A new bioinformatic analysis highlights a subpopulation of mitochondria with highly methylated DNA present in oocytes, early embryos and blood.

Whole-genome EM-seq in bovine oocytes, early embryos, and blood reveals a conserved U-shaped mtDNA methylation distribution with a small subset of highly methylated reads.

• Why it matters: Average mtDNA methylation is often reported as very low, leaving conflicting observations unresolved and masking molecule-to-molecule heterogeneity.
• What they did: The study analyzed multiple bovine datasets from whole-genome EM-seq, separating nuclear and mitochondrial reads and quantifying methylation per read at specific cytosines or across all cytosines.
• The result: Methylated-read proportions rose from oocytes to 8–16 cell embryos, then declined in blastocysts and blood, and heavily methylated reads (>50% methylation) showed conserved profiles across the 16 kb mitochondrial genome.