Evolutionary transitions from hermaphroditism to dioecy have been common in flowering plants,^1,2 but recent analysis also points to frequent reversions from dioecy to hermaphroditism.2, 3, 4 Here, we use experimental evolution to expose a mechanism for such reversions, validating an explanation for the scattered phylogenetic distribution of dioecy. We removed males from dioecious populations of the wind-pollinated plant Mercurialis annua and allowed natural selection to act on the remaining females that occasionally produced male flowers; such “leaky” sex expression is common in both males and females of dioecious plants.⁵ Over the course of four generations, females evolved a 23-fold increase in average male flower production. This phenotypic masculinization of females coincided with the evolution of partial self-fertilization, high average seed set in the continued absence of males, and a capacity to sire progeny when males were re-introduced into their populations. Our study thus validates a mechanism for the rapid dissolution of dioecy and the evolution of functional hermaphroditism under conditions that may frequently occur during periods of low population density, repeated colonization, or range expansion.^6,7 Our results illustrate the power of natural selection, acting in replicated experimental populations, to bring about transitions in the mating behavior of plants.

从雌雄同体到雌雄同体的进化转变在开花植物中很常见，^{1 , 2}但最近的分析也指出从雌雄同体到雌雄同体的频繁逆转2, 3, 4 在这里，我们使用实验进化来揭示这种逆转的机制，验证解释了雌雄异株分散的系统发育分布。我们从风授粉植物Mercurialis annua的雌雄异株种群中去除雄性，并让自然选择作用于偶尔产生雄花的剩余雌性；这种“泄漏”的性表达在雌雄异株植物的雄性和雌性中都很常见。⁵在四代的过程中，雌性雄性花的平均产量增加了 23 倍。雌性的这种表型男性化与部分自我受精的进化、在雄性持续缺席的情况下平均种子高、以及当雄性重新引入其种群时产生后代的能力相吻合。因此，我们的研究验证了在人口密度低、反复定殖或范围扩大期间可能经常发生的条件下，雌雄异体快速解体和功能性雌雄同体进化的机制。^{6 , 7}我们的结果说明了自然选择的力量，在重复的实验种群中起作用，以实现植物交配行为的转变。