7.4: Polyploidy

Cells (and their owners) are polyploid if they contain more than two haploid (n) sets of chromosomes; that is, their chromosome number is some multiple of n greater than the 2n content of diploid cells. For example, triploid (3n) and tetraploid cell (4n) cells are polyploid.

Polyploidy in plants

Polyploidy is very common in plants, especially in angiosperms. From 30% to 70% of today's angiosperms are thought to be polyploid. 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. In fact, the chromosome content of most plant groups suggests that the basic angiosperm genome consists of the genes on 7–11 chromosomes. Domestic wheat, with its 42 chromosomes, is probably hexaploid (6n), where n (the ancestral haploid number) was 7.

Some other examples:

Plant	Probable ancestral haploid number	Chromosome number	Ploidy level
domestic oat	7	42	6n
peanut	10	40	4n
sugar cane	10	80	8n
banana	11	22, 33	2n, 3n
white potato	12	48	4n
tobacco	12	48	4n
cotton	13	52	4n
apple	17	34, 51	2n, 3n

Polyploid plants not only have larger cells but the plants themselves are often larger. This has led to the deliberate creation of polyploid varieties of such plants as watermelons, marigolds, and snapdragons.

Origin of Polyploidy

Polyploidy has occurred often in the evolution of plants. The process can begin if diploid (2n) gametes are formed. These can arise in at least two ways.

• The gametes may be formed by mitosis instead of meiosis.
• Plants, in contrast to animals, form germ cells (sperm and eggs) from somatic tissues. If the chromosome content of a precursor somatic cell has accidentally doubled (e.g., as a result of passing through S phase of the cell cycle without following up with mitosis and cytokinesis), then gametes containing 2n chromosomes are formed.

Polyploidy also occurs naturally in certain plant tissues.

• As the endosperm (3n) develops in corn (maize) kernels (Zea mays), its cells undergo successive rounds (as many as 5) of endoreplication producing nuclei that range as high as 96n.
• When rhizobia infect the roots of their legume host, they induce the infected cells to undergo endoreplication producing cells that can become 128n (from 6 rounds of endoreplication).

Polyploidy can also be induced in the plant-breeding laboratory by treating dividing cells with colchicine. This drug disrupts microtubules and thus prevents the formation of a spindle. Consequently, the duplicated chromosomes fail to separate in mitosis. Onion cells exposed to colchicine for several days may have over 1000 chromosomes inside.

Polyploidy and Speciation

When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral species (a backcross), 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.

In 1928, the Russian plant geneticist Karpechenko produced a new species by crossing a cabbage with a radish. Although belonging to different genera (Brassica and Raphanus respectively), both parents have a diploid number of 18. Fusion of their respective gametes (n=9) produced mostly infertile hybrids. However, a few fertile plants were formed, probably by the spontaneous doubling of the chromosome number in somatic cells that went on to form gametes (by meiosis). Thus these contained 18 chromosomes — a complete set of both cabbage (n=9) and radish (n=9) chromosomes. Fusion of these gametes produced vigorous, fully-fertile, polyploid plants with 36 chromosomes. (They had the roots of the cabbage and the leaves of the radish.)

These plants could breed with each other, but not with either the cabbage or radish ancestors, so Karpechenko had produced a new species. The process also occurs in nature. Three species in the mustard family (Brassicaceae) appear to have arisen by hybridization and polyploidy from three other ancestral species:

• B. oleracea (cabbage, broccoli, etc.) hybridized with B. nigra (black mustard) → B. carinata (Abyssinian mustard).
• B. oleracea x B. rapa (turnips) → B. napus (rutabaga)
• B. nigra x B. rapa → B. juncea (leaf mustard)

Modern wheat and perhaps some of the other plants listed in the table above have probably evolved in a similar way.

Polyploidy in Animals

Polyploidy is much rarer in animals. It is found in some insects, fishes, amphibians, and reptiles. Until recently, no polyploid mammal was known. However, the 23 September 1999 issue of Nature reported that a polyploid (tetraploid; 4n = 102) rat has been found in Argentina. Polyploid cells are larger than diploid ones; not surprising in view of the increased amount of DNA in their nucleus. The liver cells of the Argentinian rat are larger than those of its diploid relatives, and its sperm are huge in comparison. Normal mammalian sperm heads contain some 3.3 picograms (10^-12 g) of DNA; the sperm of the rat contains 9.2 pg. Although only one mammal is known to have all its cells polyploid, many mammals have polyploid cells in certain of their organs, e.g, the liver.