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The Neoselectionist Theory of Genome Evolution

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The Neoselectionist Theory of Genome Evolution The neoselectionist theory of genome evolution Giorgio Bernardi* Molecular Evolution Laboratory, Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Naples, Italy Edited by Tomoko Ohta, National Institute of Genetics, Mishima, Japan, and approved April 4, 2007 (received for review February 22, 2007) The vertebrate genome is a mosaic of GC-poor and GC-rich isoch- of chance in evolution. A very significant modification of the ores, megabase-sized DNA regions of fairly homogeneous base neutral theory was the nearly neutral theory of Ohta (11, 12), composition that differ in relative amount, gene density, gene who proposed that a substantial fraction of changes are caused expression, replication timing, and recombination frequency. At by random fixation of nearly neutral changes, a class that the emergence of warm-blooded vertebrates, the gene-rich, mod- ‘‘includes intermediates between neutral and advantageous, as erately GC-rich isochores of the cold-blooded ancestors underwent well as between neutral and deleterious classes’’ (13). The first a GC increase. This increase was similar in mammals and birds and four schemes of Fig. 1 display a qualitative picture of the classical was maintained during the evolution of mammalian and avian theories just mentioned. orders. Neither the GC increase nor its conservation can be ac- All classical theories proposed that natural selection acted on counted for by the random fixation of neutral or nearly neutral the ‘‘phenotype’’ [called the ‘‘classical phenotype’’ by Bernardi single-nucleotide changes (i.e., the vast majority of nucleotide and Bernardi (14), to distinguish it from the ‘‘genome pheno- substitutions) or by a biased gene conversion process occurring at type’’ presented below]. Whether visualized through hereditary random genome locations. Both phenomena can be explained, morphological traits (by Darwin), genetic characters (by the however, by the neoselectionist theory of genome evolution that neo-Darwinians), or protein and DNA sequences (by Kimura), is presented here. This theory fully accepts Ohta’s nearly neutral the classical phenotype is the result of the expression of single view of point mutations but proposes in addition (i) that the genes or of a small number of genes, and its evolution was seen AT-biased mutational input present in vertebrates pushes some as essentially due to single-nucleotide mutations. DNA regions below a certain GC threshold; (ii) that these lower GC In contrast, the ‘‘neoselectionist theory’’ arose by comparing levels cause regional changes in chromatin structure that lead to the genome phenotypes (14), namely the ‘‘compositional pat- deleterious effects on replication and transcription; and (iii) that terns’’ or ‘‘compositional landscapes’’ of whole genomes. The the carriers of these changes undergo negative (purifying) selec- ‘‘compositional approach’’ was initially made possible by a tion, the final result being a compositional conservation of the high-resolution analysis (see refs. 15–18) of eukaryotic genomes, original isochore pattern in the surviving population. Negative in which DNA was fractionated by ultracentrifugation in Cs2SO4 selection may also largely explain the GC increase accompanying density gradients in the presence of sequence-specific ligands the emergence of warm-blooded vertebrates. In conclusion, the (Agϩ or, later, BAMD [bis(acetato-mercuri-methyl)dioxane]). neoselectionist theory not only provides a solution to the neutral- Although experimentally sophisticated, this approach was con- ist/selectionist debate but also introduces an epigenomic compo- ceptually simple, because it concerned the frequency of ligand- nent in genome evolution. binding oligonucleotides, a property correlated with the base composition of DNA. This compositional approach was ex- ͉ ͉ ͉ base composition isochores nearly neutral theory neutral theory tended by computer work to sequences of genes and whole genomes as soon as they became available. he most famous sentence from The Origin of Species (1), The compositional approach led to the following: (i) the T‘‘This preservation of favourable variations and the rejection discovery of compositional compartmentalization, composi- of injurious variations I call Natural Selection,’’ suggests a tional correlations, and compositional evolution; (ii) evidence dichotomy in the fate of ‘‘variations’’ and is generally interpreted that genome evolution does not only proceed by point mutations, accordingly. This sentence was immediately followed, however, but also by regional changes; and (iii) the development of the by another one that still is only exceptionally quoted: ‘‘Variations neoselectionist theory of genome evolution. The three points neither useful nor injurious would not be affected by natural under (i) will be summarized in the sections below by using our selection.’’ I.e., Darwin distinguished not two, but three kinds of results on the vertebrate genome. variations: advantageous, deleterious, and neutral. Whereas advantageous variations expand in the progeny (by positive, or Compositional Compartmentalization: Isochores and Darwinian, selection), the deleterious ones tend to disappear (by Chromosomal Bands, and Gene Distribution negative, or purifying, selection), and the neutral ones may come The genomes of mammals are compositionally compartmental- out of their limbo to be fixed (like the advantageous variations), ized (16), in that they are ‘‘mosaics of isochores’’ (18), fairly or to disappear (like the deleterious ones). Incidentally, the homogeneous regions, originally estimated as longer than 300 kb concept of neutral variations is absent in Wallace (2). (the upper limit that could be measured by the approach used at Neutral variations were simply obliterated by the selectionists that time; see previous section). These regions could be assigned (the neo-Darwinians) (3, 4), although not by all of them (see ref. to five compositionally narrow families that cover a very wide 5). They were, however, resurrected by the neutral theory, which GC range (34–59% GC in the human genome). This discontin- broke the long predominance of the selectionist theories (see ref. uous compartmentalization was in sharp contrast with the EVOLUTION 6). Indeed, Kimura (7, 8) claimed that ‘‘the main cause of evolutionary change at the molecular level—changes in the genetic material itself—is random fixation of selectively neutral Author contributions: G.B. designed research, performed research, analyzed data, and or nearly neutral mutants.’’ Darwin’s ‘‘survival of the fittest’’ was wrote the paper. replaced by Kimura’s ‘‘survival of the luckiest,’’ and Darwinian The author declares no conflict of interest. and neo-Darwinian evolution were substituted by ‘‘non- This article is a PNAS Direct Submission. Darwinian evolution’’ (9). Kimura’s revolutionary proposal Freely available online through the PNAS open access option. started a neutralist/selectionist debate, which is still going on *To whom correspondence should be sent. E-mail: [email protected]. (see ref. 10) and which concerns the important issue of the role © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0701652104 PNAS ͉ May 15, 2007 ͉ vol. 104 ͉ no. 20 ͉ 8385–8390 Downloaded by guest on September 30, 2021 highest-resolution bands observed by Yunis et al. (21) in early prophase, and the average size of an isochore is equal to the average size of a ‘‘site of replication’’ as defined by Ma et al. (22). It is generally accepted that the molecular basis of cytogenetic bands is not well understood. Isochores do allow one, however, to define the standard Giemsa and reverse bands at a 850- or 400-band resolution on purely compositional grounds (23), thus ending the debate on whether the chromosomal bands are due to DNA, as thought by Caspersson (see ref. 24), or to the associated proteins (25). In addition, the isochore map allowed us to detect a nested structure that concerns not only contiguous isochores but also contiguous high-resolution bands (23). The assessment of gene density in compositional DNA frac- tions (see Fig. 2) led to the discovery (20, 26, 27) that genes are not uniformly distributed in the mammalian genome, contrary to Fig. 1. Darwin postulated the existence of deleterious, advantageous, and what was previously thought. Indeed, in the human genome neutral changes. The neo-Darwinians (or selectionists) neglected neutral almost two-thirds of the protein-coding genes are concentrated changes. These were reintroduced and amplified by Kimura (7, 8), who in the GC-richest isochore families H2 and H3 (only representing developed the neutral theory of evolution (a non-Darwinian evolution, ac- 15% of the genome), which was called the ‘‘genome core’’ (28, cording to ref. 9). The nearly neutral theory was proposed by Ohta (12, 13, 102) 29). The rest is spread over the vast (Ϸ85%) GC-poor ‘‘empty to include intermediates between neutral and advantageous, as well as space,’’ or ‘‘empty quarter’’ (from the name of the Arabian between neutral and deleterious changes. In the neoselectionist theory, the desert), eventually called the ‘‘genome desert’’ (30–32), namely critical changes are responsible for the transition from point mutations to the GC-poor isochore families L1, L2, and H1. These two ‘‘gene regional changes (modified from refs. 12 and 103). spaces’’ (30) are different not only in gene density but also in a number of other basic properties, which are summarized in Fig. then-predominant view of a continuous compositional
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