Wolbachia in experimental evolution

The infection dynamics of the endosymbiont Wolbachia can be influenced by many factors, such as transmission rate, cytoplasmic incompatibility and selection. The interplay of these factors in natural populations results in heterogeneous infection patterns with substantial differences between populations and strains. The causes of these heterogeneities are not yet understood, partly due to the complexity of natural environments. We present experimental evolution as a new approach to study Wolbachia infection dynamics in replicate populations exposed to a controlled environment. A natural Drosophila melanogaster population infected with different strains of Wolbachia evolved in two different laboratory environments (hot and cold) for 1.5 years. In both treatments the rate of Wolbachia infection increased until fixation. In the hot environment the composition of the Wolbachia population remained stable over 37 generations. In the cold environment we observed marked changes in the Wolbachia composition: within 15 generations one Wolbachia clade increased more than 50% in frequency whereas the other two clades decreased in frequency, resulting in the loss of one clade. The frequency change was highly reproducible not only among replicates, but also when flies that evolved for 42 generations in the hot environment were transferred to the cold environment. This first unambiguous demonstration of habitat-specific fitness of Wolbachia strains sheds light on evolutionary processes in natural populations. Moreover, the high reproducibility suggests that experimental evolution studies are key to determine the functional basis of habitat-specific fitness among Wolbachia strains and host-symbiont interactions.