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Downloaded Publicly Available Coding Sequence GC Content and Negatively with Chromosome Size [59, Alignments of 8253 Orthologous Genes from 48 Avian Ge- 60] Bolívar et al. Genome Biology (2019) 20:5 https://doi.org/10.1186/s13059-018-1613-z RESEARCH Open Access GC-biased gene conversion conceals the prediction of the nearly neutral theory in avian genomes Paulina Bolívar1, Laurent Guéguen2, Laurent Duret2, Hans Ellegren1 and Carina F. Mugal1* Abstract Background: The nearly neutral theory of molecular evolution predicts that the efficacy of natural selection increases with the effective population size. This prediction has been verified by independent observations in diverse taxa, which show that life-history traits are strongly correlated with measures of the efficacy of selection, such as the dN/dS ratio. Surprisingly, avian taxa are an exception to this theory because correlations between life-history traits and dN/dS are apparently absent. Here we explore the role of GC-biased gene conversion on estimates of substitution rates as a potential driver of these unexpected observations. Results: We analyze the relationship between dN/dS estimated from alignments of 47 avian genomes and several proxies for effective population size. To distinguish the impact of GC-biased gene conversion from selection, we use an approach that accounts for non-stationary base composition and estimate dN/dS separately for changes affected or unaffected by GC-biased gene conversion. This analysis shows that the impact of GC-biased gene conversion on substitution rates can explain the lack of correlations between life-history traits and dN/dS. Strong correlations between life-history traits and dN/dS are recovered after accounting for GC-biased gene conversion. The correlations are robust to variation in base composition and genomic location. Conclusions: Our study shows that gene sequence evolution across a wide range of avian lineages meets the prediction of the nearly neutral theory, the efficacy of selection increases with effective population size. Moreover, our study illustrates that accounting for GC-biased gene conversion is important to correctly estimate the strength of selection. Keywords: Nearly neutral theory, Life-history traits, dN/dS, GC-biased gene conversion, Base composition, Avian genomes Background stochasticity of population genetics, and further estab- With his proposal of the neutral theory of molecular lished the relationship between sequence conservation evolution in 1968, Kimura introduced a revolutionizing and functional importance, which is key to many bio- concept to evolutionary biology, which at that time was informatics software for the identification of conserved strongly influenced by the view that evolution is driven coding as well as non-coding elements [2]. by positive Darwinian selection [1]. Instead, Kimura pro- After realizing the importance of nearly neutral (mainly posed that at the molecular level deleterious mutations slightly deleterious) mutations in molecular evolution, are common, while advantageous mutations are rare, and Tomoko Ohta extended Kimura’s theory to the nearly that in a finite population most evolutionary changes are a neutral theory of molecular evolution [3, 4]. The nearly consequence of the fixation of neutral mutations due to neutral theory of molecular evolution states that the ef- genetic drift. Kimura thus put a new emphasis on the fectiveness of selection depends on a balance between the strength of random genetic drift and the selection * Correspondence: [email protected] coefficient (s) of new mutations [5]. A measure of the 1Department of Ecology and Genetics, Uppsala University, Norbyvägen 18D, 75236 Uppsala, Sweden strength of genetic drift is the effective population size Full list of author information is available at the end of the article (Ne)[6]. At the population level, aside from the mutation © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Bolívar et al. Genome Biology (2019) 20:5 Page 2 of 13 rate, Ne and s together determine the level of genetic di- However, not all observations are in accordance with versity [7, 8]. Over larger evolutionary time scales, Ne and the prediction of the nearly neutral theory. An intriguing s determine the probability of fixation of new mutations, example is the repeated failure to find a positive correl- which ultimately affects the evolutionary rate of change ation between life-history traits and dN/dS in birds [25, [5, 8]. Therefore, the nearly neutral theory not only forms 30, 31]. Since the scenario that the nearly neutral theory the basis for a solid null-hypothesis to test for evidence of does not hold in birds is rather unlikely, there must be a positive Darwinian selection, but also allows clear predic- methodological reason or a biological phenomenon that tions to be formulated, which can be tested against data