Historic Museum Samples Provide Evidence for a Recent Replacement of Wolbachia Types in European Drosophila melanogaster.

Wolbachia is one of the most common bacterial endosymbionts, which is frequently found in numerous arthropods and nematode taxa. Wolbachia infections can have a strong influence on the evolutionary dynamics of their hosts since these bacteria are reproductive manipulators that affect the fitness and life history of their host species for their own benefit. Host-symbiont interactions with Wolbachia are perhaps best studied in the model organism Drosophila melanogaster, which is naturally infected with at least 5 different variants among which wMel and wMelCS are the most frequent ones. Comparisons of infection types between natural flies and long-term lab stocks have previously indicated that wMelCS represents the ancestral type, which was only very recently replaced by the nowadays dominant wMel in most natural populations. In this study, we took advantage of recently sequenced museum specimens of D. melanogaster that have been collected 90 to 200 yr ago in Northern Europe to test this hypothesis. Our comparison to contemporary Wolbachia samples provides compelling support for the replacement hypothesis. Our analyses show that sequencing data from historic museum specimens and their bycatch are an emerging and unprecedented resource to address fundamental questions about evolutionary dynamics in host-symbiont interactions. However, we also identified contamination with DNA from crickets that resulted in co-contamination with cricket-specific Wolbachia in several samples. These results underpin the need for rigorous quality assessments of museomic data sets to account for contamination as a source of error that may strongly influence biological interpretations if it remains undetected.