Life-Cycle of Emiliania huxleyi

John Green
Plymouth Marine Laboratory
Citadel Hill
Plymouth PL1 2PB, UK.

Sexual reproduction in all species of plants and animals is characterised by the fusion of nuclei from specialised cells (gametes - e.g. sperm and ova in mammals), derived from the parental organisms. The cell that results from the fusion of two gametes develops subsequently to form a new individual. The parental cells are diploid, having twice as much DNA as the gametes, which are haploid. In many organisms, the haploid and diploid stages of the life-cycle are different in form. Thus, in flowering plants, the male haploid stage consists of the pollen grain, and the female haploid stage consists of the embryo sac. All other parts of the familiar flower-bearing plant are diploid, and this diploid stage is the only stage that can lead an independent existence. However, within the algae, haploid and diploid generations may be equally well-developed and able to exist independently, each reproducing vegetatively (i.e. the clonal offspring having identical DNA, and the same amount of DNA as the parent). In such cases, the haploid and diploid stages may be similar or different in form.

It has been known for some time that the the life-cycle of Emiliania huxleyi involves several cell types (Klaveness, 1972) that can maintain themselves by vegetative reproduction. They include the non-motile coccolith-bearing cell (the C-cell; Fig. A), naked non-motile cells (N-cells), and motile scale-bearing swarmers (S-cells; Figs B, C). How they are related genetically is not certain, though early evidence suggested that the S-cells may be haploid relative to the C-cell (Klaveness, 1972).

(A): non-motile C-cell of Ehuxleyi showing the investment of overlapping coccoliths (scale bar = 1.0 micron); (B): scale-bearing cell (S-cell) of Ehuxleyi (scale bar = 1.0 micron); (C): body-scales from an S-cell of Ehuxleyi (scale bar = 0.2 micron). (SEMs from John Green)

Recent work has concentrated on elucidating this problem by measuring relative levels of DNA using flow cytometric analysis of fluorochrome-stained DNA in the different cell types (Green et al., 1996). In this technique, the flow cytometer measures the fluorescence produced by the stained DNA in individual cells when illuminated using light at particular wave-lengths. The fluorescence produced is proportional to the DNA-content of the cells. It was successfully demonstrated that the motile S-cells have a DNA level half that of the coccolith-bearing cells and are, therefore, haploid relative to the C-cells (Fig. D). This strongly suggests that Ehuxleyi does indeed have a life-cycle involving a sexual stage with the S-cells acting as gametes, though they themselves can reproduce vegetatively. However, fusion of gametes as part of sexual reproduction has not yet been observed. It is also important to determine what initiates the changes between the various cell forms; for example, the N-cells and S-cells may arise spontaneously from C-cells in culture, but the exact triggering conditions are not yet known. Such information is urgently needed in order to understand the way that the Ehuxleyi life-cycle operates in the oceans, and in understanding how and why blooms of this widespread and cosmopolitan organism develop.

(D): Relative DNA content of Ehuxleyi cells. (a): peaks from a culture containing C-, N- and S-cells. The principal peaks are at 30 and 60 units of DNA, with smaller peaks at 90 and 120. (b): in a culture containing principally S-cells, the main peaks are again at 30 and 60 units of DNA. (c): in a culture of C-cells only, the principal peaks are at 60 and 120 units. (d): this culture, used as a reference clone, also contained only C-cells with the main peak at 60 units and a small peak at 120 units.
It is presumed that 60 units represents a diploid complement of DNA for one cell; 30 units indicates a gamete and 120 units comes from a cell just before binary fission, or just before it releases gametes. In addition, 60 units could also represent a gamete about to divide to form 2 gametes. (Diagram by Glen Tarran)

For further details of the work above, see Green et al. (1996) and for preliminary alternative explanations of the DNA complements in different Ehuxleyi cell types, see Green et al. (1996) and Medlin et al. (1996). For a general discussion on life-cycles within the algal division Haptophyta, see Billard (1994).


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