CELL-CYCLE STUDIES OF THE DINOFLAGELLATES GONYAULAX-POLYEDRA STEIN AND GYRODINIUM-UNCATENUM HULBURT DURING ASEXUAL AND SEXUAL REPRODUCTION

A microscope-photometer system was used to quantify the relative DNA content of individual cells of the dinoflagellates Gonyaulax polyedra Stein and Gyrodinium uncatenum Hulburt during asexual reproduction and, for the latter species, during sexuality as well. The cell cycles of these two dinoflagellates are distinctly different although both include discrete periods of DNA synthesis. In G. polyedra, DNA synthesis and cell division are tightly phased to restricted segments of the photocycle, separated in time by distinct gap phases. All cells that replicate their DNA and enter G2 divide during the next division interval. G1 is the stage occupied by the cells most of the time. In G. uncatenum, cell division is more broadly distributed and occurs throughout half the photocycle. Cell division is thus phased only to the extent that it is restricted to the same half of the photocycle each day. G1 is very brief as DNA synthesis begins immediately after cell division. G2 is the dominant stage through time since it includes cells about to divide as well as those that just divided. G. uncatenum is the first free-living photosynthetic dinoflagellate to exhibit a pattern of DNA synthesis immediately after mitosis. The tight phasing of G. polyedra division and the restriction of G2 to cells that must divide at the end of the dark period should make it possible to calculate the growth rate of this species in mixed natural assemblages using these techniques. It should also be possible to distinguish sexual stages (zygotes) from vegetative cells. Since G. uncatenum vegetative cells often have two complements of DNA, as do zygotes following gamete fusion, these stages could not be distinguished from each other on the basis of DNA measurements alone. However, these data in combination with cell counts were used to define the onset of sexuality and to show that the gametes of this species result from two rapid divisions. © 1990.