Bizoux J-P, Daïnou K, Bourland N, Hardy O J, Heuertz M, Mahy G, Doucet J-L
Laboratory of Ecology, Gembloux Agricultural University, 5030 Gembloux, Belgium.
Mol Ecol. 2009 Nov;18(21):4398-408. doi: 10.1111/j.1365-294X.2009.04365.x. Epub 2009 Sep 29.
In this study, we analysed spatial genetic structure (SGS) patterns and estimated dispersal distances in Milicia excelsa (Welw.) C.C. Berg (Moraceae), a threatened wind-pollinated dioecious African tree, with typically low density (approximately 10 adults/km(2)). Eight microsatellite markers were used to type 287 individuals in four Cameroonian populations characterized by different habitats and tree densities. Differentiation among populations was very low. Two populations in more open habitat did not display any correlation between genetic relatedness and spatial distance between individuals, whereas significant SGS was detected in two populations situated under continuous forest cover. SGS was weak with a maximum S(p)-statistic of 0.006, a value in the lower quartile of SGS estimates for trees in the literature. Using a stepwise approach with Bayesian clustering methods, we demonstrated that SGS resulted from isolation by distance and not colonization by different gene pools. Indirect estimates of gene dispersal distances ranged from sigma(g) = 1 to 7.1 km, one order of magnitude higher than most estimates found in the literature for tropical tree species. This result can largely be explained by life-history traits of the species. Milicia excelsa exhibits a potentially wide-ranging wind-mediated pollen dispersal mechanism as well as very efficient seed dispersal mediated by large frugivorous bats. Estimations of gene flow suggested no major risk of inbreeding because of reduction in population density by exploitation. Different strategy of seed collection may be required for reforestation programmes among populations with different extent of SGS.
在本研究中，我们分析了非洲桃花心木（Milicia excelsa (Welw.) C.C. Berg，桑科）的空间遗传结构（SGS）模式，并估计了其扩散距离。非洲桃花心木是一种受威胁的风媒传粉雌雄异株的非洲树木，密度通常较低（约10株成年树/平方公里）。我们使用8个微卫星标记对喀麦隆4个具有不同栖息地和树木密度特征的种群中的287个个体进行了分型。种群间的分化非常低。两个处于更开阔栖息地的种群中，个体间的遗传相关性与空间距离之间没有任何关联，而在两个处于连续森林覆盖下的种群中检测到了显著的SGS。SGS较弱，最大S(p)统计量为0.006，该值处于文献中树木SGS估计值的下四分位数。使用贝叶斯聚类方法的逐步方法，我们证明SGS是由距离隔离而非不同基因库的定殖导致的。基因扩散距离的间接估计范围为σ(g) = 1至7.1公里，比文献中大多数热带树种的估计值高一个数量级。这一结果很大程度上可以由该物种的生活史特征来解释。非洲桃花心木表现出一种潜在的广泛的风媒花粉扩散机制以及由大型食果蝙蝠介导的非常有效的种子扩散。基因流估计表明，由于开发导致种群密度降低，不存在近亲繁殖的重大风险。对于具有不同SGS程度的种群，重新造林计划可能需要不同的种子采集策略。
