Influence of floral morphology on pollinator visitatation to Allionia incarnata (Nyctaginaceae)

Background/Question/Methods: Pollinators forage for floral resources based on a variety of cues, and their foraging decisions can ultimately impact plant reproductive success. In turn, plants have evolved floral morphological characteristics to direct pollinator visitation. One such characteristic is enantiomorphy, spatial separation of anthers and stigmas where stigmas are oriented laterally on a blossom. Allionia incarnata (Nyctaginaceae) blossoms exhibit enantiomorphy in each of its three fused, bilaterally symmetrical flowers which appear as a single, radially symmetrical blossom. Little is known about which species pollinate Allionia, but it is thought that its unique floral morphology would influence pollinator visitation and subsequent plant success. This study aimed to describe the reproductive ecology of Allionia, asking how the orientation of floral stigmas influences the foraging patterns of its floral visitors. Individual foraging bouts of 94 insect floral visitors to Allionia were observed in Saguaro National Park in autumn 2012, noting where they landed on the blossom and the order in which they contacted Allionia reproductive structures. Pollen deposition on 90 blossoms was measured, noting the orientation of stigmas, and which stigmas received pollen. Finally, Allionia blossoms were hand-pollinated with self-pollen to test for self-compatibility. 

Results/Conclusions: The most common floral visitors to Allionia were Hemiargus ceraunus (Lycaenidae) and Halictus bees, comprising 45% and 19% of all visits, respectively. Floral stigmas are presented in a Y-shape, with two stigmas being displayed together and one being displayed separately. This orientation was found more frequently than expected by chance, and subsequent pollinator landing patterns were consistent relative to the orientation. Halictus significantly landed away from stigmas (χ², P < 0.001). The landing patterns of Hemiargus did not differ significantly than expected by chance overall, but consistently landed on the outer edge of the blossom before approaching the nectary. The landing pattern of Allionia pollinators can inform the order in which they contact floral reproductive structures, and there was a trend in both taxa for visiting anthers first. Hemiargus contacted an anther first 71% of the time, while Halictus did so 83% of the time. These results suggest that Allionia pollinators are contributing to self-pollination more frequently than outcrossing, and preliminary evidence shows that selfed flowers will set seeds that do not differ significantly in weight from outcrossed seeds. Further research must be conducted to track pollen movement and to assess the germinability of selfed seeds.