Origin and evolution of woody sow-thistles in the Macaronesian Islands revisited: four decades after the revision of Sonchus (Asteraceae)

Abstract

The woody Sonchus alliance (Asteraceae), which includes six genera and ca. 31 species, represents a premier example of adaptive radiation in Macaronesia. This contribution reviews previous research relevant to the phylogeny and biology of this group and present several research questions that remain to be answered. Both nuclear and chloroplast gene phylogenies clearly demonstrate the monophyly of the entire alliance. Yet, the closest continental ancestor(s) remain to be determined based on additional molecular phylogenies. In the evolution of the alliance, intraisland differentiation and interisland colonization events played important roles, but evolutionary contributions of homoploid hybrid speciation need to be determined. The role of photosynthetic adaptation in exploring habitats with contrasting light regimes during rapid evolutionary radiation was assessed but additional studies are necessary to synthesize emerging patterns. Both nuclear and chloroplast gene phylogenies refute the relictual nature of the alliance and demonstrate a rather recent origin and adaptive radiation during late Miocene and early Pliocene. The genetic basis of morphological and physiological differences appears to be controlled by many genes with small effects. The genetic basis of insular woodiness has not been revealed yet. Therefore, understanding the genetic basis of woody forms and their ecological relevance remains to be determined.