Variation in pathogenicity of recombinations of Heterorhabditis and Xenorhabdus luminescens strains (1993)

Surnrnary - Xeoorhabdus luminescens isolates were exchanged between four insect pathogenic nematode strains : two different Heterorhabditis isolates from the Netherlands and two different Heterorhabditis baeteriophora isolates. Of the sixteen possible combinations three combinations, ofDutch nematodes with bacteria of H. bacteriophora, were not able to grow and multiply. Of the thirteen combinations that could be propagated the four combinations of H. bacteriopJwra nematodes with bacteria from the Dutch nematodes were not pathogenic. When waxmoth larvae were in;ected with bacteria, the four non-pathogenic combinations were able to penetrate and multiply in these larvae. This showed that the lack of pathogeniciry of these four combinations was due to the absence ofbacteria. Both H. bacteriophora strains penetrated insect larvae in lower numbers than both Dutch Heterorhabditis strains, independent what bacteriurn they were carrying. Resume - Variabilite dans la pathogenie des combinaisons entre souches d'Heterorhabditis et de Xenorhabdus lurninescens - Des isolats de Xenorhabdus luminescens ont ete echanges entre quatre souches de nematodes entomopathogenes : deux isolats differents d'Heterorhabditis provenant des Pays-Bas et deux isolats differents d' Heterorhabditis bacteriopJwra. Parmi les seize combinaisons possibles, trois - nematodes neerlandais et bacteries provenant de H. bacteriophora- sont incapables de croitre et de se multiplier. Parmi les treize combinaisons qui ont pu etre elevees, les quatre combinaisons de nematodes provenant de H. bacteriophoraet de bacteries provenant de nematodes neerlandais ne sont pas pathogenes. Si l'on injecte des bacteries dans les larves de Galleria mellonella, les quatre combinaisons non pathogenes sont capables de penetrer a leur tour dans les larves et de s'y multiplier. Cela demontre que le manque de nocuite de ces quatre combinaisons est du a l'absence de bacteries. Les deux souches de H. bacteriopJwra penetrent en moins grand nombre dans les larves d'insectes que les deux souches neerlandaises d'HeteroThabditis, et ce independamment de la bacterie qu'elles transportent. Insect pathogenic nematodes of the genera Sœineme­ ma and Helerorhabditis are symbiotically associated with bacteria of the genus Xenorhabdus (Thomas & Poinar, 1979). The infective dauerlarvae of the nematode carry the bacterial symbiont in the intestine. The nematode penetrates an insect host, moves into the haemocoel, and reJeases the bacterium. The bacteriurn starts mul­ tiplying and kills the host, helped by excretion products of the nematode that repress the immune system of the insect. Xenorhabdus further produces antibiotics to in­ hibit growth of other micro-organisms in the insect ca­ daver and provides nutrients utilized by the nematodes (Poinar & Thomas, 1966; Akhurst, 1982; Gerritsen et al., 1992). Each Steinernema species is associated with its own Xenorhabdus species (Akhurst & Boemare, 1988). AlI Helerorhabditis spp. have X. luminescens as the symbiont, but the species X. luminescens is composed of several DNA homology groups which might also be considered separare species (Smits & Ehlers, 1991). Sorne of these DNA-homologous bacteria groups are associated with one Heterorhabditis species while other groups are associated with several Heterorhabditis spe­ cies (Smits & Ehlers, 1991). Akhurst (1983) and Ehlers