Allopolyploidization-driven short-term evolution of miRNAs in Brassica A genome

Allopolyploidization is a major driver of plant evolution and crop improvement, influencing both adaptation and diversification. MicroRNAs (miRNAs), 20-24 nt endogenous noncoding RNAs, regulate post-transcriptional gene expression and influence diverse biological processes. MiRNAs regulate a variety of agronomic traits and represent an important genetic resource for crop genetic improvement. While prevalent in plant evolution, the short-term (< 10,000 years) impact of allopolyploidization on miRNA evolution remains unclear. This study systematically compared miRNAs in the A genomes of Brassica rapa, Brassica juncea, and Brassica napus to reveal the short-term effects of allopolyploidization on miRNAs. The results showed that allopolyploidization caused loss of over half of the miRNAs in the A genomes of B. juncea and B. napus and accelerated miRNA cluster loss. The subgenome dominance (LF > MF1/MF2) resulting from ancient whole-genome triplication persisted post-allopolyploidization. Following allopolyploidization, the nucleotide divergence of miRNAs did not change significantly, and the maximum nucleotide divergence was only 0.11. Multi-copy miRNA retention rates differ between B. juncea and B. napus, potentially due to the influence of B and C genomes. MiRNA retention was affected by flanking protein-coding genes, with those adjacent to multi-copy protein-coding genes more likely retained as multiple copies post-allopolyploidization. Retained single miRNAs may form miRNA clusters via tandem duplication events. Additionally, homoeologous exchanges may affect the protein-coding genes flanking miRNAs. These findings indicated that short-term allopolyploidization significantly affected miRNA retention in Brassica A genome, providing new insights into allopolyploidization impacts on miRNA evolution.