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，林下乔木与灌木贡献了该类群植物物种多样性的极大份额，在高降雨区域尤为如此。目前，催生如此极高物种多样性的核心机制仍未阐明。有假说提出，结肉质果实的林下物种因鸟类传播者的活动受限，会出现种子传播受限的情况，进而导致基因交流受阻、小尺度空间下的种群分化，最终提升异地物种形成的发生率。
研究方法：我们在巴拿马的巴罗科罗拉多岛（Barro Colorado Island, BCI）及周边两处样地，对物种多样性极高的热带灌木属九节属（Psychotria，茜草科Rubiaceae）的4个物种开展采样。我们采用扩增片段长度多态性（Amplified Fragment Length Polymorphism, AFLP）技术对各物种进行基因分型，评估种群间的遗传分化，并解析了BCI种群的精细空间遗传结构模式。本研究通过空间自相关指数与种群密度，估算了传播距离参数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。BCI样地的精细空间遗传结构强度高于多数冠层乔木，且对于经消化道传播的热带木本植物而言，估算的基因流距离异乎寻常地低，传播距离σ为9~113米。
结论：本研究结果证实，以鸟类为传播媒介的林下物种的基因交流相对受限，这支持了热带雨林植物高物种多样性形成的相关假说——该假说聚焦于地球上开花植物最大属之一的九节属类群。