Reverse taxonomy for elucidating diversity of insect-associated nematodes: a case study with termites

Advances in Migratory Plant Endoparasitic Nematode Effectors

Unlike sedentary plant-parasitic nematodes, migratory plant endoparasitic nematodes (MPENs) are unable to establish permanent feeding sites, and all developmental stages (except eggs) can invade and feed on plant tissues and can be easily overlooked because of the unspecific symptoms. They cause numerous economic losses in agriculture, forestry, and horticulture. In order to understand the pathogenetic mechanism of MPENs, here we describe research on functions and host targets focused on currently identified effectors from six MPENs, namely Radopholus similis, Pratylenchus spp., Ditylenchus destructor, Bursaphelenchus xylophilus, Aphelenchoides besseyi, and Hirschmanniella oryzae. This information will provide valuable insights into understanding MPEN effectors and for future fostering advancements in plant protection.

Taxonomy of Tobrilidae species from the Alkaline Lakes of the western Nebraska Sandhills

Six distinct COI mitochondrial Haplotype Groups (HG) are morphologically, ecologically, and genetically characterized from the aquatic nematode family Tobrilidae. Collection locations included the extreme habitats of the Alkaline Lakes in the western Nebraska Sandhills and the contaminated stream, Johnson Creek, bordering the AltEn 2021 catastrophic pesticide release near the village of Mead in eastern Nebraska. Maximum likelihood and genetic distance metrics supported the genetic integrity of the haplotype groups. Discriminant function analysis of COI haplotype group datasets of combined morphological characters and soil chemistry attributes for both male and female Tobrilidae were classified correctly in all but one case. Scanning electron microscopy revealed new details about amphid apertures, male supplements, and spicules. Partial 18S gene phylogeny suggests that the genus Semitobrilus may not be a member of the subfamily Neotobrilinae, and three specimens in the 226 tobrilid dataset provide evidence of incongruence between COI and 18S derived phylogenies. Given the strong signal provided by the environmental chemistry data, tobrilid mitochondrial haplotypes may well have value as environmental indicators.

Two strains Neocosmosporastercicola (Sordariomycetes, Nectriaceae) with high nematicidal activity, isolated from the cysts of Globodera sp. (Heteroderidae) in China

Plant-parasitic nematodes (PPNs) are significant pests that result in considerable economic losses in global crop production. Due to the high toxicity of chemical nematicides, there is a need to develop new strategies for nematode control. In this context, nematophagous fungi may offer a viable option for biological control. Two fungal strains (GUCC2212 and GUCC2232) were isolated from cysts of Globodera sp., identified as Neocosmosporastercicola. The fungal filtrates of the strains were evaluated for their nematicidal activity against three species of PPNs: Aphelenchoidesbesseyi, Bursaphelenchusxylophilus and Ditylenchusdestructor. The fermentation filtrates of two strains exhibited substantial toxicity towards the evaluated nematodes, with mortality rates reaching up to 100% within 72 h. Concurrently, N.stercicola also demonstrated predatory and parasitic behavior. The eggs of Globodera sp. were parasitized by the two strains. N.stercicola represents a newly recorded species in China and a novel nematophagous species. In conclusion, the two strains of N.stercicola show promise as biocontrol agents for PPNs management.

Plectus of the Prairie: A Case Study of Taxonomic Resolution from a Nematode Biodiversity Survey

