A firm step from water to land

Comparisons of developmental processes of air-breathing organs among terrestrial isopods (Crustacea, Oniscidea): implications for their evolutionary origins

BACKGROUND

The acquisition of air-breathing organs is one of the key innovations for terrestrialization in animals. Terrestrial isopods, a crustacean lineage, can be interesting models to study the evolution of respiratory organs, as they exhibit varieties of air-breathing structures according to their habitats. However, the evolutionary processes and origins of these structures are unclear, due to the lack of information about their developmental processes. To understand the developmental mechanisms, we compared the developmental processes forming different respiratory structures in three isopod species, i.e., 'uncovered lungs' in Nagurus okinawaensis (Trachelipodidae), 'dorsal respiratory fields' in Alloniscus balssi (Alloniscidae), and pleopods without respiratory structures in Armadilloniscus cf. ellipticus (Detonidae).

RESULTS

In N. okinawaensis with uncovered lungs, epithelium and cuticle around the proximal hemolymph sinus developed into respiratory structures at post-manca juvenile stages. On the other hand, in Al. balssi with dorsal respiratory fields, the region for the future respiratory structure was already present at manca 1 stage, immediately after hatching, where the lateral protrusion of ventral epithelium occurred, forming the respiratory structure. Furthermore, on pleopods in Ar. cf. ellipticus, only thickened dorsal cuticle and the proximal hemolymph sinus developed during postembryonic development without special morphogenesis.

CONCLUSIONS

This study shows that the respiratory structures in terrestrial isopods develop primarily by postembryonic epithelial modifications, but the epithelial positions developing into respiratory structures differ between uncovered lungs and dorsal respiratory fields. This suggests that these two types of respiratory structures do not result from simple differences in the degree of development. Future analysis of molecular developmental mechanisms will help determine whether these are the result of heterotopic changes or have different evolutionary origins. Overall, this study provides fundamental information for evolutionary developmental studies of isopod respiratory organs.

Investigating the quadrupedal abilities of Scutellosaurus lawleri and its implications for locomotor behavior evolution among dinosaurs

A reversion to secondary quadrupedality is exceptionally rare in nature, yet the convergent re-evolution of this locomotor style occurred at least four separate times within Dinosauria. Facultative quadrupedality, an intermediate state between obligate bipedality and obligate quadrupedality, may have been an important transitional step in this locomotor shift, and is proposed for a range of basal ornithischians and sauropodomorphs. Advances in virtual biomechanical modeling and simulation have allowed for the investigation of limb anatomy and function in a range of extinct dinosaurian species, yet this technique has not been widely applied to explore facultatively quadrupedal gait generation. This study places its focus on Scutellosaurus, a basal thyreophoran that has previously been described as both an obligate biped and a facultative quadruped. The functional anatomy of the musculoskeletal system (myology, mass properties, and joint ranges of motion) has been reconstructed using extant phylogenetic bracketing and comparative anatomical datasets. This information was used to create a multi-body dynamic locomotor simulation that demonstrates that whil quadrupedal gaits were physically possible, they did not outperform bipedal gaits is any tested metric. Scutellosaurus cannot therefore be described as an obligate biped, but we would predict its use of quadrupedality would be very rare, and perhaps restricted to specific activities such as foraging. This finding suggests that basal thyreophorans are still overwhelmingly bipedal but is perhaps indicative of an adaptive pathway for later evolution of quadrupedality.

Survey of biomechanical aspects of arthropod terrestrialisation - Substrate bound legged locomotion

Arthropods are the most diverse clade on earth with regard to both species number and variability of body plans. Their general body plan is characterised by variable numbers of legs, and many-legged locomotion is an essential aspect of many aquatic and terrestrial arthropod species. Moreover, arthropods belong to the first groups of animals to colonise subaerial habitats, and they did so repeatedly and independently in a couple of clades. Those arthropod clades that colonised land habitats were equipped with highly variable body plans and locomotor apparatuses. Proceeding from their respective specific anatomies, they were challenged with strongly changing environmental conditions as well as altered physical and physiological constraints. This review explores the transitions from aquatic to terrestrial habitats across the different arthropod body plans and explains the major mechanisms and principles that constrain design and function of a range of locomotor apparatuses. Important aspects of movement physiology addressed here include the effects of different numbers of legs, different body sizes, miniaturisation and simplification of body plans and different ratios of inertial and damping forces. The article's focus is on continuous legged locomotion, but related ecological and behavioural aspects are also taken into account.

