THE PRODUCTION AND INHERITANCE OF THE TRIRADIATE FORM IN COSMARIUM TURPINII

Abstract

AtLTHOUGH THE placoderm desmids have been grown in culture for almost 50 years, taxonomists of the desmids have ignored the possible use of cultures in their studies, instead relying on natural populations as a source of material. An overwhelming number of species, varieties, and forms of desmids have been described but many are highly suspect as environmental variants which have been recognized as taxonomic entities. One of the more common variants among desmid genera is an increase in the amount of radiation shown by a cell. Teiling (1950), in his discussion of desmid form, defines radiation in desmids as element of structure which is decisive in the shape of the desmids according to their vertical symmetry-planes. Thus a Cosmarium cell is said to be biradiate in end view; a simple Staurastruim, triradiate in end view. Although the genus Cosmarium is defined as bein,g biradiate, forms which are triradiate on end view have been described both from natural collections and from cultures. Fritsch (1953) pointed out the confusion among taxonomists as to the proper disposition of such triangular forms. In some cases they have been described as species of the triradiate genus Staurastrum; other specimens have been considered as forma triquetra of biradiate species of Cosmarium. Fritsch's suggestion to group such problematical forms under a separate genus to be called Cosmostaurastrum indicates the highly subjective foundations on which the taxonomy of desmids rests. Heterothallic strains of Cosmarium turpinii Br6b.2 were isolated in 1953 by the writer and since that time have been used in studies of the sexual process, germination of the zygospore, and the inheritance of mating type and a zygotic lethal factor (Starr, 1954a, b, 1955). The appearance of triradiate cells in the biradiate populations of Cosmarium turpinii provided material for the present investigation of the production of the triradiate form and its inheritance in sexual as well as asexual reproduction. The heterothallic strains of C. turpinii (no. 3, 4) are identical morphologically. A typical semicell (fig. 1, 11) in front view is angular, being broad at