Morph differences in seed output and the maintenance of the polymorphism for capitulum type and outcrossing rate in Senecio vulgaris L

Summary

The effect of a greater outcrossing rate of the radiate relative to the non-radiate morph of Senecio vulgaris on the fate of the radiate allele in polymorphic populations of the species has been analysed using a series of deterministic computer simulations. It is shown that for the radiate allele to be maintained, or for its initial spread in a population, the ‘autoselective’ advantage of the non-radiate allele, arising from a greater selfing rate of the non-radiate genotype, can be balanced by selection acting against the non-radiate genotype.

A previous study by other workers showed that in several British populations the radiate morph produced a significantly greater amount of seed per individual than the non-radiate morph. This advantage was shown to be enough to counterbalance the autoselection against the radiate allele. However, the present study reveals that in four Edinburgh populations the non-radiate morph produces either an equivalent or greater seed output per plant than the radiate morph. In discussion we comment on a possible reason for the inconsistent pattern of differences in seed output between morphs and conclude that considerable information will be required on the biology of the capitulum polymorphism m S. vulgaris if an understanding is to be gained of its evolution in Britain.