Dinoflagellates are haploid eukaryotic microalgae in which rapid proliferation causes dense blooms, with harmful health and economic effects to humans. cytometry, cell sorting, and Fluorescence In Situ Hybridization (FISH), we followed DNA content and nuclear changes in a populace of the harmful dinoflagellate that was induced to encystment. Our results first show that planozygotes behave like a populace with an encystment-independent division cycle, which is usually light-controlled and follows the same Light:Dark (T:Deb) pattern as the cycle governing the haploid mitosis. Resting cyst formation was the fate of just a small portion of the planozygotes created and was restricted to a period of strongly limited nutrient conditions. The diploid-haploid turnover between T:Deb 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 usually not haplontic, as previously considered, but biphasic, because individuals could undergo mitotic sections in both the sexual (diploid) and the asexual (haploid) phases. We also statement incomplete genome duplication processes. Our work calls for a reconsideration of the dogma of rare sex in dinoflagellates. Introduction Dinoflagellates are haploid microalgae extensively analyzed with respect to their worldwide event, toxicity, and capacity to become ecologically dominating. These organisms form an outstanding group among eukaryotes due to the many peculiarities of their physiology  and enormous genome, which may be as large as 185 Gb  and is made up of hundreds of chromosomes that lack both histones [3,4] and nucleosomes, but which are organized along a cholesteric crystal structure that ensures their maintenance in a permanently semi-condensed and visible state [5C7]. There are a bunch of dinoflagellate species explained to exhibit facultative sex [8,9]. Thus, under non-optimal conditions (generally related to nutrient deficiency) haploid vegetative cells differentiate into gametes, which partner and form diploid zygotes and, in change, benthic cysts, which are better able than vegetative cells to resist nerve-racking environmental conditions. After the cysts total their required dormancy period and if the environmental conditions are favorable for cyst germination, division of the germinated cell restores the haploid stage. However, because sexual processes in dinoflagellates are often cryptic and unstable, the comparative importance of sexuality in their life histories is usually ambiguous. Dinoflagellates are first and foremost haploid, because reductive sections are a characteristic of their zygotes. During these sections, the nucleus undergoes a highly specialized process, known as nuclear cyclosis [10,11], that results in quick chromosomal movement and is usually related to meiosis [12,13]. Early studies on chromosomal segregation patterns suggested that dinoflagellate meiosis is usually unusual , occurring in a single step in which homologous unreplicated chromosomes in a diploid cell form pairs that are then distributed among the PP242 haploid child cells. This type of meiosis differed from regular mitosis in a haploid cell only by the source of the chromatid pairs that are split after metaphase. However, it was later established that meiosis in dinoflagellates generally profits by a more standard two-step process but with a delay in the second division [15C18]. Nonetheless, this general model has been wondered. For example, zygotes often undergo planktonic division and thereby miss encystment. This behavior was first suggested by PP242 Uchida et al.  and subsequently confirmed at the nuclear level in several species belonging to the genera (at the.g., [20,21]). The haplontic model, presumed to apply to all dinoflagellates except for the diploid was recently shown to have another exception in . Accordingly, either the dinoflagellate life cycle cannot be explained in a single, general model or the model is usually more complex than originally formulated and must take into account cryptic sexuality. Sexuality is usually hard PP242 to induce in the laboratory, entails complex mating patterns (at the.g. ), and produces sexual stages that morphologically are almost identical to asexual stages (observe review in ). Therefore, in laboratory studies the sexual interactions of dinoflagellates GP9 that occur under natural conditions may be very easily overlooked (at the.g. [26,27]). In fact, recent studies of blooming dinoflagellate populations showed unexpected genetic variability in populations believed to be the result of predominantly asexual, mitotic growth (at the.g., PP242 [28C30]). In this study, we followed DNA content and nuclear/chromosomal changes through the process of sexual induction and encystment in a sexual laboratory populace of dinoflagellates using imaging.