Plasticity in leakiness

In dioecious plants, males and females frequently show ‘leaky’ sex expression, with individuals occasionally producing flowers of the opposite sex. This leaky sex expression may have enabled the colonization of oceanic islands by dioecious plant species, and it is likely to represent the sort of variation upon which selection acts to bring about evolutionary transitions from dioecy to hermaphroditism. Although leakiness is commonly reported for dioecious species, it is not known whether it has plastic component. The question is interesting because males or females with an ability to enhance their leakiness plastically in the absence of mates would have an advantage of being able to produce progeny by self-fertilization. Here, we demonstrate that leaky sex expression in the wind-pollinated dioecious herb Mercurialis annua is plastically responsive to its mating context. We compared experimental populations of females growing either with or without males. Females growing in the absence of males were leakier in their sex expression than controls growing with males, producing more than twice as many male flowers. Our results thus provide a striking instance of plasticity in the reproductive behavior of plants that is likely adaptive. We consider how females might sense their mating environment as a function of pollen availability, and we discuss possible constraints on the evolution of plasticity in sex expression when the environmental signals that individuals receive are unreliable. 

Methods
See text of paper.

在雌雄异株植物（dioecious plants）中，雌雄个体常表现出“渗漏性”性别表达（leaky sex expression），即偶会产生异性花朵。这种渗漏性性别表达或许助力了雌雄异株植物类群对大洋岛屿的拓殖，且很可能代表了选择作用于演化转变的一类变异——即从雌雄异株向雌雄同体（hermaphroditism）的进化转变。尽管渗漏性性别表达在雌雄异株物种中已有诸多报道，但目前尚不清楚其是否具备可塑性组分（plastic component）。该问题颇具研究价值：若雌雄个体能在缺乏配偶时通过可塑性方式增强自身的渗漏性性别表达，便可通过自交受精（self-fertilization）产生后代，从而获得演化优势。

本研究证实，风媒传粉（wind-pollinated）的雌雄异株草本植物一年生汞草（Mercurialis annua）的渗漏性性别表达，会对其交配环境产生可塑性响应。我们比较了两类实验种群中的雌性个体：一类伴生雄性个体，另一组则无雄性个体。结果显示，无雄性伴生的雌性个体其性别表达渗漏程度显著高于伴生雄性的对照组，产生的雄性花朵数量是对照组的两倍以上。

综上，本研究揭示了植物生殖行为可塑性的典型适应性案例。我们探讨了雌性个体如何依据花粉可获得性感知交配环境，并讨论了当个体接收的环境信号不可靠时，性别表达可塑性演化可能面临的多重约束。

研究方法
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