Molecular mechanism of cytoplasmic inheritance: Active digestion of mt- cpDNA as a driving force for maternal inheritance in Chlamydomonas reinhardtii

Abstract

Summary: The non-Mendelian inheritance of organelle genes is a phenomenon that is common to almost all sexual eukaryotes. In the isogamous alga Chlamydomonas reinhardtii, chloroplast(cp)genes are transmitted from the mating type plus(mt+)parent. In this study, the preferential disappearance of fluorescent cp nucleoids of the mating type minus(mt-)parent in living young zygotes was observed. To study the change in cpDNA molecules during this process, the cpDNA of mt+ and mt origin was labeled separately with bacterial aadA gene sequences. Then, a single zygote with or without cp nucleoids was obtained using the optical tweezers, and investigated by nested PCR. This demonstrated the active digestion of mt cpDNA during the preferential disappearance of mt cp nucleoids. To identify factors that cause the active digestion, nuclease activities in C. reinhardtii cells were surveyed. Consequently, a Ca2+-dependent nuclease(NUC-C)that is detected only in mt+ gametes was identified. Further analysis of NUC-C activity in chloroplasts of young zygotes detected NUC-C activity in mt chloroplasts60-90min after mating, which precisely corresponds to the timing of preferential digestion. These results indicate that the maternal nuclease NUC-C is the driving force for the maternal inheritance of cpDNA, which is achieved by the preferential digestion of mt cpDNA in young zygotes.