Pollen morphology of the Megamexican cycads reveals the potential of morphometrics to identify cycad genera

Abstract

Background: Although pollen morphology has always been an important tool in plant systematics, the possibility of distinguishing between related taxa is still debated. A difficult case is the cycads, where their morphology and small size is very similar. Pollen morphology has been important in plant systematics and determination at the species level is possible depending on the group. Controversial cases are the cycads, where their small pollen and smooth sculpture when observed under light microscopy present few useful characters for separation at the species level. Question: Will cycad pollen morphometry be useful to distinguish taxa? Study species: 15 species within the genera Ceratozamia, Dioon and Zamia were investigated with five species of each genus. Methods: Pollen was obtained from both live specimens from the Clavijero Botanic Garden of the Instituto de Ecología, A.C., as well as herbarium specimens. Acetolyzed pollen was used for descriptions and morphometry, and non-acetolyzed pollen for SEM images. Results: The thickness of the exine and pollen sculpture of Dioon differed from that of Ceratozamia and Zamia thus allowing its identification. The pollen morphology of Ceratozamia and Zamia is very similar and therefore difficult to distinguish between species. Conclusions: The thicker exine of Dioon allows its pollen to be distinguished from the other two genera. Morphometric methods must be applied to distinguish pollen between species of Ceratozamia and Zamia.