Pollen diversity in Neotropical representatives of Dialioideae (Fabaceae)

The subfamily Dialioideae comprises 17 genera and 83 species, having a pantropical distribution. Dialium L., the typical and most diverse genus of the subfamily, contains 34 accepted species, of which five are Neotropical. The other genera of Dialioideae are restricted to specific continents. Noteworthy, five genera are exclusively Neotropical, namely Androcalymma Dwyer, Apuleia Mart., Dicorynia Benth., Martiodendron Gleason, and Poeppigia C.Presl. This article presents a palynological study of 21 taxa belonging to six genera of the subfamily Dialioideae, including the above-mentioned five Neotropical genera and the Neotropical species of Dialium. The aim was to characterize pollen grains and thus provide support for the taxonomic circumscription of the group. Pollen grains were acetolysed, measured, described, and illustrated using light microscopy. Surface details and apertures were examined on unacetolysed pollen grains by scanning electron microscopy. The parameters evaluated were shape, size, aperture type, polarity, and exine ornamentation. The analyzed species have medium-sized pollen grains. Pollen shape ranges from isopolar to subprolate, prolate, oblate-spheroidal, prolate-spheroidal, and perprolate. The polar area is small or large, tricolporate, syncolporate, or parasyncolporate in some species, with short, wide, or narrow colpi, ornamented or slightly ornamented membrane, lalongate or lolongate endoaperture, and rugulate, microreticulate, or striate sexine. The sexine is as thick as, thicker than, or less thick than the nexine. Pollen attributes were successfully used to construct a key to distinguish genera and species. The findings show the importance of pollen morphology in the taxonomy of Dialioideae, allowing identification of taxa.