Transcriptomic Analysis of Steinernema Nematodes Highlights Metabolic Costs Associated to Xenorhabdus Endosymbiont Association and Rearing Conditions

Abstract

Entomopathogenic nematodes of the genus Steinernema have a mutualistic relationship with bacteria of the genus Xenorhabdus and together they form an antagonist partnership against their insect hosts. The nematodes (third-stage infective juveniles, or IJs) protect the bacteria from the external environmental stressors and vector them from one insect host to another. Xenorhabdus produce secondary metabolites and antimicrobial compounds inside the insect that protect the cadaver from soil saprobes and scavengers. The bacteria also become the nematodes’ food, allowing them to grow and reproduce. Despite these benefits, it is yet unclear what the potential metabolic costs for Steinernema IJs are relative to the maintenance and vectoring of Xenorhabdus. In this study, we performed a comparative dual RNA-seq analysis of IJs of two nematode-bacteria partnerships: Steinernema carpocapsae-Xenorhabdus nematophila and Steinernema. puntauvense-Xenorhbdus bovienii. For each association, three conditions were studied: (1) IJs reared in the insect (in vivo colonized), (2) colonized IJs reared on liver-kidney agar (in vitro colonized), and (3) IJs depleted by the bacteria reared on liver-kidney agar (in vitro aposymbiotic). Our study revealed the downregulation of numerous genes involved in metabolism pathways, such as carbohydrate, amino acid, and lipid metabolism when IJs were reared in vitro, both colonized and without the symbiont. This downregulation appears to impact the longevity pathway, with the involvement of glycogen and trehalose metabolism, as well as arginine metabolism. Additionally, a differential expression of the venom protein known to be secreted by the nematodes was observed when both Steinernema species were depleted of their symbiotic partners. These results suggest Steinernema IJs may have a mechanism to adapt their virulence in absence of their symbionts.

Metabonomics reveals that entomopathogenic nematodes mediate tryptophan metabolites that kill host insects

“…Xenorhabdus have been reported to utilize the fatty acids of host insects to synthesize some natural products ( Proschak et al, 2011 ; Shi and Bode, 2018 ). Many selective toxic secondary metabolites produced by entomopathogenic bacteria are associated with phenylethylamides and tryptamides ( Bode et al, 2017 ; Tse et al, 2017 ; Kusakabe et al, 2022 ; Lefoulon et al, 2022 ). For example, X. doucetiae DSM179 can synthesize large quantities of these related substances ( Bode et al, 2017 ), which can modulate quorum sensing in other bacteria and partially exhibit quorum quenching activity ( Morohoshi et al, 2008 ; Lang and Faure, 2014 ; Johnson et al, 2016 ).…”

Optimizing Entomopathogenic Nematode Genetics and Applications for the Integrated Management of Horticultural Pests

“…Such studies are essential to help boost EPN application and host specificity. Similar studies [95,97] aim at upgrading scientific assumptions for a better understanding and favorably directing insect-EPN interactions.…”

Xenorhabdus spp.: An Overview of the Useful Facets of Mutualistic Bacteria of Entomopathogenic Nematodes

“…bovienii indicated important metabolic shifts related to the rearing conditions as well as bacterial (presence versus absence). Nematode genes involved in the production of amino acid, carbohydrate, and lipid were downregulated as IJs were cultured in vitro, in the presence or absence of the symbiotic bacteria [29]. The downregulation seems to influence the longevity of the relevant metabolism pathways.…”

“…Both axenic nematode species showed a differential manifestation of the toxic protein that they secrete, i.e., absence of their symbiotic bacteria raises the expression of their secreted venom protein. The study suggested that IJs of these nematode species may have a unique mechanism to adjust infecting their hosts in the absence of their Xenorhabdus partners [29]. This does not mean, in any way, that the fundamental role played by these mutualistic bacteria in the reproduction of nematodes and even their production on a commercial scale can be neglected or may be abandoned.…”