Can Sexual Selection Cause Divergence in Mating System-Related Floral Traits?

Premise of research. The wide diversity of floral traits seen among plants is shaped by neutral and selective evolutionary processes. In outcrossing species, sexual selection from competing pollen donors is expected to be important for shaping mating system-related traits, but empirical evidence is scarce. In a previous evaluation of experimental evolution lines crossed with either one or two pollen donors (monogamous, M, or polyandrous, P, lines) at early floral stages in mixed-mating Collinsia heterophylla (Plantaginaceae), P evolved enhanced pollen competitive ability and reduced maternal seed set compared with M, in accordance with the sexually antagonistic evolution of pollen. Here, we asked whether the presence of sexual selection during pollen competition affects mating system-related floral traits in the same lines. Methodology. We compared flowering start, timing of anther-stigma contact (as an indication of timing of self-pollination), timing of stigma receptivity, and first seed set between M and P and with a source line, S (starting plants). The first three traits are later in outcrossers than in selfers of Collinsia. The last trait was expected to be earlier in P than in M, resulting from sexual selection for early seed siring of pollen. Pivotal results. Artificial polyandry for four generations resulted in later flowering start and later anther-stigma contact in P compared with M, and the latter trait was intermediate in S. Thus, P appeared more "outcrossing" than M. Timing of stigma receptivity did not differ between lines. First seed set was earlier in P than in M, as expected from sexual selection. Conclusions. Our results from the C. heterophylla experimental evolution lines suggest that a component of sexual selection during outcross pollination could enhance the patterns of floral divergence commonly found between outcrossers and selfers.