Fine-scale cospeciation betweenBrachycaudusandBuchnera aphidicola: bacterial genome helps define species and evolutionary relationships in aphids

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Abstract
Aphids harbour an obligatory symbiont,Buchnera aphidicola, providing essential amino acids not supplied by their diet. These bacteria are transmitted vertically and phylogenic analyses suggest that they have ‘cospeciated’ with their hosts. We investigated this cospeciation phenomenon at a fine taxonomic level, within the aphid genusBrachycaudus. We used DNA-based methods of species delimitation in both organisms, to avoid biases in the definition of aphid andBuchneraspecies and to infer association patterns without the presumption of a specific interaction. Our results call into question certain ‘taxonomic’ species ofBrachycaudusand suggest thatB. aphidicolahas diversified into independently evolving entities, each specific to a ‘phylogenetic’Brachycaudusspecies. We also found thatBuchneraand their hosts simultaneously diversified, in parallel. These results validate the use ofBuchneraDNA data for inferring the evolutionary history of their host. TheBuchneragenome evolves rapidly, making it the perfect tool for resolving ambiguities in aphid taxonomy. This study also highlights the usefulness of species delimitation methods in cospeciation studies involving species difficult to conceptualize—as is the case for bacteria—and in cases in which the taxonomy of the interacting organisms has not been determined independently and species definition depends on host association.