Fitness costs of the cultivable symbiont Serratia symbiotica and its phenotypic consequences to aphids in presence of environmental stressors
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Associations between symbiotic microorganisms and animals are ubiquitous and hosts may benefit from hosting microbial communities through enhanced protection to environmental stresses or resource exploitation. Like many insects, aphids are hosts of a wide diversity of heritable symbionts that can be important drivers of their evolutionary ecology. Serratia symbiotica is one of the most common symbiont associated with aphids and includes a great variety of strains whose degree of interdependence on hosts varies significantly. Among these strains, some are gut-associated and have been isolated from aphids and cultivated. One of these strains (CWBI-2.3T) confers immediate protection against parasitoids. Here, we investigated additional associated phenotypes to elucidate the implication of cultivable S. symbiotica in the aphid evolutionary ecology. We show that under benign conditions, the aphids tended to suffer from reduced survival and fecundity when harboring the symbiont. We also demonstrate that gut infection with cultivable S. symbiotica does not protect aphids from the fungal pathogen Zoophtora occidentalis and from the lethal pathogen Serratia marcescens. However, while the bacterium is costly for aphids, this effect is no longer observed in the presence of the fungus, suggesting a negative effect of S. symbiotica on the latter. Our results further demonstrate that the cultivable S. symbiotica strain does not confer benefits to its hosts after the aphids were heat-stressed. These findings exposed that cultivable S. symbiotica does not have the same fitness effects on aphids as endosymbiotic strains, highlighting the significance of considering intraspecific variation of symbionts when studying their associated extended phenotypes.
KeywordsBacterial mutualism Facultative symbiosis Aphis fabae Ecological benefits Extended phenotype
We are grateful to Christoph Vorburger who supplied the A06–407 clone Aphis fabae used in our experiments. We especially thank Richard Humber and the USDA ARS Collection of Entomopathogenic Fungal Cultures, who provided cultures of Z. occidentalis. We also thank the team of Nicolas Schtickzelle for technical facilities. This work was supported by the Fonds de la Recherche Scientifique (FNRS) through a Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA) (FRIA Grant No. 1.E014.17F). This paper is publication BRC 347 of the Biodiversity Research Center (Université catholique de Louvain).
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Conflict of interest
The authors declare that they have no conflict of interest.
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