Cell lineage analysis of the mandibular segment of the amphipod Orchestia cavimana reveals that the crustacean paragnaths are sternal outgrowths and not limbs

The Cambrian Odaraia alata and the colonization of nektonic suspension-feeding niches by early mandibulates

The diversity of cephalic morphologies in mandibulates (myriapods and pancrustaceans) was key to their evolutionary success. A group of Cambrian bivalved arthropods called hymenocarines exhibit diagnostic mandibulate traits that illustrate this diversity, but many forms are still poorly known. These include the odaraiids, typified by Odaraia alata from the Burgess Shale (Wuliuan), characterized by its unique tubular carapace and rudder-like tail fan, and one of the largest Cambrian euarthropods at nearly 20 cm in length. Unfortunately, odaraiid cephalic anatomy has been largely unknown, limiting evolutionary scenarios and putting their mandibulate affinities into question. Here, we reinvestigate Odaraia based on new specimens from the Burgess Shale and describe exquisitely preserved mandibles with teeth and adjacent structures: a hypostome, maxillae and potential paragnaths. These structures can be homologized with those of Cambrian fuxianhuiids and extant mandibulates, and suggest that the ancestral mandibulate head could have had a limbless segment but retained its plasticity, allowing for limb re-expression within Pancrustacea. Furthermore, we show the presence of limbs with spinose endites which created a suspension-feeding structure. This discovery provides morphological evidence for suspension feeding among large Cambrian euarthropods and evinces the increasing exploitation of planktonic resources in Cambrian pelagic food webs.

Aquatic stem group myriapods close a gap between molecular divergence dates and the terrestrial fossil record

Identifying marine or freshwater fossils that belong to the stem groups of the major terrestrial arthropod radiations is a longstanding challenge. Molecular dating and fossils of their pancrustacean sister group predict that myriapods originated in the Cambrian, much earlier than their oldest known fossils, but uncertainty about stem group Myriapoda confounds efforts to resolve the timing of the group's terrestrialization. Among a small set of candidates for membership in the stem group of Myriapoda, the Cambrian to Triassic euthycarcinoids have repeatedly been singled out. The only known Devonian euthycarcinoid, Heterocrania rhyniensis from the Rhynie and Windyfield cherts hot spring complex in Scotland, reveals details of head structures that constrain the evolutionary position of euthycarcinoids. The head capsule houses an anterior cuticular tentorium, a feature uniquely shared by myriapods and hexapods. Confocal microscopy recovers myriapod-like characters of the preoral chamber, such as a prominent hypopharynx supported by tentorial bars and superlinguae between the mandibles and hypopharynx, reinforcing an alliance between euthycarcinoids and myriapods recovered in recent phylogenetic analysis. The Cambrian occurrence of the earliest euthycarcinoids supplies the oldest compelling evidence for an aquatic stem group for either Myriapoda or Hexapoda, previously a lacuna in the body fossil record of these otherwise terrestrial lineages until the Silurian and Devonian, respectively. The trace fossil record of euthycarcinoids in the Cambrian and Ordovician reveals amphibious locomotion in tidal environments and fills a gap between molecular estimates for myriapod origins in the Cambrian and a post-Ordovician crown group fossil record.

Functional morphology of parasitic isopods: understanding morphological adaptations of attachment and feeding structures in Nerocila as a pre-requisite for reconstructing the evolution of Cymothoidae

Parasites significantly influence food webs and ecosystems and occur all over the world in almost every animal group. Within crustaceans there are numerous examples of ectoparasites; for example, representatives of the isopod group Cymothoidae. These obligatory parasitic isopods are relatively poorly studied regarding their functional morphology. Here we present new details of the morphological adaptations to parasitism of the cymothoiid ingroup Nerocila with up-to-date imaging methods (macro photography, stereo imaging, fluorescence photography, micro CT, and histology). Central aspects of the study were (1) the morphology of the mouthparts and (2) the attachment on the host, hence the morphology of the thoracopods. The mouthparts (labrum, mandibles, paragnaths, maxillulae, maxillae, maxillipeds) form a distinct mouth cone and are most likely used for true sucking. The mouthparts are tightly “folded” around each other and provide functional rails for the only two moving mouthparts, mandible and maxillula. Both are not moving in an ancestral-type median-lateral movement, but are strongly tilted to move more in a proximal-distal axis. New details concerning the attachment demonstrate that the angular arrangement of the thoracopods is differentiated to impede removal by the host. The increased understanding of morphological adaptation to parasitism of modern forms will be useful in identifying disarticulated (not attached to the host) fossil parasites.

