Is There an Upper Limit to Genome Size?

Eukaryote genomes range in size by a factor of 64 000, but the parts comprising genes, regulatory regions, and other functional components typically account for only a small fraction of total genome size. The huge range largely arises from differences in the amount of repetitive, parasitic, and often selfishly accumulating DNA and their degraded products. Despite the diversity, most species have small genomes, and those with giant genomes are the exception and belong to only a few phylogenetically distinct lineages. The recent reports of giant genomes in flowering plants and ferns (the largest so far for any eukaryote) join the similarly giant genomes previously noted for lungfish and salamanders. At 50-fold the size of the human genome (3 Gb), the staggeringly huge genome of 147.3 Gb recently discovered in the fern Tmesipteris obliqua is comparable in size to those of the other plant and animal record-holders (i.e., Paris japonica, a flowering plant with a genome size of 148.8 Gb, and Protopterus aethiopicus, a lungfish with a genome of 130 Gb). The synthesis of available information on giant genomes suggests that the biological limit to genome size expansion in eukaryotes may have been reached. We propose several explanations for why the genomes of ferns, flowering plants, and lungfish, all of which have independently undergone dramatic increases in genome size through a variety of mechanisms, do not exceed 150 Gb. At 50-fold the size of the human genome (3 Gb), the staggeringly huge genome of 147.3 Gb recently discovered in the fern Tmesipteris obliqua is comparable in size to those of the other plant and animal record-holders (i.e., Paris japonica, a flowering plant with a genome size of 148.8 Gb, and Protopterus aethiopicus, a lungfish with a genome of 130 Gb). The synthesis of available information on giant genomes suggests that the biological limit to genome size expansion in eukaryotes may have been reached. We propose several explanations for why the genomes of ferns, flowering plants, and lungfish, all of which have independently undergone dramatic increases in genome size through a variety of mechanisms, do not exceed 150 Gb. the amount of DNA in the unreplicated gametic nucleus. C-values are usually reported in terms of mass in picograms (pg) or the number of base pairs in gigabasepairs (Gb); 1 pg = 0.978 Gb [45Doležel J. et al.Nuclear DNA content and genome size of trout and human.Cytometry. 2003; 51A: 127-128Crossref Google Scholar]. the total amount of DNA in the nucleus of a cell. This can vary depending, for example, on the stage of the cell cycle and ploidy level [46Greilhuber J. et al.The origin, evolution and proposed stabilization of the terms ‘Genome size’ and ‘C-value’ to describe nuclear DNA contents.Ann. Bot. 2005; 95: 255-260Crossref PubMed Scopus (523) Google Scholar]. chemicals that have the capacity to fluoresce when irradiated with light of the appropriate wavelength. The fluorochromes used to estimate genome size and ploidy by flow cytometry bind specifically and quantitatively to DNA, and include, for example, propidium iodide (PI), 4′,6-diamino-2-phenylindole (DAPI), SYTOX, and PicoGreen. the presence of more than two sets of chromosomes in the nucleus (genome); for example, tetraploid (4×) = possessing four sets of chromosomes. DNA sequence motifs that are repeated hundreds or thousands of times across the genome, including tandem repeats (e.g., DNA satellites) and dispersed repeats (e.g., DNA transposons and retroelements).