To kill or not to kill: Evolution of faculative male-killing Spiroplasma in Neotropical Drosophila

• Applicant: Dr. Michael Gerth
• Subject Area: Evolution, Anthropology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552946998

Inherited symbionts are bacteria that reside within the cells of animals and are transmitted from mothers to offspring. At least half of all arthropod species harbour inherited symbionts which may profoundly impact many aspects of their host’s biology. Spiroplasma is a genus of Mollicutes, cell-wall-less bacteria that are pathogens or inherited symbionts of arthropods. As symbionts, Spiroplasma have the ability to kill male offspring in order to enhance their own spread via females. They may also provide protection from parasitoids and parasites through specific toxins. Spiroplasma was first discovered in association with neotropical Drosophila flies (species of the willistoni and saltans groups), which are models for studying the mechanisms of speciation, exchange of mobile genetic elements, and the interaction of Drosophila with inherited symbionts such as Wolbachia and Spiroplasma. Through a preliminary screen, we recently found that many neotropical Drosophila species harbour a Spiroplasma variant that differs from all previously studied strains by its very low titre, absence of induced phenotypes, and very high incidence - all tested neotropical Drosophila species appear to harbour the same strain. In preliminary experiments we found that for this strain, “typical” Spiroplasma phenotypes (male killing, high titres) can be induced by upsetting the homeostasis of hosts and endosymbionts, e.g., through heat-stress. We hypothesise that our newly discovered variant is a “systemic” strain with a high degree of adaptation to its hosts, and possibly a long evolutionary history with neotropical hosts. This would be In contrast to all previously studied Spiroplasma, that are often costly reproductive manipulators (“pathogenic”) Spiroplasma. In this project, we will comprehensively characterise the evolutionary dynamics and phenotypic potential of our newly discovered systemic Spiroplasma symbionts in neotropical Drosophila species. This will be achieved through comparison of systemic with pathogenic Spiroplasma with respect to incidence, diversity, tropism, transmission dynamics, genomic architecture and evolutionary history, and through assessing phenotypes and ability for host shifting. The project will improve our understanding of the evolutionary transition of opportunistic sex ratio distorters to covert symbionts under a high degree of host control. More broadly, our research will contribute to deciphering the complex functional and evolutionary interplay between different microbes and host physiology in the model system Drosophila through an integrative approach of field and laboratory experiments, genetics, cell biology, and experimental evolution.

• DFG Programme: Research Grants
• International Connection: Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Cooperation Partner: Professor Dr. Wolfgang Miller