Response of three cyprinid fish species to the Scavenger Deterrent Factor produced by the mutualistic bacteria associated with entomopathogenic nematodes

Abstract

The symbiotic bacteria, Photorhabdus and Xenorhabdus associated with entomopathogenic nematodes (EPNs) in the genera Heterorhabditis and Steinernema, respectively, produce a compound(s) called the Scavenging Deterrent Factor (SDF). SDF deters a number of terrestrial insect scavengers and predators and one bird species from feeding on host insects killed by the nematode-bacterium complex but has not been tested against aquatic vertebrates. Moreover, the Heterorhabditis-Photorhabdus association is believed to have evolved in an aquatic environment. Accordingly, we hypothesized that SDF will deter fish from feeding on nematode-killed insects and tested the responses of three omnivorous fresh water fish species, Devario aequipinnatus, Alburnoides bipunctatus, and Squalius pursakensis, to SDF in the laboratory. When the fish were exposed to Galleria mellonella larvae killed by the Heterorhabditis- or Steinernema-bacterium complex at 2 or 4days post-infection, all three fish species made several attempts to consume the cadavers but subsequently rejected them. However, all fish species consumed freeze-killed control larvae. In a choice test, when D. aequipinnatus or A. bipunctatus were offered a pair of nematode-killed larvae, both fish species rejected these cadavers; when offered a nematode-killed larva and a freeze-killed larva, both fish species consumed the freeze-killed larva but not the nematode-killed one. In further tests with D. aequipinnatus, there was no significant difference in the number of 2-day-old Bacillus thuringiensis subsp. kurstaki-killed (Btk) larvae consumed compared to freeze-killed larvae, but significantly fewer 4-day-old Btk-killed larvae were consumed compared to freeze-killed larvae. When D. aequipinnatus was fed G. mellonella larvae killed by the symbiotic bacteria, the fish rejected the cadavers. When given freeze-killed or nematode-killed mosquito (Aedes aegypti) larvae, the fish consumed significantly more of the former larvae (99%) compared to the latter (55%). When D. aequipinnatus was placed in a symbiotic cell-free supernatant for 18h, a significant reduction in consumption of freeze-killed larvae compared to cell-free Btk or control broth supernatant was observed. We showed that SDF protects the nematode-killed insects from being consumed by omnivorous fishes and suggests that they will have minimal effects on recycling of EPNs in the aquatic environment.

Principales microorganismos entomopatógenos de Tecia solanivora y sus mecanismos de acción

“…Otro modelo importante de bacterias con actividad entomopatógena son Xenorhabdus y Photorhabdus, las cuales establecen relaciones mutualistas con nematodos de las familias Steinernema sp y Heterorhabditis sp respectivamente [41]. Ambos nematodos tienen ciclos de vida similares, y se divide en huevo, larva, estadios juveniles (J1, J2, J3, J4), y adultos, únicamente el tercer estadio juvenil (J3) de estos nemátodos son infectivos y buscan al hospedador, detectando sus excrementos y la producción de CO 2 [42].…”

The symbiotic bacteria Alcaligenes faecalis of the entomopathogenic nematodes Oscheius spp. exhibit potential biocontrol of plant‐ and entomopathogenic fungi

“…Depending on the available resources, one or more EPN generations will occur within the cadaver, and a large number of infective juveniles are gradually released into environment for infecting other hosts and continuing their life cycle (Poinar, 1990). Since the defence system of the insect cadavers is completely lost, the symbiotic bacteria must then provide defences in order to avoid the loss of the EPN population inside the cadaver (Jones et al, 2016) from foraging insects ants (Zhou et al, 2012), cricket and wasps (Gulcu et al, 2012), beetles (Jones et al, 2017), bird (Fenton et al, 2011) and even fishes (Raja et al, 2016). However, besides the abovementioned predators, insect cadavers containing EPNs will also suffer risk of infection from entomopathogenic fungi, entomopathogenic bacteria and even competition with other soil free-living nematodes.…”

Transmission Success of Entomopathogenic Nematodes Used in Pest Control

“…A wide range of species, including various arthropods [ 166 , 167 ] and birds [ 168 ], are known to be sensitive to such factors, avoiding infected cadavers or promptly rejecting them after an attempt to consume them. Recently, laboratory tests on three cyprinid fish species showed they were also sensitive to SDFs, rejecting G. mellonella larvae killed by Heterorhabditis or Steinernema species, and preferentially feeding on freeze-killed mosquito ( Aedes aegypti ) larvae compared to nematode-killed ones [ 169 ]. Tests on the carabid beetle Carabus granulatum , a predator of the lepidopteran host of H. amazonensis , showed that larvae and adults both avoided feeding on infected larvae when they had a choice [ 170 ].…”