Examining the ecological significance of floral orientation in relation to biotic and abiotic agents of selection

Background/Question/Methods

Trait divergence between flowering species can act as a reproductive isolating barrier between the species when they co-occur in sympatry, either by facilitating pollinator differentiation between species (ethological isolation) or by decreasing the likelihood of pollen transfer even if pollinators visit both species (mechanical isolation).  Understanding the mechanisms that maintain species integrity and the selective agents driving these differences are important steps in explaining speciation within most taxa.  For co-occurring bluebell species Mertensia fusiformis and M. brevistyla (Boraginaceae), little is known about how these species differ in their floral traits and if reproductive character displacement are mediated by pollinators.  To answer these questions, we measured various reproductive traits in both species and performed cross-pollination of bagged plants to test for interspecific pollination.  We then tested floral orientation as a potential trait that may be under selection by the pollinators because M. fusiformis and M. brevistyla differ in their floral orientation (i.e., M. brevistyla flowers are more upright than that of M. fusiformis).  We selected 90 individuals per species and manipulated floral orientation into upright, horizontal, or downward orientation.  We then supplemented pollen to half of the individuals and compared seed set among orientation treatments and between open-pollinated versus pollen-supplemented flowers.

Results/Conclusions

We identified several traits that differed significantly between M. fusiformis and M. brevistyla, including number of flowers per plant, first and peak flowering date, corolla length and width, anther and style length, stigma-anther separation, and floral orientation.  These differences suggest that floral features may contribute to reproductive isolation by ethological and/or mechanical isolation.  Cross-pollination of bagged plants revealed that seed set is significantly influenced by plant species and pollination treatment (conspecific versus heterospecific pollen), and there is no evidence of autogamy in both species.  Seed set of M. brevistyla was unaffected by floral orientation treatment or pollen supplementation.  Seed set in M. fusiformis was significantly influenced by the orientation treatment (flowers oriented horizontally set more seed than those in downward or upward orientation) but not by the supplemental pollination treatment.  The lack of evidence for pollinator-mediated selection may be due to conflicting selection between pollinator attraction and effectiveness.  If so, although pollinators are more likely to visit upright flowers, they may spend longer periods of time visiting downward flowers and thus collecting more pollen.  Examining pollinator assemblages and preferences between the flowering species will explain the roles that pollinators play in trait divergence and reproductive isolation between sympatric species.