Pollen morphology and phylogeny of the Alyxieae (Apocynaceae)

The pollen of the tribe Alyxieae, including Alyxia , Chilocarpus , Condylocarpon , Lepinia , Lepiniopsis , Plectaneia and Pteralyxia , has been studied with light, scanning electron and transmission electron microscopy. It is characterised by relatively large, irregularly disposed and - unequal-sized pores. 2-porate grains are most common in the Alyxieae, and occur in all genera except for Condylocarpon . 1-, 3-, 4- and 5-porate pollen grains are less common. The orientation of the polar axis and the equatorial plane could not be determined in any of the genera. Within the Alyxieae, Chilocarpus and Plectaneia are the nearest to other Apocynaceae, showing an essentially similar exine ultrastructure. Alyxia , Lepinia , Lepiniopsis and Pteralyxia make up a derived group, characterised by large pollen grain size and a remarkable nexine structure. Pollen of Lepinia and Lepiniopsis might be derived on the basis of their fossulate-verrucate ornamentation. Condylocarpon pollen shows a deviating set of pollen features (tetrads, inaperturate, reduced exine) that suggest it to be neotenic. Pollen morphology supports preliminary results of cladistic analyses of chloroplast data, in which the tribe is a monophyletic group nested in a 3-colporate environment, Chilocarpus and Condylocarpon are sisters at the base of the tree, Plectaneia is sister to a clade made up by Alyxia , Pteralyxia , Lepinia and Lepiniopsis , of which Lepinia and Lepiniopsis are the most derived.