Perturbation of chloroplast gene transmission in diploid and triploid zygotes of Chlamydomonas reinhardi by 5-fluorodeoxyuridine

Origins of eukaryotic sexual reproduction

Sexual reproduction is a nearly universal feature of eukaryotic organisms. Given its ubiquity and shared core features, sex is thought to have arisen once in the last common ancestor to all eukaryotes. Using the perspectives of molecular genetics and cell biology, we consider documented and hypothetical scenarios for the instantiation and evolution of meiosis, fertilization, sex determination, uniparental inheritance of organelle genomes, and speciation.

Chloroplast DNA methylation and inheritance in Chlamydomonas

When Chlamydomonas reinhardtii cells mate, a zygotic maturation program is activated, part of which leads to destruction of chloroplast DNA (cpDNA) from the mating type minus (mt-) parent, and, therefore, to uniparental inheritance of mating type plus (mt+) cpDNA. A long-standing model that explains the selective destruction of mt(-) cpDNA in zygotes invokes a methylation-restriction system. We tested this model by using the potent methylation inhibitor 5-aza-2'-deoxycytidine (5adc) to hypomethylate parental cpDNA and found that the pattern of cpDNA inheritance is altered by 5adc in a manner that is consistent with the model. Surprisingly, however, hypomethylated mt+ cpDNA is not destroyed in zygotes as the methylation-restriction model predicts it should be. Destruction of mt- cpDNA is also unaffected when the parental mt+ cpDNA is hypomethylated. Instead, loss of methylation affects the relative rates of replication of residual mt- cpDNA and mt+ cpDNA in germinating zygotes. The mode of action for 5adc on cpDNA replication in germinating zygotes may be via hypomethylation of mt+ cpDNA, but is also consistent with its action as a DNA-damaging agent. Interestingly, 5adc causes reduced cpDNA replication only in germinating zygotes, not in vegetatively grown cells, indicating that cpDNA replication is qualitatively different in these two stages of the life cycle. Our results demonstrate that methylation is not necessary for protection of the mt+ cpDNA in early zygotes and uncover a novel stage of the Chlamydomonas life cycle when replication of cpDNA is highly susceptible to perturbation. Our data support a model in which differential cpDNA replication in germinating zygotes is used as a mechanism to selectively amplify intact and properly methylated cpDNA molecules.

Detection of chloroplast DNA by using fluorescent monoclonal anti-bromodeoxyuridine antibody and analysis of its fate during zygote formation in Chlamydomonas reinhardtii

A monoclonal anti-bromodeoxyuridine antibody conjugated to fluorescein was used to detect the chloroplast nucleoids after specific incorporation of bromodeoxyuridine (BUdR) into the chloroplast DNA of Chlamydomonas reinhardtii. The incorporation of BUdR was enhanced by simultaneous treatment with fluorodeoxyuridine (FUdR). The method was applied to analyze the fate of chloroplast DNA in zygotes resulting from mating between BUdR-treated gametes (mt+ or mt-) and untreated gametes of opposite mating-type. In crosses between wild-type strains, the nucleoids of mt+ origin remained in the large majority of zygotes whereas those of mt- origin most often disappeared within the first hours following copulation. In crosses of the type mat-3 mt+ x wild-type mt- (the mat-3 mutation permits a high transmission of chloroplast genes from the mt- parent), the nucleoids of mt- origin were generally not eliminated which indicates that the mat-3 mutation prevents the selective destruction of paternal chloroplast DNA in the zygote.