The evolution of invasive species in Senecio (Asteraceae)

Research of the last ca. 20 years on the evolution of the weedy Senecio viscosus and S. vulgaris is summarised. The widespread annual and self-compatible S. viscosus is most closely related to the perennial and self-incompatible Iberian endemic S. nebrodensis. In the past, S. viscosus has been regarded as an evolutionary derivative of S. nebrodensis. The phylogenetic analysis of a RAPD data set, however, implies that the two species are sister to each other. Genetic variation at the isozyme and RAPD level is much lower in S. viscosus than in S. nebrodensis. It is argued that this is not the result of the recent origin of S. viscosus from S. nebrodensis, but rather of its population history, ecology, and breeding system. The cosmopolitan S. vulgaris subsp. vulgaris var. vulgaris is interpreted as a derivative of the rare S. vulgaris subsp. denticulatus. Whereas subsp. denticulatus is a winter annual with pronounced seed dormancy, no seed dormancy is found in most material of the very short-lived var. vulgaris. Genetic analyses have shown that the presence or absence of seed dormancy, speed of development, and presence (subsp. denticulatus) or absence (var. vulgaris) of ray florets are governed by one locus each. A RAPD-QTL analysis revealed that seven different characters including speed of development are under the control of three chromosomal regions in two linkage groups. The genetic architecture of the differences between subsp. denticulatus and var. vulgaris implies that the evolution of var. vulgaris could have been fast.