Methyl methanesulfonate mutagenesis of synchronized Chlamydomonas

Methyl methanesulfonate (MMS) mutagenesis of Chlamydomonas reinhardtii at different stages of the synchronous cell-cycle revealed the following results. (1) Induction of phenotypically distinct Mendelian (nuclear), str-50 and non-Mendelian (chloroplast) str-500P, streptomycin resistant mutants was relatively high during the first portion of the cell-cycle when chloroplast DNA replication is known to occur. (2) A second and more pronounced interval of enhanced Mendelian, str-50 mutant induction was observed near the middle of the cell-cycle when the initial stages of nuclear DNA replication occur. Induction of non-Mendelian, str-500P mutants was inconsistent during this period. (3) The incidence of mutants from a second phenotypically distinct class of non-Mendelian streptomycin-resistant mutants (str-500D) was not increased over control levels at any stage of the cell-cycle examined. It is concluded that MMS, like N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), may not be the most suitable general mutagen for this alga because its enhanced mutagenesis of cells in the nuclear S phase could result in multiple closely linked mutations.