| Examination of the freshwater dinoflagellate Gloeodinium montanum KLEBS (1912) utilizing light, scanning and transmission electron microscopy resulted in new information on the vegetative life history, as well as the discovery of sexual reproduction in this organism, and a first account of its ultrastructure. Gloeodinium montanum, a member of the order Dinocapsales, is a coccoid, nonmotile organism surrounded by a thick, stratified layer of sometimes gelatinized cell coverings. Vegetative cysts measure approximately 18-28 um in diameter.; Ephemeral zoospores (swarmers), produced by vegetative cysts, and identified as belonging to the genus Hemidinium STEIN (1883), are released at the beginning of the light cycle. Zoospores develop directly into vegetative cysts. The cysts are predominantly binucleated; cytokinesis is delayed.; Zoospores and gametes are morphologically identical. Sexual reproduction occurs upon nutrient depletion. Gloeodinium montanum is a homothallic haplont with isogamous and anisogamous copulation. After development of a copulation tube gametes coalesce into each other, or one gamete acts a recipient for the other.; Meiosis is zygotic. Zygotes are surrounded by thick coverings consisting of several acetolysis-resistant membrane layers. Germination of zygotes produces four non-motile viable cysts.; The ultrastructure of G. montanum is comparable to that of other dinoflagellates with some exceptions. The cell covering of G. montanum cysts consists of a plasma-membrane series which is surrounded by a cellulose layer and several layers of acetolysis-resistant membranes. Chloroplasts, nucleus and mitochondria are similar to those of other dinoflagellates. A single, stalked pyrenoid is present, as well as numerous starch grains and lipids droplets. The accumulation body becomes very large in old cysts, occupying as much as one third of the cyst-volume. A pusule was only detected in the swarmer. Trichocysts and ocelli were absent in both swarmers and cysts. Besides the usual organelles, two types of endosymbionts were discovered. |
