Does long distance pollen dispersal preclude inbreeding in tropical trees? Fragmentation genetics of Dysoxylum malabaricum in an agro-forest landscape

Tropical trees often display long distance pollen dispersal, even in highly fragmented landscapes. Understanding how patterns of spatial isolation influence gene flow and interact with background patterns of fine scale spatial genetic structure are critical for evaluating the genetic consequences of habitat fragmentation. In the endangered tropical timber tree Dysoxylum malabaricum (Meliaceae) we apply eleven microsatellite markers with paternity and parentage analysis to directly estimate contemporary gene flow across a large area (216 km2) in a highly fragmented agro-forest landscape. A comparison of genetic diversity and genetic structure in adult and juvenile life stages indicates an increase of differentiation and fine-scale spatial genetic structure (FSGS) over time. Paternity analysis and parentage analysis demonstrate high genetic connectivity across the landscape by pollen dispersal. A comparison between mother trees in forest patches with low and high densities of adult trees shows that the frequency of short distance mating increases, as does average kinship among mates in low density stands. We interpret this as an early warning sign of potentially negative genetic consequences of forest fragmentation. Single isolated trees, in contrast, frequently receive heterogeneous pollen from distances exceeding five kilometres. We discuss the processes leading to the observed patterns of gene flow and the implications of this for conservation management of D. malabaricum and tropical trees more generally.

热带树木即便处于高度破碎化的生境中，往往也能实现长距离花粉扩散。解析空间隔离模式如何影响基因流，并与精细尺度空间遗传结构（fine-scale spatial genetic structure, FSGS）的本底模式相互作用，对于评估生境破碎化的遗传效应至关重要。本研究以濒危热带用材树种马拉巴尔樫木（Dysoxylum malabaricum，楝科Meliaceae）为研究对象，采用11个微卫星标记（microsatellite marker）结合父本分析与亲权分析，直接估算了高度破碎化农林复合景观中216平方公里大尺度范围内的当代基因流。对成年与幼龄个体的遗传多样性和遗传结构进行比较后发现，随着时间推移，种群分化程度与精细尺度空间遗传结构（FSGS）均有所提升。父本分析与亲权分析结果证实，该景观范围内通过花粉扩散实现了较高的遗传连通性。对成年树木密度高低各异的林斑中的母树进行比较后发现，低密度林分中短距离交配的频率有所上升，配偶间的平均亲缘关系也随之升高。我们将这一现象视为森林破碎化可能引发负面遗传效应的早期预警信号。与之相反，孤立单株树木往往会接收来自5公里以外的异质花粉。本研究探讨了导致观测到的基因流模式的潜在过程，以及该结果对马拉巴尔樫木（D. malabaricum）乃至热带树木整体的保护管理的启示。