The exine ultrastructure of pollen grains in Gnetum (Gnetaceae) from China and its bearing on the relationship with the ANITA Group

Abstract

Pollen grains of six species of Gnetum, G. parvifolium, G. hainanense, G. luofuense, G. pendulum, G. cleistostachyum and G. montanum, collected from China were examined using light, scanning and transmission electron microscopy. Pollen grains of Gnetum are subspheroidal or irregular-apolar, inaperturate, 11.21–22.44 µm in long axis and 9.34–20.47 µm in short axis. The exine surface is covered with spinules, 0.50(0.30–0.71) µm long spaced on average 1.12(0.81–1.46) µm apart. The exine is about 0.55 µm thick and comprises ectexine and endexine. The ectexine includes a thin tectum and an infratectal granular layer. The tectum protrudes outwards, forming the spinules. The endexine is composed of discontinuous lamellae, with lacunae between lamellae. The pollen grains of Gnetum are compared with those of Ephedra and Welwitschia, and also those of the ANITA Group of angiosperms, including Amborellaceae, Nymphaeales, Illiciales, Trimeniaceae and Austrobaileyaceae. The exine ultrastructures of Gnetum, Ephedra and Welwitschia are quite similar, consisting of tectum, granular layer and lamellated endexine. The exine ultrastructure of Gnetum is also similar to that of Nymphaea colorata (Nymphaeaceae) in the transitional region between the proximal and distal poles, but differs from that of Amborellaceae, Illicium religiosum (Illiciaceae), Schisandra (Schisandraceae), Trimeniaceae and Austrobaileyaceae. This comparison of exine ultrastructure provides new evidence for consideration of the relationship between Gnetum and the ANITA Group.