Fine structure of zoosporogenesis, zoospore germination, and early gametophyte development in Cladophora surera (Cladophorales, Chlorophyta)

The fine structure of zoosporogenesis, zoospore germination, and early gametophyte development in Cladophora surera Parodi et Caceres were studied. Zoosporogenesis started with simultaneous meiosis in all nuclei of apical initial cells. The resulting haploid nuclei duplicated in turn by successive centric, closed mitoses. Then, each initial cell divided into two short zoosporangia. Numerous vacuoles appeared around each sporic nucleus. The delimitation of uninucleate zoosporocytes occurred by cytokinetic furrows produced by the coalescence of tiny, clear vesicles, without microtubules. Final shape of the zoospore resulted from gradual expulsion of vacuoles from the cell body. Mature biflagellate zoospores exhibited a conspicuous apical papilla containing fine granular globules, the basal apparatus, and a microtubular "umbrella" formed by numerous cortical microtubules that ran backward the length of the cell body. The chloroplast showed a conspicuous eyespot. The zoosporangial wall disorganized at the pore through which the zoospores were liberated. Zoospores settled on a substrate by their anterior papilla secreting an adhesive. Germination involved retraction of the apical papilla, loss of the "umbrella" microtubules and eyespot, and the lateral absorption of the entire flagellar apparatus, i.e. basal apparatus plus axoneme, into the cytoplasm. Early gametophyte development involved the synthesis of a thin young cell wall, the development of outer peripheral vacuoles, the appearance of the marginal reticulate chloroplast, and the formation of the first central vacuoles derived from abundant endoplasmic reticulum. Close to the plasmalemma ran longitudinally oriented cortical microtubules. Eventually, the germling developed an achlorophylic, elongated rhizoidal portion.