Going Deeper into High and Low Phylogenetic Relationships of Protura

Revisiting the four Hexapoda classes: Protura as the sister group to all other hexapods

Insects represent the most diverse animal group, yet previous phylogenetic analyses based on morphological and molecular data have failed to agree on the evolutionary relationships of early insects and their six-legged relatives (together constituting the clade Hexapoda). In particular, the phylogenetic positions of the three early-diverging hexapod lineages-the coneheads (Protura), springtails (Collembola), and two-pronged bristletails (Diplura)-have been debated for over a century, with alternative topologies implying drastically different scenarios of the evolution of the insect body plan and hexapod terrestrialization. We addressed this issue by sampling all hexapod orders and experimenting with a broad range of across-site compositional heterogeneous models designed to tackle ancient divergences. Our analyses support Protura as the earliest-diverging hexapod lineage ("Protura-sister") and Collembola as a sister group to Diplura, a clade corresponding to the original composition of Entognatha, and characterized by the shared possession of internal muscles in the antennal flagellum. The previously recognized 'Elliplura' hypothesis is recovered only under the site-homogeneous substitution models with partial supermatrices. Our cross-validation analysis shows that the site-heterogeneous CAT-GTR model, which recovers "Protura-sister," fits significantly better than homogeneous models. Furthermore, the morphologically unusual Protura are also supported as the earliest-diverging hexapod lineage by other lines of evidence, such as mitogenomes, comparative embryology, and sperm morphology, which produced results similar to those in this study. Our backbone phylogeny of hexapods will facilitate the exploration of the underpinnings of hexapod terrestrialization and megadiversity.

Further evidence of low infection frequencies of Wolbachia in soil arthropod communities

Endosymbiotic Alphaproteobacteria of the genus Wolbachia are exclusively transferred maternally from mother to offspring, but horizontal transfer across species boundaries seems to be frequent as well. However, the (ecological) mechanisms of how these bacteria are transferred between distantly related arthropod hosts remain unclear. Based on the observation that species that are part of the same ecological community often also share similar Wolbachia strains, host ecology has been hypothesized as an important factor enabling transmission and a key factor in explaining the global distribution of Wolbachia lineages. In this study, we focus on the diversity and abundance of Wolbachia strains in soil arthropods, a so far rather neglected community. We screened 82 arthropod morphotypes collected in the beech forest (dominated by Fagus sp.) soil in the area of Göttingen in central Germany for the presence of Wolbachia. By performing a PCR screen with Wolbachia-MLST markers (coxA, dnaA, fbpA, ftsZ, gatB, and hcpA), we found a rather low infection frequency of 12,2%. Additionally, we performed metagenomic screening of pooled individuals from the same sampling site and could not find evidence that this low infection frequency is an artefact due to PCR-primer bias. Phylogenetic analyses of the recovered Wolbachia strains grouped them in three known supergroups (A, B, and E), with the first report of Wolbachia in Protura (Hexapoda). Moreover, Wolbachia sequences from the pseudoscorpion Neobisium carcinoides cluster outside the currently known supergroup diversity. Our screening supports results from previous studies that the prevalence of Wolbachia infections seems to be lower in soil habitats than in above-ground terrestrial habitats. The reasons for this pattern are not completely understood but might stem from the low opportunity of physical contact and the prevalence of supergroups that are less suited for horizontal transfer.

Molecular diversity of Protura in southern High Appalachian leaf litter

The higher elevations of the southern Appalachian Mountains, U.S.A., host a rich, but little-studied fauna of Proturan hexapods. Here, we publish 117 Proturan barcode sequences from this region, estimated by automated species delimitation methods to represent 72 distinct species, whereas only nine species have previously been reported from the region. Two families, Eosentomidae and Acerentomidae, co-occur at most sampling sites, with as many as five species occurring in sympatry. Most populations exhibit very low haplotype diversity, but divergences amongst populations and amongst closely-related species are very high, a finding common to other phylogeographic studies of Proturans. Though we were unable to identify any of the barcodes to species, they form a useful, if preliminary, glimpse of southern Appalachian Proturan diversity.

A review of the hexapod tracheal system with a focus on the apterygote groups

Respiratory systems are key innovations for the radiation of terrestrial arthropods. It is therefore surprising that there is still a considerable lack of knowledge. In this review of the available information on tracheal systems of hexapods (with a focus on the apterygote lineages Protura, Collembola, Diplura, Archaeognatha and Zygentoma), we summarize available data on the spiracles (number, position and morphology), the shape and variability of tracheal branching patterns including anastomoses, the tracheal fine structure and the respiratory proteins. The available data are strongly fragmented, and information for most subgroups is missing. In various cases, individual observations for one species account for the knowledge of the entire order. The available data show that there are strong differences between but also within apterygote orders. We conclude that the available data are insufficient to derive detailed conclusions on the hexapod ground plan and outline the possible evolutionary scenarios for the tracheal system in this group.

Protura in Arctic Regions, with Description of Mastodonentomon n. gen. (Acerentomidae, Nipponentominae) and a Key to Known Arctic Taxa

Protura are widespread, but their presence in the Arctic was first noted only ca. 70 years ago and is still little acknowledged. This work compiles taxonomic information on proturans in the Arctic regions and adds unpublished data from Northern Siberia. Currently, this fauna is represented by 23 species in two orders and 14 genera. The large cosmopolitan genus Eosentomon is represented by only four species, whereas Acerentomidae is much more diverse, with 19 species in 13 genera (eight Nipponentominae, five Acerentominae). Most of the Arctic species possess a larger number of setae than species living in temperate regions. Based on several unique characters, a new genus, Mastodonentomon, is erected for Nipponentomon macleani, and the species is re-described with the original description supplemented with new characters, including head chaetotaxy, seta length, and porotaxy. Proturan occurrence in the Arctic is limited to Beringia, but the majority of species have restricted distributions and none have been found in both the American Arctic and Siberia. This implies relict origins and high levels of proturan endemism in the Arctic. This emerging view on biogeographical history is, however, hampered by the limited extent of available data, which highlights the need for considerably greater survey efforts. A key to Arctic proturans is provided to facilitate further studies.