**Figure 3 | Taxon-wide recurrence and positional organization of CG and UA
dinucleotide biases in mature miRNAs.** **a,** Overall CG and UA frequencies
were compared with two permutation null models across ten mutually exclusive
taxonomic groups. The within-sequence null shuffles nucleotides separately
within each miRNA, whereas the position-wise, length-stratified null
redistributes nucleotides within sequence-length strata while preserving the
position-wise nucleotide composition of each stratum. Relative depletion was
calculated as 100 × (null mean − observed)/null mean. The cladogram is
schematic and branch lengths are not scaled. **b,c,** Position-specific
observed-minus-null frequency differences for CG (**b**) and UA (**c**) under
the position-wise, length-stratified null. Each taxon uses the same vertical
scale (2 percentage points indicated); start positions are zero-based. The
shaded band in **c** marks the 5′-terminal UA start position. **d,** UA at start
position 0 compared with both null models across taxa; values at right give the
observed UA frequency at that position. Circles denote the within-sequence
null and diamonds the position-wise, length-stratified null. Filled symbols
indicate Benjamini–Hochberg-adjusted q ≤ 0.05 and open symbols q > 0.05.
Statistics are based on 10,000 permutations per null model.

**Interpretive boundary.** The panel demonstrates recurrence and
taxon-associated variation in the estimated effects. It is not, by itself, a
formal test of an ordered evolutionary trend, because the displayed taxonomic
groups are not independent phylogenetic replicates.
