Open main menu

In genetics, an isochore is a large region of DNA (greater than 300 kb) with a high degree uniformity in guanine (G) and cytosine (C): G-C and C-G (collectively GC content).

Bernardi and colleagues first uncovered the compositional non-uniformity within vertebrate genomes using thermal melting and density gradient centrifugation.[1][2][3] The DNA fragments extracted by the gradient centrifugation were later termed "isochores",[4] which was subsequently defined as "very long (much greater than 200 KB) DNA segments" that "are fairly homogeneous in base composition and belong to a small number of major classes distinguished by differences in guanine-cytosine (GC) content".[3] Subsequently, the isochores "grew" and were claimed to be ">300 kb in size."[5][6] The theory proposed that isochore’s composition varied markedly between "warm-blooded" (homeotherm) vertebrates and "cold-blooded" (poikilotherm) vertebrates[3] and later became known as the isochore theory.

The thermodynamic stability hypothesisEdit

The isochore theory purported that the genome of "warm-blooded" vertebrates (mammals and birds) are mosaics of long isochoric regions of alternating GC-poor and GC-rich composition, as opposed to the genome of "cold-blooded" vertebrates (fishes and amphibians) that were supposed to lack GC-rich isochores.[3][7][8][9][10][11] These findings were explained by the thermodynamic stability hypothesis, attributing genomic structure to body temperature. GC-rich isochores were purported to be a form of adaptation to environmental pressures, as an increase in genomic GC-content could protect DNA, RNA, and proteins from degradation by heat. [3][4]

Despite its attractive simplicity, the thermodynamic stability hypothesis has been repeatedly shown to be in error [12][13][14] .[15][16][17][18][19] Many authors showed the absence of a relationship between temperature and GC-content in vertebrates,[17][18] while others showed the existence of GC-rich domains in "cold-blooded" vertebrates such as crocodiles, amphibians, and fish.[14][20][21]

Principles of the isochore theoryEdit

The isochore theory was the first to identify the nonuniformity of nucleotide composition within vertebrate genomes and predict that the genome of "warm-blooded" vertebrates such as mammals and birds are mosaic of isochores (Bernardi et al. 1985). The human genome, for example, was described as a mosaic of alternating low and high GC content isochores belonging to five compositional families, L1, L2, H1, H2, and H3, whose corresponding ranges of GC contents were said to be <38%, 38%-42%, 42%-47%, 47%-52%, and >52%, respectively.[22]

The main predictions of the isochore theory are that:

  • GC content of the third codon position (GC3) of protein coding genes is correlated with the GC content of the isochores embedding the corresponding genes.[22]
  • The genome organization of warm-blooded vertebrates is a mosaic of mostly GC-rich isochores.[23]


  • Genome organization of cold-blooded vertebrates is characterized by low GC content levels and lower compositional heterogeneity than warm-blooded vertebrates. Homogeneous domains do not reach the high GC levels attained by the genomes of warm-blooded vertebrates.[23][24]

The neutralist-selectionist controversyEdit

Two opposite explanations that endeavored to explain the formations of isochores were vigorously debated as part of the neutralist-selectionist controversy. The first view was that isochores reflect variable mutation processes among genomic regions consistent with the neutral model.[25][26] Alternatively, isochores were posited as a result of natural selection for certain compositional environment required by certain genes.[27] Several hypotheses derive from the selectionist view, such as the thermodynamic stability hypothesis [6][28] and the biased gene conversion hypothesis.[26] Thus far, none of the theories provides a comprehensive explanation to the genome structure, and the topic is still under debate.

The rise and fall of the isochore theoryEdit

The isochore theory became one of the most useful theories in molecular evolution for many years. It was the first and most comprehensive attempt to explain the long-range compositional heterogeneity of vertebrate genomes within an evolutionary framework. Despite the interest in the early years in the isochore model, in recent years, the theory’s methodology, terminology, and predictions have been challenged.

