Adaptive radiation and genetic differentiation in the woody Sonchus alliance (Asteraceae : Sonchinae) in the Canary Islands

The woody Sonchus alliance consists of 19 species of Sonchus subg. Dendroso nchus, one species of Sonchus subg. Sonchus and species of five genera (i.e . Babcockin, Sventenia, Taeckholmia, Lactucosonchus, Prenanthes), and is re stricted primarily to the archipelago of the Canaries in the Macaronesian p hytogeographical region. An enzyme electrophoretic study, including 13 loci , was conducted to assess genetic diversity within and divergence among spe cies of the alliance. Nei's genetic identities (distances) between genera a nd/or subgenera range from 0.490 (0.714) to 0.980 (0.013), and pairwise com parisons of all populations show relatively high genetic identities, with a mean of 0.804. The high identities further support the genetic cohesivenes s of the alliance and its single origin on the Macaronesian islands. Specie s in the alliance also show about 50% higher total genetic diversity (H-T) than the mean for other oceanic endemics. There is greater divergence betwe en endemics or species on older islands compared to those on younger island s, which suggests that time is a factor for divergence at allozyme loci. Fu rthermore, populations on older islands have higher total genetic diversiti es and lower identities than conspecific populations on younger islands. Th ese results imply early colonization, radiation, and divergence of the wood y Sonchus alliance on older islands followed by subsequent colonization to younger islands. The taxonomic distribution of alleles in the alliance indi cates lineage sorting also played a role in divergence among species. Linea ge sorting may also produce nonconcordance with either taxonomic designatio n or the pattern of variation obtained from other molecular markers such as ITS sequences of nrDNA. Timing for the origin and radiation of the allianc e agrees with the estimate based on ITS sequences, and suggests that the ea rly divergence and rapid radiation took place during the Late Tertiary on e ither Gran Canaria or Tenerife.