title: Cheaper is not always worse: strongly protective isolates of a defensive symbiont are less costly to the aphid host.
abstract: Defences against parasites are typically associated with costs to the host that contribute to the maintenance of variation in resistance. This also applies to the defence provided by the facultative bacterial endosymbiont Hamiltonella defensa, which protects its aphid hosts against parasitoid wasps while imposing life-history costs. To investigate the cost-benefit relationship within protected hosts, we introduced multiple isolates of H. defensa to the same genetic backgrounds of black bean aphids, Aphis fabae, and we quantified the protection against their parasitoid Lysiphlebus fabarum as well as the costs to the host (reduced lifespan and reproduction) in the absence of parasitoids. Surprisingly, we observed the opposite of a trade-off. Strongly protective isolates of H. defensa reduced lifespan and lifetime reproduction of unparasitized aphids to a lesser extent than weakly protective isolates. This finding has important implications for the evolution of defensive symbiosis and highlights the need for a better understanding of how strain variation in protective symbionts is maintained.
authors.affiliation: Institute of Integrative Biology, ETH Zürich, Universitätsstrasse 16, 8092 Zürich, Switzerland EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland.
authors.forename: Luis
authors.initials: L
authors.lastname: Cayetano
authors.affiliation: Institute of Integrative Biology, ETH Zürich, Universitätsstrasse 16, 8092 Zürich, Switzerland EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland.
authors.forename: Lukas
authors.initials: L
authors.lastname: Rothacher
authors.affiliation: INRA, UMR 1349 INRA-Agrocampus Ouest/Université Rennes 1, Institut de Génétique, Environnement et Protection des Plantes (IGEPP), Domaine de la Motte, 35653 Le Rheu Cedex, France.
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Simon
authors.affiliation: Institute of Integrative Biology, ETH Zürich, Universitätsstrasse 16, 8092 Zürich, Switzerland EAWAG, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland christoph.vorburger@eawag.ch.
authors.forename: Christoph
authors.initials: C
authors.lastname: Vorburger
mesh_terms: D000818:Animals; D001042:Aphids / Q000382:microbiology*; D005075:Biological Evolution; D006790:Host-Parasite Interactions / Q000235:genetics; D008018:Life Cycle Stages; D012098:Reproduction; D013559:Symbiosis
keywords: Hamiltonella defensa; cost of resistance; defensive symbiosis; lifespan; parasitoid; trade-off