A MOLECULAR CLOCK IN ENDOSYMBIOTIC BACTERIA IS CALIBRATED USING THE INSECT HOSTS

The primary endosymbionts of aphids are maternally inherited bacteria that live only within specialized host cells. Phylogenetic analysis of the 16S ribosomal DNA sequences of aphid endosymbionts reveals that t hey are a monophyletic group with a phylogeny completely concordant wi th that of their hosts, implying long-term cospeciation. Here we show that rates of base substitution are similar in the 16S ribosomal DNA o f different endosymbiont lineages. In addition, we calibrate these rat es by assigning age estimates for ancestral aphid hosts to the corresp onding endosymbionts. The resulting rate estimates (1-2% per 50 Ma) ar e among the most reliable available for prokaryotes. They are very nea r values previously conjectured by using more tenuous assumptions for dating divergence events in eubacteria. Rates calibrated using dates i nferred from fossil aphids imply that Asian and American species of th e aphid tribe Melaphidina diverged by the early Eocene; this result co nfirms an earlier hypothesis based on biogeographic evidence. Based on these rate estimates, the minimum age of this endosymbiotic associati on and the age of aphids as a whole is estimated at 160-280 Ma.