Regulation of the cell cycle is a crucial component of development, and has been linked to the execution of cell fate decisions in a wide range of developmental contexts. Although many of the molecular components involved in cell cycle progression have been identified, how these proteins are regulated and how their distribution and abundance can influence cell fate remains unclear. In this issue (p. 3119), Matthew Michael identifies cyclin B3 (CYB-3) as a key regulator of cell cycle timing in the developing C. elegans embryo. Using RNAi to knockdown CYB-3, Michael demonstrates that in the one-cell embryo CYB-3 controls not only mitotic entry but S-phase entry as well – a dual-action that is unique among cyclins. At the two-cell stage, the author shows that CYB-3 is asymmetrically distributed in a par-dependant manner such that somatic precursor cells inherit ∼2.5-fold more CYB-3 than do their germline precursor sister cells. The author uses maternal strains with varying copy numbers of cyb-3 to show how variations in the level of CYB-3 can affect the speed and synchrony of the cell cycle at the two-cell stage, suggesting a novel role for CYB-3 in regulating asynchronous cell cycling in the developing embryo. Together, these data advance our understanding of how the timing of cell division is differentially regulated in the early embryo.
