The Ultrastructure of the Early Development of Schizaea pusilla Gametophytes

Abstract

Schizaea pusilla is a fern that maintains a completely filamentous gametophyte throughout its development. In this study, we characterize young gametophytes by using light and electron microscopy. In order to gain more reliable structural information, the cryofixation techniques of high pressure freezing and propane jet freezing were used. Apical protonemal cells of young S. pusilla gametophytes are unlike apical cells of other ferns studied to date. The apical protonema was divided into four regions with zone 1 at the apex and zone 4 at the base. Zone 1 has apical vacuoles, numerous Golgi stacks, ER, and a high density of apical vesicles. Zone 2 consists of chloroplasts with large starch grains, small vacuoles, uncharacterized electron-dense inclusions, mitochondria, lipid bodies, Golgi stacks, and ER. The nucleus is located in zone 3, and zone 4 consists of a large vacuole with traversing cytoplasmic strands, which contain chloroplasts, mitochondria, Golgi stacks, and ER. The cytoskeleton and endomembrane system were particularly well-preserved in cryofixed protonemata. Novel structures observed in cryofixed cells include microfilament bundles, multivesicular bodies, and apical vesicles, which appeared to be fusing with the plasma membrane. This study is the first report to use cryofixation/freeze substitution to characterize fern gametophytes and demonstrates that these techniques can be useful in ultrastructural studies of fern development.