The life history and cell cycle of Kryptoperidinium foliaceum, a dinoflagellate with two eukaryotic nuclei.

Kryptoperidinium foliaceum is a binucleate dinoflagellate that contains an endosymbiont nucleus of diatom origin. However, it is unknown whether the binucleate condition is permanent or not and how the diatom nucleus behaves during the life history processes. In this sense, it is also unknown if there is a sexual cycle or a resting stage during the life history of this species, two key aspects necessary to understand the life history strategy of this dinoflagellate. To answer these questions, life history and cell cycle studies were performed with the following results: (i) Kryptoperidinium foliaceum has a sexual cycle and in the dinoflagellate strains studied, the binucleate condition is permanent. Sexuality in the host was confirmed by the presence of fusing gamete pairs and planozygotes in clonal cultures (revealing homothallism), but signs of meiosis in the endosymbiont were not observed. The endosymbiont nucleus likely fuses first, because fusing gamete pairs were found to have two dinoflagellate nuclei but only one endosymbiont nucleus. After complete gamete fusion, the planozygotes had apparently normal endosymbiont and dinoflagellate nuclei. (ii) Asexual division studies using flow cytometry showed that the S phase in the endosymbiont (diatom) nucleus starts 6-8h later than in the host nucleus, but there was no evidence of mitosis in the former. (iii) Sexual and asexual cysts were formed in culture. Neither cysts from natural samples nor those formed in culture exhibited a dormancy period before germination.