Table 1.

Summary of various explanations put forward to resolve the c-value paradox (for details, see text).

Explanation	Evidence	Counter evidence	Current status
Variable accumulation of ‘selfish’ or ‘junk’ or ‘parasitic’ DNA underlies the c-value paradox.	High variability in content of ‘non-coding’ intergenic DNA and constitutive heterochromatin, which was initially believed to i) lack typical protein-coding genes, ii) to be transcriptionally silent, and iii) to be enriched in highly repetitive, satellite, mid-repetitive and transposable element sequences. Together, these were considered ‘selfish’ or ‘junk’ or ‘parasitic' DNA and were suggested to variably accumulate and persist in genomes resulting in the loss of correlation between biological complexity and c-value.	Studies during the later part of 20th century, and the subsequent progresses in genomics, have established essential functions of heterochromatin and other noncoding DNA sequences.	‘Selfish’ or ‘junk’ or ‘parasitic’ DNA sequences do not exist in genomes in quantities that can explain the enormous variations in c-values.
Besides the DNA associated with genetic functions, genome also includes nucleoskeletal or nucleotypic sequences to sustain larger nuclear and cell volumes. Necessity for such DNA may explain the orders of magnitude high levels of DNA in some 'lower' taxa or in taxonomically related species.	Larger cells with associated greater nuclear DNA permit greater synthetic and storage capability. There is a wide correlation between higher c-values and larger cell and nuclear volumes, longer cell cycle duration and generation time. The additional ‘nucleoskeletal’ or ‘secondary’ or ‘nucleotypic’ DNA provides a skeletal framework for sustaining larger nuclei and cell bodies but its variable quantum leads to the variable c-values.	Many instances exist where organisms with very large body size have small cells and lower genomic DNA content. In several instances where endoreplication generates larger nuclear and cell sizes, the hereochromatin regions, presumed to also function as 'nucleoskeletal' DNA, actually remain under-replicated.	Existence of ‘nucleoskeletal’ DNA may explain some instances of high c-values but does not satisfactorily answer the question why some species within a group or in some ‘less’ evolved taxa need higher cell size and, therefore, greater ‘nucleoskeletal’ DNA.
The newly understood non-coding but regulatory functions of genomic DNA explain the high proportion of non-coding component in genomes.	A very large proportion of non-coding DNA in any genome has diverse regulatory roles and is thus essential for biological complexity. Variable quantities of such regulatory DNA determine the c-value.	Species with lower biological complexity but with very high c-value or species with much higher c-value than their relatives having comparable biological complexity are not expected to need enormously greater regulatory DNA.	Regulatory roles of the ncDNA account for its higher abundance than the coding DNA in organisms with taxa's basal genome size but cannot fully explain other features of the c-value paradox.