Mechanical properties such as pressure, viscosity and substrate stiffness are inherent components of a cell's environment. Mammalian cells respond to such mechanical cues by altering their behavior. Mechano-responsive behavior is common across several cell types and across immortalized cells, stem cells, primary cells, and cancer cells. Multicellular structures like spheroids and organoids also respond to mechanical cues. The mechanisms by which cells sense and respond to mechanical cues are not fully understood. As all cells are ultimately products of evolution and given that mechanosensing is conserved across different cell types from different species, it is possible that mechanosensing is an evolved mechanism. Just as temperature and ambient chemistry often vary over the course of the life of a cell and its recent descendants, mechanical properties may be similarly variable. Therefore, we expect that mechanical properties of a cell's environment constitute a significant agent of natural selection; but this has never been investigated before to our knowledge. In this dissertation, we sought to test the hypothesis that controlled mechanical stiffness of the micro-environment can impose selection pressure over a genetically variable population of cells. We found that (1) Natural selection imposed by a novel soft substrate selected for genetic variants in the original population that increased fitness on the novel soft substrate. (2) Cell phenotypes and associated gene expression on soft substrates tended to "return to normal". (3) The pattern of plasticity in soft-selected lines was opposite to the ancestral pattern. (4) Different evolved lines achieved similar phenotypic outcomes by means of different underlying transcriptional architectures. These results are the first demonstration that mechanical stiffness of the microenvironment can cause cellular evolution. We also explored how self-organized clusters of cells adapt to mechanical perturbations in three-dimensional culture. We report the discovery that altering mechanical homeostasis alters tissue polarity through an extraordinary spatial rearrangement at the tissue level. Our results overall reveal potential new mechanisms by which mechanical changes could contribute to altered genetic composition and tissue structure in human diseases such as cancer.
