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The rapid pace of species discovery outstrips the rate of species description in many taxa. This problem is especially acute for Caenorhabditis nematodes, where the naming of distinct species would greatly improve their visibility and usage for biological research, given the thousands of scientists studying Caenorhabditis. Species description and naming has been hampered in Caenorhabditis, in part due to the presence of morphologically cryptic species despite complete biological reproductive isolation and often enormous molecular divergence. With the aim of expediting species designations, here we propose and apply a revised framework for species diagnosis and description in this group. Our solution prioritizes reproductive isolation over traditional morphological characters as the key feature in delineating and diagnosing new species, reflecting both practical considerations and conceptual justifications. DNA sequence divergence criteria help prioritize crosses for establishing patterns of reproductive isolation among the many species of Caenorhabditis known to science, such as with the ribosomal internal transcribed spacer-2 (ITS2) DNA barcode. By adopting this approach, we provide new species name designations for 15 distinct biological species, thus increasing the number of named Caenorhabditis species in laboratory culture by nearly 3-fold. We anticipate that the improved accessibility of these species to the research community will expand the opportunities for study and accelerate our understanding of diverse biological phenomena.

Understanding how parasites evolved is crucial to understand the host and parasite interaction. The evolution of entomopathogenesis in rhabditid nematodes has traditionally been thought to have occurred twice within the phylum Nematoda: in Steinernematidae and Heterorhabditidae families, which are associated with the entomopathogenic bacteria Xenorhabdus and Photorhabdus, respectively. However, nematodes from other families that are associated with entomopathogenic bacteria have not been considered to meet the criteria for “entomopathogenic nematodes.” The evolution of parasitism in nematodes suggests that ecological and evolutionary properties shared by families in the order Rhabditida favor the convergent evolution of the entomopathogenic trait in lineages with diverse lifestyles, such as saprotrophs, phoretic, and necromenic nematodes. For this reason, this paper proposes expanding the term “entomopathogenic nematode” considering the diverse modes of this attribute within Rhabditida. Despite studies are required to test the authenticity of the entomopathogenic trait in the reported species, they are valuable links that represent the early stages of specialized lineages to entomopathogenic lifestyle. An ecological and evolutionary exploration of these nematodes has the potential to deepen our comprehension of the evolution of entomopathogenesis as a convergent trait spanning across the Nematoda. Through convergent evolution, the entomopathogeny in nematodes is widespread within the Rhaditidae families, owing to the shared ecological and evolutionary attributes raised from saprophagy, phoresis, and necromeny.

Background The first metazoan genome sequenced, that of Caenorhabditis elegans , has motivated animal genome evolution studies. To date > 50 species from the genus Caenorhabditis have been sequenced, allowing research on genome variation. Results In the present study, we describe a new gonochoristic species, Caenorhabditis niphades n. sp., previously referred as C. sp. 36, isolated from adult weevils ( Niphades variegatus ), with whom they appear to be tightly associated during its life cycle. Along with a species description, we sequenced the genome of C. niphades n. sp. and produced a chromosome-level assembly. A genome comparison highlighted that C. niphades n. sp. has the smallest genome (59 Mbp) so far sequenced in the Elegans supergroup, despite being closely related to a species with an exceptionally large genome, C. japonica . Conclusions The compact genome of C. niphades n. sp. can serve as a key resource for comparative evolutionary studies of genome and gene number expansions in Caenorhabditis species .

A new nematode species of the genus Phasmarhabditis was isolated from the body surface of a slug ( Philomycus bilineatus Benson, PB). Morphological and molecular analyses confirmed this nematode as a new species. The nematode was named Phasmarhabditis zhejiangensis sp. nov. (Nematoda: Rhabditidae) and is dioecious. In males, the open bursa with genital papillae is characterized by the formula 1-1-1-2-1-3, and the spicule length is 58μm. In female, the vulva is located approximately in the middle of the body. The nematode belongs to papillosa group because of its tail shape pointed with filiform tip. The phasmids are rod-shaped. The posterior anus is slightly swollen. P . zhejiangensis was further characterized by internal transcribed spacer (ITS), 18S rDNA and 28S rDNA sequences. After the sequencing results were compared with sequences available from the National Center for Biotechnology Information (NCBI), the maximum similarities of ITS, 18S and 28S sequences were 89.81%, 96.22% and 95.28%, respectively. Phylogenetic analyses placed Phasmarhabditis zhejiangensis sp. nov. in the genus Phasmarhabditis .

