THE OCCURRENCE AND PHYLOGENETIC SIGNIFICANCE OF A MULTILAYERED STRUCTURE IN COLEOCHAETE SPERMATOZOIDS

Spermatozoid-forming cells of Coleochaete scutata were found in packets of four arranged in concentric internal bands. Spermatozoids, which occur singly in antheridial cells, are spherical to ovoid, approximately 7 μm long by about 3.9 μm wide. As compared to relatively unspecialized zoospores, male gametes undergo a number of specialized cellular changes during development. The spherical nuclei and cytoplasm of mature spermatozoids are increased in density. Posterior plastids are reduced and contain large starch grains. Many small mitochondria are clustered near the cell anterior. The plasmalemma is covered with a layer of flattened, diamond-shaped scales, while body scales of zoospores are pyramidal. The two flagella of both zoospores and spermatozoids are covered with flattened, diamond-shaped scales and hairs. The spermatozoids contain an anterior multilayered structure (MLS) structurally similar to, though smaller than, the MLS observed in zoospores. An asymmetrical cytoskeleton consisting of a band of 30–45 microtubules extends from the MLS down one side of the spermatozoid close to the plasmalemma. An immature MLS was observed in an early stage of spermatozoid development. The finding of an MLS and asymmetrical cytoskeleton in specialized male gametes as well as relatively unspecialized zoospores of Coleochaete strengthens assumptions of homology between MLSs of green algal reproductive cells and those found in flagellated spermatozoids of archegoniate plants. The structure of the spermatozoid of Coleochaete supports the hypothesis that this alga may be relatively close to the phylogenetic line which led directly to archegoniates.