WHY ARE CAPITULUM MORPHS ASSOCIATED WITH OTHER CHARACTERS IN NATURAL-POPULATIONS OF SENECIO-VULGARIS (GROUNDSEL)

The capitulum polymorphism in Senecio vulgaris (groundsel) has previou sly been shown to have arisen in Britain following hybridization with the introduced species S. squalidus (Oxford ragwort). In natural popul ations, suites of developmental, morphological, reproductive and elect rophoretic characters are often found to be associated with the radiat e and non-radiate capitulum morphs. Here we demonstrate that the reaso ns for these character associations may vary among different populatio ns. In plants from Acomb and Morpeth, there is no evidence of genetic linkage between the capitulum locus and those controlling a range of o ther characters. It is suggested that the associations between charact ers in these populations are a result of very low outcrossing rates be tween the two capitulum morphs so that chance associations present in founder individuals are not dispersed. In a population from York, howe ver, linkage is shown to be present and provides an adequate mechanism to explain most of the character associations. York radiate plants ma y represent the product of a recent introgressive event involving S. s qualidus. On the basis of linear discriminant function scores, in all three populations, both radiate and non-radiate progeny in the F-2 gen eration from crosses derived from radiate female parents are shown to be more radiate-like than equivalent progeny derived from the reciproc als with non-radiate female parents. The reason for this bias is not c lear.