Pollen ultrastructure of the Pontederiaceae

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Abstract
Pollen of eight genera and species of the Pontedcriaceae was investigated using SEM and TEM in order to elucidate interrelationships to other monocot families. All examined Pontederiaceae are diaperturate with furrow-shaped apertures and have a distinctly verrucate non-apertural exine sculpturing (except Pontederia, which is psilate to scabrate in sculpturing). Among investigated taxa the exine wall architecture exhibits a gradation between: (1) a 1-layered exine composed of laterally appressed, basally fused baculate elements, (2) a 2-layered exine, the outer layer composed of laterally appressed baculate elements and the inner forming a thin layer or composed of discrete papillate elements, and (3) a tectate-columellate exine, differing, however, from a typical tectate-columellate architecture by having short, narrow columellae and generally baculate tectal elements. Of the palynologically investigated families presumed closely related to the Pontederiaceae, only members of the tribe Haemodoreae of the Haemodoraceae have a verrucate sculpturing similar to that of most Pontederiaceae. In addition, the exine architecture of the nontectate-columellate genera of the Pontederiaceae is strikingly similar (and presumably homologous) to members of the family Haemodoraceae. It is proposed that the similarities in pollen exine sculpturing and architecture between the Pontederiaceae and Haemodoraceae are so by homology (common evolutionary origin) and constitute evidence for the close relationship and possible sister-group status of these families.