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

Levels of outcrossing and self-compatibility were estimated in two populations of Senecio squalidus, a highly successful colonizing species in the U.K. In both populations there was a tendency for outcrossing rate, t, to be depressed below unity in early summer; however, in neither instance was the departure from unity significant. Samples collected in early autumn also exhibited outcrossing rates not significantly different from t = 1. Levels of self-compatibility measured in terms of seed set on selfing, were very low in all material tested. This finding plus the results of a crossing programme, provides evidence that British S. squalidus possesses a highly effective self-incompatibility system, most probably of the homomorphic sporophytic type. It is concluded that mating normally occurs at random in British populations of S. squalidus, although some degree of biparental inbreeding may also occur. The latter would cause inbreeding coefficients to exceed zero as recorded in a number of populations previously studied. The ability of S. squalidus to colonize new sites without engaging in uniparental sexual or asexual reproduction is briefly discussed.