Nicotra A B
Department of Ecology and Evolutionary Biology University of Connecticut, Storrs, CT 06269, USA, , , , , , US.
Oecologia. 1998 Jun;115(1-2):102-113. doi: 10.1007/s004420050496.
Populations of dioecious plant species often exhibit biased sex ratios. Such biases may arise as a result of sex-based differences in life history traits, or as a result of spatial segregation of the sexes. Of these, sex-based differentiation in life history traits is likely to be the most common cause of bias. In dioecious species, selection can act upon the sexes in a somewhat independent way, leading to differentiation and evolution toward sex-specific ecological optima. I examined sex ratio variation and spatial distribution of the tropical dioecious shrub Siparuna grandiflora to determine whether populations exhibited a biased sex ratio, and if so, whether the bias could be explained in terms of non-random spatial distribution or sex-based differentiation in life history traits. Sex ratio bias was tested using contingency tables, a logistic regression approach was utilized to examine variation in life history traits, and spatial distributions were analyzed using Ripley's K, a second-order neighborhood analysis. I found that although populations of S. grandiflora have a male-biased sex ratio within and among years, there was no evidence of spatial segregation of the sexes. Rather, the sex ratio bias was shown to result primarily from sex-based differentiation in life history traits; males reproduce at a smaller size and more frequently than females. The sexes also differ in the relationship between plant size and reproductive frequency. Light availability was shown to affect reproductive activity in both sexes, though among infrequently flowering plants, females require higher light levels than males to flower. The results of this study demonstrate that ecologically significant sex-based differentiation has evolved in S. grandiflora.
雌雄异株植物物种的种群往往呈现出偏向性的性别比例。这种偏向可能是由于生活史特征中基于性别的差异，或者是由于性别的空间隔离导致的。其中，生活史特征中基于性别的分化可能是造成偏向的最常见原因。在雌雄异株物种中，选择可以以某种独立的方式作用于两性，导致向特定性别的生态最优状态的分化和进化。我研究了热带雌雄异株灌木大花锡帕鲁纳（Siparuna grandiflora）的性别比例变化和空间分布，以确定种群是否呈现出偏向性的性别比例，如果是，这种偏向是否可以用非随机的空间分布或生活史特征中基于性别的分化来解释。使用列联表检验性别比例偏差，利用逻辑回归方法研究生活史特征的变化，并使用二阶邻域分析Ripley's K分析空间分布。我发现，尽管大花锡帕鲁纳种群在年内和年间都存在雄性偏向的性别比例，但没有证据表明存在性别的空间隔离。相反，性别比例偏差主要是由生活史特征中基于性别的分化导致的；雄性比雌性在更小的植株大小和更频繁地进行繁殖。两性在植株大小和繁殖频率之间的关系上也存在差异。光照可用性被证明会影响两性的繁殖活动，不过在开花不频繁的植物中，雌性比雄性需要更高的光照水平才能开花。这项研究的结果表明，大花锡帕鲁纳已经进化出了具有生态学意义的基于性别的分化。
