Genome Changes After Gene Duplication: Haploidy vs. Diploidy

Since genome size and the number of duplicate genes observed in genomes increase from haploid to diploid organisms, diploidy might provide more evolutionary probabilities through gene duplication. It is still unclear how diploidy promotes genomic evolution in detail. In this study, we explored the evolution of segmental gene duplication in haploid and diploid populations by analytical and simulation approaches. Results show that (1) under the double null recessive (DNR) selective model, given the same recombination rate, the evolutionary trajectories and consequences are very similar between the same-size gene-pool haploid vs. diploid populations; (2) recombination enlarges the probability of preservation of duplicate genes in either haploid or diploid large populations, and haplo-insufficiency reinforces this effect; and (3) the loss of duplicate genes at the ancestor locus is limited under recombination while under complete linkage the loss of duplicate genes is always random at the ancestor and newly duplicated loci. Therefore, we propose a model to explain the advantage of diploidy: diploidy might facilitate the increase of recombination rate, especially under sexual reproduction; more duplicate genes are preserved under more recombination by originalization (by which duplicate genes are preserved intact at a special quasi-mutation-selection balance under the DNR or haplo-insufficient selective model), so genome sizes and the number of duplicate genes in diploid organisms become larger. Additionally, it is suggested that small genomic rearrangements due to the random loss of duplicate genes might be limited under recombination.USUALLY genome size becomes larger from haploid to diploid organisms (Lynch and Conery 2003), and so does the number of duplicate genes observed in genomes (Zhang 2003). It is extensively hypothesized that diploidy might facilitate the preservation and accumulation of duplicate genes, but it is still unclear how diploidy supports the evolution of duplicate genes in detail. The superiority of diploidy is classically attributed to preventing expression of deleterious mutations (Crow and Kimura 1965), but it is also argued that the sheltering of deleterious mutations cannot adequately explain the advantages of diploidy (Perrot et al. 1991).Recombination is a common phenomenon in all three kingdoms of life, Bacteria, Eukarya, and Archaea. It has been reported that recombination influences the loss of duplicate genes (Zhang and Kishino 2004; Xue et al. 2010). In diploid organisms, if recombination between the ancestor locus and the newly duplicated locus is free, the rate of recombination is maximally 0.5, which is commonly observed especially when the two loci are located on different chromosomes. Although recombination should not be regarded as an exception in haploid organisms (Fraser et al. 2007), recombination events usually occur more frequently in diploid populations than they do in haploid populations. In other words, diploidy might facilitate the occurrence of recombination. The difference of recombination behaviors between haploid and diploid organisms is an obvious and important feature during genomic evolution.In our recent studies of genomic duplication, we proposed a new possible way of preserving and accumulating duplicate genes in genomes—originalization (Xue and Fu 2009a). As is well known, for a locus in an infinite diploid population, the frequencies of wild-type and degenerative alleles will move to an equilibrium under purifying selection and mutation, which is known as the mutation–selection balance. After genomic duplication, under two simple selective models, double null recessive (DNR, under which valid individuals require at least one active wild-type allele on the ancestor and newly duplicated loci) and haplo-insufficient (HI or partial dominant, under which valid individuals require at least two active wild-type alleles on both loci) models, a special equilibrium of allele frequencies at the ancestor and newly duplicated loci will be reached under recombination, in which the frequency of wild-type allele is kept high at both loci. Under the HI selective model this balance becomes so stable and flexible that the fixation of a degenerative allele at one of these two loci (or the balance being broken) becomes very difficult even in a modest population (Xue and Fu 2009a,b). However, if the two loci are tightly linked (recombination rate r = 0), this balance of allele frequencies does not appear. As r increases, the balance becomes more stable and the frequency of the wild-type allele at two loci becomes higher. High frequency of the wild-type allele at both loci means that duplicate genes are preserved intact in genomes, so this phenomenon was named originalization.Although many duplicate genes originated from genomic duplications in some species, such as yeast, maize, and fish (Li et al. 2005), those from segmental duplications are also very popular (Zhang et al. 2000; Leister 2004). In haploid populations, most duplication events are small segmental duplications. Therefore, to understand genomic evolution comprehensively, it is necessary to explore the evolution of segmental genomic duplication.Lynch et al. (2001) and Tanaka et al. (2009) have studied the evolution of segmental gene duplication in diploid populations theoretically. However, in this study, we further compared the evolution of segmental gene duplication in haploid vs. diploid populations by numerical and simulation approaches under the DNR and HI selective models. We observed that haploid and diploid populations with the same-size gene pool are very similar under the DNR model and the same recombination rate. Recombination enlarges the probability of preservation of duplicate genes in either haploid or diploid populations via originalization, and haplo-insufficiency reinforces this effect. The loss of duplicate genes at the ancestor locus might be limited under recombination, while under complete linkage, the loss of duplicate genes is random at the ancestor and newly duplicated loci. According to these results, we propose a model with which to explain the revolutionary genomic transition from haploidy to diploidy.     

