Pollen morphology and ultrastructure of representatives of the Thyrsacanthus clade (Acanthaceae)

The Thyrsacanthus clade belongs to the Tetramerium lineage (Justicieae-Acanthaceae) and possesses 12 species distributed among dry forests of South America and Mexico. The sporoderm of the pollen grains of seven species of the clade were analyzed by scanning and transmission electron microscopy. The pollen grains are of the 3-colporate, 6-pseudocolpate type. The exine is heterobrochate reticulate, except in Thyrsacanthus microphyllus, which possesses a perforated exine with granules distributed in a reticulated manner. The membrane of the ectoaperture and pseudoaperture are ornamented with granules, and the membrane of the endoaperture is microechinate, with the exception of Schaueria humuliflora, which possesses microechinate ecto- and endoapertures. The ultrastructure of the ectexine is semitectate or tectate perforated in T. microphyllus, with simple and/or digitate columellae. The species of the clade possess pollenkitt on the tectum and in the infratectal cavity between the columellae. The endexine consists of one or two layers. The intine is irregular and well developed under the apertures. The information provided by this study reveals important data regarding the structure of the sporoderm of the pollen grains of representatives of the Thyrsacanthus clade, and thus of Acanthaceae.