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Abstract
The two-sex model makes the assumption that there are only two sexual reproductive states: male and female. However, in land plants (embryophytes) the application of this model to the alternation of generations life cycle requires the subtle redefinition of several common terms related to sexual reproduction, which seems to obscure aspects of one or the other plant generation: For instance, the homosporous sporophytic plant is treated as being asexual, and the gametophytes of angiosperms treated like mere gametes. In contrast, the proposal is made that the sporophytes of homosporous plants are indeed sexual reproductive organisms, as are the gametophytes of heterosporous plants. This view requires the expansion of the number of sexual reproductive states we accept for these plant species; therefore, a three-sex model for homosporous plants and a four-sex model for heterosporous plants are described and then contrasted with the current two-sex model. These new models allow the use of sexual reproductive terms in a manner largely similar to that seen in animals, and may better accommodate the plant alternation of generations life cycle than does the current plant two-sex model. These new models may also help stimulate new lines of research, and examples of how they might alter our view of events in the flower, and may lead to new questions about sexual determination and differentiation, are presented. Thus it is suggested that land plant species have more than merely two sexual reproductive states and that recognition of this may promote our study and understanding of them.
Highlights
In describing the history of the study of plant sexual reproduction (Bennett 1875; von Sachs 1906; Browne 1989; Žárský and Tupý 1995), it is noted how the early model of land plants as being non-sexual was replaced by a monosexual model in which all these plants were considered to be female but reproducing without the use of fertilization, and how this view was replaced by what Taiz and Taiz (2017) refer to as the “two-sex model” in which plants were recognized as having distinct male and female sexual reproductive states
The second item of interest is that, if again one accepts the endosperm to be a multicellular “body,” this implies that while most embryophytic species are diplobiontic in terms of the types of sexually reproductive adult organisms found in their two life cycle stages, those species that carry out double fertilization and produce endosperm may be argued to be triplobiontic in terms of the total number of types of categories of plant bodies present in their life cycle
It might be that the new four-sex model, by accepting that both gametophytes and sporophytes are distinct sexual organisms, creates a better conceptual context for studies which hope to explore differences in gene expression patterns leading to sex cell formation in each of the four types of sexual reproductive states
Summary
In describing the history of the study of plant sexual reproduction (Bennett 1875; von Sachs 1906; Browne 1989; Žárský and Tupý 1995), it is noted how the early model of land plants (embryophytes) as being non-sexual was replaced by a monosexual model in which all these plants were considered to be female but reproducing without the use of fertilization, and how this view was replaced by what Taiz and Taiz (2017) refer to as the “two-sex model” in which plants were recognized as having distinct male and female sexual reproductive states. With the discovery of the alternation of generations life cycle in land plants we should expand our thinking, to consider the traits of all the adult individuals at each of these stages of the life cycle, if we wish to truly characterize the sexual reproductive features of each plant species as a whole. Land plant species are suggested to have more than just two sexual reproductive states, and full acceptance of this might help to further advance our studies of plants
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