Summary• To characterize structural diversity within Equisetum and among pteridophytes, architectural features of the sperm cell are described here in a second subgenus of Equisetum , a divergent basal group in the fern clade.• Transmission electron microscopy observations of prereleased spermatozoids of Equisetum arvense were correlated with three-dimensional scanning electron microscopy images of swimming cells.• The mature spermatozoid completes a helix of approximately 2.5 revolutions. At the cell anterior is a complex multilayered locomotory apparatus with staggered flagella. Mitochondria (elongated-rounded) are aggregated near the locomotory apparatus and organelles extend along the cell length. The spline contains up to 300 microtubules and wraps in part around the long cylindrical nucleus. In swimming sperm cells, the anterior of the cell remains tightly coiled while the posterior relaxes and extends in a trailing fashion.• Spermatozoids of Equisetum arvense are smaller than those of Equisetum hyemale but structurally similar, except for nuclear shape. Conservation of cellular features suggests recent radiation of the genus. Equisetum spermatozoids share several critical features with ferns, including Psilotum , and support monophyly of a fern-Equisetum assemblage. Entry of the male gametes of Equisetum in their entirety into the archegonial venters indicates possible biparental inheritance of chloroplast and mitochondrial genomes.

The only motile cells produced in land plants are male gametes (spermatozoids), which are reduced to non-flagellated cells in flowering plants and most gymnosperms. Although a coiled architecture is universal, the complexity of land plant flagellated cells varies from biflagellated in bryophytes to thousands of flagella per gametes in the seed plants Ginkgo and cycads. This wide diversity in number of flagella is associated with vast differences in cell size and shape. Scanning electron microscopy (SEM) has played an important role in characterizing the external form, including cell shape and arrangement of flagella, across the varied motile gametes of land plants. Because of the size and scarcity of released swimming sperm, it is difficult to concentrate them and prepare them for observation in the SEM. Here we detail an SEM preparation technique that yields good preservation of sperms cells across plant groups.

Toxic dinoflagellates gained notoriety for their devastation to coastal and island economies, causing damages totaling in the millions of dollars that included losses of harvestable resources, clean-up costs, as well as cancellation of holiday reservations in popular tourist/recreational areas.3 These red tide blooms sparked an urgency for obtaining information pertaining to the dynamics of toxin production, but understanding the mechanisms of toxin production have proven to be difficult to pin point. This study tested the hypothesis that cells grown under various light intensities undergo structural and compositional changes and served as a basis for future biochemical studies of Prorocentrumhoffmanianum.We examined ultrastructural changes to the chloroplast in cells from monocultures of Prorocentrum hoffmanianum grown in three different light intensities, via a transmission electron microscope and a fluorescent light microscope. The full or normal growth chamber intensity of ca. 5000 lux represented the control group referred to as the full light treatment.

Among land plants, lower vascular plants produce the most elaborate of all male gametes. in order to characterize the degree of variability in sperm cell architecture among ferns, this scanning electron microscope study was undertaken on three highly disparate genera: Ceratopteris(a derived leptosporangiate fern) Angiopteris(a derived eusporangiate fern) and Psilotum(a putatively basal eusporangiate fern).Mature gametophytes were placed in 0.01M phosphate buffer (pH ca. 7.2) or distilled water until spermatozoids were released. Gametophyte portions were removed and the spermatozoid suspension fixed with 2% glutaraldehyde in 0.05M phosphate buffer. Post-fixation utilized 1% aqueous osmium tetroxide for lhr, followed by rinsing and dehydration in ethanol (10 min @ 25, 50, 75 and 100%). The dehydrated pellet was suspended in 100% hexamethyldisilazane, immediately centrifuged and the pellet deposited onto clean glass coverslips. Specimens were dried at 60°C, sputtered with ca. 350 A of palladium/gold and imaged in a Hitachi S570 SEM.

The phylogenetic position of mosses, ferns and fern allies is enhanced by detailed knowlegde of gametophyte reproductive structures, in particular the spermatozoid (freeswimming sperm). The number and placement of flagella in conjunction with their ultrastructure and that of the multilayered structure (MLS) has been used to establish related groups. Herein, ultrastructural details of pre-release spermatozoids obtained via TEM are compared with the morphological appearance of fully mature (free-swimming) spermatozoids obtained via SEM. This represents the first intensive study of any plant sperm cell that correlates 3-dimensional SEM images with TEM ultrastructural features.Equisetum arvense L. is a fern ally commonly refered to as ‘horsetail’. The mature spermatozoid (Fig. 1) is twisted into approximately 3.5 revolutions, or gyres. A complex locomotory apparatus (Fig. 2), involving approximately 55 flagella, dominates the anterior region of the cell. The basal bodies subtending these flagella feature a well-defined stellate pattern and occur in 4 irregular rows staggered over approximately 1.75 gyres.