Phylogeny, Evolution, and Ecology of Sexual Systems Across the Land Plants
Land plants dominate nearly every terrestrial habitat and include some of the largest and longest-lived organisms on earth. They are well known for their sexual diversity, reflecting tremendous variation in sex expression, and elaborate reproductive structures and behaviors. Much of what we understand about plant sexual diversity comes from studies of a single group—the flowering plants. Here, we discuss our current state of knowledge about sexual systems across the land plants and how principles and concepts derived from studies of angiosperms can (or cannot) be applied to mosses, hornworts, liverworts, ferns, fern allies, and gymnosperms. First, we show how variation in the expression and lability of sexual systems across the land-plant phylogeny raises fundamental questions about sexual-system evolution. Second, we discuss selective mechanisms, focusing specifically on polyploidy as a mechanism that may either constrain or facilitate evolutionary changes in sexual systems. Finally, we compare ecological traits that are commonly associated with alternate sexual systems in angiosperms and their (not so obvious) cognates in other land-plant groups.
The authors are grateful to Janet L. Leonard for inviting our contribution and to John R. Pannell, Sean Graham, and an anonymous reviewer for providing comments on earlier versions of this manuscript.
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