Potential of Oscheius tipulae nematodes as biological control agents against Ceratitis capitata

Model nematodes as a practical innovation to promote high throughput screening of natural products for anthelmintics discovery in South Asia: Current challenges, proposed practical and conceptual solutions

With the increasing prevalence of anthelmintic resistance in animals recorded globally, and the threat of resistance in human helminths, the need for novel anthelmintic drugs is greater than ever. Most research aimed at discovering novel anthelmintic leads relies on high throughput screening (HTS) of large libraries of synthetic small molecules in industrial and academic settings in developed countries, even though it is the tropical countries that are most plagued by helminth infections. Tropical countries, however, have the advantage of possessing a rich flora that may yield natural products (NP) with promising anthelmintic activity. Focusing on South Asia, which produces one of the world's highest research outputs in NP and NP-based anthelmintic discovery, we find that limited basic research and funding, a lack of awareness of the utility of model organisms, poor industry-academia partnerships and lack of technological innovations greatly limit anthelmintics research in the region. Here we propose that utilizing model organisms including the free-living nematode Caenorhabditis elegans, that can potentially allow rapid target identification of novel anthelmintics, and Oscheius tipulae, a closely related, free-living nematode which is found abundantly in soil in hotter temperatures, could be a much-needed innovation that can enable cost-effective and efficient HTS of NPs for discovering compounds with anthelmintic/antiparasitic potential in South Asia and other tropical regions that historically have devoted limited funding for such research. Additionally, increased collaborations at the national, regional and international level between parasitologists and pharmacologists/ethnobotanists, setting up government-industry-academia partnerships to fund academic research, creating a centralized, regional collection of plant extracts or purified NPs as a dereplication strategy and HTS library, and holding regional C. elegans/O. tipulae-based anthelmintics workshops and conferences to share knowledge and resources regarding model organisms may collectively promote and foster a NP-based anthelmintics landscape in South Asia and beyond.

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.

Can Entomopathogenic Nematodes and Their Symbiotic Bacteria Suppress Fruit Fly Pests? A Review

Fruit flies (Diptera: Tephritidae) are serious pests that affect fruit production and marketing. Both third instar larvae and pupae are biological stages that persist in the soil until adult emergence. Entomopathogenic nematodes (ENs) are biological control agents that are used to control agricultural pests in greenhouse or field conditions. Several studies have been carried out under laboratory and field conditions showing how ENs can be applied within an area-wide integrated pest management approach to control fruit fly species in orchards and backyard fruit trees. In this review, we analyze how soil physical characteristics and biotic factors affect the performance of these biological control agents. Of the reviewed papers, more than half evaluated the influence of soil texture, humidity, temperature, and other factors on the performance of infective juveniles (IJs). Abiotic factors that significantly influence the performance of IJs are temperature, humidity, and texture. Among the biotic factors that affect IJs are fungi, bacteria, mites, insects, and earthworms. We conclude that ENs have the potential to be applied in the drip area of fruit trees that are infested by fruit flies and contribute to their suppression. This approach, in conjunction with an area-wide pest management approach, may contribute to pest suppression and increase the sustainability of agroecosystems.

Acinetobacter nematophilus sp. nov., Alcaligenes nematophilus sp. nov., Enterobacter nematophilus sp. nov., and Kaistia nematophila sp. nov., Isolated from Soil-Borne Nematodes and Proposal for the Elevation of Alcaligenes faecalis subsp. faecalis, Alcaligenes faecalis subsp. parafaecalis, and Alcaligenes faecalis subsp. phenolicus to the Species Level

