Data from: Spatial genetic structure in four understory Psychotria species and implications for tropical forest diversity

Premise of the study: Tropical forests are the most species-rich terrestrial communities on Earth, and understory trees and shrubs comprise a large fraction of their plant species diversity, especially at high rainfalls. The mechanisms responsible for generating such high levels of diversity remain unknown. One hypothesis is that fleshy-fruited understory species should have limited seed dispersal due to the sedentary nature of their avian dispersers, resulting in restricted gene flow, population differentiation at small spatial scales, and ultimately, high rates of allopatric speciation. Methods: We sampled four species of the hyperdiverse tropical shrub genus Psychotria (Rubiaceae) on Barro Colorado Island (BCI) and two nearby sites in Panama. We genotyped each species with AFLPs, assessed genetic differentiation among populations, and determined patterns of fine-scale spatial genetic structure in the BCI population. Measures of spatial autocorrelation and population density were used to estimate the dispersal distance parameter σ. Key results: Regionally, ΦPT values ranged from 0.13 to 0.28, reflecting local population differentiation and suggesting that Lake Gatun/Rio Chagres has posed a relatively strong barrier to gene flow. Fine-scale spatial genetic structure on BCI was stronger than in most canopy trees, and estimated distances of gene flow were unusually low for endozoochorous tropical woody plants, with dispersal distance σ = 9–113 m. Conclusions: These results demonstrate comparatively limited gene flow in bird-dispersed understory species, supporting a hypothesized mechanism for generating high levels of plant species diversity in tropical rain forests, in one of the largest genera of flowering plants on Earth.

研究背景：热带森林是地球上物种最丰富的陆地群落，林下乔木与灌木占据了该群落植物物种多样性的极大比例，在高降雨区域尤为如此。目前，维持如此高物种多样性的潜在机制仍未明确。其中一种假说认为，结肉质果实的林下物种因依赖定居性鸟类作为传播者，其种子扩散会受到限制，进而导致基因交流受限、小尺度空间下的种群分化，最终引发异域物种分化速率升高。
研究方法：我们在巴拿马的巴罗科罗拉多岛（Barro Colorado Island, BCI）以及附近两处样地，对物种超丰富的热带灌木属九节属（*Psychotria*，茜草科Rubiaceae）的4个物种开展采样。我们通过扩增片段长度多态性（Amplified Fragment Length Polymorphism, AFLP）标记对各物种进行基因分型，评估种群间的遗传分化，并解析BCI种群的精细空间遗传结构模式。本研究利用空间自相关指数与种群密度，估算得到扩散距离参数σ。
主要结果：区域尺度上，ΦPT值介于0.13至0.28之间，这反映了局域种群分化，同时表明加通湖/查格雷斯河对基因交流构成了较强的阻隔作用。BCI样地内的精细空间遗传结构强度高于多数冠层树种；对于经动物消化道传播的热带木本植物而言，本次估算得到的基因流距离异乎寻常地短，扩散距离σ为9~113米。
研究结论：本研究结果证实，依靠鸟类传播种子的林下物种存在相对受限的基因交流，为热带雨林高植物物种多样性的形成机制假说提供了支持——该假说的研究对象为地球上被子植物最大属之一的九节属类群。