Exploring the functional ecology of pea aphid symbiont strain diversity

Symbioses that involve maternally transmitted bacteria have proven to be highly prevalent in the natural world and serve as important sources of adaptive novelty. 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 impacts of heritable symbiosis on host ecology.  One symbiont, Hamiltonella defensa, when associated with a bacteriophage (APSE) provides resistance against the parasitoid wasp, Aphidius ervi.  Multiple APSE variants exist that confer varying levels of host parasitoid defense.  The most common variant found in the Eastern United States (APSE2) has reportedly 2 alleles at the toxin encoding region (cdtB1 and cdtB2).  The cdtB2 allele has recently been found in two clonal backgrounds that have intrinsic defense against A. ervi.  While rare in most populations, maintenance of this allele suggests a fitness advantage to the host under some environmental conditions.  Field cage and controlled temperature experiments are currently underway to explore alternative costs and benefits of two H. defensa-APSE strain combinations (including cdtB1 and cdtB2) under natural conditions, across temperatures and in the presence or absence of A. ervi.  Along with seasonal sampling to assess temporal variation, these experiments should provide insight in to the factors that maintain symbiont strain diversity under heterogeneous environmental conditions.