Pollen Ultrastructure of the Biovulate Euphorbiaceae

Abstract

Pollen ultrastructure of the biovulate Euphorbiaceae, including the subfamilies Phyllanthoideae and Oldfieldioideae, was investigated with light, scanning electron, and transmission electron microscopy. Pollen of Phyllanthoideae, represented by 12 species in nine genera, was prolate to oblate, almost always 3-colporate, rarely 3-porate or pantoporate, and mostly with reticulate, rarely baculate, echinate, or scabrate, sculpturing. The ektexine showed little variation in ultrastructure; almost without exception it was tectate-columellate with a homogeneous foot layer, columellae continuous with both the tectum and the foot layer, and a perforate, homogeneous tectum. The only major exception was Amanoa guianensis, which was intectate and baculate as the result of the evolutionary loss of the tectum. Pollen of Oldfieldioideae, in which we studied 28 species from 25 genera, and obtained data from the literature for two of the three remaining genera in the subfamily, was globose to oblate, with four or more, rarely only three, apertures that were either equatorial and brevicolporate to porate or pantoporate, and with echinate, rarely scabrate, sculpturing. The ektexine showed considerable variation in ultrastructure. The most widespread structural type consisted of a foot layer that is either thin and irregular or absent, an interstitium composed of irregular to columellate exine deposits that are discontinuous from the foot layer, if present, and a tectum composed of laterally appressed baculate elements that delimit fine microperforations. A second common structural type differed in having a relatively thick, homogeneous foot layer and thin, granular to tabular, microperforate tectum. Isolated genera have a reduced interstitium and/or a homogeneous tectum or show a reversal to a tectate-columellate architecture. Variation in aperture morphology, sculpturing, and exine ultrastructure is phylogenetically significant in the Oldfieldioideae.