Taxonomic resolution is a critical component of biodiversity assessments. In this case study, we examined a single taxon within a larger study of nematode diversity to evaluate the taxonomic resolution of different diversity assessment methods. The selected taxon was the microbial-feeding genus Plectus, a group considered to include multiple cosmopolitan species. The methods included a morphological evaluation by light microscopy, Sanger sequencing of PCR amplicons of COI and 18S gene regions, and 18S metabarcoding sequencing. The study sites were 15 remnant tallgrass prairie plots in eastern Nebraska. In the morphological analysis, we observed two basic morphotypes, a short-tailed form with a small amphid and a long-tailed form with a large amphid. Sanger sequencing of COI sorted Plectus diversity into six distinct clades. The largest two of these six clades keyed to P. parietinus and P. rhizophilus based on morphology. BLAST analysis with COI revealed no close matches in GenBank. Sanger sequencing of the 18S region did not differentiate the six clades. These results illustrate that the method of diversity assessment strongly influences estimates of biodiversity. An additional 95 Plectus specimens, from outside the remnant sites, added taxonomic breadth to the COI phylogenetic tree. There were no geographically widespread COI haplotypes and no evidence of cosmopolitan Plectus species.

A new strain of Volutella citrinella with nematode predation and nematicidal activity, isolated from the cysts of potato cyst nematodes in China

BACKGROUND

Plant parasitic nematodes (PPNs) are responsible for causing many plant diseases and are extremely difficult to control at present. Currently, due to the negative effects of chemical agents on the environment and human health, the development of new biological pesticides has become an important part of plant nematode control. Nematophagous fungi refers to a class of fungi that kill plant nematodes. Notably, a large number of nematophagous fungi resources remain to be studied. The objective of our study was to use in vitro screening to identify nematophagous fungi and select strains that were highly active against nematodes, providing a primary research for the development and utilization of new nematophagous fungi.

RESULTS

A new nematophagous fungal strain (GUCC2219) was isolated from cysts of possibly Globodera spp. and Heterodera spp., identified as Volutella citrinella. The hyphae of V. citrinella produced ring structures of variable size and exhibited predatory and nematicidal activity. The hyphal predation rates (in vitro) against three species of nematodes, Aphelenchoides besseyi, Bursaphelenchus xylophilus, and Ditylenchus destructor, averaged 59.45, 33.35, and 50.95%, respectively, while the fermentation broth produced by the fungus exhibited mortality rates of 100, 100, and 55.63%, respectively, after 72 h.

CONCLUSION

V. citrinella is a new strain with nematophagous properties, which are a novel discovery. At the same time, this is the first report of nematicidal and nematode predation activity in the genus Volutella.

Nutritional symbiosis and ecology of host-gut microbe systems in the Blattodea

Cockroaches and termites (Order: Blattodea) have been the subject of substantial research attention for over a century due, in part, to a subset of them having a strong propensity to cohabitate with humans and their structures. Recent research has led to numerous insights into their behavior, physiology, and ecology, as well as their ability to harbor taxonomically diverse microbial communities within their digestive systems, which include taxa that contribute to host growth and development. Further, recent investigations into the physiological and behavioral adaptations that enable recalcitrant polysaccharide digestion and the maintenance of microbial symbionts in cockroaches and termites suggests that symbionts contribute significantly to nutrient provisioning and processing.

Molecular profiling of nematode associates with Rhynchophorus ferrugineus in southern Italy

A survey of nematodes associated with the red palm weevil Rhynchophorus ferrugineus was conducted in southern Italy in 2015 and 2016 in order to create a species inventory and obtain data about nematode biodiversity. A total of 70 insect samples (pupae and adults) were collected from infested Phoenix canariensis, Phoenix dactylifera, and Chamaerops humilis palms in three Italian Regions: sampling took place at 11 locations in Apulia, 1 in Basilicata, and 1 in Sardinia regions. Individual insects were dissected to determine nematode presence, and different nematode species were also recovered from red palm weevil cocoons collected at the sites in Apulia. Individual nematodes were molecularly identified by sequencing the ITS, D2-D3 expansion domains of the 28SrRNA gene and the mitochondrial COI and inferring the phylogenetic relationships. The insect-associated nematofauna identified belonged to the families Rhabditidae, Cephalobidae, and Diplogastridae. Just two nematode species, Teratorhabditis synpapillata and Mononchoides macrospiculum, were always found in association with adult insects and cocoons taken from all sampling sites. This paper reports on the biodiversity of the nematodes associated with R. ferrugineus and on current knowledge of the specific habitat of specialized and divergent entomophilic nematodes.

