Hexaploïd

Hexaploïd, or hexaploïde in French, refers to an organism that carries six complete sets of chromosomes. In terms of cytogenetics, a hexaploid typically has a somatic chromosome number of 2n = 6x, where x is the basic chromosome set. Hexaploidy is a form of polyploidy and is most frequently observed in plants; it occurs less commonly in animals.

Formation mechanisms: It can arise by autopolyploidy, when chromosome doubling occurs within a species, or by allopolyploidy, when three diploid progenitors hybridize in a way that combines their genomes, sometimes followed by chromosome doubling. The result is a genome composed of three subgenomes, often referred to as A, B, and D in crops like wheat.

Bread wheat, Triticum aestivum, is a classical example of a hexaploid. Its genome is designated AABBDD, reflecting its three ancestral diploid genomes and the doubling that produced the six sets of chromosomes. Common oats, Avena sativa, are also hexaploid, with 2n = 6x = 42 chromosomes.

Consequences for biology: Hexaploidy influences gene dosage, cell size, and metabolism, and it poses challenges for meiosis due to potential multivalent pairing. In many allopolyploids, pairing is stabilized by genetic mechanisms that promote regular homologous pairing. Hexaploidy can confer advantages in vigor and environmental tolerance, contributing to the agricultural value of hexaploid crops.

Detection and study: Polyploid status is determined by cytogenetic analysis, flow cytometry, and increasingly by genome sequencing and comparative genomics. Understanding hexaploidy helps explain trait variation, breeding behavior, and evolutionary history in polyploid lineages.