Why are capitulum morphs associated with other characters in natural populations of Senecio vulgaris (groundsel)?

The capitulum polymorphism in Senecio vulgaris (groundsel) has previously been shown to have arisen in Britain following hybridization with the introduced species S. squalidus (Oxford ragwort). In natural populations, suites of developmental, morphological, reproductive and electrophoretic characters are often found to be associated with the radiate and non-radiate capitulum morphs. Here we demonstrate that the reasons for these character associations may vary among different populations. In plants from Acomb and Morpeth, there is no evidence of genetic linkage between the capitulum locus and those controlling a range of other characters. It is suggested that the associations between characters in these populations are a result of very low outcrossing rates between the two capitulum morphs so that chance associations present in founder individuals are not dispersed. In a population from York, however, linkage is shown to be present and provides an adequate mechanism to explain most of the character associations. York radiate plants may represent the product of a recent introgressive event involving S. squalidus. On the basis of linear discriminant function scores, in all three populations, both radiate and non-radiate progeny in the F-2 generation from crosses derived from radiate female parents are shown to be more radiate-like than equivalent progeny derived from the reciprocals with non-radiate female parents. The reason for this bias is not clear.