TOWARD A NATURAL SYSTEM OF SPECIES IN CHLOROGONIUM (VOLVOCALES, CHLOROPHYTA): A COMBINED ANALYSIS OF MORPHOLOGICAL AND rbcL GENE SEQUENCE DATA

Abstract

The taxonomy of species of Chlorogonium (Volvocales, Chlorophyta) was studied based on comparative light and electron microscopy and DNA sequence data of 23 strains from five major algal culture collections. All of the 23 strains showed pyrenoids under photoautotrophic conditions, but 17 of the 23 exhibited marked reduction in size of pyrenoids, or pyrenoids were absent under photoheterotrophic conditions. The strains could clearly be delineated into six species, C. euchlorum, C. elongatum, C. fusiforme, C. capillatum, C. neglectum, and C. kasakii on the basis of differences in cell shape, number of contractile vacuoles, number and stability of pyrenoids, and ultrastructure of pyrenoids and stigmata. This distinction of species based on morphology was also supported by analyses of rbcL gene sequences. The later strongly showed that each species, C. euchlorum (seven strains), C. elongatum (three strains), and C. capillatum (10 strains), forms a robust clade. Although some morphological differences were noted within different strains of C. euchlorum and C. capillatum, these features were regarded as strain‐specific because they were not reflected in the rbcL gene phylogenies. In addition, the rbcL gene trees strongly suggested that C. neglectum and C. kasakii are closely related to each other, consistent with the similarity of the ultrastructure of pyrenoids and stigmata between the two species. However, C. kasakii can be distinguished clearly from C. neglectum by its multiple pyrenoids in the chloroplast and acute anterior and posterior ends in the vegetative cell.