Four bacterial strains, A-IN1T, A-TC2T, E-TC7T, and K-TC2T, isolated from soil-borne nematodes of the species Oscheius tipulae and Acrobeloides bodenheimeri, were found to represent new species of the genera Acinetobacter, Alcaligenes, Enterobacter, and Kaistia, respectively. In this study, we described these new species using a polyphasic taxonomic approach that included whole-genome and whole-proteome phylogenomic reconstructions, core genome sequence comparisons, and phenotypic characterization. Phylogenomic reconstructions using whole-genome and whole-proteome sequences show that A-IN1T is closely related to Acinetobacter guillouiae DSM 590T and to Acinetobacter bereziniae LMG 1003T. The dDDH values between A-IN1T and these latest strains are 25.1 and 39.6%, respectively, which are below the 70% divergence threshold for prokaryotic species delineation. A-TC2T is closely related to Alcaligenes faecalis subsp. faecalis DSM 30030T and to Alcaligenes faecalis subsp. phenolicus DSM 16503T. The dDDH values between A-TC2T and these latest strains are 47.0 and 66.3%, respectively. In addition, the dDDH values between Alcaligenes faecalis subsp. faecalis DSM 30030T, Alcaligenes faecalis subsp. phenolicus DSM 16503T, and Alcaligenes faecalis subsp. parafaecalis are always lower than 70%, demonstrating that the three strains represent species within the genus Alcaligenes rather than subspecies within Alcaligenes faecalis. E-TC7T is closely related to Enterobacter kobei DSM 13645T, Enterobacter chuandaensis 090028T, and to Enterobacter bugandensis STN0717-56T. The dDDH values between E-TC7T and these strains are 43.5, 42.9, and 63.7%, respectively. K-TC2T is closely related to Kaistia terrae DSM 21341T and to Kaistia defluvii JCM 18034T. The dDDH values between these strains are 29.2 and 30.7%, respectively. Several biochemical tests allow to differentiate the type strains of the newly described species from the type strains of their more closely related species. Based on the results of this polyphasic taxonomic approach, the following new species are proposed: Acinetobacter nematophilus sp. nov. with A-IN1T (=CCM 9231T =CCOS 2018T) as the type strain, Alcaligenes nematophilus sp. nov. with A-TC2T (=CCM 9230T =CCOS 2017T) as the type strain, Enterobacter nematophilus sp. nov. with E-TC7T (=CCM 9232T =CCOS 2020T) as the type strain, and Kaistia nematophila sp. nov. with K-TC2T (=CCM 9239T =CCOS 2022T) as the type strain. In addition, we propose the elevation of Alcaligenes faecalis subsp. faecalis, Alcaligenes faecalis subsp. parafaecalis, and Alcaligenes faecalis subsp. phenolicus to the species level. Therefore, we propose the creation of Alcaligenes parafaecalis sp. nov. with DSM 13975T as the type strain, and Alcaligenes phenolicus sp. nov. with DSM 16503T as the type strain. Our study contributes to a better understanding of the biodiversity and phylogenetic relationships of bacteria associated with soil-borne nematodes.

Entomopathogenic potential of bacteria associated with soil-borne nematodes and insect immune responses to their infection

Soil-borne nematodes establish close associations with several bacterial species. Whether they confer benefits to their hosts has been investigated in only a few nematode-bacteria systems. Their ecological function, therefore, remains poorly understood. In this study, we isolated several bacterial species from rhabditid nematodes, molecularly identified them, evaluated their entomopathogenic potential on Galleria mellonella larvae, and measured immune responses of G. mellonella larvae to their infection. Bacteria were isolated from Acrobeloides sp., A. bodenheimeri, Heterorhabditis bacteriophora, Oscheius tipulae, and Pristionchus maupasi nematodes. They were identified as Acinetobacter sp., Alcaligenes sp., Bacillus cereus, Enterobacter sp., Kaistia sp., Lysinibacillus fusiformis, Morganella morganii subsp. morganii, Klebsiella quasipneumoniae subsp. quasipneumoniae, and Pseudomonas aeruginosa. All bacterial strains were found to be highly entomopathogenic as they killed at least 53.33% G. mellonella larvae within 72h post-infection, at a dose of 106 CFU/larvae. Among them, Lysinibacillus fusiformis, Enterobacter sp., Acinetobacter sp., and K. quasipneumoniae subsp. quasipneumoniae were the most entomopathogenic bacteria. Insects strongly responded to bacterial infection. However, their responses were apparently little effective to counteract bacterial infection. Our study, therefore, shows that bacteria associated with soil-borne nematodes have entomopathogenic capacities. From an applied perspective, our study motivates more research to determine the potential of these bacterial strains as biocontrol agents in environmentally friendly and sustainable agriculture.