Development of inflorescences, cupules, and flowers in Amphipterygium and comparison with Pistacia (Anacardiaceae)

Amphipterygium was originally placed in its own family, Julianiaceae, mainly because of its unique infructescences, which form samaroid dispersal units containing a single fertile, one-seeded fruit and three or more sterile fruits enclosed in a cupulelike structure. Its position in Anacardiaceae-Anacardioideae and a close relationship with Pistacia were suggested by structural and chemical features, and the position of both genera in Anacardioideae was recently supported by molecular phylogenetic studies. However, the development and structure of these infructescences and flowers have never been analyzed and comparatively studied. This study shows that each samaroid structure in Amphipterygium is a few-flowered cyme and that the teeth at the entrance of the cupule are the subtending bracts of the flowers. A comparison of Amphipterygium with Pistacia also shows that both genera share with Rhus and other genera of Anacardioideae a tricarpellate, pseudomonomerous gynoecium with a unilocular ovary and a single crassinucellar and (hemi)anatropous ovule with a ponticulus. However, the ovules in both Amphipterygium and Pistacia are outstanding in being unitegmic (though sometimes with traces of a second integument on the convex side) and having a massive funicle with unique lateral and median outgrowths, which becomes much larger than the ovule after anthesis. The funicle is also proportionally much larger and more complicated in shape than that of all other Anacardiaceae studied. In addition, both genera are wind pollinated and thus exhibit similar evolutionary trends, such as dioecy, reduction of perianth (lack of petals and, at least in part, also of sepals), large (bilobed) stigmas with multicellular papillae, and similar pollen. It is not yet clear whether wind pollination evolved separately in each genus or only once in their common ancestor. However, the inclusion of Amphipterygium within Anacardioideae is strongly supported by floral reproductive structures.