Abstract: Aphids are a group of phloem-feeding hemipteran insects. Due to their some unique biological features, aphids are good model organisms for understanding many significant theoretical issues in adaptive evolution. Aphids maintain an obligate endosymbiotic association with Buchnera, their primary endosymbionts, which are nutritionally and developmentally indispensable to aphids. Studies on phylogenetic relationships between aphids and Buchnera help us understand the evolution of obligate symbiosis. In this article we reviewed the advances in phylogenetic relationships between aphids and Buchnera at different taxonomic levels (from higher to lower). Available evidences indicate that parallel diversifications of aphids and Buchnera occur at lower taxonomic level. However, such relationships may not exist at higher taxonomic level, which contradicts with the traditionally accepted parallelism hypothesis. On the basis of the detailed review of previous studies, it is proposed that more taxonomic groups should be sampled and more genomic data as well as robust phylogenetic concordance tests should be used. Furthermore, Buchnera horizontal transfer and consistency of evolutionary rates of Buchnera genes among different aphid lineages should be investigated to clarify the evolutionary relationships between aphids and Buchnera.

Key words: Buchnera, endosymbiosis, parallel evolution, phylogenetic relationship, higher taxonomic level, horizontal transfer