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 patter n of division, We studied cell division in seven strains from six species b elonging to the genus Nannochloris Naumann and classified their modes of ce ll division into three types: binary fission (N, bacillaris Naumann), buddi ng (N, coccoides Naumann), and autosporulation resulting in the formation o f two to four daughter cells (N. maculata Butcher, N, sp, SAG 251-2, N, ato mus Butcher CCAP 251/7 and SAG 14.87, and N, eucaryotum [Wilhelm et al,] Me nzel and Wild). To determine the evolutionary relationships among these mul tiple modes of cell division, we investigated the strains' genome sizes, co py number of actin genes, and phylogeny, The genome sizes were determined b y counter-clamped homogeneous electric fields electrophoresis and fluorimet ry,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 (bin ary fission), N, coccoides (budding), Nannochloris sp, (two-cell type of au tosporulation), and N, eucaryotum (multicell type of autosporulation) conta in a single actin gene, whereas N, maculata (two-cell type of autosporulati on) 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.