Family Tylenchidae (Nematoda): an overview and perspectives

Molecular and Morphological Characterization of Tylenchus Zeae n. Sp. (Nematoda: Tylenchida) from Corn (Zea Mays) in South Carolina

Specimens of a tylenchid nematode were recovered in 2019 from soil samples collected from a corn field, located in Pickens County, South Carolina, USA. A moderate number of Tylenchus sp. adults (females and males) were recovered. Extracted nematodes were examined morphologically and molecularly for species identification, which indicated that the specimens of the tylenchid adults were a new species, described herein as Tylenchus zeae n. sp. Morphological examination and the morphometric details of the specimens were very close to the original descriptions of Tylenchus sherianus and T. rex. However, females of the new species can be differentiated from these species by body shape and length, shape of excretory duct, distance between anterior end and esophageal intestinal valve, and a few other characteristics given in the diagnosis. Males of the new species can be differentiated from the two closely related species by tail, spicules, and gubernaculum length. Cryo-scanning electron microscopy confirmed head bearing five or six annules; four to six cephalic sensilla represented by small pits at the rounded corners of the labial plate; a small, round oral plate; and a large, pit-like amphidial opening confined to the labial plate and extending three to four annules beyond it. Phylogenetic analysis of 18S rRNA gene sequences placed Tylenchus zeae n. sp. in a clade with Tylenchus arcuatus and several Filenchus spp., and the mitochondrial cytochrome oxidase c subunit 1 (COI) gene region separated the new species from T. arcuatus and other tylenchid species. In the 28S tree, T. zeae n. sp. showed a high level of sequence divergence and was positioned outside of the main Tylenchus-Filenchus clade.

Molecular Characterization of Filenchus cylindricus (Thorne & Malek, 1968) Niblack & Bernard, 1985 (Tylenchida: Tylenchidae) from Korea, with Comments on Its Morphology

Filenchus cylindricus (Thorne & Malek, 1968) Niblack & Bernard, 1985 was reported from the sandy rhizospheric soils of Poa pratensis and for the first time in Korea. Females and males are molecularly characterized and morphological and morphometric data supplied. Identification was made using an integrative approach considering morphological characteristics and inferences drawn from the analyses of the D2-D3 expansion segment of 28S rRNA and ITS1-5.8S-ITS2 of rRNA partial sequences. Females and males from Korea conform to the type descriptions and also to subsequent species descriptions from Iowa and Colorado USA, Sudan and Pakistan. Despite the close morphological and morphometric similarities with F. thornei (Andrássy, 1954) Andrássy, 1963, the two species can be adequately differentiated based on molecular data inference.

Increased Resilience of Peas Toward Root Rot Pathogens Can Be Predicted by the Nematode Metabolic Footprint

Legumes are important drivers of soil fertility, however, their frequent use in rotations fosters long-lived soil-borne pathogens that can seriously compromise legume root health and nitrogen fixation. To overcome this, the current study aims at improving the general soil suppressiveness toward pests and diseases by agroecosystem management that can be predicted by nematode-based bioindicators. Two long-term organically managed agroecosystems comparing plow and shallow non-inversion tillage were analyzed for free-living nematode communities. Soils out of these agroecosystems were evaluated further in a greenhouse assay for their ability to suppress pea root rot caused by Didymella pinodella, Fusarium avenaceum, and F. redolens. There was a general trend for higher levels of pea root rot disease severity following inoculations with single pathogens, however, this effect was heterogeneous among experiments and tillage systems. This was mainly due to an already very high resident population of D. pinodella in soil and the presence of seed-borne F. oxysporum determined by their high incidence in pea roots irrespective of the soil and inoculated pathogens. Additional inoculation with D. pinodella, for example, resulted in only 8.5% biomass reduction compared to the non-inoculated control, in both tillage systems. Similar biomass losses were recorded in non-inversion tilled soils inoculated with F. redolens. When analyzed across inoculation treatments, the pea root rot disease severity was only slightly reduced in non-inversion tilled soils when compared to the plough systems (11% in Exp 1 and, 9% in Exp 2), however in both experiments non-inversion tillage resulted in greater pea biomass (33 and 19% in Exp1 and 2, respectively). Furthermore, the metabolic, enrichment, and bacterivore carbon footprints of nematodes were 88, 81, and 97% higher, respectively, in the non-inversion tilled soils compared to the plough. The metabolic carbon footprint of nematodes correlated negatively with pea root rot disease severity (rho = −0.71, p = 0.047). Hence, non-inversion tillage was effective in controlling pea root rot. The use of nematode metabolic footprints for predicting soil health should be extended for various agroecosystems aiming for its general use in evaluating effects of agroecosystem management through researchers and potentially farm management advisors.

First Report of Three Tylenchidae Taxa from Southern Alberta, Canada

Members of the family Tylenchidae are highly abundant in soil habitats, including agricultural settings, where they play key ecological roles. In the present study, we identified three Tylenchidae species, namely Basiria bhabi, Coslenchus acceptus, and Filenchus vulgaris, using integrative taxonomy. The detailed morphological and morphometric characteristics, distribution, and host associations of each species were also discussed. Phylogenetic analyses of these populations with other Tylenchidae nematodes indicated the presence of divergent lineages in Filenchus and Basiria, whereas Coslenchus appeared to be a monophyletic genus. Herein, we aim to grow awareness about this common but least studied group of nematodes. The species reported in this study are new records for Canada, revealing that the identified nematode diversity in our cultivated areas is relatively underrepresented. Our analyses also provided greater taxonomic resolution and captured rare taxa that might have been missed or misidentified in prior nematode inventory surveys. These findings will add to our understanding of the nematofauna of southern Alberta, thereby providing a more complete picture of existing nematode diversity present in the fields of this highly cultivated region.