The spotted-wing drosophila, Drosophila suzukii Matsumura, is an exotic species in North America and represents a major threat to fruit production. Efforts to manage D. suzukii have focused primarily on insecticides, but such controls may, at times, be unreliable, given that D. suzukii larvae are often ensconced within fruit.The fruit interior, however, may represent suitable foraging substrates for carnivorous/entomopathogenic nematodes. In preliminary trials, a rare nematode species, Oscheius onirici Torrini et al., was shown to be highly virulent against D. suzukii when the nematodes were applied directly to fly larvae. To address the more important question of whether this nematode would be as virulent when applied to fruit, we set up assays in which blueberries were infested with D. suzukii larvae and then sprayed with O. onirici infective juveniles (IJs). Across two laboratory trials, O. onirici IJs suppressed D. suzukii puparia by 78.2%. Oscheius onirici IJs were able to search effectively within fruit substrates, find the fly larvae therein, and kill the flies before they could pupariate. Oscheius onirici, therefore, may represent a viable new bio-control agent for D. suzukii management and should be field-tested across a broader diversity of cropping systems.

Pelodera (A. Schneider, 1866) is a Clade V nematode genus of the Rhabditidae family and close relative of Caenorhabditis elegans . The genus comprises 26 species poised between free-living, commensal, and parasitic lifestyles, as well as species with underappreciated roles in soil nutrient cycling. Pelodera strongyloides , the type species, is a cause of follicular larva migrans among humans and other mammals but can be confused with hookworm folliculitis. With minimal genetic resources available, the capacity to identify and diagnose infectious species and strains of Pelodera are limited. Thus, the ecology of Pelodera species as both parasites and commensalists is likely underreported and possibly mischaracterised. Similarly, decomposition biome studies report Pelodera and other Rhabditidae as dominant taxa during decay but require greater genetic resources to classify species. To renew appreciation for these neglected model organisms, this review collates available literature to detail Pelodera associations across mammalian and invertebrate hosts and discusses traits that drive host association such as dauer formation, waving (nictation), phoresy, and tissue invasion. The potential for Pelodera as a satellite model to C. elegans is also discussed since Pelodera are readily culturable while having inducible parasitic forms. Research into Pelodera will not only improve understanding of their ecology and contribution to decomposition but may prove invaluable in identification of behaviours underpinning parasitism and parasite origins. Utilising Pelodera facultative parasites may be essential to triangulate and resolve differences between C. elegans and obligate nematode parasites with similar niches such as hookworms, lungworms, Strongyloides , and gastrointestinal nematodes of livestock. Graphical Abstract

Background The family Rhabdiasidae (Nematoda: Rhabditida) is a globally distributed group of nematode parasites, with over 110 species parasitic mainly in amphibians and reptiles. However, the systematic position of the family Rhabdiasidae in the order Rhabditida remains unsolved, and the evolutionary relationships among its genera are still unclear. Moreover, the present knowledge of the mitochondrial genomes of rhabdiasids remains limited. Methods Two rhabdiasid species: Rhabdias kafunata Sata, Takeuchi & Nakano, 2020 and R. bufonis (Schrank, 1788) collected from the Asiatic toad Bufo gargarizans Cantor (Amphibia: Anura) in China, were identified based on morphology (light and scanning electron microscopy) and molecular characterization (sequencing of the nuclear 28S and ITS regions and mitochondrial cox1 and 12S genes). The complete mitochondrial genomes of R. kafunata and R. bufonis were also sequenced and annotated for the first time. Moreover, phylogenetic analyses based on the amino acid sequences of 12 protein-coding genes (PCGs) of the mitochondrial genomes were performed to clarify the systematic position of the family Rhabdiasidae in the order Rhabditida using maximum likelihood (ML) and Bayesian inference (BI). The phylogenetic analyses based on the 28S + ITS sequences, were also inferred to assess the evolutionary relationships among the genera within Rhabdiasidae. Results The detailed morphology of the cephalic structures, vulva and eggs in R. kafunata and R. bufonis was revealed using scanning electron microscopy (SEM) for the first time. The characterization of 28S and ITS regions of R. kafunata was reported for the first time. The mitogenomes of R. kafunata and R. bufonis are 15,437 bp and 15,128 bp long, respectively, and both contain 36 genes, including 12 PCGs (missing atp8 ). Comparative mitogenomics revealed that the gene arrangement of R. kafunata and R. bufonis is different from all of the currently available mitogenomes of nematodes. Phylogenetic analyses based on the ITS + 28S data showed Neoentomelas and Kurilonema as sister lineages, and supported the monophyly of Entomelas , Pneumonema , Serpentirhabdias and Rhabdias . Mitochondrial phylogenomic results supported Rhabdiasidae as a member of the superfamily Rhabditoidea in the suborder Rhabditina, and its occurrance as sister to the family Rhabditidae. Conclusions The complete mitochondrial genome of R. kafunata and R. bufonis were reported for the first time, and two new gene arrangements of mitogenomes in Nematoda were revealed. Mitogenomic phylogenetic results indicated that the family Rhabdiasidae is a member of Rhabditoidea in Rhabditina, and is closely related to Rhabditidae. Molecular phylogenies based on the ITS + 28S sequence data supported the validity of Kurilonema , and showed that Kurilonema is sister to Neoentomelas . The present phylogenetic results also indicated that the ancestors of rhabdiasids seem to have initially infected reptiles, then spreading to amphibians. Graphical Abstract

