Evaluating possible reinforcement in campanula americana: response to postzygotic isolation by increasing selfing

Allopatry leads to postzygotic isolation through the accumulation of genetic incompatibilities between different lineages. Consequently, when lineages come into secondary contact, the fitness of hybrids between lineages will be reduced. Then a process called reinforcement may occur, where selection to avoid hybridization leads to the evolution of greater prezygotic isolation between lineages. Selfing is reported as a mechanism of prezygotic isolation between sympatric sister species that contributes to reinforcement, but whether selfing also is involved in reinforcement in the early stages of speciation is still unknown. To examine whether selfing increases in secondary contact as a mechanism of reinforcement in the early stages of speciation, I estimated the levels of selfing in populations from different lineages in contact zones and allopatry in the hermaphroditic herb, Campanula americana. This species has asymmetric cytonuclear incompatibilities between lineages that cause unfit hybrids. In addition, selfing is evolutionarily flexible for C. americana, since populations vary in the degree of autonomous selfing. Selfing was measured by determining autonomous fruit set of plants in the greenhouse as well as by estimating selfing rates from wild-collected seeds. I found that populations in contact zones do not have higher autonomous fruit set or selfing rates than allopatric populations, and populations that experience stronger cytonuclear incompatibility also do not have higher autonomous fruit set or selfing rates. These findings indicate that reinforcement is not the cause of observed variation in selfing. However, there was a linear increase of selfing rates from eastern populations to western populations, indicating that a geographical cline, rather than reinforcement, explains the variation in selfing among populations. Despite the evolutionary flexibility of autonomous selfing in C. americana, there is no evidence that selfing evolves in response to postzygotic isolation as a prezygotic isolating mechanism.