Estuarine fish and tetrapod evolution: insights from a Late Devonian (Famennian) Gondwanan estuarine lake and a southern African Holocene equivalent

The Waterloo Farm lagerstätte in South Africa provides a uniquely well-preserved record of a Latest Devonian estuarine ecosystem. Ecological evidence from it is reviewed, contextualised, and compared with that available from the analogous Swartvlei estuarine lake, with a particular emphasis on their piscean inhabitants. Although the taxonomic affinities of the estuarine species are temporally very different, the overall patterns of utilisation prove to be remarkably congruent, with similar trophic structures. Significantly, both systems show evidence of widespread use of estuaries as fish nurseries by both resident and marine migrant taxa. Holocene estuaries are almost exclusively utilised by actinopterygians which are overwhelmingly dominated by oviparous species. Complex strategies are utilised by estuarine resident species to avoid exposure of eggs to environmental stresses that characterize these systems. By contrast, many of the groups utilising Devonian estuaries were likely live bearers, potentially allowing them to avoid the challenges faced by oviparous taxa. This may have contributed to dominance of these systems by non-actinoptergians prior to the End Devonian Mass Extinction. The association of early aquatic tetrapods at Waterloo Farm with a fish nursery environment is consistent with findings from North America, Belgium and Russia, and may be implied by the estuarine settings of a number of other Devonian tetrapods. Tetrapods apparently replace their sister group, the elpistostegids, in estuaries with both groups having been postulated to be adaptated to shallow water habitats where they could access small piscean prey. Correlation of tetrapods (and elpistostegids) with fish nursery areas in the Late Devonian lends strong support to this hypothesis, suggesting that adaptations permitting improved access to the abundant juvenile fish within the littoral zone of estuarine lakes and continental water bodies may have been pivotal in the evolution of tetrapods.

Effects of fin fold mesenchyme ablation on fin development in zebrafish

The evolution of the tetrapod limb involved an expansion and elaboration of the endoskeletal elements, while the fish fin rays were lost. Loss of fin-specific genes, and regulatory changes in key appendicular patterning genes have been identified as mechanisms of limb evolution, however their contributions to cellular organization and tissue differences between fins and limbs remains poorly understood. During early larval fin development, hoxa13a/hoxd13a-expressing fin fold mesenchyme migrate through the median and pectoral fin along actinotrichia fibrils, non-calcified skeletal elements crucial for supporting the fin fold. Fin fold mesenchyme migration defects have previously been proposed as a mechanism of fin dermal bone loss during tetrapod evolution as it has been shown they contribute directly to the fin ray osteoblast population. Using the nitroreductase/metronidazole system, we genetically ablated a subset of hoxa13a/hoxd13a-expressing fin fold mesenchyme to assess its contributions to fin development. Following the ablation of fin fold mesenchyme in larvae, the actinotrichia are unable to remain rigid and the median and pectoral fin folds collapse, resulting in a reduced fin fold size. The remaining cells following ablation are unable to migrate and show decreased actinodin1 mesenchymal reporter activity. Actinodin proteins are crucial structural component of the actinotrichia. Additionally, we show a decrease in hoxa13a, hoxd13a, fgf10a and altered shha, and ptch2 expression during larval fin development. A continuous treatment of metronidazole leads to fin ray defects at 30dpf. Fewer rays are present compared to stage-matched control larvae, and these rays are shorter and less defined. These results suggest the targeted hoxa13a/hoxd13a-expressing mesenchyme contribute to their own successful migration through their contributions to actinotrichia. Furthermore, due to their fate as fin ray osteoblasts, we propose their initial ablation, and subsequent disorganization produces truncated fin dermal bone elements during late larval stages.

Co-option of a motor-to-sensory histaminergic circuit correlates with insect flight biomechanics

Nervous systems must adapt to shifts in behavioural ecology. One form of adaptation is neural exaptation, in which neural circuits are co-opted to perform additional novel functions. Here, we describe the co-option of a motor-to-somatosensory circuit into an olfactory network. Many moths beat their wings during odour-tracking, whether walking or flying, causing strong oscillations of airflow around the antennae, altering odour plume structure. This self-induced sensory stimulation could impose selective pressures that influence neural circuit evolution, specifically fostering the emergence of corollary discharge circuits. In Manduca sexta, a pair of mesothoracic to deutocerebral histaminergic neurons (MDHns), project from the mesothoracic neuromere to both antennal lobes (ALs), the first olfactory neuropil. Consistent with a hypothetical role in providing the olfactory system with a corollary discharge, we demonstrate that the MDHns innervate the ALs of advanced and basal moths, but not butterflies, which differ in wing beat and flight pattern. The MDHns probably arose in crustaceans and in many arthropods innervate mechanosensory areas, but not the olfactory system. The MDHns, therefore, represent an example of architectural exaptation, in which neurons that provide motor output information to mechanosensory regions have been co-opted to provide information to the olfactory system in moths.