Comprehensive analysis of Hox gene expression in the amphipod crustacean Parhyale hawaiensis

Hox genes play crucial roles in establishing regional identity along the anterior-posterior axis in bilaterian animals, and have been implicated in generating morphological diversity throughout evolution. Here we report the identification, expression, and initial genomic characterization of the complete set of Hox genes from the amphipod crustacean Parhyale hawaiensis. Parhyale is an emerging model system that is amenable to experimental manipulations and evolutionary comparisons among the arthropods. Our analyses indicate that the Parhyale genome contains a single copy of each canonical Hox gene with the exception of fushi tarazu, and preliminary mapping suggests that at least some of these genes are clustered together in the genome. With few exceptions, Parhyale Hox genes exhibit both temporal and spatial colinearity, and expression boundaries correlate with morphological differences between segments and their associated appendages. This work represents the most comprehensive analysis of Hox gene expression in a crustacean to date, and provides a foundation for functional studies aimed at elucidating the role of Hox genes in arthropod development and evolution.

Reconstruction of cell lineage and spatiotemporal pattern formation of the mesoderm in the amphipod crustacean Orchestia cavimana

BACKGROUND

Cell lineage studies in amphipods have revealed an early restriction of blastomere fate. The mesendodermal cell lineage is specified with the third cleavage of the egg. We took advantage of this stereotyped mode of development by fluorescently labeling the mesodermal precursors in embryos of Orchestia cavimana and followed the morphogenesis of the mesodermal cell layer through embryonic development.

RESULTS

The mesoderm of the trunk segments is formed by a very regular and stereotypic cell division pattern of the mesoteloblasts and their segmental daughters. The head mesoderm in contrast is generated by cell movements and divisions out of a mesendodermal cell mass. Our reconstructions reveal the presence of three different domains within the trunk mesoderm of the later embryo. We distinguish a cell group median to the limbs, a major central population from which the limb mesoderm arises and a dorsolateral branch of mesodermal cells.

CONCLUSIONS

Our detailed description of mesodermal development relates different precursor cell groups to distinct muscle groups of the embryo. A dorsoventral subdivision of mesoderm is prepatterned within the longitudinal mesodermal columns of the germ-band stage. This makes amphipods excellent crustacean models for studying mesodermal differentiation on a cellular and molecular level.

Axogenesis in the central and peripheral nervous system of the amphipod crustacean Orchestia cavimana

We describe the formation of the major axon pathways in the embryonic central and peripheral nervous systems of the amphipod crustacean Orchestia cavimana Heller, 1865 by means of antibody staining against acetylated alpha-tubulin. The data add to a long list of previous studies of various other aspects of development in Orchestia and provide a basis for future studies of neurogenesis on a deeper cellular and molecular level. Orchestia exhibits a tripartite dorsal brain, which is a characteristic feature of euarthropods. Its anlagen are the first detectable structures in the developing nervous system and can be traced back to distinct neuronal cell clusters in the early embryo. The development of the ventral nervous system proceeds with an anteroposterior gradient of development. In each trunk segment, the longitudinal connectives and the anterior commissure form first, followed by the intersegmental nerve, the posterior commissure and segmental nerves, respectively. A single commissure of a vestigial seventh pleonal segment is found. In the peripheral nervous system we observe a spatial and temporal pattern of leg innervation, which is strikingly similar in both limb types, the uniramous pereopods and the biramous pleopods. A proximal leg nerve splitting distally into two separated nerves probably reflects a general feature of crustaceans.

