Fitness costs of symbiont switching using entomopathogenic nematodes as a model

John G Mcmullen,John G Mcmullen,Heidi Goodrich Blair,Steven Forst,Brittany F Peterson,Brittany F Peterson,S Patricia Stock,S Patricia Stock

doi:10.1186/s12862-017-0939-6

Abstract

BackgroundSteinernematid nematodes form obligate symbioses with bacteria from the genus Xenorhabdus. Together Steinernema nematodes and their bacterial symbionts successfully infect, kill, utilize, and exit their insect hosts. During this process the nematodes and bacteria disassociate requiring them to re-associate before emerging from the host. This interaction can be complicated when two different nematodes co-infect an insect host.ResultsNon-cognate nematode-bacteria pairings result in reductions for multiple measures of success, including total progeny production and virulence. Additionally, nematode infective juveniles carry fewer bacterial cells when colonized by a non-cognate symbiont. Finally, we show that Steinernema nematodes can distinguish heterospecific and some conspecific non-cognate symbionts in behavioral choice assays.ConclusionsSteinernema-Xenorhabdus symbioses are tightly governed by partner recognition and fidelity. Association with non-cognates resulted in decreased fitness, virulence, and bacterial carriage of the nematode-bacterial pairings. Entomopathogenic nematodes and their bacterial symbionts are a useful, tractable, and reliable model for testing hypotheses regarding the evolution, maintenance, persistence, and fate of mutualisms.

Highlights

Steinernematid nematodes form obligate symbioses with bacteria from the genus Xenorhabdus
S. intermedium was capable of moderate levels of virulence with both XbkBU and Xbj resulting in 58.3% and 66.7% mortality by day seven, respectively (Table 3)
Association with XbfFL and Xbp did result in some insect mortality after 1 week, median lethal time (LT50) was not observed within the 7-day observation period, and virulence when S. intermedium was colonized by these species was not statistically distinguishable from aposymbiotic S. intermedium or those species that were completely incompetent (Table 3)

Summary

Introduction

Steinernematid nematodes form obligate symbioses with bacteria from the genus Xenorhabdus. Together Steinernema nematodes and their bacterial symbionts successfully infect, kill, utilize, and exit their insect hosts. During this process the nematodes and bacteria disassociate requiring them to re-associate before emerging from the host. Cooperative association, fitness benefits for all parties, partner fidelity and preference are key factors that influence the establishment and maintenance of mutualisms [12, 42]. In this respect, the relationship between Gram-negative Xenorhabdus bacteria and their Steinernema nematode hosts yields insights into processes that influence symbiont specificity, selection, and function. Nutrients provisioned by the nematode IJ are thought to support outgrowth and bacterial survival until the insect host [29]

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Discussion

Conclusion