allohexaploid

An allohexaploid is an organism with six complete sets of chromosomes (sixfold) derived from more than one species, classed as an allopolyploid. In an allohexaploid, the six chromosome sets are partitioned into three subgenomes that originated from distinct progenitor species, rather than six identical sets from a single species as in autopolyploids. Allohexaploids commonly arise through interspecific hybridization followed by chromosome doubling, enabling the genomes from different species to be maintained together in a single nucleus.

A prominent example is bread wheat, Triticum aestivum, a hexaploid with the genome constitution AABBDD. It is believed to have formed from hybridization events among three diploid ancestors: the A-genome donor (close to Triticum urartu), the B-genome donor related to the Aegilops speltoides lineage, and the D-genome donor, Aegilops tauschii. The resulting allohexaploid inherits traits from all three lineages, contributing to its agronomic value.

Meiotic behavior in allohexaploids can be complex because chromosomes from different subgenomes can pair as homologs or homoeologs. In bread wheat, the Ph1 locus helps ensure proper pairing between true homologs and minimizes pairing between homoeologs, stabilizing fertility and traits.

Allohexaploids occur in several plant groups, particularly among cereals and ornamentals, and can exhibit heterosis and novel trait combinations due to genome merging. Formation is typically through two successive events: hybridization between species followed by genome doubling, or the formation of unreduced gametes and subsequent fertilization.