Isozyme studies in the genusCheirolophus (Asteraceae:Cardueae-Centaureinae) in the Iberian Peninsula, North Africa and the Canary Islands

The phylogenetic and phenetic analysis of 109 RAPD polymorphisms inS. nebrodensis, a perennial and self-incompatible endemic of four mountain ranges in Spain, andS. viscosus, a self-compatible annual widespread in Europe, as well asS. lividus, S. sylvaticus andS. vulgaris revealed a sister group relationship between the first two species. This result contrasts sharply with the earlier hypothesis based on isozyme variation thatS. viscosus originated from within a paraphyleticS. nebrodensis and that the two species represent a progenitor-derivative pair. After considering possible reasons for the sister group relationship found, including the possibility of rooting artefacts, it is concluded that neither the RAPD data nor the isozyme data allow to draw safe conclusions about the mode of speciation and therefore the relative age of the two species. As a consequence, the limited genetic variation ofS. viscosus in comparison toS. nebrodensis as revealed by both the RAPD and the isozyme data may reflect its population history, geographical distribution, reproductive ecology, or mode of dispersal just as well as its recent origin from a paraphyleticS. nebrodensis. The result of this study calls for a critical reexamination of other taxon pairs postulated to have a progenitor-derivative relationship on the basis of isozyme evidence.