Genetic inference of epiphytic orchid colonization; it may only take one

Colonization of vacant habitat is a fundamental ecological process that affects the ability of species to persist and undergo range modifications in continually shifting landscapes. Thus, understanding factors that affect and limit colonization has important ecological and conservation implications. Epiphytic orchids are increasingly threatened by various factors, including anthropogenic habitat disturbance. As cleared areas (e.g. pastures) are recolonized by suitable host trees, the establishment and genetic composition of epiphytic orchid populations are likely a function of their colonization patterns. We used genetic analyses to infer the prevailing colonization pattern of the epiphytic orchid, Brassavola nodosa. Samples from three populations (i.e. individuals within a tree) from each of five pastures in the dry forest of Costa Rica were genotyped with neutral nuclear and chloroplast markers. Spatial autocorrelation and hierarchical genetic structure analyses were used to assess the relatedness of individuals within populations, among populations within pastures and among populations in different pastures. The results showed significant relatedness within populations (mean r = 0.166) and significant but lower relatedness among populations within a pasture (mean r = 0.058). Our data suggest that colonization of available habitats is by few individuals with subsequent population expansion resulting from in situ reproduction, and that individuals within a tree are not a random sample of the regional seed pool. Furthermore, populations within a pasture were likely colonized by seeds produced by founders of a neighbouring population within that pasture. These results have important ramifications for understanding conservation measures needed for this species and other epiphytic orchids.

空缺生境的定植是一项核心生态学过程，其影响物种在持续动态变化的景观中存续以及实现分布范围改变的能力。因此，解析影响并限制定植过程的关键因素，具有重要的生态学与保护生物学意义。附生兰科植物正面临各类因素的日益严峻的威胁，其中包括人为活动导致的生境破坏。当受清理的区域（例如牧场）被适宜的宿主树木重新定植后，附生兰种群的建立过程与遗传组成，大概率取决于其定植模式。本研究通过遗传分析，推断附生兰诺氏白拉索兰（Brassavola nodosa）的主导定植模式。我们对哥斯达黎加干旱森林中5个牧场的种群进行采样，每个牧场选取3个种群（即单棵树木上的个体），并利用中性核基因与叶绿体分子标记对样本进行基因分型。本研究采用空间自相关分析与层级遗传结构分析，分别评估种群内部、牧场内不同种群间，以及不同牧场种群间的个体亲缘关系。分析结果显示，种群内部存在显著的亲缘相关性（平均r值为0.166），而牧场内不同种群间同样存在显著但程度更低的亲缘相关性（平均r值为0.058）。本研究数据表明，可利用生境的定植过程由少量奠基个体完成，后续种群扩张则源于原地繁殖；同时，单棵树木上的个体并非区域种子库的随机取样结果。此外，同一牧场内的种群，大概率由该牧场内邻近种群奠基个体所产生的种子完成定植。上述研究结果，对于明确该物种以及其他附生兰科植物所需的保护策略具有重要参考价值。