C-value enigma
Definition and origin
The C-value enigma is a term used to describe the complex puzzle surrounding the extensive variation in nuclear genome size among eukaryotic species. The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term C-value (Swift 1950) in reference to haploid nuclear DNA contents. 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 term was first used by Canadian biologist Dr. T. Ryan Gregory of the University of Guelph in 2000, and was officially coined for the first time by Gregory (2001). The C-value enigma, unlike the older C-value paradox, is explictly defined as a series of independent but equally important component questions. These include: 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
Although it refers to a different problem (which has since been solved), the term "C-value paradox" is commonly used in reference to one or more of the component questions of the C-value enigma. The term C-value enigma is preferred 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 the most basic features of eukaryotic genomes (namely that they are composed primarily of non-coding DNA). It 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 the more accurate term.
References
Gregory, T.R. 2000. Nucleotypic effects without nuclei: genome size and erythrocyte size in mammals. Genome 43: 895-901.
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.
Swift, H. 1950. The constancy of desoxyribose nucleic acid in plant nuclei. Proceedings of the National Academy of Sciences of the USA 36: 643-654.
Thomas, C.A. 1971. The genetic organization of chromosomes. Annual Review of Genetics 5: 237-256.