Poles Apart: Monosporic, Bisporic, and Tetrasporic Embryo Sacs Revisited

Abstract

Bisporic embryo sacs contain nuclei derived from two members of a megaspore tetrad whereas tetrasporic embryo sacs contain nuclei derived from all four members of a tetrad. Megaspores from the same tetrad are less genetically similar than are megaspores from different tetrads. Therefore, cells derived from different megaspores in bisporic and tetrasporic embryo sacs are expected to compete with each other rather than cooperate. The tacit assumption of the plant embryological literature that bisporic and tetrasporic embryo sacs function as integrated organisms is demonstrably false, both theoretically and empirically. This competition within embryo sacs can be expressed as the formation of eggs by descendants of more than one megaspore or by the suppression of the descendants of all but one megaspore. Both phenomena have evolved multiple times. In contrast, all nuclei of monosporic female gametophytes are genetically identical. Consequently, monosporic development is predicted to be more evolutionarily stable than bisporic or tetrasporic development. The triploid endosperm produced by most monosporic gametophytes was probably derived from an ancestrally diploid endosperm and has been considered a key adaptation of the most successful lineages of flowering plants. The greater weight given to the doubled maternal genome in triploid endosperm may have facilitated a more efficient distribution of resources among a mother's seeds because it reduced the influence of competition for maternal resources among unrelated paternal genomes of endosperm.

Ovule development, megasporogenesis, and megagametogenesis of Fouquieria fasciculata (Willd. ex Roem. & Schult.) Nash (Fouquieriaceae)

“…Regarding the development of the female gametophyte in Fouquieriaceae, Johansen (1936) interpreted it as tetrasporic, which means that the meiosis is not followed by cytokinesis, and that a megaspore tetrad, of four nuclei, is formed in a shared cytoplasm (Haig 2020). However, observations by Mauritzon (1936) and Khan (1943) supported a monosproric development, which implies that the meiotic divisions are followed by cytokinesis and a linear megaspore tetrad is formed (Haig 2020).…”

“…Regarding the development of the female gametophyte in Fouquieriaceae, Johansen (1936) interpreted it as tetrasporic, which means that the meiosis is not followed by cytokinesis, and that a megaspore tetrad, of four nuclei, is formed in a shared cytoplasm (Haig 2020). However, observations by Mauritzon (1936) and Khan (1943) supported a monosproric development, which implies that the meiotic divisions are followed by cytokinesis and a linear megaspore tetrad is formed (Haig 2020). In F. fasciculata, a dyad and a linear tetrad of megaspores were observed during the meiosis; thus our study confirms the monosporic development of the female gametophyte and supports the early observations by Mauritzon (1936) and Khan (1943).…”

Evolutionary Patterns of the Internal Structures of Generative Organs in Angiosperm Plants

“…Tetrasporic embryo sacs of the Drusa, Peperomia, and Penaea types arise through a overlapping the medial and terminal deviations and terminal prolongation [12,32,72]. The structural diversity of bi-and tetrasporic types may be the result of instability and competition between macrospores participating in their ontogenesis [73]. This competition within embryo sacs can be expressed as eggcell production by descendants of more than one megaspore, or suppression of the descendants of all but one megaspore.…”

“…This competition within embryo sacs can be expressed as eggcell production by descendants of more than one megaspore, or suppression of the descendants of all but one megaspore. Both phenomena evolved several times, forming obviously an amazing variety of the structure of the embryo sacs [73].…”

MIR822 modulates monosporic female gametogenesis through an ARGONAUTE9-dependent pathway in Arabidopsis thaliana

“…While one of the binucleated cells dies without further differentiation, the second one directly incorporates both of its nuclei into a developing two-nuclear female gametophyte. Finally, in genera such as Drusa, cytokinesis is absent after meiosis II, and all four meiotically derived nuclei are incorporated into a four nuclear female gametophyte (Webb and Gunning, 1990, Huang and Russell, 1992, Haig, 2020.…”