Life cycle and functional genomics of the unicellular red alga for elucidating algal and plant evolution and industrial use.

Sexual reproduction is widespread in eukaryotes; however, only asexual reproduction has been observed in unicellular red algae, including , which branched early in Archaeplastida. possesses a small genome; it is polyextremophile, grows either photoautotrophically, mixotrophically, or heterotrophically, and is being developed as an industrial source of vitamins and pigments because of its high biomass productivity. Here, we show that exhibits a sexual life cycle, alternating between cell-walled diploid and cell wall-less haploid, and that both phases can proliferate asexually.

Cell size for commitment to cell division and number of successive cell divisions in cyanidialean red algae.

Several eukaryotic cell lineages proliferate by multiple fission cell cycles, during which cells grow to manyfold of their original size, then undergo several rounds of cell division without intervening growth. A previous study on volvocine green algae, including both unicellular and multicellular (colonial) species, showed a correlation between the minimum number of successive cell divisions without intervening cellular growth, and the threshold cell size for commitment to the first round of successive cell divisions: two times the average newly born daughter cell volume for unicellular Chlamydomonas reinhardtii, four times for four-celled Tetrabaena socialis, in which each cell in the colony produces a daughter colony by two successive cell divisions, and eight times for the eight-celled Gonium pectorale, in which each cell produces a daughter colony by three successive cell divisions. To assess whether this phenomenon is also applicable to other lineages, we have characterized cyanidialean red algae, namely, Cyanidioschyzon merolae, which proliferates by binary fission, as well as Cyanidium caldarium and Galdieria sulphuraria, which form up to four and 32 daughter cells (autospores), respectively, in a mother cell before hatching out.

Cell size for commitment to cell division and number of successive cell divisions in multicellular volvocine green algae Tetrabaena socialis and Gonium pectorale.

Volvocine algae constitute a green algal lineage comprising unicellular Chlamydomonas, four-celled Tetrabaena, eight to 32-celled Gonium, and others up to Volvox spp., which consist of up to 50,000 cells. These algae proliferate by multiple fissions with cellular growth up to several fold in size and subsequent successive cell divisions.