RELATIONSHIP BETWEEN PRESENCE OF A MOTHER CELL WALL AND SPECIATION IN THE UNICELLULAR MICROALGA NANNOCHLORIS (CHLOROPHYTA)1

The cell division mechanisms of seven strains from six species of Nannochloris Naumann were analyzed and compared with those of three species of Chlorella Beijerinck and Trebouxia erici Ahmadjian using differential interference microscopy and fluorescence microscopy. Nannochloris bacillaris Naumann divides by binary fission and N. coccoides Naumann divides by budding. Distinct triangular spaces or mother cell walls were found in the dividing autosporangia of the other five strains from four species of Nannochloris, three species of Chlorella, and T. erici. In an attempt to infer an evolutionary relationship between nonautosporic and autosporic species of Nannochloris, we constructed a phylogenetic tree of the actin genes using seven strains from six species of Nannochloris, three species of Chlorella, and T. erici. Nannochloris species were polyphyletic in the Trebouxiophyceae group. Two nonautosporic species of N. bacillaris and N. coccoides were monophyletic and positioned distally. Moreover, to determine their phylogenetic position within the Trebouxiophyceae, we constructed phylogenetic tree of 18S rRNA genes adding other species of Trebouxiophyceae. Nannochloris species were polyphyletic in the Trebouxiophyceae and appeared in two different lineages, a Chlorella–Nannochloris group and a Trebouxia–Choricystis group. The nonautosporic species, N. bacillaris and N. coccoides, and three autosporic species of Nannochloris belonged to the Chlorella–Nannochloris group. Nannochloris bacillaris and N. coccoides were also monophyletic and positioned distally in the phylogenetic tree of 18S rRNA genes. These results suggest that autosporulation is the ancestral mode of cell division in Nannochloris and that nonautosporulative mechanisms, such as binary fission and budding, evolved secondarily.     

EVOLUTIONARY RELATIONSHIPS AMONG MULTIPLE MODES OF CELL DIVISION IN THE GENUS NANNOCHLORIS (CHLOROPHYTA) REVEALED BY GENOME SIZE, ACTIN GENE MULTIPLICITY, AND PHYLOGENY

A single cell divides to multiply, but not all cells follow the same pattern of division. We studied cell division in seven strains from six species belonging to the genus Nannochloris Naumann and classified their modes of cell division into three types: binary fission ( N. bacillaris Naumann), budding ( N. coccoides Naumann), and autosporulation resulting in the formation of two to four daughter cells ( N. maculata Butcher, N . sp. SAG 251-2, N. atomus Butcher CCAP 251/7 and SAG 14.87, and N. eucaryotum [Wilhelm et al.] Menzel and Wild). To determine the evolutionary relationships among these multiple modes of cell division, we investigated the strains' genome sizes, copy number of actin genes, and phylogeny. The genome sizes were determined by counter-clamped homogeneous electric fields electrophoresis and fluorimetry. The genomes are very small and range from 12.6 Mbp ( N. maculata ) to 47.4 Mbp ( N. atomus SAG 14.87). The genomes of Nannochloris species seem to be among the smallest for free-living eukaryotes. Nannochloris bacillaris (binary fission), N. coccoides (budding), Nannochloris sp. (two-cell type of autosporulation), and N. eucaryotum (multicell type of autosporulation) contain a single actin gene, whereas N. maculata (two-cell type of autosporulation) and two strains of N. atomus (two-cell type of autosporulation) contain two actin genes. This suggests that the actin gene was duplicated in this eukaryote, which has a very small genome. Phylogenetic analyses of partial actin gene sequences suggest that autosporulation is the ancestral mode of cell division.