Sequential evolution of a symbiont inferred from the host: Wolbachia and Drosophila simulans.

This study aims to unravel the biogeography of a model symbiont/host system by exploiting the prediction that a symbiont will leave a signature of infection on the host. Specifically, a global sample of 1,442 Drosophila simulans from 33 countries and 64 sampling localities was employed to infer the phylogeography of the maternally inherited alpha-proteobacteria Wolbachia. Phylogenetic analyses, from three symbiont genes and 24 mtDNA genomes (excluding the A + T-rich region), showed that each of four Wolbachia strains infected D. simulans once. The global distribution and abundance of the Wolbachia strains and the three mtDNA haplogroups (D. simulans siI, siII and siIII) was then determined. Finally, network analyses of variable regions within siI (584 bp from seven additional lines) and siII (1,701 bp from 383 lines) facilitated a detailed biogeographic discussion. There is little variation in siIII and the haplogroup is restricted in its distribution. These data show how the history of an infection can be mapped by combining data from the symbiont and the host. They say little about the organismal history of the host because the mtDNA genome is a biased representation of the whole genome.