Because this theory was proposed in the 20th century before complete genomes were sequenced, it could not be fully tested for nearly 30 years. In the beginning of the 21st century, when the first genomes were made available it was clear that isochores do not exist in the human genome[29] nor in other mammalian genomes.[30] When failed to find isochores, many attacked the very existence of isochores.[29][31][32][33][34] The most important predictor of isochores, GC3 was shown to have no predictable power [35][36] to the GC content of nearby genomicregions, refuting findings from over 30 years of research, which were the basis for many isochore studies. Isochore-originators replied that the term was misinterpreted [22][37][38] as isochores are not "homogeneous" but rather fairly homogeneous regions with a heterogeneous nature (especially) of GC-rich regions at the 5 kb scale,[39] which only added to the already growing confusion. The reason for this ongoing frustration was the ambiguous definition of isochores as long and homogeneous, allowed some researchers to discover "isochores" and others to dismiss them, although both camps used the same data.

The unfortunate side effect of this controversy was an "arms race" in which isochores are frequently redefined and relabeled following conflicting findings that failed to reveal "mosaic of isochores." [22][31][33] The unfortunate outcomes of this controversy and the following terminological-methodological mud were the loss of interest in isochores by the scientific community. When the most important core-concept in isochoric literature, the thermodynamic stability hypothesis, was rejected, the theory lost its appeal. Even today, there is no clear definition to isochores nor is there an algorithm that detects isochores.[40] Isochores are detected manually by visual inspection of GC content curves ,[41] however because this approach lacks scientific merit and is difficult to replicate by independent groups, the findings remain disputed.

The compositional domain modelEdit

As the study of isochores was de facto abandoned by most scientists, an alternative theory was proposed to describe the compositional organization of genomes in accordance with the most recent genomic studies. The Compositional Domain Model depicts genomes as a medley of short and long homogeneous and nonhomogeneous domains.[34] The theory defines "compositional domains" as genomic regions with distinct GC-contents as determined by a computational segmentation algorithm.[34] The homogeneity of compositional domains is compared to that of the chromosome on which they reside using the F-test, which separated them into compositionally homogeneous domains and compositionally nonhomogeneous domains based on the outcome of test. Compositionally homogeneous domains that are sufficiently long (≥ 300 kb) are termed isochores or isochoric domains. These terms are in accordance with the literature as they provide clear distinction between isochoric- and nonisochoric-domains.

A comprehensive study of the human genome unraveled a genomic organization where two-thirds of the genome is a mixture of many short compositionally homogeneous domains and relatively few long ones. The remaining portion of the genome is composed of nonhomogeneous domains. In terms of coverage, only 1% of the total number of compositionally homogeneous domains could be considered "isochores" which covered less than 20% of the genome.[34]

Since its inception the theory received wide attention and was extensively used to explain findings emerging from over dozen new genome sequencing studies.[30][42][43][44][45][46][47][48][49] yet many important questions remain open, such as which evolutionary forces shaped the structure of compositional domain and how do they differ between different species?


