Absence of cytoplasmic incompatibility and high vertical Wolbachia transmission in a neotropical drosophilid

Intracellular endosymbionts such as Wolbachia are generally thought to persist in host populations by inducing reproductive phenotypes that enhance maternal transmission, often at the expense of male hosts. Here, we examined the fitness consequences of the Wolbachia strain wStv in Drosophila sturtevanti, a highly abundant Neotropical drosophilid. Samples from 2015 and 2016 showed that all individuals from the nearby sampled populations carried Wolbachia, suggesting the induction of a phenotype capable of maintaining high infection levels. We therefore established isofemale lines from a local population and used one of them in controlled crosses between infected and treated flies to assess symbiont-induced changes in reproduction within a single genetic background. Contrary to expectations, we detected no cytoplasmic incompatibility or other reproductive manipulation. Instead, infection decreased female fecundity and decreased larvae production in crosses with treated males. Additional samplings in 2019 and 2022 showed that the infection persists in the population, and wsp sequencing confirmed that all infections detected from 2015 to 2022 carried the same allele. We also found imperfect but high maternal transmission, which may help to explain both the high infection levels observed in 2015-16 and the persistence of the infection. Our findings provide a foundation for future studies seeking to understand this association more broadly. They also reveal that Neotropical host-symbiont interactions can involve unexpectedly complex dynamics, indicating that the processes traditionally used to explain Wolbachia persistence may not be sufficient in this system.