The Hidden Sexuality of Alexandrium Minutum: An Example of Overlooked Sex in Dinoflagellates.

Dinoflagellates are haploid eukaryotic microalgae in which rapid proliferation causes dense blooms, with harmful health and economic effects to humans. The proliferation mode is mainly asexual, as the sexual cycle is believed to be rare and restricted to stressful environmental conditions. However, sexuality is key to explaining the recurrence of many dinoflagellate blooms because in many species the fate of the planktonic zygotes (planozygotes) is the formation of resistant cysts in the seabed (encystment). Nevertheless, recent research has shown that individually isolated planozygotes in the lab can enter other routes besides encystment, a behavior of which the relevance has not been explored at the population level. In this study, using imaging flow cytometry, cell sorting, and Fluorescence In Situ Hybridization (FISH), we followed DNA content and nuclear changes in a population of the toxic dinoflagellate Alexandrium minutum that was induced to encystment. Our results first show that planozygotes behave like a population with an "encystment-independent" division cycle, which is light-controlled and follows the same Light:Dark (L:D) pattern as the cycle governing the haploid mitosis. Resting cyst formation was the fate of just a small fraction of the planozygotes formed and was restricted to a period of strongly limited nutrient conditions. The diploid-haploid turnover between L:D cycles was consistent with two-step meiosis. However, the diel and morphological division pattern of the planozygote division also suggests mitosis, which would imply that this species is not haplontic, as previously considered, but biphasic, because individuals could undergo mitotic divisions in both the sexual (diploid) and the asexual (haploid) phases. We also report incomplete genome duplication processes. Our work calls for a reconsideration of the dogma of rare sex in dinoflagellates.

Nuclear and cell morphological changes during the cell cycle and growth of the toxic dinoflagellate Alexandrium minutum.

Elucidation of the cell cycle of dinoflagellates is essential to understand the processes leading to their massive proliferations, known as harmful algal blooms. In this study, we used imaging flow cytometry (IFC) to monitor the changes in DNA content and nuclear and cell morphology that occur during clonal growth of the toxic species Alexandrium minutum Halim. Our results indicate that the population was in S phase (C→2C DNA content) during the light period, whereas haploid cells with a C DNA content peaked only during a short interval of the dark period. The timing of the phases, identified based on the nuclear morphology and cytoplasmic-to-nuclear (CNR) ratio of the cells, suggests that the length of the G2/M phase is regulated by nutrient levels whereas the beginning of S phase is clock controlled. In addition we found that up to 7% of individual cells achieved a DNA content higher than 2C, indicative of either zygote formation and replication (homothallism), or of double-haploid cells able to divide (polyploid forms). Cells belonging to different cell cycle phases (G1-S-G2/M) could be readily discriminated based on nuclear size. Our study provides evidence of cell-cycle plasticity during clonal growth and unambiguously characterizes the cell-cycle phases of this dinoflagellate species.