Noncorrespondence of Breeding Groups, Morphology, and Monophyletic Groups in Spirogyra (Zygnemataceae: Chlorophyta) and the Application of Species Concepts

Recent findings on the effects of polyploidy on morphology in filaments of the common green alga Spirogyra invalidate the reliance on several characters used to erect the 386 species described for this genus. Spontaneous ploidal changes have been observed that induce significant morphological changes in filaments, such that members of a series of ploidal variants would be identified as different species according to conventional taxonomy. Grouping filaments into species based on morphology (qualitative and quantitative differences between character states), breeding groups (sexual compatibility or lack thereof), or monophyly (lineages of filaments estab- lished through laboratory culture work) results in groups, or "species," that differ in membership. Strains of identical morphology are not always more closely related to each other than to morpho- logically distinct ploidal variants descended from them. Nor are such ploidal variants sexually compatible with their parental strains except to a limited degree. Thus, a single monophyletic group of filaments (descended from and including an ancestral strain) may contain sexually incompatible filaments that differ greatly in morphology. This noncorrespondence of morphology, breeding groups, and monophyly suggests that conventional taxonomic monographs that distinguish species on the basis of morphology alone describe only part of the species picture, although they are still valuable as sources of information on phenotypic variability. Additional study of these three group- ing criteria in Spirogyra is needed to develop a satisfactory species concept for this genus and others in the family Zygnemataceae.