| Summary: | <p>Abstract</p> <p>Background</p> <p>Symbioses between invertebrates and prokaryotes are biological systems of particular interest in order to study the evolution of mutualism. The symbioses between the entomopathogenic nematodes <it>Steinernema </it>and their bacterial symbiont <it>Xenorhabdus </it>are very tractable model systems. Previous studies demonstrated (i) a highly specialized relationship between each strain of nematodes and its naturally associated bacterial strain and (ii) that mutualism plays a role in several important life history traits of each partner such as access to insect host resources, dispersal and protection against various biotic and abiotic factors. The goal of the present study was to address the question of the impact of <it>Xenorhabdus </it>symbionts on the progression and outcome of interspecific competition between individuals belonging to different <it>Steinernema </it>species. For this, we monitored experimental interspecific competition between (i) two nematode species: <it>S. carpocapsae </it>and <it>S. scapterisci </it>and (ii) their respective symbionts: <it>X. nematophila </it>and <it>X. innexi </it>within an experimental insect-host (<it>Galleria mellonella</it>). Three conditions of competition between nematodes were tested: (i) infection of insects with aposymbiotic IJs (i.e. without symbiont) of both species (ii) infection of insects with aposymbiotic IJs of both species in presence of variable proportion of their two <it>Xenorhabdus </it>symbionts and (iii) infection of insects with symbiotic IJs (i.e. naturally associated with their symbionts) of both species.</p> <p>Results</p> <p>We found that both the progression and the outcome of interspecific competition between entomopathogenic nematodes were influenced by their bacterial symbionts. Thus, the results obtained with aposymbiotic nematodes were totally opposite to those obtained with symbiotic nematodes. Moreover, the experimental introduction of different ratios of <it>Xenorhabdus </it>symbionts in the insect-host during competition between <it>Steinernema </it>modified the proportion of each species in the adults and in the global offspring.</p> <p>Conclusion</p> <p>We showed that <it>Xenorhabdus </it>symbionts modified the competition between their <it>Steinernema </it>associates. This suggests that <it>Xenorhabdus </it>not only provides <it>Steinernema </it>with access to food sources but also furnishes new abilities to deal with biotic parameters such as competitors.</p>
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