conceals this relationship. To explore the lack of a posi- [9]. Specifically, the nearly neutral theory predicts that if tive correlation in more detail, the ratio of radical to Ne is small, the effect of genetic drift is strong, and slightly conservative amino acid substitutions (Kr/Kc) has been deleterious mutations are more likely to reach fixation used as an alternative proxy for the efficacy of selection than if Ne is large [5]. Consequently, species with small in avian lineages [25, 31]. This revealed a positive correl- Ne will accumulate more slightly deleterious substitu- ation between body mass and Kr/Kc in birds. This obser- tions over time than species with large Ne. Here, com- vation questions whether dN/dS is an appropriate parison of the evolutionary rate at non-synonymous measure for the efficacy of selection in birds. However, and synonymous sites allows assessment of the strength the use of the Kr/Kc ratio as a measure of the efficacy of of natural selection acting on protein-coding genes, selection is not straightforward either given that radical where low effectiveness of selection results in an ele- and conservative mutations can be both affected by ef- vated (but generally < 1), non-synonymous to synonym- fectively neutral mutations and hence vary with Ne [30]. ous substitution rate ratio (dN/dS). Therefore, to test Later, Figuet et al. [30] explored the lack of correlations the validity of the nearly neutral theory, the relationship between dN/dS and life-history traits in birds from an- between Ne and dN/dS can be explored. other angle. Specifically, they first investigated the hy- Since measuring Ne in natural populations is a challen- potheses that life-history traits are poor proxies for Ne ging task [10–12], a common alternative is to use species and that variation in life-history traits is narrow in birds. characteristics that are associated with Ne as proxies They further explored the possibility that a peculiar dis- [13–15]. For example, this includes geographic range, tribution of fitness effects of new mutations in birds where widely distributed species usually have larger Ne could be responsible for low power to detect a correl- than species with a limited geographic range, such as is- ation. However, they found evidence to reject all these land species [16]; social organization, such as eusociality hypotheses and could thus not explain the lack of a posi- versus solitarity, where eusocial species usually have tive correlation. Botero-Castro et al. [32] recently found lower Ne than solitary species [17]; and mating system, evidence that sequence alignment quality affects the cor- where selfing species usually have lower Ne than out- relation between life-history traits and dN/dS; after re- crossers [18]. It also includes life-history traits that are moving putative alignment errors, a positive correlation associated with growth, survival, and reproductive suc- between dN/dS and longevity, but not between dN/dS cess and strategy [19]. Common examples of life-history and body mass, could be recovered. Moreover, they traits include, among others, adult body mass or size, found stronger correlations when previously undetected maximum longevity, generation time, age of sexual ma- GC-rich orthologs were included in the analysis. Taken turity or age at first reproduction, and fecundity. For in- together, observations from previous studies suggest that stance, species with a small body size tend to have a dN/dS might not be an appropriate measure for the effi- short generation time, large reproductive output, and cacy of selection in birds, potentially because there is usually a large Ne [15, 20]. some force that biases estimates of dN/dS and thereby By empirically testing for a relationship between so- obscures the correlation between dN/dS and life-history cial organization, mating system or life-history traits traits in birds. and dN/dS, several studies have confirmed the prediction It is well documented that avian genomes are strongly of the nearly neutral theory. For example, it has been ob- impacted by GC-biased gene conversion (gBGC) [33– served that eusocial insects show higher dN/dS ratios than 36]. gBGC is a process associated with meiotic recom- solitary species [21]. Another study reported a general bination that leads to the preferential fixation of GC trend for higher dN/dS ratios in the chloroplasts of self- (strong: S) over AT (weak: W) alleles in GC/AT hetero- ers than of outcrossing species of plants [22]. Moreover, zygous sites close to recombination-initiating double- several studies in mammalian taxa have reported strong strand breaks [37, 38]. It acts in a manner similar to dir- positive correlations between generation time, body mass, ectional selection, increasing the probability of fixation or longevity and dN/dS in mitochondrial and nuclear of S over W alleles [39].
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