How early does the selfing syndrome arise? Associations between selfing ability and flower size within populations of the mixed-mater Collinsia verna

Premise: Widespread associations between selfing rate and floral size within and among taxa suggest that these traits may evolve in concert. Does this association develop immediately because of shared genetic and/or developmental control, or stepwise with selection shaping the evolution of one trait following the other? If the former, then association ought to appear within and across selfing populations. We explore this fundamental question in three populations of the mixed-mater Collinsia verna where autonomous selfing (AS) ability has been shown to be under selection by the pollination environment.

Methods: We grew clonal replicates of C. verna in a controlled environment to characterize broad-sense genetic correlations among traits within populations and to assess whether divergence in mating system and floral traits among these populations is consistent with their previously observed selection pressures.

Results: As predicted by their respective pollination environments, we demonstrate significant genetic divergence among populations in AS ability. However, patterns of divergence in floral traits (petal, stamen, and style size, stigmatic receptivity, and stigma-anther distance) were not as expected. Within populations, genetic variation in AS appeared largely independent from floral traits, except for a single weak negative association in one population between flower size and AS rate.

Conclusions: Together, these results suggest that associations between selfing rate and floral traits across Collinsia species are not reflected at microevolutionary scales. If C. verna were to continue evolving toward the selfing syndrome, floral trait evolution would likely follow stepwise from mating system evolution.