Functional Conservation Amid Extreme Divergence in Freshwater SAR11 Hypervariable Region 2

Freshwater Fontibacterium HVR2 sequences from Lake Biwa and Lake Zurich show high within-population diversity yet include conserved HVR2 pairs across continents.

• Why it matters: Prior work on SAR11 HVR2 focused mainly on marine clades, leaving freshwater patterns and whether HVR2 retains functional configurations unclear.
• What they did: Single-cell genomics recovered 176 Fontibacterium HVR2 sequences across multiple Lake Biwa years, with comparative analyses against 145 marine HVR2 sequences and gene-family conservation across habitats.
• The result: Conserved HVR2 sequence pairs and shared gene families, plus tRNA gene placement near 5S boundaries, suggest horizontal transfer and functional equivalence of HVR2 across marine and freshwater SAR11.

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