Plant speciation in the Namib Desert: origin of a widespread derivative species from a narrow endemic

Abstract

Background: Parapatric (or budding) speciation is increasingly recognized as an important phenomenon in plant evolution but its role in extreme (e.g. desert) environments is poorly documented. Aims: To test this speciation model in a hypothesized sister pair, the Southwest and North African disjunct Senecio flavus and its putative progenitor, the Namibian Desert endemic S. englerianus. Methods: Phylogenetic inferences were combined with niche divergence tests, morphometrics, and experimental genetic approaches. We also evaluated the potential role of an African Dry Corridor (ADC) in promoting the hypothesized northward expansion of S. flavus (from Namibia), using palaeodistribution models. Results: Belonging to an isolated (potentially relict) clade, the two morphologically distinct species show pronounced niche divergence in Namibia and signs of digenic epistatic hybrid incompatibility (based on F2 pollen fertility). The presence of connate fluked pappus hairs in S. flavus, likely increasing dispersal ability, is controlled by a single gene locus. Conclusions: Our results provide support for a rare example of budding speciation in which a wider ranged derivative (S. flavus) originated at the periphery of a smaller ranged progenitor (S. englerianus) in the Namib Desert region. The Southwest and North African disjunction of S. flavus could have been established by dispersal across intermediate ADC areas during periods of (Late) Pleistocene aridification.