Arthropod phylogeny: an overview from the perspectives of morphology, molecular data and the fossil record

Monophyly of Arthropoda is emphatically supported from both morphological and molecular perspectives. Recent work finds Onychophora rather than Tardigrada to be the closest relatives of arthropods. The status of tardigrades as panarthropods (rather than cycloneuralians) is contentious from the perspective of phylogenomic data. A grade of Cambrian taxa in the arthropod stem group includes gilled lobopodians, dinocaridids (e.g., anomalocaridids), fuxianhuiids and canadaspidids that inform on character acquisition between Onychophora and the arthropod crown group. A sister group relationship between Crustacea (itself likely paraphyletic) and Hexapoda is retrieved by diverse kinds of molecular data and is well supported by neuroanatomy. This clade, Tetraconata, can be dated to the early Cambrian by crown group-type mandibles. The rival Atelocerata hypothesis (Myriapoda+Hexapoda) has no molecular support. The basal node in the arthropod crown group is embroiled in a controversy over whether myriapods unite with chelicerates (Paradoxopoda or Myriochelata) or with crustaceans and hexapods (Mandibulata). Both groups find some molecular and morphological support, though Mandibulata is presently the stronger morphological hypothesis. Either hypothesis forces an unsampled ghost lineage for Myriapoda from the Cambrian to the mid Silurian.

Embryonic origins of hull cells in the flatworm Macrostomum lignano through cell lineage analysis: developmental and phylogenetic implications

The development of macrostomid flatworms is of interest for evolutionary developmental biology research because these taxa combine characteristics of the canonical spiral cleavage pattern with significant deviations from this pattern. One such deviation is the formation of hull cells, which surround the remaining embryonic primordium during early development. Using live observations with a 4D microscope system, histology, and 3D reconstructions, we analyzed the ontogeny of these hull cells in the macrostomid model organism Macrostomum lignano. Our cell lineage analysis allowed us to find the precursors of the hull cells in this species. We discuss the relation between macrostomid development and the development of other spiralians and the question of whether hull cells are homologous within rhabditophoran flatworms.

Comparative gene expression in the heads of Drosophila melanogaster and Tribolium castaneum and the segmental affinity of the Drosophila hypopharyngeal lobes

Drosophila melanogaster has long played an important role in debates surrounding insect and arthropod head segmentation. It is surprising, therefore, that one important feature of Drosophila head segmentation has remained controversial: namely the position of the boundary between the intercalary and mandibular segments. The Drosophila embryonic head has a pair of structures lying behind the stomodeum known as the hypopharyngeal lobes. Traditionally they have been seen as part of the intercalary segment. More recent work looking at the position of the lobes relative to various marker genes has been somewhat equivocal: segment polarity gene expression has been used to argue for a mandibular affinity of these lobes, while the expression of the anterior-most hox gene labial (lab) has supported an intercalary affinity. We have addressed the question of the segmental affinity of the hypopharyngeal lobes by conducting a detailed comparison of gene expression patterns between Drosophila and the red flour beetle Tribolium castaneum, in which the intercalary segment is unambiguously marked out by lab. We demonstrate that there is a large degree of conservation in gene expression patterns between Drosophila and Tribolium, and this argues against an intercalary segment affinity for the hypopharyngeal lobes. The lobes appear to be largely mandibular in origin, although some gene expression attributed to them appears to be associated with the stomodeum. We propose that the difficulties in interpreting the Drosophila head result from a topological shift in the Drosophila embryonic head, associated with the derived process of head involution.

Engrailed-like immunoreactivity in the embryonic ventral nerve cord of the Marbled Crayfish (Marmorkrebs)

The homeobox transcription factor Engrailed is involved in controlling segmentation during arthropod germ band formation but also in establishing individual neuronal identities during later embryogenesis. In Crustacea, most studies analysing the expression of Engrailed so far have focussed on its function as segment polarity gene. In continuation to these previous studies, we analysed the neuronal expression of the Engrailed protein by immunohistochemistry in the embryonic nerve cord of a parthenogenetic crustacean, the Marbled Crayfish (Marmorkrebs). We paid particular attention to the individual identification of Engrailed expressing putative neuroblasts in the crayfish embryos. Engrailed positive cells in the neuroectoderm were counted, measured and mapped from 38 to 65% of embryonic development. That way, several Engrailed positive putative neuroblasts and putative neurons were identified. Our findings are compared with earlier studies on Engrailed expression during germ band formation in Crustacea. Recent data on neurogenesis in an amphipod crustacean have provided compelling evidence for the homology of several identified neuroblasts between this amphipod and insects. The present report may serve as a basis to explore the question if during crustacean neurogenesis additional communalities with insects exist.