Nematode-free agricultural system of a fungus-growing termite

Fungus-growing termites forage dead plant materials from the field to cultivate symbiotic Termitomyces fungi in the nest. Termite foraging behavior and the entry of symbiotic arthropod inquilines may transfer nematodes into a nest and adversely affect fungus production. To test whether nematodes were transferred to fungus gardens by termites and inquilines, we examined the occurrence of nematodes in fungus gardens, five termite castes, and nine species of inquilines of a fungus-growing termite, Odontotermes formosanus. Our results revealed that nematodes were commonly carried by foraging termites and beetle inquilines. Numerous nematodes were found under the beetle elytra. No nematodes were found on termite larvae, eggs, and wingless inquilines. In addition, nematodes rarely occurred in the fungus garden. By observing the response of nematodes to three species of Termitomyces spp. and the fungus gardens, we confirmed that the fungus and fungus gardens are not actually toxic to nematodes. We suggest that nematodes were suppressed through grooming behavior and gut antimicrobial activity in termites, rather than through the antimicrobial activity of the fungus.

Soil nematode abundance and diversity from four vegetation types in Central Mexico

Soil nematode abundance and MOTU diversity were estimated from a geographically broad area of Mexico that included four out of seven recognised vegetation types. Vegetation types were assessed for abundance and diversity of nematode communities and inferred ecological relationships between them. Soils were sampled from tropical rainforest, tropical dry deciduous forest, temperate coniferous forest and xerophytic shrub during 2013, 2014 and 2015. Fourteen sampling sites withca10-20 samples per site from 11 localities spread across Central Mexico were assessed. Altitudes sampled ranged from 113 m a.s.l. (tropical coastal plain) to 2400 m a.s.l. (Trans-Mexican Volcanic Belt). Samples were drawn from conserved and cultivated plots from each sampling site covering an area of ⩾100 m2. A total of 13 263 individuals from 25 identified families of nematodes were collected. Family abundance and complementarity indices between sites revealed to some extent the affinities between vegetation types. Nevertheless, statistical analyses revealed no differences between nematode family abundances between sites, only between families across all sites. Molecular operational taxonomic units (MOTU) methods were employed as a framework to assess biodiversity. From these, 77 high-quality sequences for taxonomic barcoding were recovered and later identified with morphological traits. Only six sequences matched at a 98-99% level with those reported in GenBank. Sequences amounted to a total of 41 MOTU, where 100% of the MOTU from both conserved and disturbed tropical rainforest, tropical dry deciduous forest and xerophytic shrub exhibited a ⩾3% cut-off genetic identity, whilst temperate coniferous forest and disturbed temperate coniferous forest showed 73% and 70% respectively. In addition, 12.2% MOTU were shared among localities and 87.8% exhibited an apparently locality-limited distribution. The potential for a considerable diversity of nematodes, as revealed from a small sample of MOTU diversity, is discussed.

Metabarcoding of marine nematodes - evaluation of reference datasets used in tree-based taxonomy assignment approach

Background

Metabarcoding is becoming a common tool used to assess and compare diversity of organisms in environmental samples. Identification of OTUs is one of the critical steps in the process and several taxonomy assignment methods were proposed to accomplish this task. This publication evaluates the quality of reference datasets, alongside with several alignment and phylogeny inference methods used in one of the taxonomy assignment methods, called tree-based approach. This approach assigns anonymous OTUs to taxonomic categories based on relative placements of OTUs and reference sequences on the cladogram and support that these placements receive.

New information

In tree-based taxonomy assignment approach, reliable identification of anonymous OTUs is based on their placement in monophyletic and highly supported clades together with identified reference taxa. Therefore, it requires high quality reference dataset to be used. Resolution of phylogenetic trees is strongly affected by the presence of erroneous sequences as well as alignment and phylogeny inference methods used in the process. Two preparation steps are essential for the successful application of tree-based taxonomy assignment approach.

