Diploidization

Diploidization is the evolutionary process by which a genome that has become polyploid returns to a diploid-like state in terms of meiotic behavior, gene retention, and inheritance. It follows events of whole-genome duplication or polyploid speciation and results in disomic chromosome pairing and a reduction of genetic redundancy over time.

Mechanisms of diploidization include structural genome rearrangements that suppress pairing between homeologous chromosomes, biased loss of duplicated genes (fractionation), and the emergence of subgenome dominance where one ancestral genome retains more genes and activity. Epigenetic silencing and changes in gene expression reduce the functional impact of redundant genes. Over time, essential genes are retained in duplicate while others are lost or silenced, and recombination between homeologs declines, promoting stable, diploid-like meiosis.

Diploidization occurs in both autopolyploids and allopolyploids, but the dynamics differ. Allopolyploids, formed from divergent genomes, more readily achieve disomic inheritance due to reduced recombination between distinct subgenomes, whereas autopolyploids must overcome multivalent interactions. Examples include the stabilization of meiosis in bread wheat (hexaploid Triticum aestivum) and in Brassica napus (allotetraploid), as well as paleopolyploidy in maize and many other angiosperms.

Timescales span millions of years, and diploidization shapes genome structure, gene expression, and evolutionary trajectories. Understanding diploidization informs studies of plant breeding, genome evolution, and the interpretation of ancient polyploidy events.