SEXUAL REPRODUCTION IN CYMATOPLEURA

Abstract

Auxospore formation has been studied in three Edinburgh populations of Cymatopleura solea and is in all cases associated with sexual reproduction. Vegetative cells are subtly heteropolar, one pole being the site of mitosis, while the other contains the isthmus between the two plates of the single large plastid; for convenience these poles are termed proximal and distal respectively. During sexual reproduction two (rarely three) cells pair by their distal poles and undergo meiosis. Two gametes are produced in each gametangium and these fuse by their tips with the gametes in the other gametangium. Following plasmogamy, one gamete from each gametangium moves through a narrow copulation canal into the other gametangium, so that initially each elongate auxospore is contained almost completely within one of the gametangial frustules. The auxospore is dikaryotic, and its two nuclei move first to the proximal pole and then to the distal pole, where perizonium formation begins. The growth of the perizonium is unipolar and, as its bands are formed, the nuclei move down the expanding auxospore to the proximal pole. Upon completion of the perizonium the nuclei fuse and move to the centre of the cell. Later the diploid fusion nucleus moves to the distal pole and divides, prior to the formation of the initial epivalve. The initial valves are only slightly waisted and are not undulate. Previous reports of auxospore formation in Cymatopleura are analyzed and comparisons made with other raphid diatoms. In the Surirellales there are both behaviourally anisogamous forms and truly isogamous ones; perizonium growth is unipolar where known.