Completing the above mentioned preparation steps is expected to decrease the number of unassigned OTUs and thus improve the results of the tree-based taxonomy assignment approach.

Exploring the Leaf Beetle Fauna (Coleoptera: Chrysomelidae) of an Ecuadorian Mountain Forest Using DNA Barcoding

Background

Tropical mountain forests are hotspots of biodiversity hosting a huge but little known diversity of insects that is endangered by habitat destruction and climate change. Therefore, rapid assessment approaches of insect diversity are urgently needed to complement slower traditional taxonomic approaches. We empirically compare different DNA-based species delimitation approaches for a rapid biodiversity assessment of hyperdiverse leaf beetle assemblages along an elevational gradient in southern Ecuador and explore their effect on species richness estimates.

Methodology/Principal Findings

Based on a COI barcode data set of 674 leaf beetle specimens (Coleoptera: Chrysomelidae) of 266 morphospecies from three sample sites in the Podocarpus National Park, we employed statistical parsimony analysis, distance-based clustering, GMYC- and PTP-modelling to delimit species-like units and compared them to morphology-based (parataxonomic) species identifications. The four different approaches for DNA-based species delimitation revealed highly similar numbers of molecular operational taxonomic units (MOTUs) (n = 284–289). Estimated total species richness was considerably higher than the sampled amount, 414 for morphospecies (Chao2) and 469–481 for the different MOTU types. Assemblages at different elevational levels (1000 vs. 2000 m) had similar species numbers but a very distinct species composition for all delimitation methods. Most species were found only at one elevation while this turnover pattern was even more pronounced for DNA-based delimitation.

Conclusions/Significance

Given the high congruence of DNA-based delimitation results, probably due to the sampling structure, our study suggests that when applied to species communities on a regionally limited level with high amount of rare species (i.e. ~50% singletons), the choice of species delimitation method can be of minor relevance for assessing species numbers and turnover in tropical insect communities. Therefore, DNA-based species delimitation is confirmed as a valuable tool for evaluating biodiversity of hyperdiverse insect communities, especially when exact taxonomic identifications are missing.

Intraspecific variability of morphological characters in the species-rich deep-sea genus Acantholaimus Allgén, 1933 (Nematoda: Chromadoridae)

Acantholaimusis a species-rich genus of deep-sea nematodes, often with dozens of species found at the same locality but each represented by single or few individuals. Species discrimination by morphological characters in this genus is therefore often difficult due to transitional forms that may be referred to several species because of lack of data on intraspecific variability. The aim of this study was to evaluate the intraspecific variability of morphological characters that are most often used inAcantholaimustaxonomy, in order to distinguish those which are most informative for species differentiation. A reverse taxonomic approach was applied for initial species discrimination. Two loci, one each from small and large subunits of rRNA, were sequenced for 59Acantholaimusspecimens from two deep-sea locations. Twenty-seven Molecular Operational Taxonomic Units (MOTU) were identified, of which 12 were represented by more than one individual. These were then analysed for intraspecific variability in morphological characters. Some of the examined characters showed high intraspecific variability; specifically: length of cephalic setae; distance from anterior end to amphid; shape of anterior setae; position and arrangement of cervical setae. In the absence of genetic data, these characters should be used with caution for differential diagnoses or species discrimination. Other characters were more conservative within the same MOTU: body proportions; length of outer labial setae; amphidial diam.; appearance of lateral field; general arrangement of cervical setae; and shape of tail. These characters may be successfully used for species discrimination in the absence of molecular data.

