Pollen characters of Amborella trichopoda (Amborellaceae): a reinvestigation

Pollen of Amborella trichopoda was reinvestigated. Pollen is oblate to spheroidal and monoaperturate (ulcerate) at the distal pole, with a poorly defined aperture margin. The exine consists in the nonapertural region of ektexinous tectal elements (cupules), a smooth foot layer, and a thin, nonlamellate endexine. In the apertural area, the foot layer is lacking, as are most of the tectal elements. However, a thick endexine (including some ektexinous globular elements) and a conspicuous, channeled intine are present. The thick apertural endexine generally ruptures along the circular apertural area and forms an operculum. Orbicules show the same electron density as the tectal elements and recall their shape. Pollen characters of Amborella are compared with those of other basalmost angiosperms. The unique tectal elements (cupules) and the thick homogeneous endexine, which form an odd operculum in the apertural area, are unusual angiosperm pollen characters. Only Austrobaileya (Austrobaileyaceae) shares some of the Amborella apertural features, whereas the exceptionally convoluted tectal elements are found in no other (extant) angiosperm.