Reproduction in Ferns: Gametophyte Development, Sex Determination, Gametogenesis, and Embryogenesis.

Sexual reproduction in land plants has diversified over more than 500 million years of evolution. Ferns occupy a key phylogenetic position among land plants because they possess a dominant sporophyte, as in seed plants, while retaining a free-living gametophyte that produces gametes through mitotic divisions, as in bryophytes. This combination of life cycle features provides a useful framework for understanding how reproductive and developmental programs have evolved across land plants. In this review, we summarize current knowledge of fern reproduction, with particular emphasis on the model fern Ceratopteris richardii and the related species C. thalictroides. We focus on four interconnected processes: (1) gametophyte development and the meristem, which functions as a signaling center required for prothallus formation and gametangium positioning; (2) sex determination by antheridiogen, including classical genetic models, hormonal regulation, and recent studies linking meristem identity to sexual fate; (3) gametangium development and gametogenesis, covering antheridium and archegonium formation as well as the differentiation of motile sperm and egg cells; and (4) fertilization and embryogenesis, including gamete interaction, zygotic polarity, and embryonic axis formation. Recent advances in genome sequencing, transformation, genome editing, live imaging, and transcriptomic analyses are beginning to connect the rich morphological literature on ferns with molecular mechanisms. These advances make Ceratopteris a key comparative system for uncovering how reproductive structures, stem cell functions, and embryogenic programs have been conserved, modified, and redeployed during land plant evolution.