Persistence of a Wolbachia-driven sex ratio bias in an island population of Eurema butterflies

Prevalence of maternally inherited sex-ratio-distorting endosymbionts such as Wolbachia may have profound effects on reproduction and behaviour and can also select for nuclear suppressors in insect populations. In the butterfly Eurema mandarina, a Wolbachia strain wFem induces female drive, wherein all the offspring develop as female without any sibling lethality. We monitored an island population of E. mandarina, which comprises wFem-positive females, wFem-negative females and wFem-negative males, from 2005 to 2017. Despite some fluctuations, wFem appeared to be maintained for at least 12 years at a high frequency in the population, resulting in a female-biased population with a considerable number of virgin females. Because there was no evidence of suppressors against sex ratio distortion, the mechanism acting against increase/fixation of wFem infection, which would otherwise lead to the population extinction, would be either the constant immigration of wFem-negative females or the fitness cost of wFem. Suggestively, the wing size was slightly smaller in wFem-positive females than in wFem-negative females. Additionally, the discrepancies observed between sex ratios of captive individuals and sex ratios deduced from wFem infection frequencies among females can be explained by a plastic behavioural change of males and females in response to the shift of sex ratios.