Strict maternal inheritance of chloroplast and mitochondrial genomes in Bryum argenteum

Organelle inheritance is predominantly maternal in most eukaryotes, yet quantitative estimates of paternal leakage-the low-frequency transmission of paternal cytoplasmic genomes-remain scarce outside seed plants. In bryophytes, the sister group to tracheophytes, maternal inheritance of chloroplasts and mitochondria has been confirmed qualitatively in a small number of species, but no study has quantified paternal leakage rates. We assembled the complete chloroplast (122,995 bp) and mitochondrial (101,373 bp) genomes of Bryum argenteum from a Beijing population. An intraspecific cross was performed between an Antarctic maternal genotype(ma1) and a Beijing paternal genotype(XA), generating seven independent capsules from which 21 F₁ gametophytes were deep-sequenced. De novo assembly of all 21 F₁ individuals recovered exclusively the maternal haplotype at all 464 chloroplast and 96 mitochondrial diagnostic sites. Quantitative allele frequency analysis of 363 chloroplast and 58 mitochondrial diagnostic SNPs across 8,841 independent tests detected no paternal-specific allele exceeding empirically defined noise thresholds(99th percentiles of maternal background: 2.50% for chloroplasts, 2.25% for mitochondria). Principal component analysis of 421 diagnostic SNPs clustered all F₁ individuals with the maternal parent (PC1 = 98.1% of variance). The paternal leakage frequency was conservatively estimated at < 0.1% (upper 95% confidence limit 0.034%). Slightly elevated paternal-type mitochondrial signals in F₁ progeny (median 0.18% vs. maternal background 0.04%; p = 0.042) were attributable to the faithful transmission of pre-existing maternal heteroplasmy, demonstrated by a strong maternal-F₁ correlation (Pearson's r = 0.643, p = 7.31 × 10⁻5, R2 = 0.413). These results deliver quantitative evidence for conserved uniparental organelle transmission in a non-seed plant, underscoring the ancestral nature of maternal inheritance during land plant evolution.