Evaluation of pollen morphology of Passiflora species and hybrids (Passifloraceae)

Abstract

Passiflora L. is a diverse genus of mostly New World species, many of which are still evolving to overcome the challenges of competition, predation, and habitat expansion. Pollen collected from 195 Passiflora L. taxa across six subgenera; Astrophea (DC.) Mast., Decaloba (DC.) Rchb., Deidamioides (Harms) Killip, Passiflora, Tetrapathaea (DC.) P.S. Green) and Tryphostemmatoides (Harms) Killip and 40 F1 hybrids (cultivars) were examined using Scanning Electron Microscopy (SEM) and low temperature SEM. Passiflora pollen grains are divided into two major morphological groups: species with three pairs (6 apertures) of colpi found in species in subgenera Astrophea, Deidamioides, Passiflora and Tryphostemmatoides, and those with six pairs (12 apertures) of colpi found in most species from subgenus Decaloba (Tetrapathea unknown). This study shows polymorphism and surface anomalies in the morphology of pollen grains of some species. Pollen from many Passiflora F1 hybrids were examined: many have a high percentage of sterile pollen grains, others in section Dysosmia DC. and Tacsonia Juss. in subgenus Passiflora recorded a minimal percentage of sterile grains. This may be a key evolutionary advantage as several species in these groups are now recorded as invasive weeds with worldwide distribution. The term, “sticky pollen” is revealed to be the pre-germination of pollen grains without stigmatic contact. The success of some invasive species to outcompete native flora and thrive in extreme environments is in part due to the evolution of their pollen, with the absence of pollen deformation following hybridization events promoting hybrid fitness. Pollen grain morphological evidence supports keeping three endemic Australian Passiflora species in subgenus Decaloba as a unique group.