An Arabidopsis thaliana Gene That Predicts a Protein Belonging to the MSH2 Subfamily of Eukaryotic MutS Homologs

In contrast to multicellular animals, plants lack a reserved germ line; their gametes are formed late in their growth cycles by differentiation of somatic meristematic cells. Typically, the somatic precursors of gametophytes have divided many times, potentially subjecting their genomes to multiple rounds of spontaneous or environmentally induced mutagenesis (Walbot, 1985). However, plants do not seem to show extraordinarily high mutation rates. For example, long-lived trees presumably produce gametes from somatic cells that are themselves the products of many annual cycles of mitotic growth. Nevertheless, their mutation rates per zygote-to-meiosis generation, averaged over long reproductive lives, are only one order of magnitude or so higher than those of annuals (Klekowski, 1997). How do plants combat the threats to genomic stability posed by somatic mutation? Mechanisms of selection against less-fit somatic cells during growth and development have been reviewed extensively by Klekowski (1988). Diploid meristematic cells that acquire even partially dominant deleterious mutations may drop out of the actively dividing pool, which is termed "diplontic selection." Additional sieving out of recessive mutations may occur when haploid cells compete to form sperm or eggs. However, Klekowski has suggested that these mechanisms alone are not powerful enough to protect plants against rapid accumulation of extraordinary mutational loads (Klekowski, 1988, 1997). Genomic-fidelity functions must therefore be