Relations and dependencies between morphological characters

In biological classification, a character is a property of a taxon that can distinguish it from other taxa. Characters are not independent, and the relations between characters can arise from structural constraints, developmental pathways or functional constraints. That has lead to famous controversies in the history of biology. In addition, a character as a tool of data analysis has some subjective aspects. In this contribution, I develop algebraic and geometric schemes to address these issues in a mathematical framework.

Follicle Microstructure and Innervation Vary between Pinniped Micro- and Macrovibrissae

Histological data from terrestrial, semiaquatic, and fully aquatic mammal vibrissa (whisker) studies indicate that follicle microstructure and innervation vary across the mystacial vibrissal array (i.e. medial microvibrissae to lateral macrovibrissae). However, comparative data are lacking, and current histological studies on pinniped vibrissae only focus on the largest ventrolateral vibrissae. Consequently, we investigated the microstructure, medial-to-lateral innervation, and morphometric trends in harp seal (Pagophilus groenlandicus) vibrissal follicle-sinus complexes (F-SCs). The F-SCs were sectioned either longitudinally or in cross-section and stained with a modified Masson's trichrome stain (microstructure) or Bodian's silver stain (innervation). All F-SCs exhibited a tripartite blood organization system. The dermal capsule thickness, the distribution of major branches of the deep vibrissal nerve, and the hair shaft design were more symmetrical in medial F-SCs, but these features became more asymmetrical as the F-SCs became more lateral. Overall, the mean axon count was 1,221 ± 422.3 axons/F-SC and mean axon counts by column ranged from 550 ± 97.4 axons/F-SC (medially, column 11) to 1,632 ± 173.2 axons/F-SC (laterally, column 2). These values indicate a total of 117,216 axons innervating the entire mystacial vibrissal array. The mean axon count of lateral F-SCs was 1,533 ± 192.9 axons/ F-SC, which is similar to values reported in the literature for other pinniped F-SCs. Our data suggest that conventional studies that only examine the largest ventrolateral vibrissae may overestimate the total innervation by ∼20%. However, our study also accounts for variation in quantification methods and shows that conventional analyses likely only overestimate innervation by ∼10%. The relationship between axon count and cross-sectional F-SC surface area was nonlinear, and axon densities were consistent across the snout. Our data indicate that harp seals exhibit microstructural and innervational differences between their microvibrissae (columns 8-11) and macrovibrissae (columns 1-7). We hypothesize that this feature is conserved among pinnipeds and may result in functional compartmentalization within their mystacial vibrissal arrays.

Homology of the cranial vault in birds: new insights based on embryonic fate-mapping and character analysis

Bones of the cranial vault appear to be highly conserved among tetrapod vertebrates. Moreover, bones identified with the same name are assumed to be evolutionarily homologous. However, recent developmental studies reveal a key difference in the embryonic origin of cranial vault bones between representatives of two amniote lineages, mammals and birds, thereby challenging this view. In the mouse, the frontal is derived from cranial neural crest (CNC) but the parietal is derived from mesoderm, placing the CNC-mesoderm boundary at the suture between these bones. In the chicken, this boundary is located within the frontal. This difference and related data have led several recent authors to suggest that bones of the avian cranial vault are misidentified and should be renamed. To elucidate this apparent conflict, we fate-mapped CNC and mesoderm in axolotl to reveal the contributions of these two embryonic cell populations to the cranial vault in a urodele amphibian. The CNC-mesoderm boundary in axolotl is located between the frontal and parietal bones, as in the mouse but unlike the chicken. If, however, the avian frontal is regarded instead as a fused frontal and parietal (i.e. frontoparietal) and the parietal as a postparietal, then the cranial vault of birds becomes developmentally and topologically congruent with those of urodeles and mammals. This alternative hypothesis of cranial vault homology is also phylogenetically consistent with data from the tetrapod fossil record, where frontal, parietal and postparietal bones are present in stem lineages of all extant taxa, including birds. It further implies that a postparietal may be present in most non-avian archosaurs, but fused to the parietal or supraoccipital as in many extant mammals.

Animal evolution and atmospheric pO2: is there a link between gradual animal adaptation to terrain elevation due to Ural orogeny and survival of subsequent hypoxic periods?

