Environmental and Genetic Contributions to Imperfect Mel-Like Transmission and Frequency Variation.

Maternally transmitted bacteria infect about half of all insect species. They usually show imperfect maternal transmission and often produce cytoplasmic incompatibility (CI). Irrespective of CI, frequencies tend to increase when rare only if they benefit host fitness. Several , including Mel that infects , cause weak or no CI and persist at intermediate frequencies. On the island of São Tomé off West Africa, the frequencies of Mel-like infecting (Yak) and (San) fluctuate, and the contributions of imperfect maternal transmission, fitness effects, and CI to these fluctuations are unknown. We demonstrate spatial variation in Yak frequency and transmission on São Tomé. Concurrent field estimates of imperfect maternal transmission do not predict spatial variation in Yak frequencies, which are highest at high altitudes where maternal transmission is the most imperfect. Genomic and genetic analyses provide little support for effects on Yak transmission. Instead, rearing at cool temperatures reduces Yak titer and increases imperfect transmission to levels observed on São Tomé. Using mathematical models of frequency dynamics and equilibria, we infer that temporally variable imperfect transmission or spatially variable effects on host fitness and reproduction are required to explain Yak frequencies. In contrast, spatially stable San frequencies are plausibly explained by imperfect transmission, modest fitness effects, and weak CI. Our results provide insight into causes of Mel-like frequency variation in divergent hosts. Understanding this variation is crucial to explain spread and to improve Mel biocontrol of human disease in transinfected mosquito systems.