and size in the evolution of arthropod body features

Size-related changes of form in animals with periodically patterned body axes and post-embryonic growth discontinuously obtained throughout a series of moulting episodes cannot be accounted for by allometry alone. We address here the relationships between body size and number and size of appropriately selected structural units (e.g., segments), which may more or less closely approximate independent developmental units, or unitary targets of selection, or both. Distinguishing between units fundamentally involving one cell only or a small and fixed number of cells (e.g., the ommatidia in a compound eye), and units made of an indeterminate number of cells (e.g., trunk segments), we analyze and discuss a selection of body features of either kind, both in ontogeny and in phylogeny, through a review of current literature and meta-analyses of published and unpublished data. While size/number relationships are too diverse to allow easy generalizations, they provide conspicuous examples of the complex interplay of selective forces and developmental constraints that characterizes the evolution of arthropod body patterning.

Trunk anomalies in the centipede Stigmatogaster subterranea provide insight into late-embryonic segmentation

We describe and analyze naturally occurring anomalies in the segmental structures of the trunk in an isolated population of the geophilomorph centipede Stigmatogaster subterranea. Recorded anomalies include mispaired tergites, shrunk segments, variously deformed sclerites, bifurcated trunk, and defects of spiracles and sternal pore areas. One specimen has a perfect segmentally patterned trunk, but with an even number of leg-bearing segments, representing the first record of such a phenotype in adult centipedes. We interpret these anomalies as the effects of perturbation of specific morphogenetic processes in trunk segmentation, occurring at different embryonic stages. The variety of segmental anomalies found in this population provides insights into the developmental process of segmentation and its evolution in geophilomorph centipedes. Variation in dorsal mispairing anomalies demonstrates that segments, as traditionally defined in arthropod morphology, are not the effective developmental units throughout embryogenesis.

Trunk segmentation of Cambrian eodiscoid trilobites

Here we report, from Cambrian Series 2 of Chongqing, southern China, on some three-dimensionally phosphatized exoskeletons representing a series of instars of a specialized eodiscoid trilobite Badiscus spinosus gen. et sp. nov. The preservation is unusual and its protaspides (the youngest juveniles) belonging to different growth stages markedly exhibit some "embryonic trunk segments," which imply the specification of all body segments during embryogenesis. This previously unknown style of tagmosis, as being inconsistent with the conventional view of sequential segmentation during postembryonic anamorphosis, emphasizes the complexity of trilobite trunk segmentation during ontogeny and indicates for the first time that epimorphic development, like other known patterns during postembryonic development, may have already been adopted by some of the earliest trilobites.

Segmental mismatch in crustacean appendages: the naupliar antennal exopod of Artemia (Crustacea, Branchiopoda, Anostraca)

Based on traditional techniques and confocal laser scanning microscopy for external morphology, and immunohistochemistry for the muscular system, we describe here the segmental features of the antennal exopod of Artemia nauplii. Two kinds of serial elements are present, i.e. setae (with cuticular folds at their base) and ringlets (serially arranged sclerites separated by joint-like cuticular folds not extending to form complete rings around the appendage). The two series are usually not in register. The cuticular folds of the setae and of the ringlets are also sites of intermediate insertions of the three exopod muscles: as the two tegumentary structures are discordant in periodicity, this is also mirrored in the pattern of muscle insertions on the two sides of the appendage. Similar cases of segmental mismatch are known for the trunk of several arthropods, but segmental mismatch along the appendages has received very little attention. The occurrence of segmental mismatch in the naupliar appendages of both extant and fossil crustaceans is reviewed and it is suggested here to be a primitive feature of the exopods of both second antennae and mandibles. Problems in the interpretation of morphological evidence are discussed, also in relation to development and evolution of segmentation of naupliar appendages.

From embryo to adult—beyond the conventional periodization of arthropod development

The traditional framework for the description of arthropod development takes the molt-to-molt interval as the fundamental unit of periodization, which is similar to the morphological picture of the main body axis as a series of segments. Developmental time is described as the subdivision into a few major stages of one or more instars each, which is similar to the subdivision of the main body axis into regions of one to many segments each. Parallel to recent criticisms to the segment as the fundamental building block of arthropod anatomy, we argue that, while a firm subdivision of development in stages is useful for describing arthropod ontogeny, this is limiting as a starting point for studying its evolution. Evolutionary change affects the association between different developmental processes, some of which are continuous in time whereas others are linked to the molting cycle. Events occurring but once in life (hatching; first achieving sexual maturity) are traditionally used to establish boundaries between major units of arthropod developmental time, but these boundaries are quite labile. The presence of embryonic molts, the 'gray zone' of development accompanying hatching (with the frequent delivery of an immature whose qualification as 'free-embryo' or ordinary postembryonic stage is arbitrary), and the frequent decoupling of growth and molting suggest a different view. Beyond the simple comparison of developmental schedules in terms of heterochrony, the flexible canvas we suggest for the analysis of arthropod development opens new vistas into its evolution. Examples are provided as to the origin of holometaboly and hypermetaboly within the insects.