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C-values vary enormously among species. In animals they range more than 3,300-fold, and in land plants they differ by a factor of about 1,000.<ref name="Bennett2005"/><ref name="Gregory2005">{{cite book |author=Gregory TR |year=2005 |chapter=Genome size evolution in animals |title=The Evolution of the Genome |editor=T.R. Gregory |pages=3–87 |publisher=Elsevier |___location=San Diego|title-link=The Evolution of the Genome }}</ref> [[Protist]] genomes have been reported to vary more than 300,000-fold in size, but the high end of this range ([[Amoeba (genus)|''Amoeba'']]) has been called into question. Variation in C-values bears no relationship to the complexity of the organism or the number of [[genes]] contained in its genome; for example, some single-celled [[protozoa|protists]] have genomes much larger than that of [[humans]]. This observation was deemed counterintuitive before the discovery of [[non-coding DNA]]. It became known as the C-value paradox as a result. However, although there is no longer any [[paradox]]ical aspect to the discrepancy between C-value and gene number, this term remains in common usage. For reasons of conceptual clarification, the various puzzles that remain with regard to genome size variation instead have been suggested to more accurately comprise a complex but clearly defined puzzle known as the C-value enigma. C-values correlate with a range of features at the [[Cell (biology)|cell]] and organism levels, including [[cell size]], [[cell division]] rate, and, depending on the [[taxon]], body size, [[metabolic rate]], developmental rate, [[Organ (anatomy)|organ]] complexity, geographical distribution, or [[extinction]] risk (for recent reviews, see Bennett and Leitch 2005;<ref name="Bennett2005"/> Gregory 2005<ref name="Gregory2005"/>).
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 [[protozoa|protists]] have genomes much larger than that of [[humans]]. A possible answer to this enigma is provided by [[Maximum genetic diversity|Maximum Genetic Diversity]] (MGD), which posits that alleles get sorted into faster and slower mutating groups, which reflects their level of necessity for organismal function and provides an explanation for this phenomenon.<ref>{{Citation|title=Maximum genetic diversity|date=2019-11-13|url=https://en.wikipedia.org/w/index.php?title=Maximum_genetic_diversity&oldid=925980563|work=Wikipedia|language=en|access-date=2019-11-13}}</ref>
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.<ref name="kew" /> 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]], [[United Kingdom|UK]],<ref name=kew>{{Cite web |url=http://www.rbgkew.org.uk/cval/pgsm/index.html# |title=Second Plant Genome Size Discussion Meeting and Workshop |access-date=2015-04-19 |archive-url=https://web.archive.org/web/20081201130244/http://www.rbgkew.org.uk/cval/pgsm/index.html# |archive-date=2008-12-01 |url-status=dead |df= }}</ref>
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