Ultrastructural studies on zoosporogenesis of Halopteris filicina (Sphacelariales, Phaeophyta)

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Abstract
The development of the unilocular sporangia of Halopteris filicina (Phaeophyta) commences with formation of the reproductive branchlets, the apical cells of which differentiate into spore mother cells. Afterwards, meiosis occurs, which becomes evident by the formation of typical synaptonemal complexes. The four haploid nuclei divide repeatedly and produce many nuclei which, together with the organelles, are peripherally distributed, while the central area is occupied by numerous storage bodies. After nuclear multiplication, the paired centrioles accompanying each nucleus function as basal bodies and form two flagella in specific vesicles, probably of dictyosomal origin, or in some cases outside the plasmalemma. One of these flagella finally bears mastigonemes, which at first appear in vesicles of endoplasmic reticulum (ER) origin. The cleavage follows a redistribution of the nuclei-organelle complexes throughout the whole cell space and is carried out by the fusion of vesicles, some of which contain flagella. Finally, the apical wall region of the sporangium breaks down and the zoospores are liberated. Each biflagellated zoospore contains more than two plastids, large storage bodies, and lacks an eyespot. Sometimes, the zoospores are not released; instead flagella are resorbed and a typical wall is secreted.