Comparative palynomorphological study of the genus Symphoricarpos (Caprifoliaceae): exine sculpture and implications for evolution

Abstract

Recent molecular phylogenetic studies of Dipsacales place Symphoricarpos in a clade containing Leycesteria, Triosteum, and Lonicera. The aim of this research was to analyze the palynomorphological features of taxa of Symphoricarpos and to compare the results with the taxonomic classification scheme and molecular phylogenetic data. Pollen morphology of 20 herbarium specimens belonging to 12 taxa of Symphoricarpos was studied using both light microscopy and scanning electron microscopy. Pollen are 3(4)-colporate, obtate to subprolate (Polar axis (P) /Equatorial diameter (E) = 0.73-1.21) in shape, and medium- and large-sized (P=27.93-62.51 mu m, E=25.27-69.16 mu m). Three types of exine sculpture are recognized in the genus: type I - psilate-perforate, type II - microechinate-nanoechinate, and type III - echinate-microechinate-nanoechinate. Within these pollen types, seven subtypes are distinguished based on details of the exine sculpture, pollen size, details of colpi, and presence/absence and width of annuli. It has been established that the important diagnostic features at the species level for the purposes of taxonomy of Symphoricarpos are the size of pollen grains, details of the colpi and ores, presence/absence and width of the annuli, and exine sculpture. The pollen morphology of Symphoricarpos taxa allows the establishment of hypotheses regarding the evolution of exine sculpture. In particular, we assume that there is an evolutionary trend toward a decrease in the size and number of echini and microechini. Echinate-microechinate-nanoechinate exine sculpture is hypothesized to be a plesiomorphic condition within the genus Symphoricarpos.