Polymorphism, sexual reproduction, meiosis and polyploidy and their implications for desmid taxonomy.

Conjugation and zygospore germination are described for 6 Pleurotaenium, 2 Micrasterias and 1 Staurastrum species. Probable chemotaxis is demonstrated in one Pleurotaenium species and selective mating, where a plus clone actively chooses one of two minus clones, Is shown to be common in the P. mamillatum complex. The Pleurotaenium species show remarkable homogeneity in conjugation features, possession of 5-walled smooth zygospores with operculate exospores and mammillate mesospores and producing single germination products. Zygospores of M. hardyi, A laticeps and St. orbiculare var. ralfsii are spiny, 3-walled, but only the Micrasterias zygospores are operculate. All three produce 2 germination products. Meiosis is described in germinating zygospores from crosses between various karyotypes and ploidy levels of the P. mamillatum complex, between haploids and diploids and morphologically different strains for other species of Pleurotaenium and the two Micrasterias species. In the Pleurotaenium species meiosis is chiasmate, conventional in character and the chromosomes have localized centromeres. Sexual crossing, resulting in viable, fertile offspring, occurs to a surprising degree between strains with widely disparate chromosome numbers, ploidy levels and geographical location. These strains form discrete groups that are reproductively isolated from one another. The implications that the results have for desmid taxonomy are discussed.