allopolyploidization

Allopolyploidization is the formation of an allopolyploid, a polyploid organism whose chromosome sets derive from two or more distinct species. It typically results from interspecific hybridization followed by chromosome doubling, which creates a stable organism with multiple complete genomes from different species. This process can produce instant reproductive isolation and is a major mechanism of speciation in many plants.

Mechanism involves an initial hybrid between species that is often sterile because chromosomes from different genomes fail to pair properly during meiosis. Chromosome doubling or the formation of unreduced gametes can restore fertility by producing homologous chromosomes within each subgenome, while pairing between homeologous chromosomes remains suppressed or becomes stabilized over time. The resulting allopolyploid can undergo disomic inheritance, with proper chromosome pairing within each ancestral genome.

Genomic consequences include changes in gene expression, epigenetic reprogramming, and structural rearrangements. Subgenome dominance may occur, where one parental genome has greater influence on expression. Transposable elements can be activated, and overall genetic variation may increase, potentially enhancing adaptation to new environments.

Examples from crops illustrate the widespread impact of allopolyploidization. Bread wheat (Triticum aestivum) is a hexaploid with AABBDD genomes (2n=6x=42). Cotton (Gossypium hirsutum) is an AD tetraploid (2n=4x=52). Brassica napus, oilseed rape (AACC, 2n=4x=38), originated from hybridization between B. rapa (AA) and B. oleracea (CC). Allopolyploidization contrasts with autopolyploidy, which involves genome doubling within a single species.