Brassica diversity through the lens of polyploidy: Genomic evolution, introgression, and homoeologous exchange

Polyploidy, or whole-genome duplication, is an important evolutionary process that has shaped the genomes and traits of many plants, including numerous important crops. The Brassica genus, which includes diverse vegetables and oilseeds, is a key model system for studying how polyploidy affects plant diversification and domestication. This review summarizes the current understanding of how multiple rounds of ancient and more recent polyploidization events laid the foundation for the wide diversity seen in Brassica. We discuss the key outcomes through which polyploidy facilitates the accumulation of genetic variation, including genomic buffering that enables mutation retention. Furthermore, we explore the significant roles of interspecies and interploidy introgression in introducing external genetic novelty. We highlight homoeologous exchange (HE) as a critical mechanism unique to allopolyploids, driving substantial genomic rearrangements including presence-absence variations and gene dosage alterations that directly contribute to significant phenotypic innovation and adaptation in Brassica. Together, these polyploidy-associated processes have led to the extensive range of genomic variations that shaped great morphological diversification in the domestication of Brassica. By integrating insights from genomics, genetics, and evolutionary biology, this review shows how polyploidy has been central to Brassica's success and agricultural value. We also suggest future research areas to better understand polyploid evolution and improve crop breeding.