A test for associations between outcrossing rate and photosynthetic rate within populations of two mixed-mating Clarkia

Background/Question/Methods

Self-fertility in plants may be adaptive if pollinator service is inadequate (“reproductive assurance”).  Alternatively, self-fertility may evolve through genetic correlation with other traits that are adaptive. We examined evidence for this alternative hypothesis in the wildflower genus Clarkia, using two pairs of sister taxa that contrast in mating system. For both pairs, the selfing taxon flowers earlier and has a higher photosynthetic rate when compared with its mixed-mating progenitor.  High photosynthetic rates can drive rapid development and early flowering, which may be adaptive in the seasonal drought these plants experience.  Rapid development, in turn, can favor traits that confer self-fertility.  Thus, the observed association between selfing and high photosynthetic rate in these taxa may reflect a genetic correlation between selfing and adaptive life history traits that minimize drought exposure. If so, we predicted to observe similar association between self-fertilization and photosynthetic rates within populations of the two mixed-mating taxa, C. unguiculata and C. xantiana ssp. xantiana. We measured photosynthetic rates of plants in the field in two populations of each species, and estimated outcrossing rates using multiple polymorphic isozymes.

Results/Conclusions

In one of the two populations of C. unguiculata, we found no relationship between photosynthetic rate and outcrossing rate.  In the second population, plants with high photosynthetic rates had significantly lower outcrossing rates, which is similar to the pattern that exists between this mixed-mating species and its selfing sister taxon.  Thus, in one of the two populations of C. unguiculata, we observed a pattern that was consistent with our prediction.  In one of the two populations of C. x. ssp. xantiana, we found no association between photosynthetic rate and outcrossing rate.  In the second population, plants with high photosynthetic rates had significantly higher outcrossing rates, which is a pattern that opposes our predictions.  Overall, our prediction was upheld in one of the four populations, which suggests stronger support for the idea that the observed association between photosynthetic rate and outcrossing rate between taxa occurs because each evolved independently rather than through genetic correlation.