The significance of sexuality and cyst formation in the life-cycles of four marine dinoflagellate species
Abstract: Sexuality and cyst formation were studied in the marine dinoflagellate species Gymnodinium catenatum Graham, Gymnodinium nolleri Ellegaard ? Moestrup, Alexandrium catenella (Whedon et Kofoid) Balech, and Lingulodinium polyedrum (Stein) Dodge. The study included the morphological characterization of sexual stages and processes, the time and description of meiosis, the study of the length and regulation of the mandatory dormancy period of resting cysts, and the relationships among parental strains and cyst progeny at phenotypic and molecular levels. In the species A. catenella, the effect of nutritional factors on the encystment and excystment of in culture were analysed. The mobile zygotes were observed to encyst significantly more in replete medium than in media depleted or half-depleted in nitrate and phosphate levels. On the other hand, the resting stage germinated faster placed in seawater than in medium replete or half-replete in nitrate and phosphate levels. For the species L. Polyedrum was described the previously unknown sexual cycle, which revealed the existence of two kinds of zygotes, since the planozygotes encysted either as a short-lived (<4 days), or as a long-lived (several months) cyst form. The sexual cycle and mating type behaviour of G. nolleri was also reported. A Principal Component Analysis provided evidence for a pattern of inheritance of the dormancy period in cysts. Furthermore, the inheritance of a short dormancy period, dominant over long and medium periods, was shown through two subsequent generations of cysts. The PCA analysis further indicated that there was an inverse relationship among genetic distance between compatible strains (calculated by means of an AFLP analysis) and viability of the offspring, and among cyst size and cyst production. In two of the dinoflagellate species (G. nolleri and G. catenatum), which do not undergo nuclear cyclosis, a meiotic process was described for the first time, that was linked to the viable division of the mobile zygote in a cyst-producing species. In both species, the encystment of the planozygote was modulated by external nutritional factors, which also affected the viability and morphology of the resting cyst. Two different mating systems were observed in G. catenatum (homothallism and complex heterothallism).The RAPD-AFLP technique found that there was intra-clonal genetic variation in G. catenatum strains, though the degree was inversely proportional to the homothallic behaviour. This fact might explain the important changes that the sexual behaviour of the siblings underwent with time, and the variable response in both cyst production and progeny viability of these strains.
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