Formation and Longevity of Chimeric and Duplicate Genes in Drosophila melanogaster

Historically, duplicate genes have been regarded as a major source of novel genetic material. However, recent work suggests that chimeric genes formed through the fusion of pieces of different genes may also contribute to the evolution of novel functions. To compare the contribution of chimeric and duplicate genes to genome evolution, we measured their prevalence and persistence within Drosophila melanogaster. We find that ~80.4 duplicates form per million years, but most are rapidly eliminated from the genome, leaving only 4.1% to be preserved by natural selection. Chimeras form at a comparatively modest rate of ~11.4 per million years but follow a similar pattern of decay, with ultimately only 1.4% of chimeras preserved. We propose two mechanisms of chimeric gene formation, which rely entirely on local, DNA-based mutations to explain the structure and placement of the youngest chimeric genes observed. One involves imprecise excision of an unpaired duplication during large-loop mismatch repair, while the other invokes a process akin to replication slippage to form a chimeric gene in a single event. Our results paint a dynamic picture of both chimeras and duplicate genes within the genome and suggest that chimeric genes contribute substantially to genomic novelty.     

Out of the testis,into the ovary: biased outcomes of gene duplication and deletion in Drosophila

Gene turnover is a key source of adaptive variation. Yet most evolutionary studies have focused on gene duplication, dismissing gene deletion as a mechanism that simply eradicates redundancy. Here, I use genome‐scale sequence and multi‐tissue expression data from Drosophila melanogaster and Drosophila pseudoobscura to simultaneously assess the evolutionary outcomes of gene duplication and deletion in Drosophila. I find that gene duplication is more frequent than gene deletion in both species, indicating that it may play a more important role in Drosophila evolution. However, examination of several genic properties reveals that genes likely possess distinct functions after duplication that diverge further before deletion, suggesting that loss of redundancy cannot explain a majority of gene deletion events in Drosophila. Moreover, in addition to providing support for the well‐known “out of the testis” origin of young duplicate genes, analyses of gene expression profiles uncover a preferential bias against deletion of old ovary‐expressed genes. Therefore, I propose a novel “into the ovary” hypothesis for gene deletion in Drosophila, in which gene deletion may promote adaptation by salvaging genes that contribute to the evolution of female reproductive phenotypes. Under this combined “out of the testis, into the ovary” evolutionary model, gene duplication and deletion work in concert to generate and maintain a balanced repertoire of genes that promote sex‐specific adaptation in Drosophila.     

Molecular trajectories leading to the alternative fates of duplicate genes

Gene duplication generates extra gene copies in which mutations can accumulate without risking the function of pre-existing genes. Such mutations modify duplicates and contribute to evolutionary novelties. However, the vast majority of duplicates appear to be short-lived and experience duplicate silencing within a few million years. Little is known about the molecular mechanisms leading to these alternative fates. Here we delineate differing molecular trajectories of a relatively recent duplication event between humans and chimpanzees by investigating molecular properties of a single duplicate: DNA sequences, gene expression and promoter activities. The inverted duplication of the Glutathione S-transferase Theta 2 (GSTT2) gene had occurred at least 7 million years ago in the common ancestor of African great apes and is preserved in chimpanzees (Pan troglodytes), whereas a deletion polymorphism is prevalent in humans. The alternative fates are associated with expression divergence between these species, and reduced expression in humans is regulated by silencing mutations that have been propagated between duplicates by gene conversion. In contrast, selective constraint preserved duplicate divergence in chimpanzees. The difference in evolutionary processes left a unique DNA footprint in which dying duplicates are significantly more similar to each other (99.4%) than preserved ones. Such molecular trajectories could provide insights for the mechanisms underlying duplicate life and death in extant genomes.     

