Reassessing palynological characters in the subfamilies Periplocoideae and Asclepiadoideae (Apocynaceae): taxonomic and evolutionary implications

Abstract

The palynological characters of selected species of Periplocoideae and Asclepiadoideae were assessed and a taxonomic key made. We observed that translators of Periplocoideae are taxonomically more important structures than pollen tetrads. Within Periplocoideae, the flatter translators of Periploceae are evolutionarily more primitive than the spatulate translators of Cryptolepideae. The quantitative characters of pollinia are important for the genus- and species-level taxonomy in Asclepiadoideae. Pollinium length/width ratio was studied for the first time and it was observed to be a useful character for the delimitation of the tribes and sub-tribes of Asclepiadoideae. The position of attachment of caudicles to pollinia is an overlapping character amongst the tribes and sub-tribes and can be utilised only for the taxonomy of genera. True sterile margins of pollinia are present only in Ceropegieae and pseudo-sterile margins are rarely present in other tribes. Asclepiadoideae within Apocynaceae, and Ascelpiadeae within Asclepiadoideae are separate entities, and Secamonoideae is closer to Periplocoideae than Asclepiadoideae on evolutionary grounds.