Taxonomic re-examination of Nitella (Charales, Charophyceae) from Japan, based on microscopical studies of oospore wall ornamentation and rbcL gene sequences

Nine taxa of Nitella (Charales, Charophyceae) from Japan, including five Japanese or east Asian endemics, were examined to improve understanding of their taxonomic status. The approaches used were light and scanning electron microscopy (SEM) of the oospores and sequencing of the gene (rbcL) encoding the large subunit of Rubisco. The species delineated were N. pulchella (= N. dualis f. pulchella), N. furcata, N. inversa (= N. furcata f. inversa), N. gracilens (= N. furcata f. gracilens), N. pseudoflabellata, N. hyalina, N. spiciformis (= N. gracilis f. spiciformis), N. moriokae (= N. rigida L moriokae) and N. axillaris (= N. translucens var. axillaris). SEM observations showed that the oospore morphology of four taxa - N. inversa, N. gracilens, N. spiciformis and N. axillaris - was distinctly different from that of the species (N. furcata, N. furcata, N. gracilis and N. translucens, respectively) to which R.D. Wood assigned them as infraspecific taxa. Furthermore, the rbcL data showed that N. gracilens was separated phylogenetically from N. furcata and N. axillaris from N. translucens. This is the first integration of SEM oospore morphology and molecular phylogenetics in charalean taxonomy, demonstrating the efficiency of both approaches to address problems at lower taxonomic levels.