Studying adaptive radiation at the molecular level: A case study in the macaronesian crassulaceae-sempervivoideae

Abstract

Oceanic islands frequently harbour prominent examples of plant radiations. Here, we investigated the Macaronesian Crassulaceae-Sempervivoideae (MCS) including the genera Aichryson, Monanthes and Aeonium. This species-rich clade displays a large variety in morphological and ecological features, and is, therefore, often considered as adaptive radiation. We demonstrated, however, a distinct increase in the speciation rate only in the crown group of Aeonium. Analysing homologues of the selected candidate genes, APETALA1, APETALA3 and PEPC (Phosphoenolpyruvate carboxylase gene), revealed that an increase of cladogenesis is paralleled by an increased number of paralogous copies found for all these genes in Aeonium. These could be the result of gene duplications or early polyploidisation. The number of nonsynonymous mutations exceeds synonymous substitutions among sequences of MCS, thus indicating accelerated evolutionary rates. This was highest in APETALA1 followed by APETALA3 and PEPC. We conclude that reproductive isolation but also, to a lesser extent, physiological adaptation and hybridisation were major triggers in the evolution of this group.