The organization of plurisegmental mechanosensitive interneurons in the central nervous system of the wandering spider Cupiennius salei

Summary In spiders the bulk of the central nervous system (CNS) consists of fused segmental ganglia traversed by longitudinal tracts, which have precise relationships with sensory neuropils and which contain the fibers of large plurisegmental interneurons. The responses of these interneurons to various mechanical stimuli were studied electrophysiologically, and their unilateral or bilateral structure was revealed by intracellular staining. Unilateral interneurons visit all the neuromeres on one side of the CNS. They receive mechanosensory input either from a single leg or from all ipsilateral legs via sensory neurons that invade leg neuromeres and project into specific longitudinal tracts. The anatomical organization of unilateral interneurons suggests that their axons impart their information to all ipsilateral leg neuromeres. Bilateral interneurons are of two kinds, symmetric and asymmetric neurons. The latter respond to stimulation of all legs on one side of the body, having their dendrites amongst sensory tracts of the same side of the CNS. Anatomical evidence suggests that their terminals invade all four contralateral leg neuromeres. Bilaterally symmetrical plurisegmental interneurons have dendritic arborizations in both halves of the fused ventral ganglia. They respond to the stimulation of any of the 8 legs. A third class of cells, the ascending neurons have unilateral or bilateral dendritic arborizations in the fused ventral ganglia and show blebbed axons in postero-ventral regions of the brain. Their response characteristics are similar to those of other plurisegmental interneurons. Descending neurons have opposite structural polarity, arising in the brain and terminating in segmental regions of the fused ventral ganglia. Descending neurons show strong responses to visual stimulation. Approximately 50% of all the recorded neurons respond exclusively to stimulation of a single type of mechanoreceptor (either tactile hairs, or trichobothria, or slit sensilla), while the rest respond to stimulation of a variety of sensilla. However, these functional differences are not obviously reflected by the anatomy. The functional significance of plurisegmental interneurons is discussed with respect to sensory convergence and the coordination of motor output to the legs. A comparison between the response properties of certain plurisegmental interneurons and their parent longitudinal tracts suggests that the tracts themselves do not reflect a modality-specific organization.Abbreviations BPI bilateral plurisegmental interneuron - CNS central nervous system - FVG fused ventral ganglia - LT longitudinal tract - PI plurisegmental interneuron - PSTH peristimulus timehistogram - UPI unilateral plurisegmental interneuron     

Calibrating the chelicerate clock: a paleontological reply to Jeyaprakash and Hoy

Divergence times inferred for major lineages of Chelicerata (scorpions, spiders, mites, pycnogonids and xiphosurans) in a recent paper on mitochondrial phylogeny by Jeyaprakash and Hoy are compared to the known stratigraphical occurrences of these groups. Erroneous statements concerning fossil date estimates in the original study are corrected. We emphasize that the fossil record of chelicerates is more complete than is sometimes assumed, and that paleontology plays a key role in dating cladogenesis by setting minimum divergence times, which can and do falsify molecular clock estimates where the inferred divergence is substantially younger than the known fossil record. The oldest representatives of each chelicerate order are documented here, together with similar data for the major mite lineages down to family level. Through these, we hope to provide a robust framework and reference points for future molecular systematic studies of this nature.     

Complete mitochondrial genomes of the human follicle mites Demodex brevis and D. folliculorum: novel gene arrangement,truncated tRNA genes,and ancient divergence between species

Background

Follicle mites of the genus Demodex are found on a wide diversity of mammals, including humans; surprisingly little is known, however, about the evolution of this association. Additional sequence information promises to facilitate studies of Demodex variation within and between host species. Here we report the complete mitochondrial genome sequences of two species of Demodex known to live on humans—Demodex brevis and D. folliculorum—which are the first such genomes available for any member of the genus. We analyzed these sequences to gain insight into the evolution of mitochondrial genomes within the Acariformes. We also used relaxed molecular clock analyses, based on alignments of mitochondrial proteins, to estimate the time of divergence between these two species.

Results

Both Demodex genomes shared a novel gene order that differs substantially from the ancestral chelicerate pattern, with transfer RNA (tRNA) genes apparently having moved much more often than other genes. Mitochondrial tRNA genes of both species were unusually short, with most of them unable to encode tRNAs that could fold into the canonical cloverleaf structure; indeed, several examples lacked both D- and T-arms. Finally, the high level of sequence divergence observed between these species suggests that these two lineages last shared a common ancestor no more recently than about 87 mya.

