Comparative Structure and Development of Pollen and Tapetum in Pandanales

The newly recircumscribed monocot order Pandanales now includes five families: Cyclanthaceae, Pandanaceae, Stemonaceae, Triuridaceae, and Velloziaceae. We present new data and a review of pollen morphology, microsporogenesis, and tapetal characters in Pandanales in a systematic context. Probable plesiomorphic character states include pollen dispersed as monads, pollen small to medium in size with an ovoid shape, monosulcate apertures, perforate to reticulate sculpturing, a columellate exine with a foot layer, successive microsporogenesis, and a secretory tapetum. These correspond to generalized monocot or lilioid pollen and anther character states, and all occur within Dioscoreales, the sister group to Pandanales. Within Pandanales, there are apparently homoplastic trends in each family toward either reduction in aperture size (e.g., from sulci to pores) or covering the aperture with exine (e.g., from monosulcate to monosulcateoperculate), resulting in a reduction in the delicate apertural area exposed to desiccation and entry of pathogens. Apomorphic character states in Pandanales include pollen dispersed as tetrads (only in Vellozia), reniform pollen (in some Pandanaceae), ulcerate apertures (in some Cyclanthaceae and some Pandanaceae), inaperturate pollen (in some Stemonaceae, Triuridaceae, and Vellozia), monosulcate-operculate apertures (in some Velloziaceae), pollen surface with protuberances (in some Pandanaceae, some Stemonaceae, Triuridaceae, and some Velloziaceae), exine reduced to a foot layer (some Pandanaceae, Pentastemona, and Triuridaceae), columellate/granular or granular exine (only in some Stemona species), endexine lamellae absent (in Cyclanthaceae), and invasive tapetum (in Talbotia and possibly some Pandanaceae and Triuridaceae).