How body patterning might have worked in the evolution of arthropods-A case study of the mystacocarid Derocheilocaris remanei (Crustacea, Oligostraca)

Body organization within arthropods is enormously diverse, but a fusion of segments into "functional groups" (tagmatization) is found in all species. Within Tetraconata/Pancrustacea, an anterior head, a locomotory thorax region, and a posterior, mostly limbless tagma known as the abdomen is present. The posterior-most tagma in crustaceans is frequently confused with the malacostracan, for example, decapod pleon often misleadingly termed abdomen, however, its evolutionary and developmental origin continues to pose a riddle, especially the completely limbless abdomen of the "entomostracan morphotype" (e.g., fairy shrimps). Since the discovery of Hox genes and their involvement in specifying the morphology or identity of segments, tagmata, or regions along the anteroposterior axis of an organism, only a few studies have focused on model organisms representing the "entomostracan morphotype" and used a variety of dedicated Hox genes and their transcription products to shine light on abdomen formation. The homeotic genes or the molecular processes that determine the identity of the entomostracan abdomen remain unknown to date. This study focuses on the "entomostracan morphotype" representative Derocheilocaris remanei (Mystacocarida). We present a complete overview of development throughout larval stages and investigate homeotic gene expression data using the antibody FP6.87 that binds specifically to epitopes of Ultrabithorax/Abdominal-A proteins. Our results suggest that the abdomen in Mystacocarida is bipartite (abdomen I + abdomen II). We suggest that the limbless abdomen is an evolutionary novelty that evolved several times independently within crustaceans and which might be the result of a progressive reduction of former thoracic segments into abdominal segments.

The velvet worm brain unveils homologies and evolutionary novelties across panarthropods

BACKGROUND

The evolution of the brain and its major neuropils in Panarthropoda (comprising Arthropoda, Tardigrada and Onychophora) remains enigmatic. As one of the closest relatives of arthropods, onychophorans are regarded as indispensable for a broad understanding of the evolution of panarthropod organ systems, including the brain, whose anatomical and functional organisation is often used to gain insights into evolutionary relations. However, while numerous recent studies have clarified the organisation of many arthropod nervous systems, a detailed investigation of the onychophoran brain with current state-of-the-art approaches is lacking, and further inconsistencies in nomenclature and interpretation hamper its understanding. To clarify the origins and homology of cerebral structures across panarthropods, we analysed the brain architecture in the onychophoran Euperipatoides rowelli by combining X-ray micro-computed tomography, histology, immunohistochemistry, confocal microscopy, and three-dimensional reconstruction.

RESULTS

Here, we use this detailed information to generate a consistent glossary for neuroanatomical studies of Onychophora. In addition, we report novel cerebral structures, provide novel details on previously known brain areas, and characterise further structures and neuropils in order to improve the reproducibility of neuroanatomical observations. Our findings support homology of mushroom bodies and central bodies in onychophorans and arthropods. Their antennal nerve cords and olfactory lobes most likely evolved independently. In contrast to previous reports, we found no evidence for second-order visual neuropils, or a frontal ganglion in the velvet worm brain.

CONCLUSION

We imaged the velvet worm nervous system at an unprecedented level of detail and compiled a comprehensive glossary of known and previously uncharacterised neuroanatomical structures to provide an in-depth characterisation of the onychophoran brain architecture. We expect that our data will improve the reproducibility and comparability of future neuroanatomical studies.

The brain and the corresponding sense organs in calanoid copepods - Evidence of vestiges of compound eyes

Copepoda is one of the crustacean taxa with still unresolved phylogenetic relationships within Tetraconata. Recent phylogenomic studies place them close to Malacostraca and Cirripedia. Little is known about the morphological details of the copepod nervous system, and the available data are sometimes contradictory. We investigated several representatives of the subgroup Calanoida using immunohistochemical labeling against alpha-tubulin and various neuroactive substances, combining this with confocal laser scanning analysis and 3D reconstruction. Our results show that the studied copepods exhibit only a single anterior protocerebral neuropil which is connected to the nerves of two protocerebral sense organs: the frontal filament organ and a photoreceptor known as the Gicklhorn's organ. We suggest, on the basis of its position and the innervation it provides, that Gicklhorn's organ is homologous to the compound eye in arthropods. With regard to the frontal filament organ, we reveal detailed innervation to the lateral protocerebrum and the appearance of spherical bodies that stain intensely against alpha tubulin. A potential homology of these bodies to the onion bodies in malacostacan crustaceans and in Mystacocarida is suggested. The nauplius eye in all the examined calanoids shows the same basic pattern of innervation with the middle cup sending its neurites into the median nerve, while the axons of the lateral cups proceed into both the median and the lateral nerves. The early development of the axonal scaffold of the nauplius eye neuropil from the proximal parts of the nauplius eye nerves follows the same pattern as in other crustaceans. In our view, this specific innervation pattern is a further feature supporting the homology of the nauplius eye in crustaceans.

Crustacean olfactory systems: A comparative review and a crustacean perspective on olfaction in insects

Malacostracan crustaceans display a large diversity of sizes, morphs and life styles. However, only a few representatives of decapod taxa have served as models for analyzing crustacean olfaction, such as crayfish and spiny lobsters. Crustaceans bear multiple parallel chemosensory pathways represented by different populations of unimodal chemosensory and bimodal chemo- and mechanosensory sensilla on the mouthparts, the walking limbs and primarily on their two pairs of antennae. Here, we focus on the olfactory pathway associated with the unimodal chemosensory sensilla on the first antennal pair, the aesthetascs. We explore the diverse arrangement of these sensilla across malacostracan taxa and point out evolutionary transformations which occurred in the central olfactory pathway. We discuss the evolution of chemoreceptor proteins, comparative aspects of active chemoreception and the temporal resolution of crustacean olfactory system. Viewing the evolution of crustacean brains in light of energetic constraints can help us understand their functional morphology and suggests that in various crustacean lineages, the brains were simplified convergently because of metabolic limitations. Comparing the wiring of afferents, interneurons and output neurons within the olfactory glomeruli suggests a deep homology of insect and crustacean olfactory systems. However, both taxa followed distinct lineages during the evolutionary elaboration of their olfactory systems. A comparison with insects suggests their olfactory systems ö especially that of the vinegar fly ö to be superb examples for "economy of design". Such a comparison also inspires new thoughts about olfactory coding and the functioning of malacostracan olfactory systems in general.