Infectious speciation revisited: impact of symbiont-depletion on female fitness and mating behavior of Drosophila paulistorum

The impact of endosymbionts on the evolution of host sex-determination mechanisms

The past years have revealed that inherited bacterial endosymbionts are important sources of evolutionary novelty for their eukaryotic hosts. In this review we discuss a fundamental biological process of eukaryotes influenced by bacterial endosymbionts: the mechanisms of sex determination. Because they are maternally inherited, several endosymbionts of arthropods, known as reproductive parasites, have developed strategies to convert non-transmitting male hosts into transmitting females through feminization of genetic males and parthenogenesis induction. Recent investigations have also highlighted that endosymbionts can impact upon host sex determination more subtly through genetic conflicts, resulting in selection of host nuclear genes resisting endosymbiont effects. Paradoxically, it is because of their selfish nature that reproductive parasites are such powerful agents of evolutionary change in their host sex-determination mechanisms. They might therefore represent excellent models for studying transitions between sex-determining systems and, more generally, the evolution of sex-determination mechanisms in eukaryotes.

Rapid fluorescence-based screening for Wolbachia endosymbionts in Drosophila germ line and somatic tissues

Wolbachia is a globally distributed bacterial endosymbiont present in arthropods and nematodes. The advent of sensitive PCR-based approaches has greatly facilitated the identification of Wolbachia-infected individuals and analysis of population infection levels. Here, a complementary visual fluorescence-based Wolbachia screening approach is described. Through the use of the fluorescent dye Syto-11, Wolbachia can be efficiently detected in various Drosophila tissues, including ovaries. Syto-11 also stains Wolbachia in other insects. Because Wolbachia is inherited through the maternal germ line, bacteria reside in the ovaries of flies in infected populations. An advantage of this staining approach is that it informs about Wolbachia titer as well as its tissue and cellular distribution. Using this method, the infection status of insect populations in two central California locations was determined, and variants with unusually low or high Wolbachia titers were isolated. In addition, a variant with ovarioles containing both infected and uninfected egg chambers was identified. Syto-11 staining of Cardinium- and Spiroplasma-infected insects was also analyzed.

Complete bacteriophage transfer in a bacterial endosymbiont (Wolbachia) determined by targeted genome capture

Bacteriophage flux can cause the majority of genetic diversity in free-living bacteria. This tenet of bacterial genome evolution generally does not extend to obligate intracellular bacteria owing to their reduced contact with other microbes and a predominance of gene deletion over gene transfer. However, recent studies suggest intracellular coinfections in the same host can facilitate exchange of mobile elements between obligate intracellular bacteria—a means by which these bacteria can partially mitigate the reductive forces of the intracellular lifestyle. To test whether bacteriophages transfer as single genes or larger regions between coinfections, we sequenced the genome of the obligate intracellular Wolbachia strain wVitB from the parasitic wasp Nasonia vitripennis and compared it against the prophage sequences of the divergent wVitA coinfection. We applied, for the first time, a targeted sequence capture array to specifically trap the symbiont's DNA from a heterogeneous mixture of eukaryotic, bacterial, and viral DNA. The tiled array successfully captured the genome with 98.3% efficiency. Examination of the genome sequence revealed the largest transfer of bacteriophage and flanking genes (52.2 kb) to date between two obligate intracellular coinfections. The mobile element transfer occurred in the recent evolutionary past based on the 99.9% average nucleotide identity of the phage sequences between the two strains. In addition to discovering an evolutionary recent and large-scale horizontal phage transfer between coinfecting obligate intracellular bacteria, we demonstrate that “targeted genome capture” can enrich target DNA to alleviate the problem of isolating symbiotic microbes that are difficult to culture or purify from the conglomerate of organisms inside eukaryotes.