The Coexistence of Bicellular and Tricellular Pollen Might Be the Third Type of Pollen Cell Number: Evidence from Annonaceae

Anther is thought to release either bicellular or tricellular pollen when mature. Though a few species had been found to shed both bicellular and tricellular pollen, due to their low frequency, they had been overlooked as special cases of bicellular or tricellular pollen in previous phylogenetic studies. In the present work, the pollen cytologies of 89 species from 26 genera of Annonaceae were observed using the overall transparency method and paraffin sectioning method. The results show that 73 species from 25 genera distribute bicellular pollen, while 16 species from 10 genera shed both bicellular and tricellular pollen. This is the first time that so many species with both types of pollen has been observed in the same family. Combined with reports from other families, the plants that were known to shed both types of pollen included 15 families, 40 genera, and 52 species. Our results indicate that the coexistence of bicellular and tricellular pollen might be the third type of pollen cell number. And the systematic relationship among them is needed to be reanalyzed.