Taxonomic re-examination of six species of Nitella (Charales, Charophyceae) from Asia, and phylogenetic relationships within the genus based on rbcL and atpB gene sequences

Abstract

Six taxa of Nitella (Charales, Charophyceae), collected from Asia, were investigated using light and scanning electron microscopy (SEM) for the oospores, and sequencing of the gene encoding the large subunit of Rubisco (rbcL), in order to improve our understanding of their taxonomic status. Our SEM observations demonstrated that the oospore morphology of four taxa belonging to the subgenus Tieffa1lenia [N. megaspora (J. Groves) Sakayama, comb. nov. (= N. pseudoflabellata f. megaspora), N. tumulosa (= N. furcata f. tumulosa), N. gracillima (= N. gracilis f. gracillima) and N. axilliformis (= N. translucens f. axilliformis)] is distinctly different from that of the species N. pseudoflabellata, N. furcata, N. gracilis and N. translucens, respectively, to which R.D. Wood assigned them as infraspecific taxa. Our rbcL sequence data showed that N. megaspora is separated phylogenetically from N. pseudoflabellata, N. tumulosa from N. furcata and N. axilliformis from N. translucens. In addition, to re-examine the taxonomic system of Nitella proposed by R.D. Wood, who treated oospore wall ornamentations as diagnostic at the infraspecific level, we carried out molecular phylogenetic analyses using the combined sequence data set for the gene encoding the beta subunit of ATP synthase (atpB) and the rbcL gene of these six species, as well as eleven species of Nitella studied previously. The combined sequence data resolved fiverobust clades within the subgenus Tieffallenia that were characterized by differences in oospore wall ornamentation. However, the species and sectional diagnoses of R.D. Wood, such as the form and cell number of dactyls in vegetative thalli, were variable within the clades. Therefore, R.D. Wood's taxonomic system appears unnatural, at least within the subgenus Tieffallenia.