Routes of origin of two recently evolved hybrid taxa: Senecio vulgaris var. Hibernicus and York radiate groundsel (Asteraceae)

The possible pathways of origin of two recently arisen introgressant forms of Senecio vulgaris (i.e.. var. hibernicus and York radiate groundsel) were investigated in experimental crosses between tetraploid S. vulgaris var. v ulgaris and the normally diploid S. squalidus. Comparison of the morphology of synthesized hybrid progeny with established taxa, by discriminant funct ion analysis, revealed that fertile hybrid offspring similar in morphology to S. vulgaris var. hibernicus and York radiate groundsel could be synthesi zed: (1) following formation of genomically stable diploid gametes by the t riploid hybrid; (2) through the production of unreduced gametes by diploid S. squalidus: and (3) when a tetraploid form of S, squalidus acted as one o f the parents. It was evident that hybrid offspring similar in morphology t o the two introgressant taxa were more often produced in backcrosses to S. vulgaris than in segregating F2 or F3 generations (53% as opposed to 36%), and that fertile hybrid progeny were formed within two generations. Because hybridization between S. vulgaris and S. squalidus occurs regularly, altho ugh at very low frequency, in natural mixed populations in the British Isle s, there is the potential for multiple origins to occur in the wild of both S. vulgaris var. hibernicus and York radiate groundsel.