Interrelationships Among Inbreeding Depression, Plasticity in the Self-incompatibility System, and the Breeding System of Campanula rapunculoides L. (Campanulaceae)

The evolution of breeding systems in plants is often viewed as a balance between the adverse consequences of selfing (inbreeding depression and the loss of opportunities to sire seeds on conspecifics) and the benefits of selfing (a genetic transmission advantage and reproductive assurance when cross pollen limits seed production). In this paper we examine the genetic and environmental causes of variation in the expression of self-incompatibility (SI) in Campanula rapunculoides and explore the consequences of this variation on the breeding system. Campanula rapunculoides has an S-RNase based SI system similar to that described in the Solanaceae. However, our studies of plants from two natural populations have revealed that the flowers of most individuals are self-incompatible when they first open but become more self fertile as the flowers age. Moreover, when both cross and self pollen are deposited onto the stigmas of older flowers, the cross pollen tubes grow faster and sire a disproportionate number of the seeds. In short, self-fertilization occurs only after most opportunities for outcrossing have occurred. We also found that there is significant heritable genetic variation in the population for the strength of SI in young and old flowers and for the amount of breakdown in SI indicating that natural selection could operate on the strength of SI and its breakdown. In a multigenerational study, we used controlled crosses to create families of plants with a range of inbreeding coefficients (0, 0.25, 0.5 and 0.75). We found that fitness declined significantly over the range of inbreeding coefficients and that the decline in fitness was less for families derived from weak SI phenotypes. Consequently, it is only advantageous for C. rapunculoides to produce selfed seed when seed production is limited by the availability of cross pollen. Because of plasticity in the SI system,C. rapunculoides has a breeding system that combines the best of both worlds.