Cytoplasmic deletion processes during spermatogenesis in mosses

Comparative ultrastructural observations reveal that cytoplasmic deletion during spermatogenesis in Sphagnum and other mosses (Bryopsida) has two distinct phases. In young spermatids, Golgi-derived vesicles produce the mucopolysaccharide sheaths in which the gametes are liberated. Golgi bodies, however, play no part in removal of cytoplasm during gamete maturation. Rounding off of the cells during this process results in a 50% reduction in volume. Mid-spermatid stages in Sphagnum are characterised by the sequential loss of Golgi bodies and endoplasmic reticulum (ER) but no further diminution of the cytoplasm. The final stages of nuclear metamorphosis and chromatin condensation, in late spermatids, are marked by the sudden appearance, in the otherwise featureless central cytoplasm, of a membrane vesicle complex (MVC) comprising cisternae, tubules, and smooth and coated vesicles. Following repositioning of the MVC beneath the plasma membrane, rapid shrinkage of the cytoplasm is associated with the presence of vesicle fusion profiles at the cell surface. The MVC is considered to be intimately involved in cytoplasmic breakdown and loss. Acid phosphatase activity can be detected throughout spermatogenesis. Spermatogenous cells and young spermatids possess relatively low levels of the enzyme, restricted to the ER and perinuclear space, but particularly high levels occur in the MVC region of late spermatids of Sphagnum. The deletion process in Bryopsida is much more gradual than that of Sphagnum. Mid-spermatids contain sheets of ER, Golgi with small vesicles, and irregular cisternae associated with coated vesicles. Vacuoles derived either from dilation of the ER or the coated vesicle complexes gradually increase in size and number at the expense of the cytoplasm. During the early stages of chromatin condensation, a large central vacuole opens onto the anterior face of the gametes. Further discharge of vesicles continues throughout gamete maturation. A comparative survey of spermatogenesis in land plants indicates that cytoplasmic deletion is achieved in different ways in different groups. We speculate that the spermatozoids of the common ancestor of archegoniate plants probably possessed large amounts of cytoplasm. The deletion mechanisms may have originated from a contractile vacuole apparatus.