C-value paradox
Definition
The C-value paradox is a term used to describe the discrepancy between nuclear genome size and the number of genes among eukaryotic species.
Origin of the term
In 1948, Vendrely and Vendrely reported a "remarkable constancy in the nuclear DNA content of all the cells in all the individuals within a given animal species" (translated from the original French), which they took as evidence that DNA, rather than protein, was the substance of which genes are composed. The term C-value reflects this observed constancy. However, it was soon found that C-values (genome sizes) vary enormously among species and that this bears no relationship to the presumed number of genes (as reflected by the complexity of the organism). For example, the cells of some salamanders may contain 40 times more DNA than those of humans. Given that C-values were assumed to be constant because DNA is the stuff of genes, and yet bore no relationship to presumed gene number, this was understandably considered paradoxical; the term C-value paradox was used to describe this situation by C.A. Thomas, Jr. in 1971.
The solution to the paradox
The discovery of non-coding DNA in the early 1970s resolved the C-value paradox. It is no longer a mystery why genome size does not reflect gene number in eukaryotes: most eukaryotic (but not prokaryotic) DNA is non-coding and therefore does not consist of genes, and as such total DNA content is not determined by gene number in eukaryotes. The human genome, for example, is comprised of only about 1.5% protein-coding genes, with the other 98.5% being various types of non-coding DNA (especially transposable elements) (International Human Genome Sequencing Consortium 2001).
Terminological revision
The term C-value paradox remains in common usage, even though it implies a lack of understanding of one of the most basic features of genomes: namely that they are composed primarily of non-coding DNA. It also has the unfortunate consequence of leading authors to seek simple, one-dimensional solutions to the question of why some species contain a great deal more non-coding DNA than others. More recently, it has been suggested that the term C-value paradox should be abandoned in favor of the more appropriate term C-value enigma, which frames the issue as a complex puzzle consisting of several components rather than a simple paradox (Gregory 2001, 2005). In 2003, this change in terminology was endorsed at the Second Plant Genome Size Discussion Meeting and Workshop at the Royal Botanic Gardens, Kew, UK, and an increasing number of authors have begun adopting the more accurate term.
References
Gregory, T.R. 2001. Coincidence, coevolution, or causation? DNA content, cell size, and the C-value enigma. Biological Reviews 76: 65-101.
Gregory, T.R. 2005. Genome size evolution in animals. In The Evolution of the Genome (ed. T.R. Gregory), pp. 3-87. Elsevier, San Diego.
International Human Genome Sequencing Consortium. 2001. Initial sequencing and analysis of the human genome. Nature 409: 860-921.
Thomas, C.A. 1971. The genetic organization of chromosomes. Annual Review of Genetics 5: 237-256.
Vendrely, R. and C. Vendrely. 1948. La teneur du noyau cellulaire en acide désoxyribonucléique à travers les organes, les individus et les espèces animales: Techniques et premiers résultats. Experientia 4: 434-436.