Two mitochondrial genome configurations found in a model green alga

The mitochondrial genome of a widely studied green alga may be more structurally complex than its longstanding textbook description suggests, according to a new preprint.

What the study found

He and colleagues used high-accuracy long-read sequencing to examine mitochondrial DNA in Chlamydomonas reinhardtii. In their bioRxiv preprint, they report two coexisting genome configurations: the previously described 15.8-kilobase version and an expanded 19-kilobase version. The latter has terminal inverted repeats that duplicate two genes, cob and nad4.

The authors report no evidence for the conventional telomeric structures previously described at the ends of the genome. Their single-molecule analysis also identified linked, multimeric DNA structures joined by a conserved 17-base sequence. Digital PCR measurements showed substantial differences in the relative abundance of the two configurations across the CC-5325 lineage and additional field-isolated strains.

Why it matters

The findings challenge the longstanding description of the alga’s mitochondrial genome as a single linear molecule capped by inverted telomeres. The authors propose that its structural features support a recombination-dependent replication model, potentially explaining how mitochondrial DNA could replicate without conventional telomeres or reverse transcriptase-like genes.

They also identified mitochondrial DNA segments in the nuclear genome. These segments can complicate amplification and measurement of mitochondrial configurations, making them important to distinguish in future analyses.

Limits and unknowns

The replication mechanism is a model supported by the reported structural features, not a demonstrated process. Finding no evidence for canonical telomeres is not proof that every possible telomeric structure is absent. The abstract does not provide sample sizes, detailed measurement results or information about the functional consequences of the configurations. The study is a preprint and has not been established here as peer reviewed.

Read the research

Authors: He, Z., Bachvaroff, T. R., Gallaher, S. D., Li, Y.

Published: 2026-10-30

Source basis: Based on the abstract only

Original 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.

This article is an AI-generated adaptation, not a substitute for the original research. Preprints may not have been peer reviewed; AI can make mistakes.