Pollen morphology of the Boschniakia–Epifagus clade (Orobanchaceae): implications for taxonomy and phylogeny

Molecular phylogenetic studies have revealed an entirely holoparasitic Boschniakia–Epifagus clade within Orobanchaceae. Pollen morphology of seven species, four studied for the first time, belonging to five currently recognised genera was studied using both light and scanning electron microscopy. Pollen grains are (2 –) 3 (– 4)-colpate, rarely 3-colporate and inaperturate (reported in Epifagus for the first time); suboblate to prolate (P/E = 0.77 – 1.56) in shape; medium and rarely small-sized: Polar axis = 14.63 – 34.58 µm, Equatorial diameter = 14.63 – 30.59 µm. Colpi are long, medium-length or short, wide or narrow, and acute, obtuse, or with indistinct ends. Exine sculpture is rather diverse: reticulate, microreticulate, rugulate-granulate, granulate-perforate, rugulate-perforate, and granulate. The data obtained demonstrate that pollen grains of the studied taxa are morphologically rather diverse and can, therefore, be distinguished at the levels of genera and species. The diagnostic features include shape and size of pollen grains, length and width of colpi, exine thickness in mesocolpia and apocolpia, and exine sculpture. Palynomorphological data are interpreted within the current phylogenetic framework. An UPGMA dendrogram, based on quantitative characters of the pollen grains, supports the status of Xylanche as a genus.