Considering evolution of terrestrial animals as something happening only on flat continental plains seems wrong. Many mountains have arisen and disappeared over the geologic time scale, so in all periods some areas of high altitude existed, with reduced oxygen pressure (pO2) and increased aridity. During orogeny, animal species of the raising terrain can slowly adapt to reduced oxygen levels.This review proposes that animal evolution was often driven by atmospheric oxygen availability. Transitions of insect ancestors and amphibians out of water are here interpreted as events forced by the lack of oxygen in shallow and warm water during Devonian. Hyperoxia during early Carboniferous allowed giant insects to be predators of lowlands, forcing small amphibians to move to higher terrains, unsuitable to large insects due to reduced pO2. In arid mountainous habitats, ascended animals evolved in early reptiles with more efficient lungs and improved circulation. Animals with alveolar lungs became the mammalian ancestors, while those with respiratory duct lungs developed in archosaurs. In this interpretation, limb precursors of wings and pneumatised bones might have been adaptations for moving on steep slopes.Ural mountains have risen to an estimated height of 3000 m between 318 and 251 Mya. The earliest archosaurs have been found on the European Ural side, estimated 275 Myr old. It is proposed that Ural orogeny slowly elevated several highland habitats within the modern Ural region to heights above 2500 m. Since this process took near 60 Myr, animals in these habitats fully to adapted to hypoxia.The protracted P-Tr hypoxic extinction event killed many aquatic and terrestrial animals. Devastated lowland areas were repopulated by mammaliaformes that came down from mountainous areas. Archosaurs were better adapted to very low pO2, so they were forced to descend to the sea level later when the lack of oxygen became severe. During the Triassic period, when the relative content of O2 reduced to near 12%, archosaurs prevailed as only animals that could cope with profound hypoxia at the sea level. Their diverse descendants has become dominant terrestrial animals, until the K-Pg extinction due to meteor impact.

Engineering Xenopus embryos for phenotypic drug discovery screening

Many rare human inherited diseases remain untreatable despite the fact that the disease causing genes are known and adequate mouse disease models have been developed. In vivo phenotypic drug screening relies on isolating drug candidates by their ability to produce a desired therapeutic phenotype in whole organisms. Embryos of zebrafish and Xenopus frogs are abundant, small and free-living. They can be easily arrayed in multi-well dishes and treated with small organic molecules. With the development of novel genome modification tools, such a zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR/Cas, it is now possible to efficiently engineer non-mammalian models of inherited human diseases. Here, we will review the rapid progress made in adapting these novel genome editing tools to Xenopus. The advantages of Xenopus embryos as in vivo models to study human inherited diseases will be presented and their utility for drug discovery screening will be discussed. Being a tetrapod, Xenopus complements zebrafish as an indispensable non-mammalian animal model for the study of human disease pathologies and the discovery of novel therapeutics for inherited diseases.

More than just a bump: cam-type femoroacetabular impingement and the evolution of the femoral neck

Recent orthopaedic literature has implicated femoroacetabular impingement, the pathologic abutment of structural aberrancies in the proximal femur and acetabular rim, as an important cause of groin pain in young individuals and a potential factor in early idiopathic osteoarthritis. The etiology and risk factors for developing cam-type morphology are still unknown. The osseous anatomy of the proximal femur in humans is the culmination of nearly 400 million years of evolution. Coxa recta and coxa rotunda are the two predominant morphologies in modern animals. While the former, characterized by a straight head-neck junction, is often present in cursorial creatures, the latter, [corrected] distinguished by high offset at this junction, is exemplified in most humans. Based on the ontology and phylogeny of the proximal femur, coxa rotunda probably developed from a more primitive coxa recta. We believe that cam-type morphology is neither a redevelopment of coxa recta nor a malformation such as slipped capital epiphysis. The aspherical osteocartilaginous bump is associated with an extended physis and has been noted to appear during mid-adolescence. While this protuberance may contribute to future pathology, the authors feel that increased loading of the hip, not impingement activities, during late childhood and early adolescence predispose patients to develop this morphology.

Sequences, stratigraphy and scenarios: what can we say about the fossil record of the earliest tetrapods?

