PHYLOGENETIC IMPLICATIONS OF POLLEN ULTRASTRUCTURE IN THE OLDFIELDIOIDEAE (EUPHORBIACEAE)

The pollen structure of members of Euphorbiaceae subfamilies Oldfieldioideae and Pbyllanthoideae was studied using scanning and transmission electron microscopy in order to assess taxonomic relationships. We identified 10 palynological characters that appear to have systematic significance. We also identified 37 characters of vegetative morphology and anatomy, mostly based on data obtained by Hayden, and five characters of reproductive morphology, based on data in the literature. Cladistic analysis of the Oldfieldioideae along with selected genera in the Phyllanthoideae, using various putative relatives of the Euphorbiaceae as outgroups, provided great insight into the phylogeny of the Oldfieldioideae. Synapomorphies of this subfamily are all palynological: brevicolporate to pororate or porate apertures, echinate sculpturing, and exine with the interstitium consisting of columellae discontinuous from the foot-layer (where present). With the exception of Croizatia, the basal member of the subfamily, all Oldfieldioideae also share the synapomorphies of four or more pollen apertures and no petals; two homoplastic synapomorphies, an exinous foot-layer that is irregular to absent and a caruncle on the seeds, also characterize these genera. A clade consisting of Paradrypetes and Podocalyx is supported by the synapomorphies of an extremely reduced pollen exine interstitium and sclerified crystalliferous axial xylem parenchyma, whereas the remaining Oldfieldioideae share the synapomorphies of a ''microperforate/baculate'' tectum consisting of numerous closely appressed rod-shaped elements, some of which are continuous with the columellae and/or echinae; uniformly simple perforation plates; and alternate intervascular pitting. These Oldfieldioideae represent two clades, taxonomically recognized as the tribes Picrodendreae and Caletieae. The Picrodendreae are diagnosed by the synapomorpby of alternate, circular vessel-ray pitting, and except for Tetracoccus, the basal member of the clade, also share four additional synapomorphies, all foliar: compound or unifoliolate leaves, well-developed bundle sheath extensions, fimbriate marginal ultimate venation, and well-developed areoles. Within the Picrodendreae, synapomorphies distinguish a South American clade and an African/Madagascan/Indian clade, but do not resolve the position of Oldfieldia within the Picrodendreae. The Caletieae share the synapomorphy of stomatal subsidiary cells that have a crenulate (rather than straight) wall nearest the stomatal pore. Within the tribe, the African genus Hyaenanche is the basal member, with the remaining genera, all of which are Austyalasian, being united by the synapomorphv of chambered foliar epidermal cells. The Australasian genera comprise two major clades, but the relationships among these clades and Petalostigma are ambiguous. In addition to clarifying relationships within the Oldfieldioideae, these results (1) support the transfer of Croizatia and Paradrypetes from the Phyllanthoideae to the Oldfieldioideae, (2) support inclusion of Scagea, despite its single ovule/locule, in the Oldfieldioideae, (3) demonstrate that Androstachys and Stachyandra are bona fide members of the Oldfieldioideae rather than belonging in their own family, and (4) indicate that Neoroepera, as currently circumscribed, is diphyletic. We propose a revised classification of the subfamily in which all suprageneric taxa are monophyletic according to this phylogeny.