To identify structural modifications that accompany cell motility in plants, we compared mature pre-released sperm cells of Lygodium japonicum with cells that have been actively swimming. Anatomical changes between the two conditions were characterized in the transmission electron microscope and these were correlated with images taken in the light and scanning electron microscopes. Non-motile sperm cells still contained within the antheridium are streamlined and coil over three revolutions. The cylindrical nucleus is compacted and spirals along the cell length. Approximately 70 flagella are attached along the anterior-most coils. A band of microtubules encircles the outside of the cell, and numerous mitochondria and starch-filled plastids are positioned in a large cytoplasmic mass that fills the inner nuclear coils. Once released from the antheridium and upon motility, the cell stretches in length but exhibits no major structural modifications except that the cytoplasmic mass may be shed. When the cell enters the viscous mucilage that is exuded by and surrounds the archegonium, the mass of excess cytoplasm, including plastids, is invariably eliminated. The only organelles remaining in sperm cells adjacent to the egg are those of the locomotory complex (including flagella), the nucleus and numerous mitochondria. These findings have important implications for fertilization and inheritance of DNA; since no plastids are contained in the sperm cell at fertilization, then plastids cannot be contributed by the male but must be inherited exclusively from the female (in the egg). Conversely, the multiple mitochondria in sperm and egg cells at the time of fertilization indicate that mitochondria may be inherited from both parents.

Key words: fertilization, Lygodium, morphology, organelles, sperm cell, ultrastructure