Polyploidy
Polyploidy in plants Polyploidy is very common in plants, especially in angiosperms. 30% to 70% of angiosperms may be polyploid. Chromosome content of most plant groups suggests that the basic angiosperm genome consists of the genes on 7–11 chromosomes.
Polyploidy in crop plants Domestic wheat, with 42 chromosomes, is probably hexaploid (6n) The ancestral haploid number (n) was 7. Species of coffee plant with 22, 44, 66, and 88 chromosomes are known. This suggests that the ancestral condition was a plant with a haploid (n) number of 11 and a diploid (2n) number of 22, from which evolved the different polyploid descendants.
Ploidy levels in other crops Plant Probable ancestral haploid number Chromosome number Ploidy level domestic oat 7 42 6n peanut 10 40 4n sugar cane 80 8n banana 11 22,33 2n,3n white potato 12 48 tobacco cotton 13 52 apple 17 34,51
Polyploidy and speciation When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral species, triploid offspring are formed. These are sterile because they cannot form gametes with a balanced assortment of chromosomes. However, the tetraploid plants can breed with each other. So in one generation, a new species has been formed. Polyploidy even allows the formation of new species derived from different ancestors.
Polyploidy in Brassica hybrids Three species in the mustard family appear to have arisen by hybridization and polyploidy from three other ancestral species: B. oleracea x B. nigra → B. carinata Cabbage, etc. Black mustard Abyssinian mustard B. oleracea x B. campestris → B. napus Cabbage, etc. Turnips Rutabaga B. nigra x B. campestris → B. juncea Black mustard Turnips Leaf mustard
Triangle of U (the “U” is from a mis-translation of Woo Jang-Choon, who discovered the relationship)
Polyploidy in animals Polyploidy is much rarer in animals Examples: some insects, fishes, amphibians, reptiles. Until recently, no polyploid mammal was known A polyploid (4n=102) rat has been found in Argentina. Polyploid cells are larger than diploid ones More DNA in their nucleus.