CHLOROPLAST DNA AND ISOZYME EVIDENCE ON THE EVOLUTION OF SENECIO-VULGARIS (ASTERACEAE)

Chloroplast DNA (cpDNA) and isozyme variation were analyzed over a ran ge of populations of two infraspecific taxa of the tetraploid Senecio vulgaris. The isozyme data were supportive of the hypothesis that the weedy and cosmopolitan S. vulgaris var. vulgaris is an evolutionary de rivative of S. vulgaris subsp. denticulatus from the coasts of W Europ e and montane altitudes in S Spain and Sicily. The two taxa exhibited a very high genetic identity with subsp. denticulatus containing sligh tly more isozyme diversity than was found in var. vulgaris. - Three cp DNA haplotypes (A, B, C) already known from other Mediterranean diploi d species of Senecio were resolved in var. vulgaris, and an additional fourth haplotype (E) was found in subsp. denticulatus. Two alternativ e hypotheses were chosen to account for the origin and maintenance of the observed cpDNA composition of S. vulgaris. It either reflects (1) the retention of an ancestral polymorphism which stems from the recurr ent and polytopic formation of ancestral tetraploid lineages; or (2) S . vulgaris originally was characterized by haplotype E, and haplotypes A, B and C were acquired through repeated introgressive hybridization with related diploid species. The finding that very low levels of nuc lear (isozyme) diversity were present in both taxa of S. vulgaris exam ined supports the second of these two hypotheses; however, more detail ed analysis of nuclear genetic diversity is required before a firm con clusion can be reached on this matter.