The Separation of Generations: Biology and Biogeography of Long-Lived Sporophyteless Fern Gametophytes

Premise of research.Ferns (monilophytes) and lycophytes are unique among land plants in having two independent life stages: the gametophyte generation, which is generally small, cordiform, and short-lived, senescing after fertilization, and the sporophyte generation, which is considered the dominant, long-lived portion of the life cycle produced following fertilization. In many species of epiphytic ferns, however, the gametophyte generation is capable of sustained vegetative growth, and some are able to reproduce asexually via gemmae. These two characteristics have increased the independence of these gametophytes, so much so that some species never produce sporophytes at all, while other species produce sporophytes only in parts of their geographic range, a trend we term here the separation of generations.Pivotal results.Long-lived fern gametophytes have evolved independently in several families and can be found around the world. We present a comprehensive review of the long-lived fern gametophytes that are able to forgo the production of a sporophyte, including accounts of their discovery, taxonomy, biology, ecology, and biogeography. We also present several hypotheses concerning why these species do not produce sporophytes, identify gaps in our knowledge about these organisms, and suggest areas of future study.Conclusions.While several populations of independent gametophytes have been identified and characterized in temperate regions, it is likely that the bulk of species with spatially separated generations occur in the tropics, where little work has been done. Additionally, virtually no studies have been undertaken that attempt to determine the underlying factors inhibiting sporophyte production in ferns. As 2017 marks the fiftieth anniversary of the first comprehensive study published on independent fern gametophytes, we can think of no better time for a review on their biology and an assessment of the work that still needs to be done.