SEXUAL REPRODUCTION AND HYPNOZYGOTE FORMATION IN THE FRESHWATER DINOFLAGELLATE PERIDINIUM BIPES STEIN

The optimum growth condition for Peridinium bipes in synthetic MCV medium was at 18℃, pH 7.4 and 5000 lx. Under this condition, the doubling time was about 6 days. Sexual reproduction was induced by transferring vegetative cells into nitrogen-free MCV medium. The maximum frequency of gamete induction was achieved at late lag stage and late exponential growth stage of batch culture, and the rate of hypuozygote formation was as high as 70%. The optimum condition for zygote formation was almost the same as that for growth. Mating types of 70 clones were identified. Two of them were homothallic. Forty eight clones were heterothallic; among these, 23 were "plus" and 25 "minus". In the remaining 20 clones, sexual reproduction was not observed. During the process of hypnozygote germination, one to several "tubular structure" appeared in the space between mesospore wall and endospore wall. Obviously, this structure could help the pre-meiocyte rupturing the thick mesospore wall by offerring mechanical force.