Summary
A bioRxiv preprint reports that a butterfly population’s female-heavy sex ratio returned to near parity in 2024–2025 as the frequency of feminizing Wolbachia fell. The population had reached 93.1% females in 2022.
A preprint on bioRxiv reports a sharp reversal in the sex ratio of an island population of the butterfly Eurema hecabe: after becoming strongly female-biased, it returned to near parity in 2024–2025. The change coincided with a sharp decline in the frequency of wFem, a feminizing strain of the maternally inherited bacterium Wolbachia.
The authors report that females made up approximately 50% of the population in 2015–2018, then 93.1% in 2022. Continued monitoring of the same population found the return toward an even sex ratio in 2024–2025. The work is presented as a preprint, and the reported findings come from population monitoring.
A rapid shift in the population
Wolbachia are bacteria passed from mother to offspring through the egg cytoplasm. Some strains can affect their hosts’ reproduction. The authors describe wFem as feminizing: its presence is associated with a greater proportion of females in this butterfly population.
The new observations are notable for the speed and reversal of the change. The female proportion rose from roughly half to 93.1% over the period covered by the authors’ earlier observations, then moved back toward parity as wFem became less frequent. This pattern suggests that interactions between a host population and a reproductive-manipulating symbiont can accompany substantial shifts in sex ratio over a few years.
The two trends occurred together, but population monitoring alone cannot determine the direction or mechanism of causation. The findings support a link between wFem frequency and the changing sex ratio; further detail on the underlying processes would be needed to explain why the strain declined and how that decline relates to the reversal.