Abstract
The importance of host-specialization to speciation processes in obligate host-associated bacteria is well known, as is also the ability of recombination to generate cohesion in bacterial populations. However, whether divergent strains of highly recombining intracellular bacteria, such as Wolbachia, can maintain their genetic distinctness when infecting the same host is not known. We first developed a protocol for the genome sequencing of uncultivable endosymbionts. Using this method, we have sequenced the complete genomes of the Wolbachia strains wHa and wNo, which occur as natural double infections in Drosophila simulans populations on the Seychelles and in New Caledonia. Taxonomically, wHa belong to supergroup A and wNo to supergroup B. A comparative genomics study including additional strains supported the supergroup classification scheme and revealed 24 and 33 group-specific genes, putatively involved in host-adaptation processes. Recombination frequencies were high for strains of the same supergroup despite different host-preference patterns, leading to genomic cohesion. The inferred recombination fragments for strains of different supergroups were of short sizes, and the genomes of the co-infecting Wolbachia strains wHa and wNo were not more similar to each other and did not share more genes than other A- and B-group strains that infect different hosts. We conclude that Wolbachia strains of supergroup A and B represent genetically distinct clades, and that strains of different supergroups can co-exist in the same arthropod host without converging into the same species. This suggests that the supergroups are irreversibly separated and that barriers other than host-specialization are able to maintain distinct clades in recombining endosymbiont populations. Acquiring a good knowledge of the barriers to genetic exchange in Wolbachia will advance our understanding of how endosymbiont communities are constructed from vertically and horizontally transmitted genes.
Highlights
The increasing availability of genomic data for closely related strains and species enables bacterial population sizes and structures to be explored in far greater detail than was possible until now
Speciation in sexual organisms is defined as the inability of two populations to get viable offspring
We have sequenced the genomes from the Wolbachia strains wNo and wHa. These are interesting since they belong to different supergroups yet co-occur as a doubleinfection in natural populations of Drosophila simulans
Summary
The increasing availability of genomic data for closely related strains and species enables bacterial population sizes and structures to be explored in far greater detail than was possible until now. A major question is whether asexually reproducing bacterial cells are organized into ‘‘clusters’’ that contain genetic diversity, yet are distinguishable from each other [1,2,3,4]. Such clusters can arise through geographic isolation or extreme habitat specialization [5]. A recent study of thermophilic archaea indicated ongoing speciation and suggested that these species are maintained by ecological differentiation within hot springs [9]. Studying the mechanisms and selective forces that influence the organization of genetic diversity in unicellular organisms is important for our understanding of speciation processes
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