Formation pattern in five types of pollen tetrad in Pseuduvaria trimera (Annonaceae)
In basal angiosperms, there are several types of permanent tetrad but their formation pattern remains elusive. Pseuduvaria trimera has five types of tetrads and is the species with the most abundant tetrad types in Annonaceae. In order to interpret the formation pattern of different tetrad types, pollen development was investigated from the microspore mother cell stage to the bicellular pollen stage and the ultrastructure of pollen wall in the five tetrad types using light microscopy, transmission electron microscopy, and confocal laser scanning microscopy. Both successive and intermediate cytokinesis were observed within the same anther. The nucleus location of the microspores together with cytokinesis determine the number and the spatial arrangement of callose plates, and further have an effect on the tetrad types. The anthers with or without septation and the arrangement of microsporocytes might be also related to the tetrad type. The individual pollen grains within the tetrads are connected with each other by crosswall cohesion and cytoplasmic channels at localized points in the proximal walls. The various types of tetrads, cytokinesis, and cohesion in P. trimera reflect the high diversity in pollen development, which enhances the dramatic variety in pollen morphology in this family. Our observations of the development of tetrads provided some new insights for interpreting the factors influencing the types of tetrads, and reported the existence of a cytoplasmic channel in Annonaceae for the first time.
KeywordsCytokinesis Pollen development Microsporogenesis Tetrad·cohesion Pollen tetrad Pseuduvaria trimera
We are grateful to Dr. Louis Ronse De Craene for helping with the manuscript revision and two anonymous reviewers for their helpful suggestions. We also thank Senior Engineer Xinlan Xu and Junior Rufang Deng (South China Botanical Garden, Chinese Academy of Science) for their technical assistance with the TEM and Dr. Guifamg Yang for providing Fig. 2g–j. This work was financially supported by the National Natural Science Foundation of China (grant number 31270227), the Science and Technology Project of Guangdong Province (grant number 2017A030303062), and the Science and Technology Project of Guangzhou City (grant number 201804010100).
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Conflict of interest
The authors declare that they have no conflict of interest.
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