Palynological study of Asian Thismia (Thismiaceae: Dioscoreales) reveals an unusual pollen type

The mycoheterotrophic genus Thismia shows a great number of structural and ecological traits which are rare or unique for angiosperms. Pollen morphology of this genus is still poorly known. Pollen of Thismia was reported to be porate with one to several pores. Position of the pores is unknown, and therefore, the pollen type has not been established to date. Information on sporoderm structure in the genus is scarce, as high-quality images of pollen grains are available for several species only. In our comprehensive investigation of pollen morphology of Thismia, we involved nine Asian species of the genus and employed an integrative approach, which included data from light microscopy, scanning electron microscopy and transmission electron microscopy. All studied species were shown to possess monoporate pollen of asymmetric-elliptic planoconvex shape. Using TEM investigation of immature anthers, we demonstrated that pollen grains are organized in tetragonal tetrads, and the pore occupies an equatorial position. We argue that Asian species of Thismia along with Burmannia from the same order Dioscoreales are the only known lineages of seed plants with a single equatorial aperture. The sporoderm surface is psilate or perforate with perforations of various size and density in the studied species. We discuss the interspecific variation of sporoderm morphology in the light of the phylogenetic relationships between Asian species of Thismia. We demonstrate that the pollen morphology of Thismia fails to conform to the idea of entomophily, which is believed to prevail in this genus. Finally, we describe an unexpected striking difference in sporoderm structure between the specimens belonging to T. javanica species group.