Forest is Life, TERRA Teaching and Research Centre, Gembloux Agro-Bio Tech, University of Liège, 2 Passage des Déportés, B-5030, Gembloux, Belgium.
Evolutionary Biology and Ecology Unit, CP 160/12, Faculté des Sciences, Université Libre de Bruxelles, 50 avenue F. D. Roosevelt, B-1050, Brussels, Belgium.
Am J Bot. 2020 Mar;107(3):498-509. doi: 10.1002/ajb2.1449.
Few studies have addressed the evolutionary history of tree species from African savannahs. Afzelia contains economically important timber species, including two species widely distributed in African savannahs: A. africana in the Sudanian region and A. quanzensis in the Zambezian region. We aimed to infer whether these species underwent range fragmentation and/or demographic changes, possibly reflecting how savannahs responded to Quaternary climate changes.
We characterized the genetic diversity and structure of these species across their distribution ranges using nuclear microsatellites (SSRs) and genotyping-by-sequencing (GBS) markers. Six SSR loci were genotyped in 241 A. africana and 113 A. quanzensis individuals, while 2800 high-quality single nucleotide polymorphisms (SNPs) were identified in 30 A. africana individuals.
Both species appeared to be mainly outcrossing. The kinship between individuals decayed with the logarithm of the distance at similar rates across species and markers, leading to relatively small Sp statistics (0.0056 for SSR and 0.0054 for SNP in A. africana, 0.0075 for SSR in A. quanzensis). The patterns were consistent with isolation by distance expectations in the absence of large-scale geographic gradients. Bayesian clustering of SSR genotypes did not detect genetic clusters within species. In contrast, SNP data resolved intraspecific genetic clusters in A. africana, illustrating the higher resolving power of GBS. However, these clusters revealed low levels of differentiation and no clear geographical entities, so that they were interpreted as resulting from the isolation by distance pattern rather than from past population fragmentation.
These results suggest that populations have remained connected throughout the large, continuous savannah landscapes. The absence of clear phylogeographic discontinuities, also found in a few other African savannah trees, indicates that their distribution ranges have not been significantly fragmented during the climatic oscillations of the Pleistocene, in contrast to patterns commonly found in African rainforest trees.
很少有研究涉及来自非洲热带稀树草原的树种的进化历史。非州榄木属包含经济上重要的木材物种，其中包括两种广泛分布在非洲热带稀树草原的物种：分布在苏丹地区的 A. africana 和分布在赞比西河流域的 A. quanzensis。我们旨在推断这些物种是否经历了范围的碎片化和/或种群变化，这可能反映了热带稀树草原对第四纪气候变化的反应。
我们使用核微卫星（SSR）和基因分型测序（GBS）标记，对这些物种在其分布范围内的遗传多样性和结构进行了描述。在 241 个 A. africana 和 113 个 A. quanzensis 个体中，我们对 6 个 SSR 位点进行了基因分型，而在 30 个 A. africana 个体中，我们鉴定了 2800 个高质量的单核苷酸多态性（SNP）。
两种物种似乎主要是异交的。个体之间的亲缘关系随距离的对数以相似的速度衰减，导致相对较小的 Sp 统计量（SSR 在 A. africana 中为 0.0056，SNP 为 0.0054，SSR 在 A. quanzensis 中为 0.0075）。这些模式与无大地理梯度的距离隔离期望一致。SSR 基因型的贝叶斯聚类没有在种内检测到遗传聚类。相比之下，SNP 数据在 A. africana 中解析出种内遗传聚类，说明了 GBS 的更高分辨率。然而，这些聚类显示出低水平的分化，没有明确的地理实体，因此可以解释为是由距离隔离模式而不是过去的种群碎片化造成的。
这些结果表明，在广阔的连续热带稀树草原景观中，种群一直保持着联系。与在少数其他非洲热带稀树草原树种中发现的情况一样，没有明显的系统地理不连续性，表明它们的分布范围在更新世的气候波动中没有显著碎片化，与非洲雨林树种中常见的模式形成对比。
