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Recent investigations have revealed hitherto unsuspected roles for small, non-coding RNA molecules and regulated pre-mRNA splicing during meiosis in fission yeast.

In the fission yeast Schizosaccharomyces pombe , nutritional reduction of growth rate by supplying poor nitrogen, carbon or phosphate sources causes a decrease in cell size. The effect on cell division following three different nutritional shifts-up has been investigated. In all cases, about 20% of the cells divide at the original cell length, and then cell division stops for a period. Cell division then resumes at the new faster rate, cell length at division being characteristic of the new medium. Further investigation reveals that the first effect of the shift is to inhibit nuclear division rapidly and completely. These results are strongly suggestive of the operation of a cell size requirement for entry into nuclear division. The cell size necessary for nuclear division is set, or modulated, by the prevailing growth conditions. This model is confirmed by a nutritional shift-down, where nuclear division and cell division are stimulated after the shift. Cell length at division falls rapidly until the new shorter length is attained, when a new steady state is assumed at a slower growth rate. The control system is compared with that in bacteria, and its implications for various models proposed for the control of timing of mitosis are discussed.

Strains of Schizosaccharomyces pombe carrying the wee 1 mutation divide at a reduced cell size compared with the wild-type. In this paper, we investigate the mechanism which determines the time of division and cell size at division in wee 1 strains, using three experimental approaches. The evidence suggests that the wild-type control (a cell size control over entry into nuclear division) is absent in wee 1 strains. Instead, a mechanism operates which comprises a cell size control over the initiation of S phase plus a minimum incompressible period in G2 (“timer”) from S phase to nuclear division. The elements of this second control mechanism exist in wild-type cells, though the control is not normally expressed. In particular, the G2 interval in wild-type cells is normally longer than that in wee 1 cells, but can be reduced to this minimum value by delaying S phase. Thus there are two independent controls over entry into nuclear division, one of which operates in wild-type, and the other in wee 1 cells.

One of the most exciting aspects of scientific research is the convergence of several apparently quite diverse types of investigation. Such events are rare, and the resulting cross-fertilization demands that findings derived from different areas are substantially reassessed. A handful of recent papers 1–6 , each concerned with control of the initiation of mitosis, have brought together observations from cell types that were previously regarded as possessing quite different control systems.