Eur. J. Entomol. 91 (1): 79-83, 1994
Use of DNA sequences to reconstruct the history of the association between members of the Sternorrhyncha (Homoptera) and their bacterial endosymbionts
The suborder Sternorrhyncba (Insecta: Homoptera) includes aphids, whiteflies, psyllids and scale insects; these are all large diverse groups of herbivorous insects that feed on plant sap and that include many of the most damaging agricultural pests. All of these insects are dependent on obligately intracellular procaryotic symbionts for their survival. In this collaborative project between Nancy Moran and Carol von Dohlen, both insect biologists, and Paul Baumann, a bacteriologist, molecular phylogenetic methods have been used to explore the evolutionary history of these mutualistic interactions. Using PCR amplification with procaryote-specific primers, DNA sequences have been obtained for the 16S ribosomal gene of the endosymbionts from approximately 20 species of Sternorrhyncha, including 13 aphids. On the basis of these sequences as well as published 16S sequences from representative procaryotes, a phylogenetic tree has been constructed, using parsimony methods. Results indicate that the primary endosymbionts of aphids belong to a single distinctive clade that has descended from a single infection of a common ancestor of aphids; this conclusion is based on complete congruence between the phylogenetic tree for the endosymbionts and that for the aphid hosts. Based on fossils that give minimal ages for certain of the aphid lineages, the association is at least 80 million years old. The rate of change of the 16S sequence is roughly constant among lineages within the aphid primary endosymbiont clade. Again using fossil dating, we have estimated absolute rates of substitution, and estimates are consistent for three different aphid clades. These rates have been used to test the hypothesis based on a biogeographic argument that the divergence between the Asian and American members of the Melaphidina dates to the Eocene (approximately 50 MY ago): the molecular data are highly supportive of the hypothesis. The 16S genes of secondary endosymbionts of aphids and of endosymbionts of mealybugs and whiteflies have also been sequenced, and results indicate that, for each insect group, endosymbiotic bacteria are descendants of independent infections by other members of the gamma-subdivision of the Proteobacteria for each insect group.
Keywords: Aphidoidea, Coccoidea, Aleyrodoidea, Proteobacteria, 16S rDNA sequences, conspeciation, phylogeny
Accepted: February 10, 1994; Published: April 15, 1994