GENETIC-DIVERGENCE AMONG MEDITERRANEAN AND MACARONESIAN GENERA OF THESUBTRIBE CHRYSANTHEMINAE (ASTERACEAE)

Genetic variation at 17 isozyme loci was used to assess divergence amo ng the four genera comprising subtribe Chrysantheminae (Anthemideae: A steraceae). The Macaronesian endemic genus Argyranthemum is supported as monophyletic and is about equally divergent at isozyme loci from th e other three genera of the Chrysantheminae, Chrysanthemum, Heteranthe mis, and Ismelia. Chrysanthemum is native to the Mediterranean whereas Heteranthemis occurs in southern Iberia and Morocco, and Ismelia is e ndemic to Morocco. The genera Chrysanthemum and Ismelia have a genetic identity of 0.9283, which is comparable to values often seen For cong eneric species and indicates that they should be treated as one genus. The isozyme data indicate that three lines consisting of Argyranthemu m, Chrysanthemum-Ismelia, and Heteranthemis radiated rapidly from a co mmon ancestor. Divergence times estimated from isozyme data suggest th at the initial radiation of the three lines occurred 2.5-3.0 mya. If t his is so, then Argyranthemum or its ancestor arrived in Macaronesia a fter all the islands except La Palma and El Hierro were formed. The ev olutionary history of the subtribe is discussed in relation to the cli matic and geological events that took place in the Western Mediterrane an between the Tertiary and Quaternary, i.e., the first Northern Hemis phere glaciation and desertification of the Sahara region. The high me an genetic identities between species of Argyranthemum suggest that it might have subsequently undergone a second more recent radiation in t he Macaronesian Islands. Also, the high mean identity (0.860) between populations in Chrysanthemum and Ismelia suggest that these continenta l genera might be in the early stages of secondary speciation.