Value Paradox

There appears to be a great deal more DNA in eukaryotic organisms than is actually needed to code for the number of genes estimated for a specific species. This discrepancy is known as the C value paradox.

Genome size seems to bear little relationship to organismal complexity or the number of genes encoded. For example, genome size varies widely among insect species, with up to 250-fold differences in C values known (Petrov et al. 2000). The locust Schistocerca gregaria has a C value of 9,300,000 kilobases (kb), 52-fold more than Drosophila melanogaster, but is unlikely to have 52 times as many genes (Wagner et al. 1993). Among 37 species of tenebrionid beetles, nuclear DNA content varies by a factor of 5 (Juan and Petitpierre 1991).

Genome size also can vary within species. For example, diploid cells in the mosquito Aedes albopictus contain 0.18 to 6 picograms (pg) of DNA, and C values vary by a factor of 3 (from 0.62 to 1.6 pg) among different populations of A. albopictus (Kumar and Rai 1990). The amount of DNA in insect cells is difficult to measure because many tissues are polyploid, with different tissues having different degrees of ploidy.

Polyploidy occurs when the amount of DNA in an organism increases over the usual diploid (2n) amount, usually by duplicating the number of chromosomes, perhaps to 3n or 4n or more. Polyploidy can occur throughout an organism's cells or in just some tissues. A few insects are polyploid in all tissues (Otto and Whitton 2000), but many insects have polyploid tissues within a diploid body. For example, the diploid blood cells of Bombyx mori contain 1 pg of DNA/blood cell, but a polyploid silk gland cell in the same insect contains 170,000 pg of DNA.

DNA content within cells also varies with developmental stage. At metamorphosis, the amount of DNA in B. mori declines by 81% after adults emerge from the pupal stage, which is probably due to histolysis of the polyploid larval silk glands and other polyploid cells.

Noncoding DNA can constitute 30% to more than 90% of the insect genome. This noncoding DNA has been called junk, parasitic, or selfish. There are several hypotheses to explain its persistence in genomes. One suggests that the noncoding DNAperforms essential functions, such as global regulation of gene expression. According to this hypothesis, the junk DNA is functional and deletions of such DNA would have a deleterious effect. A second hypothesis is that the noncoding DNA is useless, but is maintained because it is linked physically to functional genes; the excess DNA is not eliminated because it does not affect fitness of the organism and can be maintained indefinitely in the population. A third hypothesis suggests that the noncoding DNA is a functionless parasite that accumulates and is actively maintained by selection. A fourth hypothesis is that the DNA has a structural function, perhaps for compartmentalizing genes within the nucleus, or for maintaining a structural organization (nucleoskeleton) within the nucleus (Manuelidis 1990, Manuelidis and Cher 1990). Of course, all these hypotheses could be correct.

The lack of correlation between genome size and complexity or gene number (C value paradox) remains a topic of study because, unless the noncoding DNA has a function, such DNA constitutes a "load" upon the insect and should be lost over evolutionary time. Petrov et al. (1996) provided evidence that nonessential DNA is lost at a higher rate in Drosophila species than in mammalian species, suggesting that differences in genome size may result from persistent differences between organisms in the rate of loss of nonessential DNA. Petrov et al. (2000) provided additional support for this hypothesis by comparing DNA loss in two insect genera (Laupala crickets and Drosophila) with different genome sizes.

The crickets have a genome size an order of magnitude larger than that of Drosophila and eliminate nonessential DNA one-fortieth as quickly.

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