Evaluating the African arid corridor hypothesis: A meta-analysis including the phylogenetic and biogeographical...

We examined the African arid corridor (AAC) disjunction pattern of vascular plants between northeastern and southwestern Africa in the context of geological and climatic events since the late Miocene. We developed a phylogenetic and biogeographical framework for the arid-adapted genus Sesamothamnus (Pedaliaceae), a classic example of the AAC disjunction pattern.

A phylogenetic tree based on next-generation sequencing of 512 nuclear genes and entire plastomes for all species of Sesamothamnus was time calibrated. Parsimony analyses were employed for morphological data. We gleaned the literature for examples of the AAC disjunction that were time calibrated or could be dated and compared them to climatic and geological changes from the Miocene to Pleistocene. Habit, dispersal mechanism, arid-adapted features, and geographic source area were collected.

The nuclear and plastome data provide congruent phylogenies for Sesamothamnus, indicating that the species restricted to northeastern Africa are sister to the southwestern species and diverged about 8.4 million years ago (Ma). Time calibrated splits for the AAC disjunct pattern were obtained for 73 vascular plant clades with dates ranging from 34 Ma to the Pleistocene, with the majority dated to the late Miocene, Pliocene, and Pleistocene.

Most clades exhibiting the AAC disjunction pattern, including Sesamothamnus, appear associated with several waves of aridification affecting both northeastern, eastern, and southwestern Africa involving the rise of C₄ and CAM photosynthesis, succulence, spiny vegetation, and bovid herbivory. The AAC disjunction pattern is complex, appears to involve migration, rather than simple vicariance, and exhibits a strong southwest to northeast migration bias.