OUTCROSSING RATE AND SELF-INCOMPATIBILITY IN THE COLONIZING SPECIES SENECIO-SQUALIDUS

Levels of outcrossing and self-compatibility were estimated in two pop ulations of Senecio squalidus, a highly successful colonizing species in the U.K. In both populations there was a tendency for outcrossing r ate, t, to be depressed below unity in early summer; however, in neith er instance was the departure from unity significant. Samples collecte d in early autumn also exhibited outcrossing rates not significantly d ifferent from t = 1. Levels of self-compatibility measured in terms of seed set on selfing, were very low in all material tested. This findi ng plus the results of a crossing programme, provides evidence that Br itish S. squalidus possesses a highly effective self-incompatibility s ystem, most probably of the homomorphic sporophytic type. It is conclu ded that mating normally occurs at random in British populations of S. squalidus, although some degree of biparental inbreeding may also occ ur. The latter would cause inbreeding coefficients to exceed zero as r ecorded in a number of populations previously studied. The ability of S. squalidus to colonize new sites without engaging in uniparental sex ual or asexual reproduction is briefly discussed.