Cell cycle control by timer and sizer in Chlamydomonas

Conservation of cell cycle control mechanisms is indicated by the presence of functionally homologous division control genes in unrelated yeasts and by the nonspecific action of oncogenes, but it remains uncertain what property of a growing cell results in the initiation of events leading to division. Response to a critical size is indicated by the longer growth period of smaller cells prior to division, which is consistent with deferment of division events until a minimum size is attained; however, in the same cell types faster growing cells are larger and this is more easily explained if division follows a timed period during which faster growing cells grow more, as is postulated for mammalian cells. Therefore, either time- or size-dependent controls might be the sole significant mechanism; we report here, however, that both controls do function in Chlamydomonas since cycle duration is under timer control and cell size determines the number of division rounds committed at the end of each cycle, and hence whether 2, 4, 8 or 16 daughter cells are formed.