Genetic diversity for esterases in the recently evolved stabilized introgressant, seneciovulgaris l. Var. hibernicus syme, and itsparental 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, 5. vulgaris var. vulgaris and 5. squalidus, over three esterase loci — αEst-1, βEst-l and βEst-3. It was established that var. hibernicus contained much less genetic diversity at the αEst-1 and βEst-3 loci than was present in var. vulgaris.This was evident irrespective of whether var. vulgaris was sampled from populations containingboth var. hibernicus and var. vulgaris or populations monomorphic for var. vulgaris. Moreover, theβEst-l locus which was present in 5. squalidus, but absent from var. vulgaris, was also absent fromvar. hibernicus, while the (βEst-3b allele which occurred at high frequency (92 per cent) in 5.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 germplasmfrom 5. 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 ofintervariant crossing known to occur in such populations. The possibility of a post-mating breedingbarrier existing between the two variants of 5. vulgaris which might maintain genetic differences isbriefly discussed.