Rapid spread of a vertically transmitted symbiont induces drastic shifts in butterfly sex ratio

Sex ratio dynamics constitutes a pivotal subject in evolutionary biology ¹ . Under conditions of evolutionary equilibrium, the male-to-female ratio tends to be approximately 1:1; however, this equilibrium is susceptible to distortion by selfish genetic elements exemplified by driving sex chromosomes and cytoplasmic elements ^2,3 . While previous studies have substantiated instances of these genetic elements distorting the sex ratio, studies specifically tracking the process with which these distorters spread within populations, leading to a transition from balanced parity to a skewed, female-biased state, are notably lacking. Herein, we present compelling substantiation regarding the rapid spread of the cytoplasmic endosymbiont Wolbachia within a localized population of the pierid butterfly Eurema hecabe (Figure 1A). This resulted in a shift in the sex ratio from near parity to an exceedingly skewed state overwhelmingly biased toward females, reaching 94.4% within a remarkably brief period of 4 years.