Overview of the Genus Boleodorus and First Reports of Boleodorus thylactus and B. volutus from Southern Alberta, Canada

Simple Summary

Two Boleodorus species were detected in cultivated areas of southern Alberta. The aim of the present work was to characterize the discovered populations of Boleodorus using morphological and molecular methods. Boleodorus is the least studied genus in family Tylenchidae, with very few species reported after formal descriptions and outside their type locality. To date, Boleodorus species are not considered nematode pest species, rather they can serve as environmental indicators. Therefore, it is important to quantify and monitor the population densities of these species for soil health management studies. The current study encompasses the distribution and host association of all described Boleodorus species. In addition, morphometrical characters of all valid species are listed for their prompt identification.

Abstract

The present study provides the morphological and molecular characterization of Boleodorus thylactus and B. volutus populations, recovered from agricultural fields of southern Alberta. Despite a significant abundance of this group of nematodes, none of the Boleodorus species were previously reported in Canada. Therefore, representative adult specimens of each population were photographed and examined morphometrically. Phylogenetic trees were reconstructed using partial D2–D3 expansion segments of the 28S and 18S rDNA sequences to understand the relationships of Boleodorus species with Tylenchidae-related genera. Boleodorus species are relevant to soil ecological studies and therefore we summarized the important morphological and morphometric characters in tabular form for easy and efficient species identification. Moreover, we discuss the associated hosts and the distribution of all described Boleodorus species. This study will serve as a guide and basic framework for species diagnostics in the genus Boleodorus and will aid in filling the gaps in our knowledge of the species present in our cultivated lands.

Barcoding a can of worms: testing cox1 performance as a DNA barcode of Nematoda

Accurate taxonomic identifications and species delimitations are a fundamental problem in biology. The complex taxonomy of Nematoda is primarily based on morphology, which is often dubious. DNA barcoding emerged as a handy tool to identify specimens and assess diversity, but its applications in Nematoda are incipient. We evaluated cytochrome c oxidase subunit I (cox1) efficiency as a DNA barcode for nematodes scrutinising 5241 sequences retrieved from BOLD and GenBank. The samples included genera with medical, agricultural, or ecological relevance: Anguillicola, Caenorhabditis, Heterodera, Meloidogyne, Onchocerca, Strongyloides, and Trichinella. We assessed cox1 performance through barcode gap and Probability of Correct Identification (PCI) analyses, and estimated species richness through Automatic Barcode Gap Discovery (ABGD). Each genus presented distinct gap ranges, mirroring the evolutionary diversity within Nematoda. Thus, to survey the diversity of the phylum, a careful definition of thresholds for lower taxonomic levels should be considered. PCIs were around 70% for both databases, highlighting operational biases and challenges in nematode taxonomy. ABGD inferred higher richness than the taxonomic labels informed by databases. The prevalence of specimen misidentifications and dubious species delimitations emphasise the value of integrative approaches to nematode taxonomy and systematics. Overall, cox1 is a relevant tool for integrative taxonomy of nematodes.

Cephalenchus driekieae n. sp. (Nematoda: Tylenchidae) from South Africa, a new member of the genus with a long pharyngeal overlap

Cephalenchus driekieae n. sp. is described and illustrated based on its morphological, morphometric, and molecular characteristics. This new species is mainly characterized by its short stylet 11.5 to 13.0 μm, and 13.5 to 17.5 μm long pharyngeal overlap extending over the intestine. It could further be delimited by 451 to 526 μm long females with a prominently annulated cuticle, dorso-ventral amphidial openings as shown using scanning electron microscopy (SEM), four lines in the lateral field, anchor-shaped stylet knobs, empty spermatheca, elongate conoid tail with finely rounded tip and males absent. The shortest stylet and long pharyngeal overlap, distinguish this new species from previously described members and update the characteristics of the genus. With four lines in the lateral field, this new species was morphologically compared with four previously described species with this feature and another species with a short stylet. Molecular phylogenetic analyses using the partial small and large subunit ribosomal DNA gene (SSU and LSU rDNA D2-D3) sequences showed that it was clustered with other Cephalenchus spp. in both SSU and LSU trees, retaining the monophyly of the genus. This new species from South Africa updates the biogeography of the genus.

Mitochondrial COI gene is valid to delimitate Tylenchidae (Nematoda: Tylenchomorpha) species

Tylenchidae is a widely distributed soil-inhabiting nematode family. Regardless their abundance, molecular phylogeny based on rRNA genes is problematic, and the delimitation of taxa in this group remains poorly documented and highly uncertain. Mitochondrial Cytochrome Oxidase I (COI) gene is an important barcoding gene that has been widely used species identifications and phylogenetic analyses. However, currently COI data are only available for one species in Tylenchidae. In present study, we newly obtained 27 COI sequences from 12 species and 26 sequences from rRNA genes. The results suggest that the COI gene is valid to delimitate Tylenchidae species but fails to resolve phylogenetic relationships.