Evolution of a polymorphism for outcrossing rate in Senecio vulgaris: Influence of germination behavior

Abstract

A difference in germination behavior between the highly selfing, nonradiate variant of Senecio vulgaris and the more outcrossing, radiate form had a large effect on the relative female fitness of these two morphs raised in mixed stands under conditions similar to those experienced in the wild. Of particular significance was the finding that female fitness differences between morphs were reversed in early- and late-sown plots. This was because a greater proportion of nonradiate than radiate seed germinated directly after sowing, and in early-sown plots a relatively large proportion of early-germinated nonradiate seedlings survived winter to produce large, highly fecund plants the following summer, that contributed greatly to the total female fitness of the nonradiate morph. In contrast, in late-sown plots (i.e., sown two weeks later), survivorship of early germinated seedlings was much reduced, and the radiate morph had an advantage because most radiate seed delayed germination until spring, therefore avoiding seedling mortality during winter. The effect of the association between morph type and germination behavior on morph female fitness in S. vulgaris is clearly important in regard to the evolution of the polymorphism for outcrossing rate in the species. This, in turn, emphasizes the point that an understanding of factors responsible for the evolution and maintenance of polymorphisms for outcrossing rate in the wild should be based on a detailed examination of the ecological genetics of such polymorphisms that extend beyond traditional studies of pollen discounting and inbreeding depression.