Understanding the Systematic Implications of Pollinium Morphology in the Aeridinae Subtribe (Orchidaceae)

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Abstract— Molecular phylogenetic studies have been the basis for major clades proposed for the Aeridinae subtribe of the Orchidaceae family. However, systematic morphological characterization has yet to be conducted. We carried out this study on Aeridinae pollinium morphologies with an emphasis on aperture type and pollinium sculpturing to determine the systematic significance of these characteristics in the context of molecular phylogenetics. The results indicated that most Aeridinae species have cleft type pollinium apertures and that few have porate type or no aperture. The shapes of most of the pollinia were flat but this was not consistent at the genus level. Pollinium length ranged from 204 to 2606 µm, and the width from 178 to 1524 µm. Long and wide pollinia were commonly found in primitive groups, while short and narrow pollinia were commonly found in evolutionarily derived groups. The shapes and sizes of the individual units of the pollinia varied from uniform to variable, with a few exceptions. The individual units of most the pollinia were tetrads, and individual pollen grains were easily distinguished within the tetrad. The degree to which individual pollen grains could be differentiated is constant at the clade level except for Aerides rosea. The exine sculpturing characteristics were consistent in each clade except in the Cleisostoma and Phalaenopsis clades. The pollinium sculpturing has a somewhat clear pattern of change that was consistent with the molecular phylogeny, with a few exceptions. In conclusion, these pollinium characteristics themselves are not systematic, but they can serve as useful characteristics for determining the major systematic groups in the subtribe Aeridinae.