Polymorphism for outcrossing frequency at the ray floret locus in senecio vulgaris L. I. Evidence

SUMMARY In two Edinburgh populations of Senecio vulgaris showing the ray floret poly-morphism, large differences in "female" outcrossing frequency were recorded between radiate and non-radiate plants. During peaks of flowering, radiate plants exhibited "female" outcrossing rates of between 13—20 per cent while the level of "female" outcrossing shown by "non-radiate" plants never exceeded one per cent. It is postulated that the possession of pistillate ray florets by capitula of radiate plants, leading to the occurrence of functional protogyny in such capitula, greatly boosts the outcrossing frequency of radiate plants relative to that of non-radiate plants. It is suggested that the polymorphism for outcross-ing frequency at the ray floret locus of S. vulgaris could be usefully employed as a "model situation" in studies aimed at improving our understanding of the maintenance of major gene variation for outcrossing frequency in plant popula-tions.