POLLEN MORPHOLOGY AND PHYLOGENY OF THE TRIBE PLUKENETIEAE (EUPHORBIACEAE)

A scanning electron microscopy and light microscopy survey of pollen morphology in the Plukenetieae (Euphorbiaceae) was undertaken to help elucidate phylogenctic relationships within the tribe. Pollen is medium to large, spheroidal to suboblate, and tricolpate, inaperturate, or with poorly defined apertures. Subtribe Plukenetiinae is characterized by tricolpate pollen with uneven-margined colpi and a perforate to reticulate tectum. Pollen evidence supports a division betweeri genera having ari aborescent habit (Angostyles, Astrococcus, and Haematostemon) and those with a scandent habit (Plukenetia and Romanoa). The synonymy of the monotypic genera Vigia and Eleutherostigma with Plukenelia is also supported. Subtribe Tragiinae is exceptionally diverse in pollen morphology. Aperture condition ranges from tricolpate, the plesiomorphic and most common state, to weakly aperturate and inaperturate; islands, fragments, or strands of sexine are usually present on the apcrtural membrane, and aperture margins are uneven and often indistinct. Exirie sculpture is punctate, foveolate, reticulate, rugulate, or baculate. The large genus Tragia includes seven distinct pollen types, with most sections (e.g., Bia, Ctenomeria, Leptobotrys, Tragia, and Zuckertia, and also subgenus Mauroya) characterized by a uniform and unique pollen morphology, supporting the sectional classification of Tragia. The other Tragiinae genera have pollen distinct from Tragia, with the exception of Tragiella, which closely reserribtes sections Tagira and Lassia. Pollen evidence supports Cnesmone and Megistostigma as sister taxa, and suggests a close relationship with Pachystylidium. 4cidoton includes two different pollen types; the inaperturate type closely resembles pollen of Platygyna, suggesting that Acidoton may not be monophyletic and the tricolpate species perhaps represents a district genus. Pollen, together with floral morphological evidence, supports the hvpothesis of section Zuckertia as a plesiomorphic rriember of Tragia, and suggests that Tragia is paraphyletic and that the smaller Tragiinae genera are derived from Tragia.