Polyploidy

Definition

The condition of possessing more than two complete sets of homologous chromosomes in somatic cells. Polyploidy arises through whole-genome duplication, either via autopolyploidy (duplication of a single genome) or allopolyploidy (hybridization between species combined with chromosome doubling). While most animals are strictly diploid, polyploidy occurs sporadically in insects—particularly in parthenogenetic lineages such as some stick insects (Phasmatodea) and weevils (Curculionidae)—and has been experimentally induced in silkworms and other economically important species. In arachnids, polyploidy is documented in certain mites and ticks with thelytokous reproduction. Polyploid individuals typically exhibit increased cell size and altered physiological traits; in arthropods, this often correlates with gigantism or modified developmental rates. The condition is generally stable in plants but frequently lethal or sterile in animals due to disrupted sex chromosome dosage and meiotic pairing failures.

Etymology

From Greek poly- (many) + ploid (fold, from ploos, fold) + -y (condition), referring to multiple chromosome sets.

Example

Certain parthenogenetic populations of the stick insect Bacillus atticus are triploid (3n), having originated through hybridization between diploid sexual species, allowing these all-female lineages to persist through ameiotic reproduction that bypasses the meiotic problems typically associated with odd ploidy levels.

Synonyms

• whole-genome duplication
• chromosome doubling

Related Terms

• diploidy
• Haplodiploidy
• aneuploidy
• Parthenogenesis
• thelytoky
• autopolyploidy
• allopolyploidy
• karyotype
• chromosome number
• gynogenesis

Usage Notes

Distinguished from aneuploidy, which involves gain or loss of individual chromosomes rather than whole sets. In entomological literature, 'polyploidy' sometimes appears loosely for any elevated chromosome number; specialists reserve it for exact multiples of the basic (x) chromosome set. Haplodiploidy in Hymenoptera (diploid females, haploid males) is not polyploidy. Induced polyploidy in silkworm breeding produces tetraploid (4n) strains used for genetic mapping, but these typically require artificial maintenance due to reduced fertility.