Patterns of asexual reproduction in Nannochloris bacillaris and Marvania geminata (Chlorophyta, Trebouxiophyceae)

Non-flagellated vegetative green algae of the Trebouxiophyceae propagate mainly by autosporulation. In this manner, the mother cell wall is shed following division of the protoplast in each round of cell division. Binary fission type Nannochloris and budding type Marvania are also included in the Trebouxiophyceae. Phylogenetic trees based on the actin sequences of Trebouxiophyceae members revealed that the binary fission type Nannochloris bacillaris and the budding type Marvania geminata are closely related in a distal monophyletic group. Our results suggest that autosporulation is the ancestral mode of cell division in Trebouxiophyceae. To elucidate how non-autosporulative mechanisms such as binary fission and budding evolved, we focused on the cleavage of the mother cell wall. Cell wall development was analyzed using a cell wall-specific fluorescent dye, Fluostain I. Exfoliation of the mother cell wall was not observed in either N. bacillaris or M. geminata. We then compared the two algae by transmission electron microscopy with rapid freeze fixation and freeze substitution; in both algae, the mother cell wall was cleaved at the site of cell division, but remained adhered to the daughter cell wall. In N. bacillaris, the cleaved mother cell wall gradually degenerated and was not observed in the next cell cycle. In contrast, M. geminata daughter cells entered the growth phase of the next cell cycle bearing the mother and grandmother cell walls, causing the uncovered portion of the plane of division to bulge outward. Such a delay in the degeneration and shedding of the mother cell wall probably led to the development of binary fission and budding.