Pea aphid symbiont and enemy dynamics not so neat and tidy under natural conditions

Symbioses that involve maternally transmitted bacteria have proven to be highly prevalent in the natural world and serve as important sources of adaptive novelty.  Many of these symbionts are facultative or found at intermediate frequencies within host populations and confer unique phenotypes to the host that favor both the host and bacterium under certain conditions.  High levels of symbiont diversity in some host populations in nature are likely maintained by a heterogeneous environment over space and time.  Understanding adaptive evolution will require identification of the environmental forces that shape symbiont-, and by extension, phenotypic- variation in the wild. The pea aphid, Acyrthosiphon pisum, and its diverse microbiome serve as a model to study the dynamics and diversity of heritable symbiosis.  All seven symbionts have been implicated, to some extent, in defense against natural enemies.  Despite these highly beneficial defensive phenotypes most pea aphid bacterial endosymbionts remain at intermediate or low frequencies in natural populations.  To understand the dynamics between pea aphid defensive symbionts and their natural enemies we have taken this mostly lab based system in to the field. Seasonal tracking of symbionts and their natural enemies over the course of two years and on different host races reveals that natural enemies are not the only factor driving symbiont spread under natural conditions.  Greater understanding of the interplay between hosts, symbionts and the multiple environmental conditions they face will be necessary to determine what maintains symbiont diversity over time and space.