Meristem development and activity in gametophytes of the model fern, Ceratopteris richardii

Ceratopteris richardii is a model fern species widely used to analyze various developmental processes and their regulation in gametophytes. The form of mature C. richardii gametophytes depends on the activity of the marginal meristem, but knowledge on meristem formation and structure is limited. Therefore, we analyzed cellular events accompanying the development of gametophytes using cell lineage and proliferation analyses to explain the establishment and functioning of the marginal meristem. We show that: i) gametophytes are devoid of the apical initial cell or the apical cell-based meristem in the early developmental stages; ii) the cells that are predestined to form the marginal meristem divide according to a stable pattern; iii) only one transient initial cell is present in the marginal meristem, and the selection of a new functioning initial cell is related to a stable sequence of its divisions. Our results contribute to a better understanding of the developmental events underlying gametophyte growth and marginal meristem functioning in Ceratopteris. The principles, which were established in this study and enabled the identification of functioning initial cells, can be applied to analyze genetic and/or physiological mechanism(s) governing meristem maintenance in vascular plants, both in developmental and evolutionary contexts.