The C-value enigma or C-value paradox is the complex puzzle surrounding the extensive variation in nuclear genome size among eukaryotic species. At the center of the C-value enigma is the observation that genome size does not correlate with organismal complexity; for example, some single-celled protists have genomes much larger than that of humans.
C-value paradox history 
In 1948, Roger and Colette Vendrely reported a "remarkable constancy in the nuclear DNA content of all the cells in all the individuals within a given animal species", 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 discovery of non-coding DNA in the early 1970s resolved the main question of the C-value paradox: genome size does not reflect gene number in eukaryotes since most of their DNA is non-coding and therefore does not consist of genes. The human genome, for example, comprises less than 2% protein-coding regions, with the remainder being various types of non-coding DNA (especially transposable elements).
C-value enigma origin 
The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "C-value" (Swift 1950) in reference to haploid nuclear DNA contents. The term was coined by Canadian biologist Dr. T. Ryan Gregory of the University of Guelph in 2000/2001. In general terms, the C-value enigma relates to the issue of variation in the amount of non-coding DNA found within the genomes of different eukaryotes.
The C-value enigma, unlike the older C-value paradox, is explicitly defined as a series of independent but equally important component questions, including:
- What types of non-coding DNA are found in different eukaryotic genomes, and in what proportions?
- From where does this non-coding DNA come, and how is it spread and/or lost from genomes over time?
- What effects, or perhaps even functions, does this non-coding DNA have for chromosomes, nuclei, cells, and organisms?
- Why do some species exhibit remarkably streamlined chromosomes, while others possess massive amounts of non-coding DNA?
Puzzle versus paradox 
Some prefer the term C-value enigma because it explicitly includes all of the questions that will need to be answered if a complete understanding of genome size evolution is to be achieved (Gregory 2005). Moreover, the term paradox implies a lack of understanding of one of the most basic features of eukaryotic genomes: namely that they are composed primarily of non-coding DNA. Some have claimed that the term paradox also has the unfortunate tendency to lead authors to seek simple one-dimensional solutions to what is, in actuality, a multi-faceted puzzle. For these reasons, in 2003 the term "C-value enigma" was endorsed in preference to "C-value paradox" 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 this term.
See also 
- Animal Genome Size Database
- Cell nucleus
- Comparative genomics
- Genome size
- Human genome
- Junk DNA
- Noncoding DNA
- Plant DNA C-values Database
- Selfish DNA
- Transposable elements
- Vendrely R and Vendrely C (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.
- "Animal Genome Size Database". Retrieved 14 May 2013.
- Elgar, G.; Vavouri, T. (2008). "Tuning in to the signals: Noncoding sequence conservation in vertebrate genomes". Trends in Genetics 24 (7): 344–352. doi:10.1016/j.tig.2008.04.005. PMID 18514361.
- Second Plant Genome Size Discussion Meeting and Workshop
- Gregory TR (2005). "Genome size evolution in animals". In T.R. Gregory. The Evolution of the Genome. San Diego: Elsevier. pp. 3–87. ISBN 0-12-301463-8.
- Gregory TR (2001). "Coincidence, coevolution, or causation? DNA content, cell size, and the C-value enigma". Biological Reviews 76 (1): 65–101. doi:10.1017/S1464793100005595. PMID 11325054.
- Gregory TR (2000). "Nucleotypic effects without nuclei: genome size and erythrocyte size in mammals". Genome 43 (5): 895–901. PMID 11081981.
- Thomas CA Jr (1971). "The genetic organization of chromosomes". Annual Review of Genetics 5: 237–256. doi:10.1146/annurev.ge.05.120171.001321. PMID 16097657.
- Swift H (1950). "The constancy of desoxyribose nucleic acid in plant nuclei". Proc Natl Acad Sci USA 36 (11): 643–654. doi:10.1073/pnas.36.11.643. PMC 1063260. PMID 14808154.
- Vendrely R, Vendrely C (1948). "La teneur du noyau cellulaire en acide désoxyribonucléique à travers les organes, les individus et les espèces animales: Techniques et premiers résultats". Experientia (in French) 4: 434–436.