Evolution toward larger cell size reduces internal cellular density

Abstract

Cellular density depends on the balance between cell volume and the amount of material contained within that volume. Cells generally maintain this balance during growth, across the cell cycle, and in response to environmental changes. But is density also maintained when cell size changes through evolution? We selected barcoded populations of Saccharomyces cerevisiae for increased cell size over approximately 160 generations and asked whether cellular material increased proportionally to preserve density. Evolved lineages became larger through multiple genetic routes, yet repeatedly converged on the same physiological outcome: larger evolved cells had lower internal density. Optical diffraction tomography showed that internal density declined as cell size increased, indicating that cellular material did not increase proportionally with cell size. Lower density was also observed in clonally propagated evolved isolates, indicating that it was a stable, heritable feature rather than a temporary response to selection. We next asked whether the same relationship extends over longer evolutionary timescales. Across published dry-mass and volume measurements from 68 species, larger cells contained more total material, but material again increased more slowly than volume. Consequently, cellular density declined with increasing cell size across species, paralleling the pattern observed during short-term experimental evolution. In sum, these results show that early adaptation to larger size can repeatedly uncouple volume expansion from material accumulation. Although cells found multiple genetic routes to become larger, volume repeatedly increased faster than cellular material, and that same scaling relationship is evident across species. Reduced density may therefore be a common consequence of evolving larger cell size.