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

A difference in germination behavior between the highly selfing, nonradiate variant of Senecio vulgaris and the more outcrossing, radiate form had a l arge effect on the relative female fitness of these two morphs raised in mi xed stands under conditions similar to those experienced in the wild. Of pa rticular significance was the finding that female fitness differences betwe en morphs were reversed in early- and late-sown plots. This was because a g reater proportion of nonradiate than radiate seed germinated directly after sowing, and in early-sown plots a relatively large proportion of early-ger minated nonradiate seedlings survived winter to produce large, highly fecun d plants the following summer, that contributed greatly to the total female fitness of the nonradiate morph. In contrast, in late-sown plots (i.e., so wn two weeks later), survivorship of early germinated seedlings was much re duced, and the radiate morph had an advantage because most radiate seed del ayed germination until spring, therefore avoiding seedling mortality during winter. The effect of the association between morph type and germination b ehavior on morph female fitness in S. vulgaris is clearly important in rega rd to the evolution of the polymorphism for outcrossing rate in the species . This, in turn, emphasizes the point that an understanding of factors resp onsible for the evolution and maintenance of polymorphisms for outcrossing rate in the wild should be based on a detailed examination of the ecologica l genetics of such polymorphisms that extend beyond traditional studies of pollen discounting and inbreeding depression.