Forest fragmentation genetics in a formerly widespread island endemic tree: Vateriopsis seychellarum (Dipterocarpaceae)

Habitat fragmentation and changed land use have seriously reduced population size in many tropical forest tree species. Formerly widespread species with limited gene flow may be particularly vulnerable to the negative genetic effects of forest fragmentation and small population size. Vateriopsis seychellarum (Dipterocarpaceae) is a formerly widespread canopy tree of the Seychelles, but is now reduced to 132 adult individuals distributed in eleven sites. Using ten microsatellite loci, a genetic inventory of all adult trees and a sample of 317 progeny, we demonstrate that despite its restricted range, overall genetic diversity was relatively high (HE: 0.56). The juvenile cohort, however, had significantly lower allelic richness (adults RS: 3.91; juveniles RS: 2.83) and observed heterozygosity than adult trees (adults HO: 0.62; juveniles HO: 0.48). Rare alleles were fewer and kinship between individuals was stronger in juveniles. Significant fine-scale spatial genetic structure was observed in remnant adults, and parentage analysis indicated that more than 90% of sampled progeny disperse <25 m and pollen dispersed <50 m. The molecular data confirmed that two populations were derived entirely from self-fertilized offspring from a single surviving mother tree. These populations produce viable offspring. Despite this extreme genetic bottleneck, self-compatibility may provide V. seychellarum with some resistance to the genetic consequences of habitat fragmentation, at least in the short term. We discuss our findings in the context of other rare and threatened dipterocarp species which are vulnerable to mis-management of genetic resources and population fragmentation.

生境破碎化与土地利用方式变更已严重缩减诸多热带林木的种群规模。原本分布广泛但基因流受限的物种，可能尤其易受森林破碎化与种群规模过小带来的负面遗传效应影响。塞舌尔韦氏香（Vateriopsis seychellarum，龙脑香科(Dipterocarpaceae)）是塞舌尔地区原本广泛分布的冠层树种，如今仅存132株成年个体，分布于11个样地中。本研究利用10个微卫星位点（microsatellite loci），对所有成年个体及317个子代样本开展遗传普查，结果显示：尽管该物种分布范围受限，其整体遗传多样性仍处于较高水平（HE：0.56）。然而，幼龄群的等位基因丰富度与观测杂合度均显著低于成年个体：成年个体等位基因丰富度RS为3.91、幼体为2.83，成年个体观测杂合度HO为0.62、幼体为0.48。幼龄群中的稀有等位基因数量更少，个体间的亲缘关系也更强。残存的成年个体中检测到显著的精细空间遗传结构（fine-scale spatial genetic structure）；亲权分析（parentage analysis）结果显示，90%以上的采样子代的扩散距离小于25米，花粉传播距离则小于50米。分子数据证实，有两个种群完全来自单株存活母树的自交子代，且这两个种群可产生具有生活力的子代。尽管遭遇了极端的遗传瓶颈（genetic bottleneck），自交亲和性（self-compatibility）至少在短期内可帮助塞舌尔韦氏香抵御生境破碎化带来的遗传后果。本研究结合其他易受遗传资源管理不当与种群破碎化影响的珍稀濒危龙脑香科物种的相关研究，对本次研究结果展开了讨论。