Conclusions

Among Acariformes, rearrangements involving tRNA genes tend to occur much more often than those involving other genes. The truncated tRNA genes observed in both Demodex species would seem to require the evolution of extensive tRNA editing capabilities and/or coevolved interacting factors. The molecular machinery necessary for these unusual tRNAs to function might provide an avenue for developing treatments of skin disorders caused by Demodex. The deep divergence time estimated between these two species sets a lower bound on the time that Demodex have been coevolving with their mammalian hosts, and supports the hypothesis that there was an early split within the genus Demodex into species that dwell in different skin microhabitats.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1124) contains supplementary material, which is available to authorized users.     

Role of musculature during defecation in a particle‐feeding arachnid,Archegozetes longisetosus (Acari,Oribatida)

Most extant Chelicerata are characterized by external digestion and the ingestion of fluid food. Exceptions include the marine taxa, most Opiliones, and the mite groups Opilioacarida (Parasitiformes) and Sarcoptiformes (Acariformes), which ingest particulate food. This leads to different physiological and morphological adaptations for food processing, including the production and extrusion of solid fecal pellets, which are rather large in sarcoptiform mites. Few studies have investigated the defecation of such large fecal pellets, and available information is contradictory. We use a combination of non invasive microscopical techniques and in vivo examination to investigate the complex functional morphology of the anal region of the oribatid mite Archegozetes longisetosus Aoki. The opening of the anus is at least initiated by indirect muscular action via an increase of hemolymph pressure, through the action of dorsoventral muscles (dvm). Extrusion of the fecal pellet is accomplished by the prerectal muscle collar, with full opening of the anus and rotation and bowing of the plates probably resulting from pressure of the pellet. The sequential nature of these actions was demonstrated by many observations in which the anus opened only partially; these were concomitant with dmv contraction but pellets were not being extruded. All muscles directly connected to the anal and adanal regions assist in keeping the anus closed; they are antagonists to hydrostatic forces that are necessary for normal activity. Based on the literature, no obvious similarities were noted with defecation musculature in other particle‐feeding chelicerates, but most muscles can be homologized with those of more specialized oribatid mites. The function of the outer anal muscles has been modified in both Euphthiracaridae and Brachypylina to assist in providing general hemolymph pressure. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Heme-binding storage proteins in the Chelicerata

Lipoglycoproteins in the Chelicerata that bind and store heme appear to represent a unique evolutionary strategy to both mitigate the toxicity of heme and utilize the molecule as a prosthetic group. Knowledge of heme-binding storage proteins in these organisms is in its infancy and much of what is known is from studies with vitellogenins (Vg) and more recently the main hemolymph storage protein in ixodid ticks characterized as a hemelipoglyco-carrier protein (CP). Data have also been reported from another arachnid, the black widow spider, Latrodectus mirabilis, and seem to suggest that the heme-binding capability of these large multimeric proteins is not a phenomenon found only in the Acari. CP appears to be most closely related to Vg in ticks in terms of primary structure but post-translational processing is different. Tick CP and L. mirabilis high-density lipoprotein 1 (HDL1) are similar in that they consist of two subunits of approximate molecular masses of 90 and 100 kDa, are found in the hemolymph as the dominant protein, and bind lipids, carbohydrates and cholesterol. CP binds heme which may also be the case for HDL1 since the protein was found to contain a brown pigment when analyzed by native polyacrylamide gel electrophoresis. Vgs in ticks are composed of multiple subunits and are the precursor of the yolk protein, vitellin. The phylogeny of these proteins, regulation of gene expression and putative functions of binding and storing heme throughout reproduction, blood-feeding and development are discussed. Comparisons with non-chelicerate arthropods are made in order to highlight the mechanisms and putative functions of heme-binding storage proteins and their possible critical function in the evolution of hematophagy.     

Molecular phylogeny of extant horseshoe crabs (Xiphosura, Limulidae) indicates Paleogene diversification of Asian species

Horseshoe crabs are marine invertebrates well known for their exceptionally low rates of diversification during their entire evolutionary history. Despite the low species diversity in the group, the phylogenetic relationships among the extant species, especially among the three Asian species are still unresolved. Here we apply a new set of molecular genetic data in combination with a wide geographic sampling of the intra-specific diversity to reinvestigate the evolutionary history among the four living limulid xiphosurans. Our analysis of the intraspecific diversity reveals low levels of connectivity among Carcinoscorpius rotundicauda lineages, which can be explained by the estuarine-bound ecology of this species. Moreover, a clear genetic break across the Thai-Malay Peninsula suggests the presence of cryptic species in C. rotundicauda. The limulid phylogeny finds strong support for a monophyletic genus Tachypleus and a diversification of the three Asian species during the Paleogene period, with speciation events well separated in time by several million years. The tree topology suggests that the three Asian species originated in central South East Asia from a marine stem group that inhabited the shallow coastal waters between the Andaman Sea, Vietnam, and Borneo. In this region C. rotundicauda probably separated from the Tachypleus stem group by invading estuarine habitats, while Tachypleus tridentatus most likely migrated northeast along the Southern coast of China and towards Japan.     

