Differential methylation of chloroplast DNA regulates maternal inheritance in a methylated mutant of Chlamydomonas

Abstract

In Chlamydomonas, the maternal inheritance of chloroplast genes correlates with the differential methylation of chloroplast DNA (chlDNA) in females (mt+) but not in males (mt-). Previous studies have supported the methylation-restriction model in which the maternal transmission is accounted for by the differential methylation in gametes which protects female but not male chlDNA from degradation during zygote formation. In the mutant me-1 chlDNA of vegetative cells of both mating types is heavily methylated even before gametogenesis; nonetheless, maternal inheritance occurs in mutants as in wild type. To investigate the mechanism of maternal inheritance in the me-1 mutant, restriction fragment patterns were compared after agarose gel electrophoresis of chlDNA from mutant vegetative cells and gametes with those from wild type, by using a set of 32 restriction enzymes of which 17 were methylation-sensitive in this system. Additional methylation occurs during gametogenesis in the mutant female (mt-+) but not in the corresponding male (mt-). Thus, gamete-specific, mating-type-specific methylation occurs in the me-1 mutant as in the wild type, consistent with the methylation-restriction model. In the me-1 mutant, gametic methylation occurs on a background of vegetative cell methylation not present in wild-type cells and irrelevant to the regulation of chloroplast inheritance. Comparison of the me-1 mutation with the mat-1 mutation [Sager et al. R., 1981] provides evidence for the existence of 2 different chlDNA methylation control systems: mat-1, linked to the mating type locus and regulating the mating-type-specific methylation that correlates with maternal inheritance, and me-1, unlinked to the mating type locus and unrelated to the regulation of maternal inheritance.