Models of cell division initiation in Chlamydomonas: A challenge to the consensus view

We develop and compare two models for division initiation in cells of the unicellular green alga Chlamydomonas reinhardtii, a topic that has remained controversial in spite of years of empirical work. Achieving a better understanding of C. reinhardtii cell cycle regulation is important because this species is used in studies of fundamental eukaryotic cell features and in studies of the evolution of multicellularity. C. reinhardtii proliferates asexually by multiple fission, interspersing rapid rounds of symmetric division with prolonged periods of growth. Our Model 1 reflects major elements of the current consensus view on C. reinhardtii division initiation, with cells first growing to a specific size, then waiting for a particular time prior to division initiation. In Model 2, our proposed alternative, growing cells divide when they have reached a growth-rate-dependent target size. The two models imply a number of different empirical patterns. We highlight these differences alongside published data, which currently fall short of unequivocally distinguishing these differences in predicted cell behavior. Nevertheless, several lines of evidence are suggestive of more Model 2-like behavior than Model 1-like behavior. Our specification of these models adds rigor to issues that have too often been worked out in relation to loose, verbal models and is directly relevant to future development of informative experiments.