Previously, a facultative symbiont was identified in pea aphid which was associated with host plant specialization - there was a dramatic increase in fecundity on clover, coupled with failure to survive on alfalfa. In a study by Leonardo in the June issue of Ecology Letters, antibiotic treatment was used to selectively remove the specialization-associated symbiont, and found that the aphids remained specialized in its absence. In combination with genetic data presented in the paper, this implies that the aphid, rather than symbiont, genome is responsible for causing the observed specialization. Specialization is symbiont associated because closely related aphids tend to share the same symbionts.
Interestingly, other recent work (Science 303: 1989) has found that this same symbiont increased aphid fecundity on clover when injected into a novel host lineage. Together, these experiments suggest the symbiont may have played an initial role in host plant specialization, but that it is no longer necessary for maintaining specialization in some parts of the aphid's range.