FIELD AND LABORATORY INVESTIGATIONS OF SPIROGYRA (CHLOROPHYTA, ZYGNEMATACEAE), WITH SPECIAL REFERENCE TO A POLYPLOID SPECIES COMPLEX (ARIZONA).

Abstract

On the basis of three morphological characters (e.g., filament, width, chloroplast number, and type of cell end wall), six filament types of Spirogyra were collected along Bear Creek in the Santa Catalina Mountains near Tucson, Arizona. The occurrence and distribution of filament types showed seasonal and geographical patterns. Filaments were more frequently collected in early summer from pools at lower elevation. Growth of Spirogyra may be influenced by water temperature, pH, and water amount. Most filaments occurred more abundantly while water temperture and pH were relatively high. The number of filament types was greatest at sites with a semi-permanent water supply rather than in temporary and permanent pools. Of the six types of Spirogyra, Type V showed morphological and genetic changes through vegetative growth and sexual reproduction in a clonal culture in the laboratory. After 33 months culturing, a narrower filament-width group (Group II, 22.0 ± 1.1 μm) was produced in the original clone (Group I, 30.9 (+OR-) 0.7 μm). Groups I and II were homothallic and sexually compatible. Zygospores from the cross of I x II yielded germlilngs of Groups I, II, III (27.2 ± 1.0 μm) and a binucleate IV (44.9 ± 0.8 μm). Chromosome counts were: Group I (24), Group II (12), Group III (18), and Group IV (24, one nucleus). Relative nuclear-DNA fluorescence values increased as filament width and chromosome number increased. Cytologically, Group I is a tetraploid, Group II a diploid, and Group III a triploid. Systematically, Groups I, II and III key out to pirogyra singularis, S. communis, and S. fragilis, respectively, using Transeau's monograph on Zygnemataceae. These species are interpreted to be a species complex of S. communis (whose name has priority) with the ancestral haploid (x = 6) missing. Five years after isolation of the original strain in this species complex filaments corresponding to Groups I and II were found at the original collection site. The two field-collected groups were indistinguishable from the laboratory species complex in morphology and chromosome number. Homothallic conjunction within two field width groups yielded progeny similar to those from homothallic conjunction of groups in the laboratory species complex. Filament widths of progeny were generally within the width limits of respective parental groups. The four intergroup crosses between laboratory and field width groups were successful and yielded progeny representing Groups I, II, and III. The similarities in morphology, chromosome numbers, and reproductive behavior of laboratory and field width groups imply that the laboratory species complex of S. communis has a natural counterpart in the field.