Does size matter? Pollination biology of Aristolochia sessilifolia (Aristolochiaceae)

Background and aims – The morphology of flowers with specialized pollination acts as a pollinator filter, resulting in compatibility between the flower and its visitors. These characteristics can influence plant diversity, and studying these interactions provides important models for understanding ecological and evolutionary processes. This is the case with Aristolochia flowers, which act as a trap, temporarily imprisoning their pollinators. This study aimed to investigate the reproductive biology of Aristolochia sessilifolia , a grassland species from South America, by addressing the following questions: (i) What is the duration of anthesis, and how is dichogamy expressed throughout this period? (ii) Which floral visitors are present, and which ones act as effective pollinators? (iii) Does fly body size influence pollen-carrying capacity? (iv) Is there a relationship between perianth dimensions and the presence of pollen on flies? (v) What is the natural fruit set rate? Material and methods – We collected 50 flowers (10 for each anthesis stage), we defined the anthesis period, identified the floral visitors and measured the height of all flies found, the smallest diameter of the tube, and the distance from the utricle wall and the gynostemium. We evaluated whether these measures influence the fly’s ability to transport pollen. In addition, we evaluated natural fruiting by marking flower buds. Key results – Flowers exhibited a prolonged anthesis (up to five days) and clearly defined protogynous dichogamy. The main pollinators identified were flies from the family Chloropidae. The presence of pollen on fly bodies was associated with body height, the internal diameter of the floral tube, and the distance between the utricle wall and the gynostemium. Natural fruit set reached 50%, a relatively high value compared to other species of the genus. Conclusion – These results support the hypothesis that compatibility between floral size and pollinator body size is necessary for effective pollination, emphasizing the specificity and complexity of Aristolochia flowers.