Ploidy

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The Ploidy of a biological cell refers to the number of complete sets of chromosomes. In humans, the gametes (sperm and egg) are haploid and they contain one set of chromosomes (23 chromosomes). A diploid zygote is formed by fertilization and its cells have a pair of each chromosome, in this case one set from each parent (46 chromosomes). It is possible to have higher levels of ploidy and such cells are described as being polyploid. Tetraploidy describes an organism with four sets of chromosomes and is a type of polyploidy fairly common in plants, amphibians, reptiles, and various species of insects.

Terminology

Euploidy describes a cell or organism having an integral multiple of the basic set of chromosomes. For example, a typical human cell has 46 chromosomes (diploidy or 2n), which is an integral multiple of the basic set of 23 chromosomes found in the sperm and eggs (haploidy or n). Any increases in the number of chromosome sets in an organism is considered euploidy and this can be detrimental, even lethal. Plants are more tolerant of large scale chromosome changes and this has even been important for the domestication of crops. Banana's and seedless watermelons are both examples of euploids with 3 sets of chromosomes (triploidy or 3n)

Aneuploidy is the state of not having euploidy and these conditions are usually described by using the suffix -somy. Examples in a normally diploid organism, such as humans, are a single extra chromosome (trisomy or 2n + 1), or missing a chromosome (monosomy or 2n - 1). Down syndrome is also known as trisomy 21 since the extra chromosome is number 21. In genetic notation this is sometimes written as 47, 21+ to indicate the number of chromosomes, 47 instead of 46, and to clarify the specific chromosome gained. In human development most cases of trisomy have severe abnormalities and will result in a miscarriage, but this is not a general rule. Many plants can tolerate the loss or gain of chromosomes and thus aneuploids are common in some species.

Haploid and monoploid

During meiosis, germ cell precursors have their number of chromosomes halved by randomly separating the homologous pairs, resulting in haploid gametes (sperm or eggs). The haploid number (n) is the number of chromosomes in a gamete of an individual, and this is distinct from the monoploid number (x) which is the number of unique chromosomes in a single complete set. For diploid organisms the haploid and monoploid numbers are synonymous but for some polyploids they can be distinct. For humans, x = n = 23; a diploid human cell contains 46 chromosomes: 2 complete haploid sets, or 23 homologous chromosome pairs. The hexaploid wheat has six sets of chromosomes in most cells and three sets of chromosomes in the gametes (n=3x).

Some organisms have life cycles where the haploid state plays a more prominent role. All plants and many fungi and algae switch between a haploid and a diploid state, called alternation of generations, with one of the stages emphasized over the other. Most fungi and algae are haploid during the principal stage of their life cycle. Male bees, wasps, and ants are haploid organisms because of the way they develop from unfertilized egg cells. Theoretically, the lowest possible haploid value is 1 and an example is the Australian bulldog ant, Myrmecia pilosula.

Haploidisation is the process of creating a haploid cell (usually from a diploid cell). A laboratory procedure called haploidisation forces a normal cell to expel half of its chromosomal complement. In mammals this renders this cell chromosomally equal to sperm or egg. This was one of the procedures used by Japanese researchers to produce Kaguya, a fatherless mouse. Haploidisation sometimes occurs in plants when meiotically reduced cells (usually egg cells) develop by parthenogenesis.

Diploid

Diploid (indicated by 2x) cells have two homologous copies of each chromosome, usually one from the mother and one from the father. The exact number of chromosomes may be one or two different from the 2 number yet the cell may still be classified as diploid (although with aneuploidy). Nearly all mammals are diploid organisms (the viscacha rats Pipanacoctomys aureus and Tympanoctomys barrerae are the only known exceptions as of 2004), although all individuals have some small fraction of cells that display polyploidy. Human diploid cells have 46 chromosomes and human haploid gametes (egg and sperm) have 23 chromosomes.

Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid. Examples include human foamy virus, human T-lymphotropic virus, and HIV.

Polyploidy

Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set. These may be from the same species or from closely related species. In the latter case these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from the hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreduced gametes, and not by diploid–diploid hybridization followed by chromosome doubling. The so-called Brassica triangle is an example of allopolyploidy, where three different parent species have hybridized in each pair combination to produce three new species.

Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms many somatic cells are polyploid due to a process called endoreduplication where duplication of the genome occurs without mitosis (cell division).

The extreme in polyploidy occurs in the fern-ally genus Ophioglossum, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or in at least one case, well over one thousand. Interestingly, these plants seem to have simplified structures in their phenotype.

Variable or indefinite ploidy

Bacteria are usually thought of having only one copy of their genome but depending on growth conditions, prokaryotes may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because under logarithmic growth conditions the cells are able to replicate their DNA faster than they can divide. Even when bacteria grow more slowly it is possible for them to gain extra copies of genes by transformation, conjugation or transduction. A specific term for this is merodiploid although it is used less often than a synonymous term merozygote or more simply, partial diploid.

Organelles such as chloroplasts and mitochondria share many characteristics with bacteria but their DNA content can have a high copy number, sometimes more than one hundred copies. This is similar to the concept of ploidy but the term is not used to describe this phenomena.

Mixoploidy

Mixoploidy refers to the presence of two cell lines, one diploid and one polyploid. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes.

Dihaploidy and Polyhaploidy

Dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i.e., by halving the chromosome constitution.

Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion.

The term “dihaploid” was coined by Bender to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense, but it has also been used for doubled monoploids or doubled haploids, which are homozygous and used for genetic research.

References

• Griffiths, A. J. et al. 2000. An introduction to genetic analysis, 7th ed. W. H. Freeman, New York ISBN 0-7167-3520-2