A Specialized Combination: The Relationship between Reproductive Structure Arrangement and Breeding Systems in oil-rewarding Calceolaria Species (Calceolariaceae)

Plants have different strategies to avoid selfing and buffer its negative consequences on plant fitness. One strategy is the arrangement of petals and the disposition of the reproductive structures (RS) inside the flowers, allowing the development of different pollination mechanisms. In Calceolaria L. species two possible floral phenotypes can be found: short RS protected by the upper corolla lip (nototribic flowers) and long RS resting in the lower corolla lip (sternotribic flowers), the latter being hypothesized to favor selfing.

We selected 13 Calceolaria taxa and characterized their floral phenotype as nototribic or sternotribic, measured RS length and herkogamy, and performed hand-pollination treatments to determine the number of seeds produced by self- and cross-pollination to test whether floral phenotype influences inbreeding. GLMs analysis was performed to determine the differences between the sizes of RS and both floral phenotypes, and LMM was performed to evaluate the relationship between the RS and inbreeding with both floral phenotypes.

We found a relationship between stamen length and herkogamy in both floral phenotypes, where sternotribic flowers have a higher stamen length and lower herkogamy, whereas the opposite occurred in taxa with nototribic morphology. Stamen length significantly influences the inbreeding with sternotribic flowers having a higher inbreeding depression by geitonogamous self-pollination than nototribic ones.

Our results suggest that plants may evolve different reproductive mechanisms to cope with pollination unreliability. Thus, floral phenotype may favor the development of geitonogamy selfing, which may explain the two floral phenotypes present in this specialized oil-secreting genus.