Chloroplast DNA and isozyme evidence on the evolution of Senecio vulgaris (Asteraceae)

Chloroplast DNA (cpDNA) and isozyme variation were analyzed over a range 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 derivative of S. vulgaris subsp. denticulatus from the coasts of W Europe and montane altitudes in S Spain and Sicily. The two taxa exhibited a very high genetic identity with subsp. denticulatus containing slightly more isozyme diversity than was found in var. vulgaris. - Three cpDNA haplotypes (A, B, C) already known from other Mediterranean diploid species of Senecio were resolved in var. vulgaris, and an additional fourth haplotype (E) was found in subsp. denticulatus. Two alternative 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 recurrent 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 nuclear (isozyme) diversity were present in both taxa of S. vulgaris examined supports the second of these two hypotheses; however, more detailed analysis of nuclear genetic diversity is required before a firm conclusion can be reached on this matter.