Heritable gene expression variability governs clonal heterogeneity in circadian period

A ubiquitous feature of circadian clocks across life forms is its organization as a network of coupled cellular oscillators. Individual cellular oscillators of the network often exhibit a considerable degree of heterogeneity in their intrinsic periods. While the interaction of coupling and heterogeneity in circadian clock networks is hypothesized to influence clocks entrainability, our knowledge of mechanisms governing network heterogeneity remains elusive. In this study, we aimed to explore the principles that underlie inter-cellular period variation in circadian clock networks (clonal period-heterogeneity). To this end, we employed a laboratory selection approach and derived a panel of 25 clonal cell populations exhibiting circadian periods ranging from 22 h to 28 h. We report that while a single parent clone can produce progeny clones with a wide distribution of circadian periods, heterogeneity is not entirely stochastically driven but has a strong heritable component. By quantifying the expression of 20 circadian clock and clock-associated genes across our panel, we found that inheritance of different expression patterns in at least three clock genes might govern clonal period-heterogeneity in circadian clock networks. Furthermore, we provide preliminary evidence suggesting that epigenetic variation might underlie such gene expression variation.