C-value enigma

The C-value enigma or C-value paradox is a term used to describe 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, many plant species and some single-celled protists, have genomes much larger than that of humans.

C-value paradox history
In 1948, Thomas and 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 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, comprises 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). It is unclear why some species have a remarkably higher amount of non-coding sequences than others of the same level of complexity. Non-coding DNA may have many functions yet to be discovered. Though now it is known that only a fragment of the genome consists of genes, the paradox remains unsolved.

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The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "C-value" (Swift 1950) in reference to haploid nuclear DNA contents.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:


 * 1) What types of non-coding DNA are found in different eukaryotic genomes, and in what proportions?
 * 2) From where does this non-coding DNA come, and how is it spread and/or lost from genomes over time?
 * 3) What effects, or perhaps even functions, does this non-coding DNA have for chromosomes, nuclei, cells, and organisms?
 * 4) 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.