  1. ^ Macaya, Thiery, and Bernardi (1976). "An approach to the organization of eukaryotic genomes at a macromolecular level". Journal of Molecular Biology. 108 (1): 237–254. doi:10.1016/S0022-2836(76)80105-2. PMID 826644.CS1 maint: multiple names: authors list (link)
  2. ^ Thiery, Macaya, and Bernardi (1976). "An analysis of eukaryotic genomes by density gradient centrifugation". Journal of Molecular Biology. 108 (1): 219–235. doi:10.1016/S0022-2836(76)80104-0. PMID 826643.CS1 maint: multiple names: authors list (link)
  3. ^ a b c d e Bernardi; Olofsson, Birgitta; Filipski, Jan; Zerial, Marino; Salinas, Julio; Cuny, Gerard; Meunier-Rotival, Michele; Rodier, Francis; et al. (1985). "The mosaic genome of warm-blooded vertebrates". Science. 228 (4702): 953–958. Bibcode:1985Sci...228..953B. doi:10.1126/science.4001930. PMID 4001930.
  4. ^ a b Cuny; Soriano, P; MacAya, G; Bernardi, G; et al. (1981). "The major components of the mouse and human genomes: Preparation, basic properties and compositional heterogeneity". European Journal of Biochemistry. 115 (2): 227–233. doi:10.1111/j.1432-1033.1981.tb05227.x. PMID 7238506.
  5. ^ Salinas; Zerial, M; Filipski, J; Crepin, M; Bernardi, G; et al. (1987). "Nonrandom distribution of MMTV proviral sequences in the mouse genome". Nucleic Acids Res. 15 (7): 3009–3022. doi:10.1093/nar/15.7.3009. PMC 340712. PMID 3031617.
  6. ^ a b Bernardi (1993). "The vertebrate genome: isochores and evolution". Molecular Biology and Evolution. 10 (1): 186–204. doi:10.1093/oxfordjournals.molbev.a039994. PMID 8450755.
  7. ^ Bernardi (1995). "The human genome: organization and evolutionary history". Annual Review of Genetics. 29: 445–476. doi:10.1146/ PMID 8825483.
  8. ^ Bernardi, Hughes, and Mouchiroud (1997). "The major compositional transitions in the vertebrate genome". Journal of Molecular Evolution. 44 Suppl 1 (S1): S44–51. Bibcode:1997JMolE..44S..44B. doi:10.1007/PL00000051. PMID 9071011.CS1 maint: multiple names: authors list (link)
  9. ^ Robinson, Gautier, and Mouchiroud (1997). "Evolution of isochores in rodents". Molecular Biology and Evolution. 14 (8): 823–828. doi:10.1093/oxfordjournals.molbev.a025823. PMID 9254920.CS1 maint: multiple names: authors list (link)
  10. ^ Galtier and Mouchiroud; Mouchiroud, D (1998). "Isochore evolution in mammals: a human-like ancestral structure". Genetics. 150 (4): 1577–1584. PMC 1460440. PMID 9832533.
  11. ^ Oliver; Bernaola-Galván, P; Carpena, P; Román-Roldán, R; et al. (2001). "Isochore chromosome maps of eukaryotic genomes". Gene. 276 (1–2): 47–56. CiteSeerX doi:10.1016/S0378-1119(01)00641-2. PMID 11591471.
  12. ^ Aota and Ikemura; Ikemura, T (1986). "Diversity in G + C content at the third position of codons in vertebrate genes and its cause". Nucleic Acids Res. 14 (16): 6345–6355. doi:10.1093/nar/14.16.6345. PMC 311650. PMID 3748815.
  13. ^ Galtier and Lobry; Lobry, J.R. (1997). "Relationships Between Genomic G+C Content, RNA Secondary Structures, and Optimal Growth Temperature in Prokaryotes". Journal of Molecular Evolution. 44 (6): 632–636. Bibcode:1997JMolE..44..632G. doi:10.1007/PL00006186. PMID 9169555.
