Pollen of Southeast Asian Alchornea (Euphorbiaceae), with an overview of the pollen fossil record

In order to evaluate pollen morphological descriptions of Alchornea in the literature, which are almost completely based on African and American species, the pollen of eight Southeast Asian species of Alchornea was investigated, using light and scanning electron microscopy. Very little variation appeared to be present in the Asian material. Slightly deviating from the scabrate ornamentation type are A. kelungensis (psilate) and A. rugosa (striate-rugulate). The scabrate type is also found in A. castaneaefolia (Brazil), A. hirtella (Liberia) and A. obovata (Colombia). The operculate Alchornea pollen type, which can be easily recognised using light microscopy, seems to represent a diagnostic character for the tribe Alchornieae (pollen of Bossera unknown). Its characteristic appearance resulted in a relatively extensive fossil record. The earliest records are from the Middle Eocene of Venezuela and Nigeria, while records for Australia and Borneo date from the mid-Tertiary and the Neogene (Miocene–Pliocene), respectively. These records suggest that the tribe Alchornieae has an African– American Gondwanic origin, and reached its pantropic distribution at least in the mid-Tertiary.