Diversity of genome size and chromosome number in homothallic and heterothallic strains of the Closterium peracerosum-strigosum-littorale complex (desmidiales, zygnematophyceae, streptophyta)

Members of the Closterium peracerosum-strigosum-littorale (C. psl.) complex are unicellular zygnematophycean algae, which are suggested to be closely related to land plants. A zygospore is typically formed as a result of conjugation between mating-type plus (mt+) and mating-type minus (mt-) cells during sexual reproduction in heterothallic strains. On the other hand, zygospores are formed between genetically identical cells in homothallic strains. In this study, we isolated novel homothallic strains in the C. psl. complex. Phylogenetic analysis revealed the polyphyly of homothallic strains, suggesting multiple transitions between homothallism and heterothallism in the C. psl. complex. We measured the 1C genome size of the C. psl. complex by using flow cytometry after staining nuclei with propidium iodide, which ranged from 0.53 to 1.42 Gbp. We counted chromosome numbers using confocal microscope images, finding that two homothallic strains had fewer chromosomes than four heterothallic strains. Genome size positively correlated with both the cell size and chromosome number. Chromosome numbers differed even within the same mating group, suggesting a mechanism tolerating chromosomal rearrangements during meiosis in the C. psl. complex.