Origin of African Physacanthus (Acanthaceae) via Wide Hybridization

Gene flow between closely related species is a frequent phenomenon that is known to play important roles in organismal evolution. Less clear, however, is the importance of hybridization between distant relatives. We present molecular and morphological evidence that support origin of the plant genus Physacanthus via “wide hybridization” between members of two distantly related lineages in the large family Acanthaceae. These two lineages are well characterized by very different morphologies yet, remarkably, Physacanthus shares features of both. Chloroplast sequences from six loci indicate that all three species of Physacanthus contain haplotypes from both lineages, suggesting that heteroplasmy likely predated speciation in the genus. Although heteroplasmy is thought to be unstable and thus transient, multiple haplotypes have been maintained through time in Physacanthus. The most likely scenario to explain these data is that Physacanthus originated via an ancient hybridization event that involved phylogenetically distant parents. This wide hybridization has resulted in the establishment of an independently evolving clade of flowering plants.

近缘物种间的基因流是一种常见现象，已知其在生物演化中发挥重要作用。然而，远缘类群间杂交的重要性仍不甚明确。我们提供分子与形态学证据，证实植物山蓝属(Physacanthus)起源于爵床科(Acanthaceae)中两个远缘支系类群间的“远缘杂交(wide hybridization)”事件。这两个支系的形态特征差异显著，但令人惊讶的是，山蓝属同时兼具二者的特征。对6个位点的叶绿体(chloroplast)序列分析显示，山蓝属全部3个物种均携带来自两个支系的单倍型(haplotype)，这表明异质性(heteroplasmy)很可能在该属的物种形成之前就已存在。尽管学界普遍认为异质性具有不稳定性，因此是短暂存在的，但山蓝属中却长期保留了多种单倍型。解释这些数据最合理的情景是：山蓝属起源于一次涉及系统发育远缘亲本的古老杂交事件。此次远缘杂交最终形成了一个独立演化的被子植物演化支。