Developmental morphology of the typical cordate gametophyte of a homosporous leptosporangiate fern, Lygodium japonicum (Lygodiaceae), focusing on the initial cell behavior of two distinct meristems

PREMISE OF THE STUDY: Understanding the origin and early evolution of vascular plants requires thorough consideration of the gametophyte generation of ferns and lycophytes. Unfortunately, information about this generation is quite limited. To reveal the origin and evolution of varied gametophyte shapes, we used comparative morphological studies of meristem behavior of gametophytes of Lygodium japonicum, which exhibit the typical cordate shape. METHODS: Microscopic images of epi-illuminated growing gametophytes cultured from spores were captured periodically using a metallurgical microscope equipped with a digital camera to analyze the cell lineage in the meristem. KEY RESULTS: Gametophytes form from two meristems: the apical-cell-based meristem and the multicellular meristem. The triangular apical cell produces six to eight derivatives from two lateral facets, then disappears. Subsequently, the multicellular meristem, with a row of several rectangular cells, forms in the notch. These rectangular cells divide asynchronously in the periclinal and anticlinal walls to produce cells to both lateral sides and downward. Usually two, and sometimes three, cells located at the center of the meristem divide at a slower pace in the periclinal and anticlinal planes than others at the periphery. The cells at the periphery are pushed away and become involved in the wing base. CONCLUSIONS: The triangular apical cell behaves as a permanent initial cell. In the multicellular meristem, however, two or three central cells behave as initial cells that are transient and regulated in a position-dependent manner. The organization and behavior of both meristems are shared with the ribbon-shaped gametophytes of Colysis.