Le pollen des Crotonoideae Apétales (Euphorbiaceae): Ultrastructure de l'exine. Pollen exine ultrastructure of the apetalous Crotonoideae

Abstract

The crotonoid pollen type defines the subfamily Crotonoideae in the Euphorbiaceae. The apetalous Crotonoideae are composed of 5 tribes (Micrandreae, Manihoteae, Elateriospermeae, Adenoclineae, Gelonieae), and 1 subtribe, Neoboutonineae (tribus Aleuritideae). In the pollen of these Crotonoideae, the exine is basically tectate. The tectum is of the tilioid type. The more or less distinct reticulum is supratectal. The muroid ridge supports ornamented excrescences, clavae or gemmulae, regularly arranged around a circular area. Although morphologically analogous, the columellae are not in fact all homologous in the exine. They are of two types. The very short ones, sometimes reduced to spheroidal grains rest on the foot-layer. The others are elongated and in two rows, supported by the tectum in the meshes. In the first case, they concern a part of the real infratectum and in the second, the columellae are a tectal formation. In the Elateriospermeae and in the Neoboutonineae, where the tectum is reduced to some clavae or verrucae in the meshes, only the tectal columellae are present and constitute a new form of infratectum. The foot-layer is continued or reduced to masses and the endexine is partly or totally lamellate. According to the nexine structure, the pollen grains of apetalous Crotonoideae are classified in two groups: Group I, Micrandreae, Glycydendron (Adenoclineae), Manihoteae, Elateriospermeae; Group II, Adenoclineae (incl. Cladogelonium; excl. Glycydendron), Gelonieae, Neoboutonineae (Aleuritideae). In the two groups, the pollen types with a maximum of primitive characters are subequatorial; group I is South American and Southeast Asiatic and group II is pantropical and principally palaeotropical. The oldest fossil pollen dates from the Late Cretaceous of the tropics and their pollen type is derived. The diversity of the pollen types of the apetalous Crotonoideae may correspond to regression associated with a new organisation of the exinous structure that is well developed in the petaliflorous Crotonoideae and in the Euphorbioideae. The apetalous Crotonoideae show some relationships with many palaeotropical Phyllanthoideae such as Martretia, Protomegabarea, Savia. In comparison with the other Euphorbiaceae, the crotonoid pollen type cited in many Angiosperms would only be used to describe the pollen ornamentation independently of the structure, which varies considerably with the taxa. This ornamentation is found in many families without close relationships. In contrast, the study of the different exinous structures in the Crotonoideae demonstrates certain relationships with other Dicotyledones such as Malvales (Tiliaceae, Bombacaceae), Dilleniales (Dipterocarpaceae), but less certain affinities with Buxaceae and Thymeleaceae.