Tapetum structure and ontogeny in Victoria (Nymphaeaceae)

The tapetum is critical for successful pollen development. Innovations in tapetum ontogeny and in the composition and role of tapetal exudate are hypothesised to have been advantageous in early angiosperm evolution. Early-diverging angiosperm lineages exhibit considerable variation in tapetum development, indicating that the earliest angiosperms exhibited evolutionary lability in tapetum ontogeny. However, little or no data on tapetum development exist for many basal flowering plant taxa, including members of Nymphaeales. Here, tapetum ontogeny in the giant water lily Victoria is described along with the development of associated microspore characters. The tapetum in Victoria is secretory. Orbicules are present along both the inner tangential and radial plasmalemmae of the tapetal cells. The timing of orbicule production is synchronous with the timing of microchannel appearance within the exine, supporting hypotheses that exine microchannels function in transport and storage of tapetal exudate. Prior to tapetum degradation late in microspore development, tapetal cells extend into the locule and contact the developing pollen grains, which are held together in permanent tetrads. In a phylogenetic context, the presence of a secretory tapetum in Victoria, rather than an invasive type, indicates that the invasive tapetum likely arose at least twice in Nymphaeaceae. Thus, tapetum ontogeny may be even more labile in Nymphaeales than previously thought.