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Based on transmission electron microscopy observation, zoosporogenesis in the siphonous multinucleate green alga  Derbesia tenuissima  can be divided into five stages. In vegetative sporophytes and Stage 1 sporangia, the cytoplasm was distinguished as two regions, the inner region of chloroplasts and the outer region of nuclei. In Stage 1 sporangia, the chloroplasts differentiated into amyloplasts and contained large starch grains. In Stage 2 sporangia, several meiotic nuclei with synaptonemal complexes were observed in the apical and the subapical portions of a sporangium. The width of a synaptonemal complex was 103±5 (mean ± standard deviation) nm (n = 20). In Stage 3 sporangia, the plug complex that was composed of 4 layers as observed with transmission electron microscopy, was formed between the sporangium and the main axis. In Stage 4 sporangia, the following conspicuous characters were observed: 1) differentiation of nuclei into two types, 2)  de novo  appearance of centrioles (flagellar basal bodies) on the type 2 nuclei (large), 3) decrease in the number of the type 1 nuclei (small), 4) development of cleavage furrows and vesicles, 5) formation and elongation of flagellar axonemes, and 6) development of three characteristic bands on the flagellar basal bodies. In Stage 5 sporangia, differentiation of flagella and flagellar bands was completed. Each stephanokont zoospore of  D. tenuissima  had about 80 flagella, about 16 nuclei and many chloroplasts.

Ultrastructural Study of Zoosporogenesis in the Siphonous Green Alga Derbesia tenuissima (Chlorophyta)