  14. ^ a b Hughes, Zelus, and Mouchiroud (1999). "Warm-blooded isochore structure in Nile crocodile and turtle". Molecular Biology and Evolution. 16 (11): 1521–1527. doi:10.1093/oxfordjournals.molbev.a026064. PMID 10555283.CS1 maint: multiple names: authors list (link)
  15. ^ Eyre-Walker and Hurst; Hurst, LD (2001). "The evolution of isochores". Nat Rev Genet. 2 (7): 549–555. doi:10.1038/35080577. PMID 11433361.
  16. ^ Hurst and Merchant; Merchant, AR (2001). "High guanine-cytosine content is not an adaptation to high temperature: a comparative analysis amongst prokaryotes". Proceedings of the Royal Society B. 268 (1466): 493–497. doi:10.1098/rspb.2000.1397. PMC 1088632. PMID 11296861.
  17. ^ a b Belle, Smith, and Eyre-Walker (2002). "Analysis of the phylogenetic distribution of isochores in vertebrates and a test of the thermal stability hypothesis". Journal of Molecular Evolution. 55 (3): 356–363. Bibcode:2002JMolE..55..356B. doi:10.1007/s00239-002-2333-1. PMID 12187388.CS1 maint: multiple names: authors list (link)
  18. ^ a b Ream, Johns, and Somero (2003). "Base Compositions of Genes Encoding {alpha}-Actin and Lactate Dehydrogenase-A from Differently Adapted Vertebrates Show No Temperature- Adaptive Variation in G + C Content". Molecular Biology and Evolution. 20 (1): 105–110. doi:10.1093/molbev/msg008. PMID 12519912.CS1 maint: multiple names: authors list (link)
  19. ^ Belle; Duret, L; Galtier, N; Eyre-Walker, A; et al. (2004). "The decline of isochores in mammals: an assessment of the GC content variation along the mammalian phylogeny". Journal of Molecular Evolution. 58 (6): 653–660. Bibcode:2004JMolE..58..653B. CiteSeerX doi:10.1007/s00239-004-2587-x. PMID 15461422.
  20. ^ Hughes, Friedman, and Murray (2002). "Genomewide pattern of synonymous nucleotide substitution in two complete genomes of Mycobacterium tuberculosis". Emerging Infectious Diseases. 8 (11): 1342–1346. doi:10.3201/eid0811.020064. PMC 2738538. PMID 12453367.CS1 maint: multiple names: authors list (link)
  21. ^ Costantini, Auletta, and Bernardi (2007). "Isochore patterns and gene distributions in fish genomes". Genomics. 90 (3): 364–371. doi:10.1016/j.ygeno.2007.05.006. PMID 17590311.CS1 maint: multiple names: authors list (link)
  22. ^ a b c d Bernardi (2001). "Misunderstandings about isochores. Part 1". Gene. 276 (1–2): 3–13. doi:10.1016/S0378-1119(01)00644-8. PMID 11591466.
  23. ^ a b Bernardi (2000). "Isochores and the evolutionary genomics of vertebrates". Gene. 241 (1): 3–17. doi:10.1016/S0378-1119(99)00485-0. PMID 10607893.
  24. ^ a b Bernardi (2000). "The compositional evolution of vertebrate genomes". Gene. 259 (1–2): 31–43. doi:10.1016/S0378-1119(00)00441-8. PMID 11163959.
  25. ^ Wolfe, Sharp, and Li; Sharp; Li (1989). "Mutation rates differ among regions of the mammalian genome". Nature. 337 (6204): 283–285. Bibcode:1989Natur.337..283W. doi:10.1038/337283a0. PMID 2911369.CS1 maint: multiple names: authors list (link)
  26. ^ a b Galtier; Piganeau, G; Mouchiroud, D; Duret, L; et al. (2001). "GC- content evolution in mammalian genomes: the biased gene conversion hypothesis". Genetics. 159 (2): 907–911. PMC 1461818. PMID 11693127.
  27. ^ Matassi, Sharp, and Gautier (1999). "Chromosomal location effects on gene sequence evolution in mammals". Current Biology. 9 (15): 786–791. doi:10.1016/S0960-9822(99)80361-3. PMID 10469563.CS1 maint: multiple names: authors list (link)
