Abstract
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.
Highlights
Dinoflagellates are haploid microalgae extensively studied with respect to their worldwide occurrence, toxicity, and capacity to become ecologically dominant
The flow cytometry histograms for the four L:D cycles performed in the cross (S1 and S2 Figs) were used to quantify the various DNA content groups according to the peaks obtained in the control, which lacked the S phase [34] and were used to determine the position of the C
The opposite was true for C!2C and 2C!4C cells: Minima of cells in 2C stages were coincident with maxima of cells in C!2C stages whereas maxima and minima of 4C-stage cells were nearly coincident with the maxima and minima of cells in 2C!4C stages
Summary
Dinoflagellates are haploid microalgae extensively studied with respect to their worldwide occurrence, toxicity, and capacity to become ecologically dominant. Studies on chromosomal segregation patterns suggested that dinoflagellate meiosis is unusual [14], occurring in a single step in which homologous unreplicated chromosomes in a diploid cell form pairs that are distributed among the haploid daughter cells. This type of meiosis differed from ordinary mitosis in a haploid cell only by the origin of the chromatid pairs that are split after metaphase. It was later established that meiosis in dinoflagellates generally proceeds by a more conventional two-step process but with a delay in the second division [15,16,17,18]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
