Understanding how population-size homeostasis emerges from stochastic individual cell behaviors remains a challenge in biology.^{1,2,3,4,5,6,7} The unicellular green alga Chlamydomonas reinhardtii (Chlamydomonas) proliferates using a multiple fission cell cycle, where a prolonged G1 phase is followed by $n$ rounds of alternating division cycles (S/M) to produce $2^n$ daughters. A “Commitment” sizer in mid-G1 phase ensures sufficient cell growth before completing the cell cycle. A mitotic sizer couples mother-cell size to division number ($n$) such that daughter size distributions are uniform regardless of mother size distributions. Although daughter size distributions were highly robust to altered growth conditions, ∼40% of daughter cells fell outside of the 2-fold range expected from a “perfect” multiple fission sizer.^7,8 A simple intuitive power law model with stochastic noise failed to reproduce individual division behaviors of tracked single cells. Through additional iterative modeling, we identified an alternative modified threshold (MT) model, where cells need to cross a threshold greater than 2-fold their median starting size to become division-competent (i.e., Committed), after which their behaviors followed a power law model. The Commitment versus mitotic size threshold uncoupling in the MT model was likely a key pre-adaptation in the evolution of volvocine algal multicellularity. A similar experimental approach was used in size mutants mat3/rbr and dp1 that are, respectively, missing repressor or activator subunits of the retinoblastoma tumor suppressor complex (RBC). Both mutants showed altered relationships between Commitment and mitotic sizer, suggesting that RBC functions to decouple the two sizers.

n slate-walked=“e2585745-0354-4aa9-9396-e4f216ff1908”>了解种群大小稳态如何从随机单个细胞行为中出现，仍然是生物学中的一个挑战。^1234567 单细胞绿藻莱茵衣藻（衣藻）使用多裂变细胞周期增殖，其中延长的 G1 期之后是 $n$ 轮交替分裂周期 （S/M） 以产生 $2^n$ 个女儿。G1 中期的“Commitment”分子量测定器可确保在完成细胞周期之前充分生长细胞。有丝分裂分子量测定器将母细胞大小与分裂数 （$n$） 耦合，这样无论母细胞大小分布如何，子细胞大小分布都是均匀的。尽管子细胞大小分布对改变的生长条件具有高度稳健性，但 ∼40% 的子细胞超出了“完美”多重裂变测定器预期的 2 倍范围。⁷⁸ 具有随机噪声的简单直观幂律模型无法再现跟踪单个细胞的个体分裂行为。通过额外的迭代建模，我们确定了另一种修改后的阈值 （MT） 模型，其中细胞需要超过其中位起始大小的 2 倍以上的阈值才能成为具有除法能力的（即 Committed），之后它们的行为遵循幂律模型。 MT 模型中的承诺与有丝分裂大小阈值解偶联可能是 volvocine 藻类多细胞性进化的关键预适应。类似的实验方法用于大小突变体 mat3/rbr 和 dp1，它们分别是视网膜母细胞瘤肿瘤抑制因子复合物 （RBC） 的缺失阻遏因子或激活因子亚基。两个突变体都显示 Commitment 和有丝分裂大小调整器之间的关系发生了变化，表明 RBC 功能使两个调整大小器解耦。