Description of Phasmarhabditis californica n. sp. and first report of P. papillosa (Nematoda: Rhabditidae) from invasive slugs in the USA

A new species ofPhasmarhabditisand the known speciesP. papillosawere isolated from cadavers of invasive slugs in California.Phasmarhabditis papillosais the type of the genus and has not previously been reported from the Americas. Both species are characterised based on morphology, morphometrics and molecular data. Molecular phylogenies were inferred from concatenated DNA sequence alignments of nearly complete SSU and the D2-D3 domains of the LSU rDNA.Phasmarhabditis californican. sp. is characterised by a robust body, mature egg-laying specimens almost spindle-shaped when relaxed, 1.5 (1.3-1.8) mm long, tapering to a bluntly rounded anterior end with stoma about as long as lip region diam., six inner labial papillae and four outer cephalic papillae, pharynx with rounded to pyriform basal bulb, vulva located halfway along the body, hermaphroditic, didelphic, amphidelphic, and short, conoid tail constricted at one-third its length with prominent phasmids. Not a single male was found among five strains.Phasmarhabditis papillosais gonochoristic and has a longer isthmus, pyriform basal bulb and longer, dome-shaped spicate female tail constricted halfway along its length. Sequence analysis revealed unambiguous autapomorphies forP. papillosaandP. californican. sp. Phylogenetic analyses placed these two species in a monophyletic clade comprisingPhasmarhabditisspecies and other gastropod-parasitic taxa. Morphology, genetic distance, reproductive strategy and nucleotide autapomorphies support the new taxon.

Description of Aphelenchoides huntensis sp. n. (Nematoda: Aphelenchoididae) isolated from Pinus sylvestris in western Iran

Aphelenchoides huntensissp. n. is described and illustrated from wood and bark samples of a dead scots pine in Kermanshah Province, western Iran. The new species has a body length of 507-673 μm (females) and 636-640 μm (males). The cuticle is weakly annulated with four lateral lines. Lip region offset. The stylet is 9.5 (9.0-10.0) μm long with small basal swellings. The excretory pore is locatedcaone body diam. posterior to the metacorpus valve. The spicules are large with the dorsal limb 37 or 38 μm long (n = 2), and have a prominent rostrum and rounded apex. The female tail is conical, the terminus having a complicated step-like projection, usually with many tiny nodular protuberances. The male tail bears six (2 + 2 + 2) caudal papillae and a well-developed mucron. The new species belongs to the Group 2 category ofAphelenchoidesspecies. Phylogenetic analysis based on the 18S and 28S D2-D3 regions of rDNA confirmed its status as a new species.

A reverse taxonomic approach to assess macrofaunal distribution patterns in abyssal Pacific polymetallic nodule fields

Heightened interest in the exploitation of deep seafloor minerals is raising questions on the consequences for the resident fauna. Assessing species ranges and determination of processes underlying current species distributions are prerequisites to conservation planning and predicting faunal responses to changing environmental conditions. The abyssal central Pacific nodule belt, located between the Clarion and Clipperton Fracture Zones (CCZ), is an area prospected for mining of polymetallic nodules. We examined variations in genetic diversity and broad-scale connectivity of isopods and polychaetes across the CCZ. Faunal assemblages were studied from two mining claims (the eastern German and French license areas) located 1300 km apart and influenced by different productivity regimes. Using a reverse taxonomy approach based on DNA barcoding, we tested to what extent distance and large-scale changes in environmental parameters lead to differentiation in two macrofaunal taxa exhibiting different functions and life-history patterns. A fragment of the mitochondrial gene Cytochrome Oxidase Subunit 1 (COI) was analyzed. At a 97% threshold the molecular operational taxonomic units (MOTUs) corresponded well to morphological species. Molecular analyses indicated high local and regional diversity mostly because of large numbers of singletons in the samples. Consequently, variation in composition of genotypic clusters between sites was exceedingly large partly due to paucity of deep-sea sampling and faunal patchiness. A higher proportion of wide-ranging species in polychaetes was contrasted with mostly restricted distributions in isopods. Remarkably, several cryptic lineages appeared to be sympatric and occurred in taxa with putatively good dispersal abilities, whereas some brooding lineages revealed broad distributions across the CCZ. Geographic distance could explain variation in faunal connectivity between regions and sites to some extent, while assumed dispersal capabilities were not as important.

