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Guanine and cytosine (GC) content is a fundamental component of genetic diversity and essential for phylogenetic analyses. However, the GC content of the ribosomal internal transcribed spacer 2 (ITS2) remains unknown, despite the fact that ITS2 is a widely used phylogenetic marker. Here, the ITS2 was high-throughput sequenced from 29 Corydalis species, and their GC contents were comparatively investigated in the context of ITS2’s characteristic secondary structure and concerted evolution. Our results showed that the GC contents of ITS2 were 131% higher than those of their adjacent 5.8S regions, suggesting that ITS2 underwent GC-biased evolution. These GCs were distributed in a heterogeneous manner in the ITS2 secondary structure, with the paired regions being 130% larger than the unpaired regions, indicating that GC is chosen for thermodynamic stability. In addition, species with homogeneous ITS2 sequences were always GC-rich, supporting GC-biased gene conversion (gBGC), which occurred with ITS2’s concerted evolution. The RNA substitution model inferred also showed a GC preference among base pair transformations, which again supports gBGC. Overall, structurally based GC investigation reveals that ITS2 evolves under structural stability and gBGC selection, significantly increasing its GC content.

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