Interspecific competition between entomopathogenic nematodes (Steinernema) is modified by their bacterial symbionts (Xenorhabdus)

Abstract

Background: 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.

Results: 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.

Conclusion: 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.

Figure 1

Percentile distribution of the percentages of X. nematophila within Xenorhabdus sp. found in the hemolymph of the insects 48 h post-infection. Results are given as box plots, where the horizontal line indicate the median (50^th of the data), the bottom and the top of the box indicate the first quartiles (25^th of the data) and the third quartiles (75^th of the data). The whiskers the range of the data (10^th of the data and 90^th of the data). Others dots are outliers which are under the 10^th percentile of the data or over 90^th percentile of the data. The abscissa shows the condition of the initial infection.

Figure 2

Percentile distribution of the ratio of S. carpocapsae within the total number of (A) nematode males and (B) nematode females found in the insect 120 h post-infection. Results are given as box plots, where the horizontal line indicate the median (50^th of the data), the bottom and the top of the box indicate the first quartiles (25^th of the data) and the third quartiles (75^th of the data). The whiskers the range of the data (10^th of the data and 90^th of the data). Others dots are outliers which are under the 10^th percentile of the data or over 90^th percentile of the data. The abscissa shows the condition of the initial infection.

Figure 3

Migration of the PCR-RFLP products on agarose gel. The wells annotated (Apo) contained PCR-RFLP products coming from DNA extraction of pools of 500 IJs having emerged from competitions between aposymbiotic nematodes without the injection of bacteria. The well annotated 100% s.c contained 500 IJs of S. carpocapsae. The wells annotated 10% s.c and 25% s.c. respectively contained 10% of S. carpocapsae and 90% of S. scapterisci and 25% of S. carpocapsae and 75% of S. scapterisci.

Figure 4

Percentile distribution of the percentages of IJs harbouring GFP labelled X. nematophila. Results are given as box plots, where the horizontal line indicate the median (50^th of the data), the bottom and the top of the box indicate the first quartiles (25^th of the data) and the third quartiles (75^th of the data). The whiskers the range of the data (10^th of the data and 90^th of the data). Others dots are outliers which are under the 10^th percentile of the data or over 90^th percentile of the data. The abscissa shows the condition of the initial infection.