Allodiplogaster josephi n. sp. and A. seani n. sp. (Nematoda: Diplogastridae), associates of soil-dwelling bees in the eastern USA

Two commensal associates of bees,Allodiplogaster josephin. sp. from the Dufour’s gland of a cellophane bee (Colletes thoracicus) from Maryland, USA, andA. seanin. sp. from the abdominal glands of an andrenid bee (Andrena alleghaniensis) from New York, USA, are described and illustrated. Both species were collected as dauers from their respective hosts and cultured on bacteria on tryptic soy broth (TSB) or NGM agar.Allodiplogaster josephin. sp. andA. seanin. sp. are morphologically closer to each other than to other species ofAllodiplogaster, which was recently revised to include 37 valid species. However, the two new species are distinguished by reproductive isolation, shape of the spicule manubrium, host associations and molecular characters, the latter in sequences of the near-full length small subunit (SSU) rRNA gene, D2-D3 expansion segments of the large subunit (LSU) rRNA gene and partial mitochondrial COI. Morphological characterisation was supplemented by scanning electron microscopy (SEM), which revealed furcation of both v5 and v6 male genital papillae, consistent with previous reports for species of thehenrichaegroup ofAllodiplogaster.

A review of the taxonomy, phylogeny, distribution and co-evolution of Schistonchus Cobb, 1927 with proposal of Ficophagus n. gen. and Martininema n. gen. (Nematoda: Aphelenchoididae)

The purposes of this paper are to clarify the taxonomic status of the fig-pollinating wasp associateSchistonchussensu lato(Nematoda: Aphelenchoididae) and to suggest directions for future research on the systematics, life history and ecology of the group. Molecular phylogenetic analyses suggest thatSchistonchus s.l.is polyphyletic, and the composition of the three major clades is outlined, together with information on nematode morphology, plant host species, associated pollinating wasp species, and distribution. Biological information and collection data is presented forSchistonchus s.l.fromFicussycones (Moracea) in Africa, Australia, Asia and Central America, and its putative phylogeny is discussed based on molecular and morphological evidence. Both wasps and figs are millions of years old and have worldwide distribution in tropical areas,i.e., opportunities forSchistonchus s.l.-like nematodes to have evolved could have occurred more than once. In addition, figs and their pollinating wasps have variable life histories, which could have provided opportunities forSchistonchus s.l.to also develop different life histories. However, these histories occur inside fig sycones and in association with wasps, which has apparently led to evolutionary convergence and extreme morphological conservatism. Diagnostic characters and their states, derived from examination of described species and morphospecies ofSchistonchus s.l.and informed by molecular phylogenetic inferences, are discussed and illustrated.Schistonchus sensu strictois redefined, andFicophagusn. gen. andMartinineman. gen. are proposed.Schistonchus s.s.is morphologically characterised by having the excretory pore opening in the region of, or posterior to, the metacorpus;Ficophagusn. gen. by having the excretory pore opening very near the cephalic region; andMartinineman. gen. by having it opening at the anterior end of the metacorpus. Several species ofSchistonchus s.s.have a labial disc, but there is no evidence of this in eitherFicophagusn. gen. orMartinineman. gen.

Phylogenetic analysis of entomoparasitic nematodes, potential control agents of flea populations in natural foci of plague

