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Annonaceae, comprising approximately 107 genera and 2400 species, is the largest family among early-divergent Magnoliales. Previous studies have concentrated on the binding mechanism that holds together the four members of tetrads in Annonaceae. However, the development mechanisms of different tetrad types remain largely unknown. Mitrephora tomentosa was found to exhibit five permanent tetrad types, with two or three of them existing in the same microsporangium, which is ideal for studying the formation mechanisms of different permanent tetrad pollens in a single microsporangium and explaining the relationship between cytokinesis and pollen tetrad types. The ontogenetic development of the different tetrads in M. tomentosa was investigated using electron microscopy technologies, histochemical staining, and immunocytochemistry. During meiosis, pollen mother cells produce decussate and tetragonal tetrads by successive cytokinesis and produce tetrahedral and rhomboidal tetrads by simultaneous cytokinesis. Bidirectional callose deposition was observed in tetrahedral, tetragonal, rhomboidal, and decussate tetrads. The variations in the process of microsporogenesis randomly accumulate and manifest as different combinations of cytokinesis and callose deposition, leading to the formation of differently shaped tetrads. In mature permanent tetrad pollens, four microspores are connected by both simple cohesion and cytoplasmic channels, which also play an important role in maintaining the synchronization of the tetrad members. [ABSTRACT FROM AUTHOR]

Quadripollis krempiiwas described by Drugg 1967 from Maastrichtian deposits in California and has been reported from the Albian to Paleocene of the western United States and Canada. A revised description is supported by illustrations of specimens found in outcrops of the Straight Cliffs Formation in southern Utah.Q. krempiiis unique among fossil tetrads and the tetrahedral tetrad is apparently inaperturate. We observe that the thinning and occasional rupturing of the exine observed by Drugg is a vestigial aperture, herein described as a tenuitas.Q. krempiihas been thought to be either gymnospermous or angiospermous. No modern gymnosperms produce pollen in obligate tetrads but at least 23 angiosperm families do. Our review revealed possible families with some similarities toQ. krempii, mainly members of the Juncaceae which exhibit an ulcerated or thinned exine in the distal areas of the monads. A relationship to bryophytes cannot be ruled out. [ABSTRACT FROM PUBLISHER]