The rise and fall of feminizing Wolbachia in a butterfly

In sexually reproducing animals, sex ratios generally evolve toward parity1 but can be distorted by selfish genetic elements, including sex chromosome-linked meiotic drivers and cytoplasmically inherited symbionts such as Wolbachia2. We previously documented the rapid spread of the feminizing Wolbachia strain wFem in an island population of the butterfly Eurema hecabe3. Within just four years, the proportion of females increased from approximately 50% in 2015-2018 to 93.1% in 2022. Here, continued monitoring of the same population revealed an equally striking reversal: the sex ratio returned to near parity in 2024-2025, accompanied by a sharp decline in the frequency of wFem. These observations suggest that host-symbiont conflict can drive major shifts in population sex ratio over remarkably short timescales.