Architecture of the sperm cell of Psilotum

In this correlated SEM (scanning electron microscope) and TEM (transmission electron microscope) investigation, we describe architectural details of t he multiflagellated sperm cell of Psilotum nudum. Comparisons with other pt eridophytes are made to (1) assess the placement of Psilotum among pteridop hyte taxa and (2) evaluate structural modifications of sperm cells during l and plant evolution. The released spermatozoid of Psilotum coils 2.0 revolu tions and is outlined by a parallel band of up to 190 microtubules. The elo ngated nucleus is highly compacted and parallels the cellular coils with nu merous mitochondria and starch-laden plastids distributed along its length. Along the anterior coil is an elaborate locomotory apparatus that includes similar to 36 flagella that are inserted into the cell by basal bodies, Su btending the basal bodies is the multilayered structure, which consists of a long narrow lamellar strip and an overlying band of microtubules. An elon gated anterior mitochondrion underlies the multilayered structure. Addition al amyloplasts and mitochondria are aggregated along the anterior coil in a ssociation with the locomotory apparatus, while a fibrous band encircles th e leading edge of the cell. Salient features of this cell, including detail s of the locomotory apparatus, structure and position of organelles, and ar rangement of the spline, are shared by spermatozoids of Equisetum and ferns (including eusporangiate and leptosporangiate taxa). Thus, this study prov ides morphological support for the hypothesis that Psilotum nudum is a memb er of an assemblage that includes ferns and Equisetum. However, the less st reamlined architecture of Psilotum gametes and the lack of architectural fe atures shared with any specific taxon examined to date suggest that Psilotu m is an early divergent fern, with relatively remote affinities to Ophioglo ssaceae and Equisetaceae.