Comparative pollen morphology in the early-divergent angiosperm family Hydatellaceae reveals variation at the infraspecific level

The small aquatic family Hydatellaceae was recently assigned to the early-divergent angiosperm order Nymphaeales. Pollen morphology is described using both SEM and LM for all 12 species of Hydatellaceae, and using TEM for one species (T. submersa). These observations are compared with pollen data from the other two families of Nymphaeales, Nymphaeaceae and Cabombaceae, including original observations for both genera of Cabombaceae. No significant interspecific variation in pollen morphology occurs in Hydatellaceae, though both of the perennial species (Trithuria inconspicua and T. filamentosa), which could be apomictic, possess at least partially sterile and collapsed pollen. Pollen of Trithuria is small, oblong or rounded (viewed from the distal pole) and monosulcate. The aperture has a distinct margin and extends the full length of the pollen grain. The exine is tectate-columellate. The tectum is perforate: discontinuous with numerous small perforations randomly scattered over the entire non-apertural surface. The exine surface is microechinate. In most species of Trithuria, a small percentage of pollen grains possess a trichotomosulcate aperture. In two species, rare atypical grains with striate or partially striate exine sculpturing were found. This striate pattern is very similar to exine sculpturing in Gymnotheca (Saururaceae, Piperales). The presence of anasulcate pollen does not contradict placement of Hydatellaceae within Nymphaeales, but this pollen type is also common among other early-divergent angiosperms and monocots. Exine sculpturing differs between Cabombaceae, Hydatellaceae and Nymphaeaceae, but this character also differs between the two genera of Cabombaceae and among genera of Nymphaeaceae. Compared with other families of Nymphaeales, Hydatellaceae are relatively uniform in pollen morphology.