Xenorhabdus aichiensis sp. nov., Xenorhabdus anantnagensis sp. nov., and Xenorhabdus yunnanensis sp. nov., Isolated from Steinernema Entomopathogenic Nematodes

Taxonomic and molecular characterization of a new entomopathogenic nematode species, Heterorhabditis casmirica n. sp., and whole genome sequencing of its associated bacterial symbiont

BACKGROUND

Nematodes of the genus Heterorhabditis are important biocontrol agents as they form a lethal combination with their symbiotic Photorhabdus bacteria against agricultural insect pests. This study describes a new species of Heterorhabditis.

METHODS

Six Heterorhabditis nematode populations were recovered from agricultural soils in Jammu and Kashmir, India. An initial examination using mitochondrial and nuclear genes showed that they belong to a new species. To describe this new species, a variety of analyses were conducted, including reconstructing phylogenetic relationships based on multiple genes, characterizing the nematodes at the morphological and morphometric levels, performing self-crossing and cross-hybridization experiments, and isolating and characterizing their symbiotic bacteria.

RESULTS

The newly discovered species, Heterorhabditis casmirica n. sp., shares 94% mitochondrial cytochrome C oxidase subunit I gene (COI) sequence identity with Heterorhabditis bacteriophora and Heterorhabditis ruandica, and 93% with Heterorhabditis zacatecana. Morphologically, it differs from H. bacteriophora in its infective juvenile phasmids (present vs. inconspicuous) and bacterial pouch visibility in the ventricular portion of the intestine (invisible vs. visible); genital papilla 1 (GP1) position (at manubrium level vs. more anterior), and in its b ratio (body length/neck length), c ratio (tail length/bulb width), and D% [(excretory pore/neck length) × 100]. Other morphological differences include anterior end to the nerve ring distance (77-100 vs. 121-130 μm), V% [(anterior end of vulva/body length) × 100] (46-57 vs. 41-47) in hermaphroditic females; rectum size (slightly longer than the anal body diameter vs. about three times longer), phasmids (smaller vs. inconspicuous), body length (0.13-2.0 vs. 0.32-0.39 mm), body diameter (73-150 vs. 160-220 μm), anterior end to the excretory pore distance (135-157 vs. 174-214 μm), and demanian ratios in amphimictic females. Morphological differences with H. ruandica and H. zacatecana were also observed. Furthermore, H. casmirica n. sp. did not mate or produce fertile progeny with other Heterorhabditis nematodes reported from India. It was also discovered that H. casmirica n. sp. is associated with Photorhabdus luminescence subsp. clarkei symbiotic bacteria.

CONCLUSIONS

The discovery of H. casmirica n. sp. provides novel insights into the diversity and evolution of Heterorhabditis nematodes and their symbiotic bacteria. This new species adds to the catalog of entomopathogenic nematodes in India.

Morphological and molecular profiling of an entomopathogenic nematode Steinernema feltiae: Unlocking its biocontrol potential against vegetable insect pests

A population of entomopathogenic nematodes, belonging to the Feltiae-clade and labelled J13, was discovered in the agricultural soils of the hilly regions of the Union territory of Jammu and Kashmir, India. Based on morphological, morphometric, and molecular analyses, the nematodes were identified as Steinernema feltiae. The J13 nematode isolate was tested in a laboratory assay for its pathogenicity against six major pests of vegetable crops: Pieris brassicae, Plutella xylostella, Helicoverpa armigera, Agrotis iplison, Trichoplusia ni, and Exelastis atomosa. The morphology of the isolated nematode closely matched the original description, except for the adult females, which had prominent epiptygmata instead of the weakly developed, double-flapped epiptygmata described in the original report. Analysis of the internal transcribed spacer and large subunit rRNA data from the J13 nematodes showed 100% similarity to sequences of the type population, indicating that they are conspecific. The virulence assays revealed that the nematode caused 100% mortality in the tested insect pests within 4872 hours, even at the lowest concentration of 50 infective juveniles per insect. The calculated median lethal concentration varied among the pests, with the lowest number of infective juveniles needed to achieve 50% larval killing being 117 for P. xylostella, 181.74 for P. brassicae, 226.35 for H. armigera, and 202.07 for T. ni at 24 hours post-inoculation. These findings suggest that S. feltiae isolated during the present investigation, may be a viable option for the biocontrol of these insect pests in Kashmir valley, India.