Two genomes in one mitochondrion: mitochondrial heteroplasmy and mitogenome dynamics in Chlamydomonas reinhardtii

Abstract

The mitochondrial genome of the model green alga Chlamydomonas reinhardtii has long been described as a 15.8-kb linear molecule capped by inverted telomeres. By leveraging high-accuracy long-read sequencing to overcome limitations associated with conventional genome algorithm biases, we demonstrated that this classical paradigm was incomplete. Our analyses provided no evidence for canonical telomeric structures, instead revealed widespread structural heteroplasmy characterized by the coexistence of two distinct mitochondrial genome configurations. In addition to the historical 15.8-kb configuration (designated as config 2), we identified an expanded 19-kb configuration (config 1) containing 4,001-bp terminal inverted repeats that duplicate the cob and nad4 genes. Single-molecule long-read profiling captured these configurations as a dynamic network of multimeric mitochondrial DNA concatemers bridged with a conserved 17-bp homologous motif. Together, these structural features support a recombination-dependent replication (RDR) model that could enable mitochondrial DNA replication in the absence of conventional telomeric structures and reverse transcriptase-like (rtl) genes. We further identify widespread nuclear mitochondrial DNA segments (NUMTs), which complicate targeted amplification and quantification of these mitogenome configurations. Specifying NUMTs enabled configuration-specific digital PCR measurements across the CC-5325 lineage and additional field-isolated Chlamydomonas strains, revealing substantial variation in the relative abundance of the two mitogenome configurations. These findings revise a four-decade-old model of the C. reinhardtii mitogenome and provided a new framework for investigating mitochondrial genome architecture, structural heteroplasmy, and replication dynamics in photosynthetic eukaryotes.