Environmental and Genetic Contributions to Imperfect wMel-Like Wolbachia Transmission and Frequency Variation

Area of Science:

• Microbiology and Evolutionary Biology: Focuses on the symbiotic relationship between bacteria and insects, and its evolutionary implications.

Background:

• * Wolbachia bacteria are maternally inherited and infect approximately half of all insect species globally.
• * These bacteria often cause cytoplasmic incompatibility (CI) and exhibit imperfect maternal transmission, influencing their population dynamics.
• * Wolbachia frequencies typically increase when rare if they benefit host fitness, but some strains persist at intermediate frequencies with weak or no CI.

Purpose of the Study:

• * To investigate the factors driving the fluctuating frequencies of wMel-like Wolbachia (wYak and wSan) in Drosophila yakuba and Drosophila santomea on São Tomé island.
• * To determine the contributions of imperfect maternal transmission, host fitness effects, and cytoplasmic incompatibility to these frequency fluctuations.
• * To understand the causes of wMel-like frequency variation in divergent hosts for broader implications in biocontrol.

Main Methods:

• * Field surveys to assess spatial variation in wYak frequency and transmission rates on São Tomé.
• * Laboratory experiments to investigate the effects of temperature on wYak titer and transmission efficiency.
• * Genomic and genetic analyses to explore host-bacteria interactions influencing transmission.
• * Mathematical modeling of Wolbachia frequency dynamics to infer equilibrium conditions.

Main Results:

• * Spatial variation in wYak frequency and transmission was observed on São Tomé, with highest frequencies at high altitudes.
• * Imperfect maternal transmission did not consistently predict spatial wYak frequency variations.
• * Cool rearing temperatures reduced wYak bacterial titers and increased transmission imperfections, mimicking field observations.
• * Mathematical models suggested that temporally variable imperfect transmission or spatially variable fitness effects are necessary to explain wYak frequencies.
• * Spatially stable wSan frequencies were explained by imperfect transmission, modest fitness effects, and weak CI.

Conclusions:

• * Environmental factors, particularly temperature-dependent transmission, play a significant role in regulating wMel-like Wolbachia frequencies in natural populations.
• * The dynamics of wMel-like Wolbachia in divergent hosts are complex, influenced by a combination of transmission fidelity, host fitness, and environmental conditions.
• * Understanding these variations is critical for predicting Wolbachia spread and optimizing the use of Wolbachia for biocontrol strategies in disease-carrying mosquitoes.