Genetic control of self-incompatibility in Senecio squalidus L. (Asteraceae): a successful colonizing species

Senecio squalidus (Oxford ragwort) is a well-known introduction to the Brit ish flora that has proved to be an extremely successful colonist over the l ast 150 years. Unusually for a colonizing species, S. squalidus is self-inc ompatible (SI). Being a member of the Asteraceae, SL in S. squalidus is exp ected to be sporophytic. This paper presents genetic data showing that the SI system of S. squalidus is indeed sporophytic and is controlled by a sing le multiallelic S locus, alleles of which show the dominance/recessive rela tionships characteristic of sporophytic SI (SSI). Early indications are tha t the number of S alleles in populations is low because only four different S alleles were identified in a sample of four plants from two distinct pop ulations; one S allele, S-1, a pollen/stigma recessive allele, was present in all four plants. Forced inbreeding, using salt-treatment to overcome SI, was shown to generate 'pseudo-self-compatible' individuals with weakened S I and a loss/reduction in stigmatic S-specific discrimination. Relatively h igh frequencies of unpredictable compatible crossing 'anomalies' suggest th at a 'gametophytic element' may influence the outcome of crosses in certain genetic backgrounds so as to increase levels of compatibility when S allel es are shared. Together, these findings indicate a genetic 'flexibility' in the SSI system of S. squalidus that could be crucial to its success as a c olonizer.