Towards a molecular clock of Wolbachia

Timing the evolutionary history of Wolbachia is a complex and virtually never tried task. Classical molecular clock techniques have the potential to answer this question, but they are complicated by a general lack of priors needed to calibrate substitutions: absence of fossil records, inapplicable replacement rates, and host co-divergences that cannot be trusted because of frequent lateral transfer. Regardless of these problems, we are attempting for the first time a molecular clock of Wolbachia using different approaches. In a first approach we are comparing genomes of Wolbachia harboured by closely related sister species hosts: replacements between Wolbachia genomes are calibrated with the divergence of the hosts, and the resulting rates are used to calibrate a genome-scaled phylogeny of Wolbachia. In a second approach we are calibrating a small dataset of housekeeping genes using known divergence of distant outgroups such as gamma-proteobacteria. We are also attempting to use previously suggested replacement rates for intra-specific Drosophila host populations, as well trying to indirectly infer mutation rate by comparing levels of polymorphism in co-hosted mitochondria and Wolbachia populations, and by studying the mutation rate of Wolbachia genomes integrated in the host genomes. Although our analyses are still ongoing, we aim at discussing our approaches and some preliminary results with the Wolbachia community.