RAPD EVIDENCE FOR A SISTER GROUP RELATIONSHIP OF THE PRESUMED PROGENITOR-DERIVATIVE SPECIES PAIR SENECIO-NEBRODENSIS AND S-VISCOSUS (ASTERACEAE)

The phylogenetic and phenetic analysis of 109 RAPD polymorphisms in S. nebrodensis, a perennial and self-incompatible endemic of four mounta in ranges in Spain, and S. viscosus, a self-compatible annual widespre ad in Europe, as well as S. lividus, S. sylvaticus and S. vulgaris rev ealed a sister group relationship between the first two species. This result contrasts sharply with the earlier hypothesis based on isozyme variation that Si viscosus originated from within a paraphyletic S. ne brodensis and that the two species represent a progenitor-derivative p air. After considering possible reasons for the sister group relations hip found, including the possibility of rooting artefacts, it is concl uded that neither the RAPD data nor the isozyme data allow to draw saf e conclusions about the mode of speciation and therefore the relative age of the two species. As a consequence, the limited genetic variatio n of S. viscosus in comparison to S. nebrodensis as revealed by both t he RAPD and the isozyme data may reflect its population history, geogr aphical distribution, reproductive ecology, or mode of dispersal just as well as its recent origin from a paraphyletic S. nebrodensis. The r esult of this study calls for a critical reexamination of other taxon pairs postulated to have a progenitor-derivative relationship on the b asis of isozyme evidence.