  28. ^ Bernardi and Bernardi; Bernardi, G (1986). "Compositional constraints and genome evolution". Journal of Molecular Evolution. 24 (1–2): 1–11. Bibcode:1986JMolE..24....1B. doi:10.1007/BF02099946. PMID 3104608.
  29. ^ a b Jean; Linton, LM; Birren, B; Nusbaum, C; Zody, MC; Baldwin, J; Devon, K; Dewar, K; et al. (2001). "Initial sequencing and analysis of the human genome" (PDF). Nature. 409 (6822): 860–921. Bibcode:2001Natur.409..860L. doi:10.1038/35057062. PMID 11237011.
  30. ^ a b Elsik; Elsik, Christine G.; Tellam, Ross L.; Worley, Kim C.; Gibbs, Richard A.; Elsik, Christine G.; Tellam, Ross L.; Gibbs, Richard A.; Muzny, Donna M.; et al. (2009). "The genome sequence of taurine cattle: a window to ruminant biology and evolution". Science. 324 (5926): 522–528. Bibcode:2009Sci...324..522A. doi:10.1126/science.1169588. PMC 2943200. PMID 19390049.
  31. ^ a b Nekrutenko and Li; Li, WH (2000). "Assessment of compositional heterogeneity within and between eukaryotic genomes". Genome Research. 10 (12): 1986–1995. doi:10.1101/gr.10.12.1986. PMC 313050. PMID 11116093.
  32. ^ Häring and Kypr; Kypr, J (2001). "No Isochores in the Human Chromosomes 21 and 22?". Biochemical and Biophysical Research Communications. 280 (2): 567–573. doi:10.1006/bbrc.2000.4162. PMID 11162557.
  33. ^ a b Cohen; Dagan, T; Stone, L; Graur, D; et al. (2005). "GC composition of the human genome: in search of isochores". Molecular Biology and Evolution. 22 (5): 1260–1272. doi:10.1093/molbev/msi115. PMID 15728737.
  34. ^ a b c d Elhaik; Graur, D; Josić, K; Landan, G; et al. (2010). "Identifying compositionally homogeneous and nonhomogeneous domains within the human genome using a novel segmentation algorithm". Nucleic Acids Res. 38 (15): e158. doi:10.1093/nar/gkq532. PMC 2926622. PMID 20571085.
  35. ^ Elhaik, Landan, and Graur (2009). "Can GC Content at Third- Codon Positions Be Used as a Proxy for Isochore Composition?". Molecular Biology and Evolution. 26 (8): 1829–1833. doi:10.1093/molbev/msp100. PMID 19443854.CS1 maint: multiple names: authors list (link)
  36. ^ Tatarinova; Alexandrov, NN; Bouck, JB; Feldmann, KA; et al. (2010). "GC3 biology in corn, rice, sorghum and other grasses". BMC Genomics. 11: 308. doi:10.1186/1471-2164-11-308. PMC 2895627. PMID 20470436.
  37. ^ Li; Bernaola-Galvan, Pedro; Carpena, Pedro; Oliver, Jose L; et al. (2003). "Isochores merit the prefix 'iso'". Computational Biology and Chemistry. 27 (1): 5–10. arXiv:physics/0209080. Bibcode:2002physics...9080L. doi:10.1016/S1476-9271(02)00090-7. PMID 12798034.
  38. ^ Clay and Bernardi (2005). "How Not to Search for Isochores: A Reply to Cohen et al". Molecular Biology and Evolution. 22 (12): 2315–2317. doi:10.1093/molbev/msi231. PMID 16093569.
  39. ^ Romiguier; Ranwez, V; Douzery, EJ; Galtier, N; et al. (2010). "Contrasting GC-content dynamics across 33 mammalian genomes: Relationship with life-history traits and chromosome sizes". Genome Research. 20 (8): 1001–1009. doi:10.1101/gr.104372.109. PMC 2909565. PMID 20530252.
  40. ^ Elhaik, Graur, and Josic (2010). "Comparative testing of DNA segmentation algorithms using benchmark simulations". Molecular Biology and Evolution. 27 (5): 1015–1024. doi:10.1093/molbev/msp307. PMID 20018981.CS1 maint: multiple names: authors list (link)
