Phylogenetic Comparative Analysis of Single-Cell Transcriptomes Reveals Constrained Accumulation of Gene Expression Heterogeneity during Clonal Expansion

In the same way that a phylogeny summarizes the evolutionary history of species, a cell lineage tree describes the process of clonal expansion, in which gene expression differences between cells naturally accrue as a result of stochastic partitioning and imperfect expression control. How is functional homeostasis, a key factor in the biological function of any population of cells, maintained in the face of such continuous accumulation of transcriptomic heterogeneity remains largely unresolved. To answer this question, we experimentally determined the single-cell transcriptomes and lineage relationships of up to 50% cells in single-HEK293–seeded colonies. Phylogenetic comparative analyses of the single-cell transcriptomes on the cell lineage tree revealed three lines of evidence for the constrained accumulation of transcriptome heterogeneity among cells, including rapid saturation of transcriptomic heterogeneity upon four cell divisions, reduced expression differences within subtrees closer to expression boundaries, and cofluctuations among genes. Our analyses showcased the applicability of phylogenetic comparative methods in cell lineage trees, demonstrated the constrained accumulation of transcriptomic heterogeneity, and provided novel insight into the functional homeostasis of cell populations.