INTROGRESSIVE ORIGIN OF THE RADIATE GROUNDSEL, SENECIO-VULGARIS L VAR HIBERNICUS SYME - AAT-3 EVIDENCE

A survey of allelic variation at the Aat-3 locus in Senecio squalidus and S. vulgaris revealed that the Aat-3c allele, which was present at high frequency in British populations of S. squalidus, was also common in British radiate groundsel (S. vulgaris var. hibernicus) but was rare among individuals of the non-radiate groundsel S. vulgaris var. vulgaris) which co-occurred with var. hibernicus and was absent from British, Irish and mainland European populations monomorphic for var. vulgaris. This evidence is taken as confirmation of an introgressive origin of S. vulgaris var. hibernicus across a chromosome barrier following hybridization between S. vulgaris var. vulgaris (2n = 40) and radiate S. squalidus (2n = 20) and backcrossing to S. vulgaris var. vulgaris. Genetic analysis showed that the Aat-3 locus, which is duplicated in S. vulgaris is not linked to the ray floret locus controlling capitulum type. It is suggested that the close association between the Aat-3c allele and the radiate allele in populations of S. vulgaris polymorphic for capitulum type may be maintained by selection favouring a co-adapted complex of genes introgressed from S. squalidus, although alternative explanations are not ruled out. The introgression of the Aat-3c allele and associated genetic material from S. squalidus into S. vulgaris is likely to have enhanced the level of genetic variation present within S. vulgaris and may have been a factor that has favoured the spread of S. vulgaris var. hibernicus in Britain following its origin last century.