The unique type of pollen grain of Dasistoma (Macranthera-Agalinis clade; Orobanchaceae): implications for taxonomy

Abstract Pollen morphology of six species from the Macranthera-Agalinis clade (Orobanchaceae) was studied using both light microscopy and scanning electron microscopy. Pollen grains are (2-)3(4-)-colpate, 3-brevicolpate and 4-, rarely 6-porate, obtate to prolate (P/E = 0.72–1.52) in shape; medium-sized and rarely large-sized (P = 26.60–50.54 µm, E = 22.61–47.88 µm). Three pollen types are found and described based on the pollen grain size, length of the colpi, exine thickness, exine sculpture, columellae length and thickness and nanogemmae size. Type III contains three subtypes differentiated by pollen grain size, length of the colpi, exine thickness, exine sculpture, columellae length and nanogemmae size. Unweighted pair group method with arithmetic mean (UPGMA) dendrograms based on palynological data support that Dasistoma macrophylla has unique morphological and morphometric characters. In D. macrophylla we observed a transition from the 3-brevicolpate type to the 4-porate and occasionally 6-porate type. This polymorphism with respect to apertures may provide an advantage for D. macrophylla to survive under different ecological conditions and on a wide range of hosts. Palynomorphological data are interpreted in the existing phylogenetic framework. The 3-colpate type of apertures and a nanogemmate exine sculpture are hypothesised to be the plesiomorphic condition within the Macranthera-Agalinis clade.