Past research on the emergence of digit-bearing tetrapods has led to the widely accepted premise that this important evolutionary event occurred during the Late Devonian. The discovery of convincing digit-bearing tetrapod trackways of early Middle Devonian age in Poland has upset this orthodoxy, indicating that current scenarios which link the timing of the origin of digited tetrapods to specific events in Earth history are likely to be in error. Inspired by this find, we examine the fossil record of early digit-bearing tetrapods and their closest fish-like relatives from a statistical standpoint. We find that the Polish trackways force a substantial reconsideration of the nature of the early tetrapod record when only body fossils are considered. However, the effect is less drastic (and often not statistically significant) when other reliably dated trackways that were previously considered anachronistic are taken into account. Using two approaches, we find that 95 per cent credible and confidence intervals for the origin of digit-bearing tetrapods extend into the Early Devonian and beyond, spanning late Emsian to mid Ludlow. For biologically realistic diversity models, estimated genus-level preservation rates for Devonian digited tetrapods and their relatives range from 0.025 to 0.073 per lineage-million years, an order of magnitude lower than species-level rates for groups typically considered to have dense records. Available fossils of early digited tetrapods and their immediate relatives are adequate for documenting large-scale patterns of character acquisition associated with the origin of terrestriality, but low preservation rates coupled with clear geographical and stratigraphic sampling biases caution against building scenarios for the origin of digits and terrestrialization tied to the provenance of particular specimens or faunas.

Tri-phasic expression of posterior Hox genes during development of pectoral fins in zebrafish: implications for the evolution of vertebrate paired appendages

During development of the limbs, Hox genes belonging to the paralogous groups 9-13 are expressed in three distinct phases, which play key roles in the segmental patterning of limb skeletons. In teleost fishes, which have a very different organization in their fin skeletons, it is not clear whether a similar patterning mechanism is at work. To determine whether Hox genes are also expressed in several distinct phases during teleost paired fin development, we re-analyzed the expression patterns of hox9-13 genes during development of pectoral fins in zebrafish. We found that, similar to tetrapod Hox genes, expression of hoxa/d genes in zebrafish pectoral fins occurs in three distinct phases, in which the most distal/third phase is correlated with the development of the most distal structure of the fin, the fin blade. Like in tetrapods, hox gene expression in zebrafish pectoral fins during the distal/third phase is dependent upon sonic hedgehog signaling (hoxa and hoxd genes) and the presence of a long-range enhancer (hoxa genes), which indicates that the regulatory mechanisms underlying tri-phasic expression of Hox genes have remained relatively unchanged during evolution. Our results suggest that, although simpler in organization, teleost fins do have a distal structure that might be considered comparable to the autopod region of limbs.

The concept of correlated progression as the basis of a model for the evolutionary origin of major new taxa

Understanding the evolutionary processes responsible for the long treks through morphospace associated with the origin of new higher taxa is hampered by the lack of a realistic and usable model that accounts for long-term phenotypic evolvability. The systems-related concept of correlated progression, in which all the traits are functionally linked and so constrained to evolve by small increments at a time in parallel with each other, provides the basis for such a model. Implications for the process of evolution at high taxonomic level are that: the evolving traits must be considered together as a system, and the exact sequence of incremental changes in characters is indeterminable; there are no identifiable key innovations; selection acts on the phenotype as a whole rather than on individual traits; and the selection force is therefore multidimensional. Application of the model to the pattern of evolution of traits and trait states as revealed by the fossil record of the stem groups of such taxa as mammals, turtles and tetrapods generates realistic testable hypotheses about how such groups evolved.

Evolutionary biology of CD1

The recognition more than a decade ago that lipids presented by CD1 could function as T cell antigens revealed a startling and previously unappreciated complexity to the adaptive immune system. The initial novelty of lipid antigen presentation by CD1 has since given way to a broader perspective of the immune system's capacity to sense and respond to a diverse array of macromolecules. Some immune recognition systems such as Toll-like receptors can trace their origins back into the deep history of sea urchins and arthropods. Others such as the major histocompatibility complex (MHC) appear relatively recently and interestingly, only in animals that also possess a jaw. The natural history of CD1 is thus part of the wider story of immune system evolution and should be considered in this context. Most evidence indicates that CD1 probably evolved from a classical MHC class I (MHC I) gene at some point during vertebrate evolution. This chapter reviews the evidence for this phylogenetic relationship and attempts to connect CD1 to existing models of MHC evolution. This endeavor is facilitated today by the recent availability of whole genome sequence data from a variety of species. Investigators have used these data to trace the ultimate origin of the MHC to a series of whole genome duplications that occurred roughly 500 million years ago. Sequence data have also revealed homologs of the mammalian MHC I and MHC II gene families in virtually all jawed vertebrates including sharks, bony fishes, reptiles, and birds. In contrast, CD1 genes have thus far been found only in a subset of these animal groups. This pattern of CD1 occurrence in the genomes of living species suggests the emergence of CD 1 in an early terrestrial vertebrate.