Phylogenetic comparative analysis of microsporogenesis in angiosperms with a focus on monocots.

This paper presents the first broad overview of three main features of microsporogenesis (male meiosis) in angiosperms: cytokinesis (cell division), intersporal wall formation, and tetrad form. A phylogenetic comparative approach was used to test for correlated evolution among these characters and to make hypotheses about evolutionary trends in microsporogenesis. The link between features of microsporogenesis and pollen aperture type was examined. We show that the pathway associated with successive cytokinesis (cytoplasm is partitioned after each meiotic division) is restricted to wall formation mediated by centrifugally developing cell plates, and tetragonal (or decussate, T-shaped, linear) tetrads. Conversely, much more flexibility is observed when cytokinesis is simultaneous (two meiotic divisions completed before cytoplasmic partitioning). We suggest that the ancestral type of microsporogenesis for angiosperms, and perhaps for all seed plants, associated simultaneous cytokinesis with centripetal wall formation, resulting in a large diversity in tetrad forms, ranging from regular tetrahedral to tetragonal tetrads, including rhomboidal tetrads. From this ancestral pathway, switches toward successive cytokinesis occurred among basal angiosperms and monocots, generally associated with a switch toward centrifugal intersporal wall formation, whereas eudicots evolved toward an almost exclusive production of regular tetrahedral tetrads. No straightforward link is found between the type of microsporogenesis and pollen aperture type.

Post-meiotic cytokinesis and pollen aperture pattern ontogeny: comparison of development in four species differing in aperture pattern.

Pollen aperture patterns vary widely in angiosperms. An increasing number of studies indicate that aperture pattern ontogeny is correlated with the way in which cytokinesis that follows male meiosis is completed. The formation of the intersporal callose walls that isolate the microspores after meiosis was studied in four species with different aperture patterns (two monocots, Phormium tenax and Asphodelus albus, and two eudicots, Helleborus foetidus and Protea lepidocarpodendron). The way in which post-meiotic cytokinesis is performed differs between all four species, and variation in callose deposition appears to be linked to aperture pattern definition.