Multiple phenotypes conferred by a single insect symbiont are independent

Abstract

Many microbial symbionts have multiple phenotypic consequences for their animal hosts. However, the ways in which different symbiont-mediated phenotypes combine to affect fitness are not well understood. We investigated whether there are correlations between different symbiont-mediated phenotypes. We used the symbiont <i>Spiroplasma</i>, a striking example of a bacterial symbiont conferring diverse phenotypes on insect hosts. We took 11 strains of <i>Spiroplasma</i> infecting pea aphids (<i>Acyrthosiphon pisum</i>) and assessed their ability to provide protection against the fungal pathogen <i>Pandora neoaphidis</i> and the parasitoids <i>Aphidius ervi</i> and <i>Praon volucre</i>. We also assessed effects on male offspring production for five of the <i>Spiroplasma</i> strains. All but one of the <i>Spiroplasma</i> strains provided very strong protection against the parasitoid <i>P. volucre</i>. As previously reported, variable protection against <i>P. neoaphidis</i> and <i>A. ervi</i> was also present; male-killing was likewise a variable phenotype. We find no evidence of any correlation, positive or negative, between the different phenotypes, nor was there any evidence of an effect of symbiont phylogeny on protective phenotype. We conclude that multiple symbiont-mediated phenotypes can evolve independently from one another without trade-offs between them.