Chloroplast, mating behaviour, and auxosporulation are described for Amphora cf. proteus Gregory. The interphase chloroplast is butterfly-shaped and lies with its central part, containing a flat discoid pyrenoid, pressed against the ventral side of the girdle. Chloroplast division is longitudinal and begins and ends before mitosis and cytokinesis. Over the cell cycle, no rotations or lateral translocations of the chloroplast were noticed. A. cf. proteus is the first species of this genus in which sexual reproduction and breeding behaviour have been studied in clonal cultures. The cells isolated exhibited strictly dioecious behaviour, reproducing only in mixed cultures of sexually compatible clones. Sexual reproduction in A. cf. proteus is morphologically and behaviourally isogamous. Each gametangium forms two functional gametes, which do not rearrange within the gametangium, and two small aborted cells. The aborted cells are produced after an unequal cytokinesis accompanying meiosis II; the second cytokinesis during gametogenesis represents a phenomenon hitherto unknown in pennate diatoms. Auxospore development takes place by bipolar expansion. The expansion is unequal on the two long sides of the auxospore, so that the auxospore becomes curved and dorsiventral. The auxospore forms a transverse and longitudinal perizonium.
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