Meiosis, nuclear cyclosis, and auxospore formation inNavicula sensu stricto(Bacillariophyta)

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Abstract
The sexual reproduction of Navicula oblonga is described. Cells aggregate actively and lie side-by-side, touching each other as they enter meiotic prophase. In contrast to some other species traditionally included in Navicula, the gametangia do not subsequently separate and no copulation envelope is produced. Each gametangium forms two gametes, which may or may not become rearranged within the gametangium before its dehiscence. Navicula oblonga is morphologically and behaviourally isogamous. The newly formed auxospores become spherical and secrete a complex tripartite wall, containing a wide central layer of mucilage. This and a thin outer layer separate into two halves as the auxospore begins to expand, remaining as caps over the auxospore apices; a silicified perizonium is produced covering the remainder of the auxospore. Following meiosis, all four haploid nuclei survive in each gametangium, so that the gametes are binucleate; only after plasmogamy do the superfluous nuclei degenerate, the two surviving gametic nuclei fusing to form the diploid zygotic nucleus. The characteristics of reproduction in N. oblonga and other lineolate Navicula species (i.e. Navicula sensu stricto) are listed to provide a basis for future systematic work. During synapsis the chromosomes contract into a synizetic knot, visible in the living cell, which rotates around the interior of the much expanded ellipsoidal nucleus. This is the first report of nuclear cyclosis during diatom meiosis but its presence in dinoflagellates and mammalian spermatocytes suggests that it may be a universal feature of zygotene and early pachytene. Structural heterozygosity seems to be present in some N. oblonga.