The entomopathogenic nematode Heterorhabditis bacteriophora (Nematoda: Rhabditidae) is used in biological insect control. Their dauer juveniles (DJs) are free-living and developmentally arrested, invading host insects. They carry cells of their bacterial symbiont Photorhabdus spp. in the intestine. Once inside the insect´s hemolymph the DJs perceive a food signal, triggering them to exit the DJ stage and regurgitate the Photorhabdus cells into the insect’s haemocoel, which kill the host and later provide essential nutrients for nematode reproduction. The exit from the DJ stage is called “recovery”. For commercial pest control, nematodes are industrially produced in monoxenic liquid cultures. Artificial media are incubated with Photorhabdus before DJs are added. In absence of the insect’s food signal, DJs depend on unknown bacterial food signals to trigger exit of the DJ stage. A synchronized and high DJ recovery determines the success of the industrial in vitro production and can significantly vary between nematode strains, inbred lines and mutants. In this study, fourteen bacterial strains from H. bacteriophora were isolated and identified as P. laumondii , P. kayaii and P. thracensis . Although the influence of bacterial supernatants on the DJ recovery of three inbred lines and two mutants differed significantly, the bacterial impact on recovery has a subordinate role whereas nematode factors have a superior influence. Recovery of inbred lines decreased with age of the DJs. One mutant (M31) had very high recovery in bacterial supernatant and spontaneous recovery in Ringer solution. Another mutant (M88) was recovery defective.

Nematodes in the genus Oscheius (Rhabditidae) have traditionally been regarded as free-living bacteriophagous or necromenic associates of insects. Over the past two decades, however, multiple Oscheius species and isolates have been shown to express facultative pathogenicity toward insects and, in some cases, parasitism of mollusks. This has stimulated interest in Oscheius as a complementary group of biological control agents that may function under conditions limiting classical entomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis. Here, we synthesize current knowledge on Oscheius taxonomy and diversity, life-history strategies, bacterial associations and virulence mechanisms, evidence for control of insect and mollusk pests, and recent advances in chemo-ecology relevant to host finding. We emphasize that Oscheius represents a continuum of ecological strategies, and we adopt conservative terminology in which “entomopathogenic” is reserved for Oscheius species/isolates that meet operational criteria of insect pathogenicity. Finally, we highlight key barriers to wider implementation—strain variability, bacterial partner instability, non-target and community effects, and production/quality control needs—and propose research priorities for the development of robust, field-reliable Oscheius-based biocontrol.

An insect parasitic nematode belonging to the genus Oscheius was recovered from the agricultural soils from the Hapur district in western Uttar Pradesh, India. Morphological studies on this species exhibited its high resemblance with two Pakistani species: Oscheius siddiqii and O. niazii. No molecular data are available for these taxa but, morphologically, both species do not differ significantly from our strains and each other. Hence, these nematodes can be considered conspecific, and the correct name for this taxon is O. siddiqii, the first described species. The phylogenetic analyses of the ITS-, 18S-, and the 28S rDNA sequences showed that O. siddiqii is a sister taxon to the group formed by Oscheius microvilli, O. myriophilus, O. safricanus, and several unidentified Oscheius species. Additionally, our analyses show that based on molecular and morphological data, the species Oscheius rugaoensis and O. microvilli cannot be distinguished from O. chongmingensis and O. myriophilus, respectively, and are thus considered junior synonyms of these taxa. Furthermore, the available data are not sufficient to evaluate the status of Oscheius basothovii and O. safricanus, which are, in consequence, considered species inquirendae. These findings highlight the necessity of the proper morphological and molecular characterisation of the described Oscheius species.