Comparative study of microgametogenesis in members of Cyperaceae and Juncaceae: a shift from permanent pollen tetrads to pseudomonads

During microsporogenesis in Cyperaceae, an asymmetric tetrad of microspores is formed, establishing a pseudomonad. Three microspores undergo programmed cell death, whereas the functional microspore goes though microgametogenesis. A full understanding of this process remains elusive, but knowledge of its stages, cell death meaning and coexistence of multiple cell lineages occurring in such restricted spaces is of great cytological interest. Therefore, a comparative study was made in Cyperaceae and Juncaceae to characterize pollen features and development using light and electron microscopy and cytochemical tests. Evidence from sporopollenin suggests that pseudomonads are derived from pollen tetrads like those in Juncaceae, and data collected allowed the establishment of five pseudomonad development stages. In summary, the late meiosis cytokinesis and precocious pollen mitosis I seem to be associated with pseudomonad formation. Vacuolation occurs later, with continuous autophagy of degenerative microspores, suggesting cell death to the benefit of pollen fitness. Reserves are accumulated as starch, depending on environmental conditions. During microgametogenesis, different cellular lineages appear to be isolated by callose, giving each cell metabolic autonomy. This process of pseudomonad establishment allows the formation of a longevous, rapidly germinating, tricellular pollen, which could be key to the great adaptative success of Cyperaceae.