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

BRIEF SUMMARYSex ratio dynamics constitutes a pivotal subject in evolutionary biology1. 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 elements2,3. 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.