Entomoparasitic nematodes are natural control agents for many insect pests, including fleas that transmit Yersinia pestis, a causative agent of plague, in the natural foci of this extremely dangerous zoonosis. We examined the flea samples from the Volga-Ural natural focus of plague for their infestation with nematodes. Among the six flea species feeding on different rodent hosts (Citellus pygmaeus, Microtus socialis, and Allactaga major), the rate of infestation varied from 0 to 21%. The propagation rate of parasitic nematodes in the haemocoel of infected fleas was very high; in some cases, we observed up to 1,000 juveniles per flea specimen. Our study of morphology, life cycle, and rDNA sequences of these parasites revealed that they belong to three distinct species differing in the host specificity. On SSU and LSU rRNA phylogenies, these species representing three genera (Rubzovinema, Psyllotylenchus, and Spilotylenchus), constitute a monophyletic group close to Allantonema and Parasitylenchus, the type genera of the families Allantonematidae and Parasitylenchidae (Nematoda: Tylenchida). We discuss the SSU-ITS1-5.8S-LSU rDNA phylogeny of the Tylenchida with a special emphasis on the suborder Hexatylina.

Description of two Pseudaphelenchus species (Tylenchomorpha: Aphelenchoididae) associated with Asian termites and proposal of Tylaphelenchinae n. subfam

Two new Pseudaphelenchus species were discovered from subterranean and arboreal termites from the East Asian subtropics. Pseudaphelenchus sui n. sp. isolated from Coptotermes formosanus was collected from Miyako Isl., Okinawa, Japan, and P. scheffrahni n. sp. was isolated from Nasutitermes takasagoensis collected from the Kenting National Park in Taiwan. The two new species and P. vindai, previously described from Panamanian termites, are close to each other, and are not easily distinguished morphologically, i.e., these three are almost ‘cryptic species’. However, they can be distinguished based on minor morphological differences, including shape of male bursa (clearest in P. vindai, vague in P. sui n. sp. and intermediate in P. scheffrahni n. sp.), male tail tip (P. sui n. sp. often have a small mucron but the others do not), and in the character of the female tail (the two new species have stronger ventral curvature than P. vindai, and further, P. sui n. sp. has clear annulation at distal part, while the other species do not). Molecular phylogenetic analysis based upon near-full-length sequences of the small subunit of the ribosomal RNA gene suggested that Pseudaphelenchus and Tylaphelenchus form a strongly-supported clade at the base of the family Aphelenchoididae, and that Tylaphelenchus is included in Pseudaphelenchus as an inner clade. Therefore, based on their phylogenetic status and common morphological characters, e.g., small body and spherical median bulb, a subfamily, Tylaphelenchinae n. subfam. is proposed to embrace these two genera. In addition, Ruehmaphelenchus ipidicola n. comb. (= Tylaphelenchus ipidicola) is proposed.

Genetic and morphological divergences in the cosmopolitan deep-sea amphipod Eurythenes gryllus reveal a diverse abyss and a bipolar species

Eurythenes gryllus is one of the most widespread amphipod species, occurring in every ocean with a depth range covering the bathyal, abyssal and hadal zones. Previous studies, however, indicated the existence of several genetically and morphologically divergent lineages, questioning the assumption of its cosmopolitan and eurybathic distribution. For the first time, its genetic diversity was explored at the global scale (Arctic, Atlantic, Pacific and Southern oceans) by analyzing nuclear (28S rDNA) and mitochondrial (COI, 16S rDNA) sequence data using various species delimitation methods in a phylogeographic context. Nine putative species-level clades were identified within E. gryllus. A clear distinction was observed between samples collected at bathyal versus abyssal depths, with a genetic break occurring around 3,000 m. Two bathyal and two abyssal lineages showed a widespread distribution, while five other abyssal lineages each seemed to be restricted to a single ocean basin. The observed higher diversity in the abyss compared to the bathyal zone stands in contrast to the depth-differentiation hypothesis. Our results indicate that, despite the more uniform environment of the abyss and its presumed lack of obvious isolating barriers, abyssal populations might be more likely to show population differentiation and undergo speciation events than previously assumed. Potential factors influencing species' origins and distributions, such as hydrostatic pressure, are discussed. In addition, morphological findings coincided with the molecular clades. Of all specimens available for examination, those of the bipolar bathyal clade seemed the most similar to the 'true' E. gryllus. We present the first molecular evidence for a bipolar distribution in a macro-benthic deep-sea organism.