CONICET | Buscador de Institutos y Recursos Humanos

Oedogonium macrandrium Wittrok is a dioecious freshwater green alga that presents sexual dimorphism. The complete development of the epiphytic dwarf males, from androspore germination to the liberation of the mature male gametes, was studied. The task was carried out with both light and transmission electron microscopes. Similar studies have been performed in multicellular dwarf males of Bulbochaete hiloensis (Nordstedt) Tiffanyand pseudo-unicellular dwarf males of O. pluviale Nordstedt. The material came from a pond located in the Parque de Mayo, Bahía Blanca, Argentina. For light microscopy observations, the thalli were fixed with glacial acetic acid/alcohol absolute (1:3) for 24 hours at room temperature. The samples were stained with the following dyes: a) a solution of 45% propionic acid, 2% iron hematoxylin and 1% iron citrate b) aceto-iron hematoxylin/chloral hydrate. For observations with transmission electron microscopy, the samples were fixed with 1% glutaraldehyde and postfixed with 1% osmium tetroxide. Then, they were dehydrated with acetone and embedded in Spurr resin. The sections were contrasted with uranyl acetate and lead citrate. Light microscopy. The newly settled androspore formed a finger-like holdfast and had a nucleus located in a half-lateral position. Then, the germling developed an apical division ring to whose level the nucleus migrated. Immediately afterwards, the nucleus underwent a markedly asymmetric vegetative mitosis, and then 1) a transverse septum was developed and 2) the division ring was expanded. As a result, the dwarf male became composed of two uninucleate cells: a basal cell and an antheridium. Successive similar mitosis and cytokinesis conducted to the development of 2 (3) new antheridia. Each antheridial nucleus suffered then a gametic mitosis and two gametes per antheridium were differentiated. The antheridia maturation was basipetal, so that the first mature gametes were located at the most apical antheridium, which were released through the opening of an operculum. Transmission electron microscopy. The dwarf males initial nucleus exhibited a large nucleolus. The cytoplasm showed large vacuoles, abundant rough endoplasmic reticulum, especially in the holdfast, and dictyosomes. There was also a reticulate chloroplast with starch grains among thylakoids and around the pyrenoid. Around the apical division ring under development smooth endoplasmic reticulum and numerous dictyosomes appeared surrounding mitochondrial profiles. The cell wall was thick, electron dense and with an external layer of mucilage. There was a clear fracture zone at the level of the division ring. During vegetative mitoses the nucleolus disappeared and numerous perinuclear dictyosomes appeared. After cytokinesis the thin wall between the basal cell and the contiguous antheridium presented plasmodesmata. The side walls of the antheridia were thinner than the traverse walls. The antheridial nucleus lacked a nucleolus and the antheridial cytoplasm showed abundant perinuclear dictyosomes and a reticulate mitochondrion. During gametogenesis both gametes gradually decreased their cell volume simultaneously. The antheridial space was filled with mucilage, which became condensed and electron dense between the two gametes. Mature gametes had an apical dome with numerous spherical vesicles and a flagellar apparatus with 8-10 axonemic basal bodies. The cytoplasm showed a large reticulate mitochondrion and a reduced chloroplast. The whole process in O. macrandrium is similar to that of Bulbochaete hiloensis with a remarkable difference. In the latter, a phycoplast and a cell plate occur between the gametic nuclei of each antheridium and a true wall between gametes is produced. In O. macrandrium and O. pluviale only condensed mucilage appeared between gametes.

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