The clonal composition of biramous and uniramous arthropod limbs

We present the first comparative cell lineage analysis of uniramous and biramous limbs of an arthropod, the crustacean Orchestia cavimana. Via single cell labelling of the cells that are involved in limb development, we are able to present the first complete clonal composition of an arthropod limb. We show that the two main branches of crustacean limbs, exopod and endopod, are formed by a secondary subdivision of the growth zone of the main limb axis. Additional limb outgrowths such as exites result from the establishment of new axes. In contrast to general belief, uniramous limbs in Orchestia are not formed by the loss of the exopod but by suppression of the split into exopod and endopod. Our results offer a developmental approach to discriminate between the different kinds of branches of arthropod appendages. This leads to the conclusion that a 'true' biramous limb comprising an endopod and an exopod might have occurred much later in euarthropod evolution than has previously been thought, probably either in the lineage of the Mandibulata or that of the Tetraconata.

Filling the gap between identified neuroblasts and neurons in crustaceans adds new support for Tetraconata

The complex spatio-temporal patterns of development and anatomy of nervous systems play a key role in our understanding of arthropod evolution. However, the degree of resolution of neural processes is not always detailed enough to claim homology between arthropod groups. One example is neural precursors and their progeny in crustaceans and insects. Pioneer neurons of crustaceans and insects show some similarities that indicate homology. In contrast, the differentiation of insect and crustacean neuroblasts (NBs) shows profound differences and their homology is controversial. For Drosophila and grasshoppers, the complete lineage of several NBs up to formation of pioneer neurons is known. Apart from data on median NBs no comparable results exist for Crustacea. Accordingly, it is not clear where the crustacean pioneer neurons come from and whether there are NBs lateral to the midline homologous to those of insects. To fill this gap, individual NBs in the ventral neuroectoderm of the crustacean Orchestia cavimana were labelled in vivo with a fluorescent dye. A partial neuroblast map was established and for the first time lineages from individual NBs to identified pioneer neurons were established in a crustacean. Our data strongly suggest homology of NBs and their lineages, providing further evidence for a close insect-crustacean relationship.

An epipodite-bearing crown-group crustacean from the Lower Cambrian

Crown-group crustaceans (Eucrustacea) are common in the fossil record of the past 500 million years back to the early Ordovician period, and very rare representatives are also known from the late Middle and Late Cambrian periods. Finds in Lower Cambrian rocks of the Phosphatocopina, the fossil sister group to eucrustaceans, imply that members of the eucrustacean stem lineage co-occurred, but it remained unclear whether crown-group members were also present at that time. 'Orsten'-type fossils are typically tiny embryos and cuticle-bearing animals, of which the cuticle is phosphatized and the material is three-dimensional and complete with soft parts. Such fossils are found predominantly in the Cambrian and Ordovician and provide detailed morphological and phylogenetic information on the early evolution of metazoans. Here we report an Orsten-type Konservat-Lagerstätte from the Lower Cambrian of China that contains at least three new arthropod species, of which we describe the most abundant form on the basis of exceptionally well preserved material of several growth stages. The limb morphology and other details of this new species are markedly similar to those of living cephalocarids, branchiopods and copepods and it is assigned to the Eucrustacea, thus representing the first undoubted crown-group crustacean from the early Cambrian. Its stratigraphical position provides substantial support to the proposition that the main cladogenic event that gave rise to the Arthropoda was before the Cambrian. Small leaf-shaped structures on the outer limb base of the new species provide evidence on the long-debated issue of the origin of epipodites: they occur in a set of three, derive from setae and are a ground-pattern feature of Eucrustacea.