A power law for cells

Darwin observed that multiple, lowly organized, rudimentary, or exaggerated structures show increased relative variability. However, the cellular basi s for these laws has never been investigated. Some animals, such as the nem atode Caenorhabditis elegans, are famous for having organs that possess the same number of cells in all individuals, a property known as eutely. But f or most multicellular creatures, the extent of cell number variability is u nknown. Here we estimate variability in organ cell number for a variety of animals, plants, slime moulds, and volvocine algae. we find that the mean a nd variance in cell number obey a power law with an exponent of 2, comparab le to Taylor's law in ecological processes. Relative cell number variabilit y, as measured by the coefficient of variation, differs widely across taxa and tissues, but is generally independent of mean cell number among homolog ous tissues of closely related species. We show that the power law for cell number variability can be explained by stochastic branching process models based on the properties of cell lineages. We also identify taxa in which t he precision of developmental control appears to have evolved. We propose t hat the scale independence of relative cell number variability is maintaine d by natural selection.