|Polyploidy and aneuploidy|
For each species of higher plants and animals, the base number of nuclear chromosomes is called the haploid number, denoted as n. Individuals of most species are diploid, having double the haploid number of chromosomes (2n) in each somatic cell. Aneuploid and polyploid organisms have abnormal numbers of whole chromosomes.
Strictly speaking, aneuploidy refers to any number of chromosomes in a cell or organism that is not an exact multiple of the haploid number. However, in common practice the termis used to refer specifically to situations in which an organism or cell has only one chromosome or a few chromosomes added or missing.
In animals, aneuploidy is usually lethal and so is rarely encountered. In the plant kingdom, on the other hand, the addition or elimination of a small number of individual chromosomes may be better tolerated.
Nullisomy is the aneuploid condition in which two homologous chromosomes are missing, so that the organism has 2n – 2 chromosomes. Monosomy refers to the absence of a single chromosome, giving 2n – 1 total chromosomes.
Aneuploidy is caused by nondisjunction, which occurs when a pair of homologous chromosomes fail to separate during cell division. If nondisjunction occurs in the first stage of meiosis, all four resulting gametes will be abnormal.
Two of them will have no copy of the given chromosome, and two, correspondingly, will have one extra copy each. If nondisjunction occurs in the second stage of meiosis, one of the four resulting gametes will have no copy of the given chromosome, another will have an extra copy, and two will be normal.
Polyploidy is caused by the addition of one or more complete chromosome sets to the normal diploid complement. In the animal kingdom polyploidy is lethal in nearly every case, but it is relatively common in plants. It is estimated that between 30 percent and 70 percent of extant angiosperms are polyploid.
The process of sex determination is more sensitive to polyploidy in animals than in plants, and because many plants undergo self-fertilization, those with an even number of chromosome sets (such as those that are tetraploid) may still produce fertile gametes.
This fact points to the crucial factor that determines whether a polyploid plantmay be fertile:whether it has an even or an odd number of chromosome sets. Plant swith an odd number of chromosome sets are almost always sterile. Because they always have an unpaired chromosome of each type, it is extremely unlikely for them to produce viable balanced gametes.
On the other hand, there is potential for a polyploid plant with an even number of chromosome sets to produce a balanced gamete if multiple sets of conspecific chromosomes pair during meiosis.
Autopolyploidy and Allopolyploidy
Polyploid plants exist in two categories. Autopolyploids have a genome comprising multiple sets of chromosomes that are all fromone species. In allopolyploids, themultiple sets of chromosomes come from multiple (usually related) species.
Autopolyploidy can arise from situations in which a defect in meiosis creates a diploid or triploid gamete. If such a gamete is fused with a typical haploid gamete fromthe same species, the union leads to a polyploid zygote.
The most common such pairing, of a diploid and a haploid gamete, produces an autotriploid. As the previous section suggests, they are usually sterile.
However, some sterile autotriploids that can be cultivated through vegetative propagation (by planting cuttings) are attractive food crops because they lack robust fertile seeds. For example, the cultivated banana is an effectively seedless (and therefore sterile) autotriploid; it cannot reproduce without human intervention.
Allopolyploidy arises through the interbreeding of different species. An allodiploid formed by the union of two haploid gametes from separate species will be sterile because there are no matching chromosomes to pair at meiosis.
However, if the two sets of chromosomes become doubled within a cell, the result will be a potentially fertile allotetraploid.
An organism in this condition is basically a double-diploid, in which homologous pairs of conspecific chromosomes can join at meiosis to produce a viable gamete. This type of polyploidy has played an important role in the natural history of many plants, including wheat, the most widely cultivated cereal in the world.
Allopolyploidy and Speciation
Contemporary domesticated wheat species can be identified as belonging to one of three groups, based on their number of chromosomes. One group has fourteen chromosomes, another twenty-eight, and a third has forty-two chromosomes.
These groups form a series of polyploids based on a haploid number, n, equal to seven chromosomes. It is believed that these groups of domesticated wheat evolved in two major steps.
First, members of a diploid genus Triticum (2n = fourteen chromosomes) may have hybridized with one of the diploid goat grasses Aegilops (2n = fourteen chromosomes) to form allotetraploid species of emmer and durum wheats (4n = twenty-eight chromosomes).
|Allopolyploidy and Speciation|
Then, it is believed, these species underwent a second round of hybridization with separate goat grass species to form the allohexaploid (6n = forty-two chromosomes) species that is now known as bread wheat, or Triticum aestivum.
Bread wheat, which probably appeared around eight thousand years ago, combines desirable qualities of all three of its diploid relatives, including nonshattering grains, a high protein content in the endosperm, and good tolerance to various environmental conditions.
Allohexaploid wheat can reproduce, thanks to normal meiosis, in which homologous chromosomes pair to form a triploid gamete with twenty-one chromosomes.
The possibility of speciation by allopolyplidy is one danger of introducing plants to new regions. The American species of salt marsh grass Spartina alterniflora was introduced to the south coast of England around 1870, probably in ships’ ballastwater.
It crossed with the native salt marsh grass Spartina maritima to form an allotetraploid species, Spartina anglica, which overran the native grass in the twentieth century and colonized coastal flats so aggressively as to create a floral monoculture that has proved inadequate for wintering populations of wading birds and wildfowl.
Like wheat, many of today’s other crops are polyploid. By creating polyploid lines, plant breeders can introduce desirable traits cumulatively.
Among major polyploid crops are dietary staples, such as the white potato (4n = forty-eight chromosomes), the domestic oat (6n = forty-two chromosomes), the peanut (4n = forty chromosomes), textile-producing plants such as cotton (4n = fifty-two chromosomes), and the cash crops tobacco (4n = forty-eight chromosomes) and coffee, of which existing species range from diploid to octoploid (8n = eighty-eight chromosomes).
As well as the aforementioned domesticated banana, which, as an autotriploid, is sterile and correspondingly seedless, some varieties of cultivated apple are triploid species.
Beyond engineering particular traits, another reason it is desirable to cultivate polyploid species is that polyploid plant cells are usually larger than the corresponding diploid cells.
Consequently, the polyploid plants themselves are usually larger. Species of particularly large watermelons, marigolds, and snapdragons have been created through cultivation of polyploid lines.
Plant polyploidy is induced in the laboratory by treating dividing cells with the drug colchicine. It prevents the formation of a spindle during mitosis by disrupting the microtubules, causing the duplicated chromosomes to fail to separate. The most commonmethod of application is to place the roots of a plant in a colchicine solution.
Because colchicine inhibits the actual division of cells without affecting the duplication of chromosomes, when full rounds of mitosis commence upon removing the roots from the colchicine, the resulting cells contain an extra set of chromosomes.