The '
or ' is the complex puzzle surrounding the extensive variation in nuclear
genome size among
eukaryotic species. At the center of the enigma is the observation that 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.
Protist genomes have been reported to vary more than 300,000-fold in size, but the high end of this range (
Amoeba) has been called into question. Variation in C-values has no bearing on the complexity of an organism or the number of
genes contained in its genome; for example, some single-celled
protists have genomes much larger than that of
humans. This observation was deemed counterintuitive before the discovery of
repetitive DNA. It became known as the C-value paradox as a result. However, although there is no longer any
paradoxical 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 and organism levels, including
cell size,
cell division rate, and, depending on the
taxon, body size,
metabolic rate, developmental rate,
organ complexity, geographical distribution, or
extinction risk (for recent reviews, see Bennett and Leitch 2005; and an increasing number of authors have begun adopting this term.
C-value paradox 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 genetic information is encoded by DNA, 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 repetitive DNA in the late 1960s resolved the main question of the C-value paradox:
genome size does not reflect
gene number in
eukaryotes since most of the excess DNA in many species appears to be
junk DNA. The
human genome, for example, contains about 10% functional elements and the remaining 90% is thought to be junk. Species with larger genomes are thought to contain a higher proportion of junk DNA.
C-value enigma 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? ==Calculating C-values==