Epigenetic inheritance of elongated phenotypes between generations revealed by individual-cell-based direct observation

A cellular phenotype is considered to be determined not only by genetic information but also by convoluted information on past states of a cell and its ancestors, i.e. hysteresis. This 'hysteretic effect' forms the basis of epigenetic phenomena. To understand these phenomena by which cells transmit certain phenotypes to descendants, it is necessary to observe individual cells and compare the phenotypes of each between generations under stringently controlled environmental conditions. We, therefore, did an individual-cell-based differential assay using Escherichia coli as a model organism. We observed normal-sized isolated cells change into elongated phenotypes, and subsequently measured the transmission of their characteristics between generations. This change occurred when the final length of the normal cells exceeded their cell-length boundary, i.e., 10 νm with 5% probability. Once a cell became elongated, it divided unequally, producing two daughter cells; one was elongated and the other was normal. The elongated daughter transmitted the elongated phenotype to one lineage of the descendants by repeating unequal cell divisions with an average interdivision time half that of the normal phenotype, whereas the normal daughter retained normal phenotypic characteristics. The results suggest one possible non-genetic inheritance of cellular characteristics where phenotypic differences can only be inherited by geometrical information, independent of specific gene regulation.