GENETIC DIVERSITY FOR ESTERASES IN THE RECENTLY EVOLVED STABILIZED INTROGRESSANT, SENECIO-VULGARIS L VAR HIBERNICUS SYME, AND ITS PARENTAL TAXA S-VULGARIS L VAR VULGARIS L AND S-SQUALIDUS L

The level of genetic diversity in the recently evolved stabilized introgressant Senecio vulgaris var. hibernicus was compared to that within its parental taxa, S. vulgaris var. vulgaris and S. squalidus, over three esterase loci - alpha-Est-1, beta-Est-1 and beta-Est-3. It was established that var. hibernicus contained much less genetic diversity at the alpha-Est-1 and beta-Est-3 loci than was present in var. vulgaris. This was evident irrespective of whether var vulgaris was sampled from populations containing both var. hibernicus and var. vulgaris or populations monomorphic for var. vulgaris. Moreover, the beta-Est-1 locus which was present in S. squalidus, but absent from var. vulgaris, was also absent from var. hibernicus, while the beta-Est-3b allele which occurred at high frequency (92 per cent) in S. squalidus was present at a low frequency (8 per cent) in var. hibernicus. It is concluded that var. hibernicus has gained no increased genetic diversity for esterases via introgression of germplasm from S. squalidus. The maintenance of a low level of genetic diversity for esterases in var. hibernicus, relative to var. vulgaris in populations containing both variants, was surprising in view of the level of intervariant crossing known to occur in such populations. The possibility of a post-mating breeding barrier existing between the two variants of S. vulgaris which might maintain genetic differences is briefly discussed.