Sicard Mathieu, Hinsinger Julie, Le Brun Nathalie, Pages Sylvie, Boemare Noël, Moulia Catherine
Laboratoire Génome, Populations, Interactions, Adaptation UMR 5171 CNRS, Université de Montpellier 2, Place Eugène Bataillon cc, 63, 34095 Montpellier, France.
BMC Evol Biol. 2006 Sep 5;6:68. doi: 10.1186/1471-2148-6-68.
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 Steinernema and their bacterial symbiont Xenorhabdus 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 Xenorhabdus symbionts on the progression and outcome of interspecific competition between individuals belonging to different Steinernema species. For this, we monitored experimental interspecific competition between (i) two nematode species: S. carpocapsae and S. scapterisci and (ii) their respective symbionts: X. nematophila and X. innexi within an experimental insect-host (Galleria mellonella). 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 Xenorhabdus symbionts and (iii) infection of insects with symbiotic IJs (i.e. naturally associated with their symbionts) of both species.
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 Xenorhabdus symbionts in the insect-host during competition between Steinernema modified the proportion of each species in the adults and in the global offspring.
We showed that Xenorhabdus symbionts modified the competition between their Steinernema associates. This suggests that Xenorhabdus not only provides Steinernema with access to food sources but also furnishes new abilities to deal with biotic parameters such as competitors.
无脊椎动物与原核生物之间的共生关系是研究互利共生进化的特别有趣的生物系统。昆虫病原线虫斯氏线虫属（Steinernema）与其细菌共生体嗜线虫致病杆菌属（Xenorhabdus）之间的共生关系是非常易于处理的模型系统。先前的研究表明：（i）每种线虫菌株与其天然相关的细菌菌株之间存在高度专业化的关系；（ii）互利共生在每个伙伴的几个重要生活史特征中发挥作用，如获取昆虫宿主资源、扩散以及抵御各种生物和非生物因素。本研究的目的是解决嗜线虫致病杆菌属共生体对属于不同斯氏线虫物种的个体之间种间竞争的进程和结果的影响问题。为此，我们在实验昆虫宿主（大蜡螟，Galleria mellonella）内监测了（i）两种线虫物种：小卷蛾斯氏线虫（S. carpocapsae）和斯氏短体线虫（S. scapterisci）以及（ii）它们各自的共生体：嗜线虫致病杆菌（X. nematophila）和内氏嗜线虫致病杆菌（X. innexi）之间的实验性种间竞争。测试了线虫之间的三种竞争条件：（i）用两种物种的脱共生感染期幼虫（即没有共生体）感染昆虫；（ii）在存在两种嗜线虫致病杆菌共生体的可变比例的情况下，用两种物种的脱共生感染期幼虫感染昆虫；（iii）用两种物种的共生感染期幼虫（即与其共生体天然相关）感染昆虫。
我们发现昆虫病原线虫之间种间竞争的进程和结果都受到其细菌共生体的影响。因此，用脱共生线虫获得的结果与用共生线虫获得的结果完全相反。此外，在斯氏线虫之间的竞争期间，在昆虫宿主中实验性引入不同比例的嗜线虫致病杆菌共生体改变了成虫和全球后代中每个物种的比例。
我们表明嗜线虫致病杆菌共生体改变了它们的斯氏线虫伙伴之间的竞争。这表明嗜线虫致病杆菌不仅为斯氏线虫提供获取食物来源的途径，还赋予其应对诸如竞争者等生物参数的新能力。
