Inbreeding depression in Campanula rapunculoides L. I. A comparison of inbreeding depression in plants derived from strong and weak self-incompatibility phenotypes

The evolution of selfing,a taxa from outcrossing ancestors has occurred rep eatedly and is the subject of many theoretical models, yet few empirical st udies have examined the immediate consequences of inbreeding in a populatio n with variable expression of self-incompatibility. Because self-incompatib ility breaks down with floral age in Campanula rapunculoides, we were able to mate outbred and self ed maternal plants in a crossing design which prod uced progeny with inbreeding coefficients of 0, 0.25, 0.50 and 0.75. Cumula tive inbreeding depression in plants that were self ed for one generation w as very high in families derived from strongly self-incompatible plants (av erage delta = 0.98), and somewhat lower in families derived from plants wit h weaker expression of self-incompatibility (average delta = 0.90). Relativ e to outbred progeny, inbred progeny produced fewer seeds; had lower rates of germination, less vegetative growth and fewer flowers per plant. Inbred progeny also took longer to germinate, and longer to produce a first leaf a nd to flower. Interestingly, inbred plants also produced 40% fewer seeds th an outcrossed plants (t-test P < 0.001) even when mated to the same, unrela ted pollen donor, suggesting that inbreeding can produce profound maternal effects. Most importantly, our results demonstrate that progeny derived fro m plants with stronger expression of self-incompatibility exhibited greater levels of inbreeding depression than progeny from plants with weaker expre ssion of self-incompatibility. Moreover, the decline in fitness (cumulative , In-transformed) over the four inbreeding levels was steeper for the proge ny of the strongly self-incompatible lineages. These empirical results sugg est that inbreeding depression and mating system phenotype have the potenti al to coevolve.