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

It is generally believed that when maternally inherited sex ratio distorters become predominant, either the host population goes extinct or nuclear suppressors evolve in the host. Here, we show an empirical case where all-female-producing Wolbachia is likely to be stably maintained at a high frequency. On an island population of the butterfly Eurema mandarina, a Wolbachia strain wFem, which makes female hosts produce all-female offspring without sibling lethality (female drive), is highly prevalent. We found that, with some fluctuations, wFem appeared to be stably maintained for at least 12 years at a high frequency, resulting in the existence of an abnormally high number of virgin females. Interestingly, comparison between sex ratios of captive individuals and sex ratios deduced from wFem frequencies suggested a plastic behavioral change of males and females in response to the shift of sex ratios. wFem presence does not affect brood size but has a slightly negative effect on body size. Stable coexistence of wFem-positive and -negative females in the population may be explained via mate choice by males, which keeps wFem in check. Taken together, this butterfly population is an attractive model for future studies on the population dynamics of sex ratios and mating behavior.