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Antheridiogen systems, whereby notch-bearing, archegoniate gametophytes induce maleness in ameristic neighbors, have been detected in many core leptosporangiate ferns. Previous studies have failed to detect an antheridiogen system in Osmundales, which is sister to all other extant leptosporangiates; hence, antheridiogen systems are thought to have evolved after their divergence. Detailed studies of morphological development and patterns of gender expression in Osmundales and other basal leptosporangiate clades are needed to clarify how antheridiogen systems evolved. Here, we tracked the development and gender expression of gametophytes of Osmundastrum cinnamomeum grown in isolation and multispore populations exposed to basal media (control), gibberellic acid (GA3), and an older generation of gametophytes. A notch-producing apical meristem invariably preceded antheridia and archegonia production in O. cinnamomeum. Exposure to either GA3 or an older generation of gametophytes delayed growth rates and prolonged asexual and male status, whereas the multispore control possessed significantly greater proportions of females relative to isolates. Our observations con?rm the existence of a putative antheridiogen system in Osmundales and a mechanism by which male-first expression is bypassed by a subset of the population. The evolution of fully developed antheridiogen systems in core leptosporangiate families may have involved the decoupling of the formation of a notch meristem from the production of antheridia in an ancestor with an Osmundales-type pattern of gender expression. [ABSTRACT FROM AUTHOR]