Polyploidy

Polyploidy is a genetic condition in which an organism possesses more than two complete sets of chromosomes. It is especially common in plants and has played a major role in plant evolution, diversification, and adaptation, though it also occurs in some animal groups.

There are two main routes to polyploidy. Autopolyploidy arises when an individual inherits additional chromosome sets from the same species, often through unreduced (diploid) gametes or genome duplication during mitosis. Allopolyploidy results from hybridization between two distinct species followed by chromosome doubling, giving a stable complementary chromosome complement from both parents. Both routes produce organisms with even multiples of a base chromosome number, though odd-numbered polyploids can occur and are usually sterile because of pairing problems during meiosis.

Polyploidy influences cellular and organismal traits. Polyploids typically have larger cells, which can alter tissue and organ size, physiology, and development. They often exhibit novel gene expression patterns and can harbor increased genetic material for variation, enabling ecological tolerance and diversification. Fertility tends to be higher in even-numbered polyploids and lower in odd-numbered ones due to meiotic pairing issues. Polyploidization can drive immediate reproductive isolation and speciation, and it is a major mechanism of plant speciation.

Notable plant examples include bread wheat, which is hexaploid (AABBDD) and 42 chromosomes; cotton, tetraploid (AADD) with 52 chromosomes; Brassica napus, an allotetraploid (AACC) with 38 chromosomes; and the cultivated strawberry, Fragaria × ananassa, an allo-octoploid (8x). Polyploidy is also studied in crops and ornamental plants, where induced chromosome doubling using chemicals like colchicine is a common tool in plant breeding.