Parallel evolution of truncated transfer RNA genes in arachnid mitochondrial genomes

The cloverleaf secondary structure of transfer RNA (tRNA) is highly conserved across all forms of life. Here, we provide sequence data and inferred secondary structures for all tRNA genes from 8 new arachnid mitochondrial genomes, including representatives from 6 orders. These data show remarkable reductions in tRNA gene sequences, indicating that T-arms are missing from many of the 22 tRNAs in the genomes of 4 out of 7 orders of arachnids. Additionally, all opisthothele spiders possess some tRNA genes that lack sequences that could form well-paired aminoacyl acceptor stems. We trace the evolution of T-arm loss onto phylogenies of arachnids and show that a genome-wide propensity to lose sequences that encode canonical cloverleaf structures likely evolved multiple times within arachnids. Mapping of structural characters also shows that certain tRNA genes appear more evolutionarily prone to lose the sequence coding for the T-arm and that once a T-arm is lost, it is not regained. We use tRNA structural data to construct a phylogeny of arachnids and find high bootstrap support for a clade that is not supported in phylogenies that are based on more traditional morphological characters. Together, our data demonstrate variability in structural evolution among different tRNAs as well as evidence for parallel evolution of the loss of sequence coding for tRNA arms within an ancient and diverse group of animals.     

Embryonic expression patterns of Wnt genes in the RTA-clade spider Cupiennius salei

Spiders represent widely used model organisms for chelicerate and even arthropod development and evolution. Wnt genes are important and evolutionary conserved factors that control and regulate numerous developmental processes. Recent studies comprehensively investigated the complement and expression of spider Wnt genes revealing conserved as well as diverged aspects of their expression and thus (likely) function among different groups of spiders representing Mygalomorphae (tarantulas), and both main groups of Araneae (true spiders) (Haplogynae/Synspermiata and Entelegynae). The allegedly most modern/derived group of entelegyne spiders is represented by the RTA-clade of which no comprehensive data on Wnt expression were available prior to this study. Here, we investigated the embryonic expression of all Wnt genes of the RTA-clade spider Cupiennius salei. We found that most of the Wnt expression patterns are conserved between Cupiennius and other spiders, especially more basally branching species. Surprisingly, most differences in Wnt gene expression are seen in the common model spider Parasteatoda tepidariorum (a non-RTA clade entelegyne species). These results show that data and conclusions drawn from research on one member of a group of animals (or any other organism) cannot necessarily be extrapolated to the group as a whole, and instead highlight the need for comprehensive taxon sampling.     

Gene silencing in the spider mite <Emphasis Type="Italic">Tetranychus urticae</Emphasis>: dsRNA and siRNA parental silencing of the <Emphasis Type="Italic">Distal-less</Emphasis> gene

A major prerequisite to understanding the evolution of developmental programs includes an appreciation of gene function in a comparative context. RNA interference (RNAi) represents a powerful method for reverse genetics analysis of gene function. However, RNAi protocols exist for only a handful of arthropod species. To extend functional analysis in basal arthropods, we developed a RNAi protocol for the two-spotted spider mite Tetranychus urticae focusing on Distal-less (Dll), a conserved gene involved in appendage specification in metazoans. First, we describe limb morphogenesis in T. urticae using confocal and scanning electron microscopy. Second, we examine T. urticae Dll (Tu-Dll) mRNA expression patterns and correlate its expression with appendage development. We then show that fluorescently labeled double-stranded RNA (dsRNA) and short interfering RNA (siRNA) molecules injected into the abdomen of adult females are incorporated into the oviposited eggs, suggesting that dsRNA reagents can be systemically distributed in spider mites. Injection of longer dsRNA as well as siRNA induced canonical limb truncation phenotypes as well as the fusion of leg segments. Our data suggest that Dll plays a conserved role in appendage formation in arthropods and that such conserved genes can serve as reliable starting points for the development of functional protocols in nonmodel organisms.     

A trigonotarbid arachnid from the Lower Devonian of Tredomen, Wales

A new trigonotarbid (Arachnida: Trigonotarbida) Arianrhoda bennetti gen. et sp. nov. is described from the Lower Devonian (Lochkovian) of a quarry near Tredomen, Powys, mid Wales, UK. This relatively complete specimen is the first record of a pre-Carboniferous arachnid from Wales, one of only a handful of early Devonian arachnids, and the second oldest trigonotarbid recorded. Based on the rounded prosomal dorsal shield and the relatively narrow, elongate opisthosoma we refer this new fossil to the family Anthracosironidae. A distinct flange-like ornament on the leg 4 tibia in the new fossil is unique among trigonotarbids and is the primary autapomorphy for the new genus.