  41. ^ Costantini; Clay, O; Auletta, F; Bernardi, G; et al. (2006). "An isochore map of human chromosomes". Genome Research. 16 (4): 536–541. doi:10.1101/gr.4910606. PMC 1457033. PMID 16597586.
  42. ^ Overall, .; Robinson, Gene E.; Gibbs, Richard A.; Weinstock, George M.; Weinstock, George M.; Robinson, Gene E.; Worley, Kim C.; Evans, Jay D.; et al. (2006). "Insights into social insects from the genome of the honeybee Apis mellifera". Nature. 443 (7114): 931–949. Bibcode:2006Natur.443..931T. doi:10.1038/nature05260. PMC 2048586. PMID 17073008.
  43. ^ Sodergren; Weinstock, George M.; Davidson, Eric H.; Cameron, R. Andrew; Gibbs, Richard A.; Weinstock, George M.; Angerer, Robert C.; Angerer, Lynne M.; Arnone, Maria Ina; et al. (2006). "The genome of the sea urchin Strongylocentrotus purpuratus". Science. 314 (5801): 941–952. Bibcode:2006Sci...314..941S. doi:10.1126/science.1133609. PMC 3159423. PMID 17095691.
  44. ^ Richards, S.; Gibbs, Richard A.; Weinstock, George M.; Brown, Susan J.; Denell, Robin; Beeman, Richard W.; Gibbs, Richard; Beeman, Richard W.; et al. (2008). "The genome of the model beetle and pest Tribolium castaneum". Nature. 452 (7190): 949–955. Bibcode:2008Natur.452..949R. doi:10.1038/nature06784. PMID 18362917.
  45. ^ Kirkness; Haas, BJ; Sun, W; Braig, HR; Perotti, MA; Clark, JM; Lee, SH; Robertson, HM; Kennedy, RC; et al. (2010). "Genome sequences of the human body louse and its primary endosymbiont provide insights into the permanent parasitic lifestyle". Proceedings of the National Academy of Sciences of the United States of America. 107 (27): 12168–12173. Bibcode:2010PNAS..10712168K. doi:10.1073/pnas.1003379107. PMC 2901460. PMID 20566863.
  46. ^ Werren; Richards, S; Desjardins, CA; Niehuis, O; Gadau, J; Colbourne, JK; Nasonia Genome Working, Group; Werren, JH; Richards, S; et al. (2010). "Functional and evolutionary insights from the genomes of three parasitoid Nasonia species". Science. 327 (5963): 343–348. Bibcode:2010Sci...327..343.. doi:10.1126/science.1178028. PMC 2849982. PMID 20075255.
  47. ^ Smith; Zimin, A.; Holt, C.; Abouheif, E.; Benton, R.; Cash, E.; Croset, V.; Currie, C. R.; Elhaik, E.; et al. (2011). "Draft genome of the globally widespread and invasive Argentine ant (Linepithema humile)". Proceedings of the National Academy of Sciences of the United States of America. 108 (14): 5673–5678. Bibcode:2011PNAS..108.5673S. doi:10.1073/pnas.1008617108. PMC 3078359. PMID 21282631.
  48. ^ Smith; Smith, C. D.; Robertson, H. M.; Helmkampf, M.; Zimin, A.; Yandell, M.; Holt, C.; Hu, H.; Abouheif, E.; et al. (2011). "Draft genome of the red harvester ant Pogonomyrmex barbatus". Proceedings of the National Academy of Sciences of the United States of America. 108 (14): 5667–5672. Bibcode:2011PNAS..108.5667S. doi:10.1073/pnas.1007901108. PMC 3078412. PMID 21282651.
  49. ^ Suen; Teiling, C; Li, L; Holt, C; Abouheif, E; Bornberg-Bauer, E; Bouffard, P; Caldera, EJ; Cash, E; et al. (2011). Copenhaver, Gregory (ed.). "The genome sequence of the leaf-cutter ant Atta cephalotes reveals insights into its obligate symbiotic lifestyle". PLoS Genetics. 7 (2): e1002007. doi:10.1371/journal.pgen.1002007. PMC 3037820. PMID 21347285.