Paleo-environmental data show that the distribution of African rain forests was affected by Quaternary climate changes. In particular, the Dahomey Gap (DG) – a 200 km wide savanna corridor currently separating the West African and Central African rain forest blocks and containing relict rain forest fragments – was forested during the mid-Holocene and possibly during previous interglacial periods, whereas it was dominated by open vegetation (savanna) during glacial periods. Genetic signatures of past population fragmentation and demographic changes have been found in some African forest plant species using nuclear markers, but such events appear not to have been synchronous or shared across species. To better understand the colonization history of the DG by rain forest trees through seed dispersal, the plastid genomes of two widespread African forest legume trees, Anthonotha macrophylla and Distemonanthus benthamianus, were sequenced in 47 individuals for each species, providing unprecedented phylogenetic resolution of their maternal lineages (857 and 115 SNPs, respectively). Both species exhibit distinct lineages separating three regions: 1. Upper Guinea (UG, i.e. the West African forest block), 2. the area ranging from the DG to the Cameroon volcanic line (CVL), and 3. Lower Guinea (LG, the western part of the Central African forest block) where three lineages co-occur. In both species, the DG populations (including southern Nigeria west of Cross River) exhibit much lower genetic diversity than UG and LG populations, and their plastid lineages originate from the CVL, confirming the role of the CVL as an ancient forest refuge. Despite the similar phylogeographic structures displayed by A. macrophylla and D. benthamianus, molecular dating indicates very contrasting ages of lineage divergence (UG diverged from LG since c. 7 Ma and 0.7 Ma, respectively) and DG colonization (probably following the Mid Pleistocene Transition and the Last Glacial Maximum, respectively). The stability of forest refuge areas and repeated similar forest shrinking/expanding events during successive glacial periods might explain why similar phylogeographic patterns can be generated over contrasting timescales.

古环境数据表明，非洲雨林的分布受到第四纪气候变化的影响。特别是，达荷美峡（Dahomey Gap，DG）–一条200公里宽的稀树草原走廊，目前分隔西非和中非雨林块，并含有残存的雨林碎片–在全新世中期和可能在前冰期之间就已造林，在冰川期以开放植被（稀树草原）为主。已经使用核标记在一些非洲森林植物物种中发现了过去种群破碎和人口变化的遗传特征，但这种事件似乎并非是同步的或在物种间共享。为了通过种子扩散更好地了解雨林树木对DG的定殖历史，大花按蚊和本生线虫，对每个物种的47个个体进行了测序，从而为其母系谱系（分别为857和115个SNP）提供了前所未有的系统发育分辨率。两种物种都具有区分三个区域的独特世系：1.上几内亚（UG，即西非森林块），2.从DG到喀麦隆火山线（CVL）的范围，以及3.下几内亚（LG，中非森林街区的西部）三个世系并存。在这两个物种中，DG种群（包括克罗斯河以西的尼日利亚南部）的遗传多样性都比UG和LG种群低得多，它们的质系来自CVL，证实了CVL作为古代森林避难所的作用。尽管A. macrophylla和A.展示了相似的植物学结构D. benthamianus，分子测年表明谱系发散（分别从7 Ma和0.7 Ma以来UG与LG分开）和DG定植（可能分别在中更新世过渡和最后一次冰期最大之后）的年龄形成了鲜明的对比。森林避难区的稳定性以及在连续冰川期反复发​​生类似的森林收缩/扩张事件，可能解释了为什么在不同的时间尺度上会产生相似的系统地理格局。