Data from: Persistence of long-distance, insect-mediated pollen movement for a tropical canopy tree species in remnant forest patches in an urban landscape

As deforestation and urbanization continue at rapid rates in tropical regions, urban forest patches are essential repositories of biodiversity. However, almost nothing is known about the effects of urbanization on gene flow of forest-dependent tree species. Buildings, roads and other “impervious” substrates considerably constrict the movement of small animals, and for many plant species the restricted movement of seed or pollen dispersers could have far-reaching genetic consequences. In this study, we investigated gene flow in the insect-pollinated, wind-dispersed tropical tree Koompassia malaccensis in and among three remnant forest patches in the urbanized landscape of Singapore. Most mature trees in the study had reproduced; parentage analysis assigned 93% of adults to at least one seedling, which likely contributed to the high allelic richness in the recruit cohort. Spatial genetic structure (SGS) of the recruit and adult cohorts was significant. Supporting this result, parentage analysis showed that 97% of recruits were within 100 m of their mother tree, and there was a high frequency of relatively short distance pollen dispersal (median 143–187 m) — a distance similar to the nearest-neighbour distances (median 105–127 m). Despite these similarities among patches, smaller patches with fewer adults displayed higher self-fertilization. The smallest patch was entirely surrounded by >2.5 km of “impervious” habitat; the SGS in this patch was significantly different from the recruits of the other patches and the adult cohort, with high levels of selfing (30.9%), low within-patch outcrossed parent pairs (n = 7), and the highest pollen and seed extra-patch immigration estimates of the three patches (seed: 2.0–6.7%; pollen 13.8–48.3%). Hence, contrary to our hypothesis, insect-mediated gene flow persists across >2.5 km of urban “impervious” substrate, and several of our results also parallel key findings from insect-pollinated canopy trees sampled in mixed agricultural-forest landscapes.

随着热带地区森林砍伐与城市化进程持续加速，城市森林斑块（urban forest patches）已然成为生物多样性的重要储存库。然而，目前学界几乎对城市化如何影响依赖森林生存的树种的基因流（gene flow）知之甚少。建筑物、道路及其他“不透水”基质（impervious substrates）会显著限制小型动物的活动；对于多数植物物种而言，种子或花粉传播者的活动受限可能会带来深远的遗传后果。本研究针对新加坡城市化景观内三处残存森林斑块中的虫媒传粉、风播热带树种马来西亚膜荚豆（Koompassia malaccensis）的基因流展开了调查。本研究中多数成熟树木均完成了繁殖；亲本分析（parentage analysis）将93%的成年个体至少匹配到1株幼苗，这或许解释了新生幼苗队列中较高的等位基因丰富度（allelic richness）。新生幼苗与成年个体队列的空间遗传结构（spatial genetic structure, SGS）均表现出显著特征。亲本分析结果也佐证了这一点：97%的新生幼苗与其母树的距离在100米以内，且花粉传播多为较短距离（中位数143~187米）——该距离与树木最近邻株间距（中位数105~127米）相近。尽管各斑块间存在上述相似性，但成年个体更少的小型斑块表现出更高的自交率。其中最小的斑块完全被超过2.5公里的“不透水”生境所环绕；该斑块内的空间遗传结构与其他斑块的新生幼苗队列及成年个体队列均存在显著差异，表现为高水平自交（30.9%）、斑块内异交亲本对数量极少（n=7），且是三个斑块中花粉与种子跨斑块迁入率最高的（种子：2.0%~6.7%；花粉：13.8%~48.3%）。因此，与我们的假说相悖，昆虫介导的基因流能够跨越超过2.5公里的城市“不透水”基质；此外，本研究的多项关键结果也与农业-森林混合景观中虫媒传粉冠层树木的相关研究结论一致。