EMS and UV-light-induced colony sectoring and delayed mutation in Chinese hamster cells

Purpose: To review studies of mutagen-induced colony sectoring which demonstrate that UV light and EMS produce delayed mutational events in Chinese hamster ovary cells. Methods and results: Since the late 1940s, it has been known that the treatment of a single bacterial or yeast cell with mutagenic agents produces complete mutant colonies (pures) and colonies composed of both mutant and non-mutant cell types (mosaics) with various sectored patterns. A similar sectoring phenomenon has been observed in Chinese hamster ovary cells (CHO) using the DNA alkylating agent ethyl methane sulphonate (EMS) or ultraviolet light. However, unlike bacteria and yeast, a significant fraction of CHO mutant colonies contained sectors of less than 1/2; i.e. 1/4, 1/8 and 1/16 sectors, suggesting a delayed production of mutations. Using various colony-replating approaches, it was found that these mutagenic agents produced the ratio of mutant to wild-type cells expected for a delayed mutational process which produces mutant even...