Evolution of vertebrate central nervous system is accompanied by novel expression changes of duplicate genes

The evolution of the central nervous system (CNS) is one of the most striking changes during the transition from invertebrates to vertebrates. As a major source of genetic novelties, gene duplication might play an important role in the functional innovation of vertebrate CNS. In this study, we focused on a group of CNS-biased genes that duplicated during early vertebrate evolution. We investigated the tempo-spatial expression patterns of 33 duplicate gene families and their orthologs during the embryonic development of the vertebrate Xenopus laevis and the cephalochordate Brachiostoma belcheri. Almost all the identified duplicate genes are differentially expressed in the CNS in Xenopus embryos, and more than 50% and 30% duplicate genes are expressed in the telencephalon and mid-hindbrain boundary, respectively, which are mostly considered as two innovations in the vertebrate CNS. Interestingly, more than 50% of the amphioxus orthologs do not show apparent expression in the CNS in amphioxus embryos as detected by in situ hybridization, indicating that some of the vertebrate CNS-biased duplicate genes might arise from non-CNS genes in invertebrates. Our data accentuate the functional contribution of gene duplication in the CNS evolution of vertebrate and uncover an invertebrate non-CNS history for some vertebrate CNS-biased duplicate genes.     

Gene Duplicability of Core Genes Is Highly Consistent across All Angiosperms

Gene duplication is an important mechanism for adding to genomic novelty. Hence, which genes undergo duplication and are preserved following duplication is an important question. It has been observed that gene duplicability, or the ability of genes to be retained following duplication, is a nonrandom process, with certain genes being more amenable to survive duplication events than others. Primarily, gene essentiality and the type of duplication (small-scale versus large-scale) have been shown in different species to influence the (long-term) survival of novel genes. However, an overarching view of “gene duplicability” is lacking, mainly due to the fact that previous studies usually focused on individual species and did not account for the influence of genomic context and the time of duplication. Here, we present a large-scale study in which we investigated duplicate retention for 9178 gene families shared between 37 flowering plant species, referred to as angiosperm core gene families. For most gene families, we observe a strikingly consistent pattern of gene duplicability across species, with gene families being either primarily single-copy or multicopy in all species. An intermediate class contains gene families that are often retained in duplicate for periods extending to tens of millions of years after whole-genome duplication, but ultimately appear to be largely restored to singleton status, suggesting that these genes may be dosage balance sensitive. The distinction between single-copy and multicopy gene families is reflected in their functional annotation, with single-copy genes being mainly involved in the maintenance of genome stability and organelle function and multicopy genes in signaling, transport, and metabolism. The intermediate class was overrepresented in regulatory genes, further suggesting that these represent putative dosage-balance-sensitive genes.     

Rapid subfunctionalization accompanied by prolonged and substantial neofunctionalization in duplicate gene evolution

Gene duplication is the primary source of new genes. Duplicate genes that are stably preserved in genomes usually have divergent functions. The general rules governing the functional divergence, however, are not well understood and are controversial. The neofunctionalization (NF) hypothesis asserts that after duplication one daughter gene retains the ancestral function while the other acquires new functions. In contrast, the subfunctionalization (SF) hypothesis argues that duplicate genes experience degenerate mutations that reduce their joint levels and patterns of activity to that of the single ancestral gene. We here show that neither NF nor SF alone adequately explains the genome-wide patterns of yeast protein interaction and human gene expression for duplicate genes. Instead, our analysis reveals rapid SF, accompanied by prolonged and substantial NF in a large proportion of duplicate genes, suggesting a new model termed subneofunctionalization (SNF). Our results demonstrate that enormous numbers of new functions have originated via gene duplication.     

Evolutionary Patterns of Gene Families Generated in the Early Stage of Vertebrates

In this paper we have analyzed 49 vertebrate gene families that were generated in the early stage of vertebrates and/or shortly before the origin of vertebrates, each of which consists of three or four member genes. We have dated the first (T₁) and second (T₂) gene duplications of 26 gene families with 3 member genes. The means of T₁ (594 mya) and T₂ (488 mya) are largely consistent to a well-cited version of two-round (2R) genome duplication theory. Moreover, in most cases, the time interval between two successive gene duplications is large enough that the fate of duplicate genes generated by the first gene duplication was likely to be determined before the second one took place. However, the phylogenetic pattern of 23 gene families with 4 members is complicated; only 5 of them are predicted by 2R model, but 11 families require an additional gene (or genome) duplication. For the rest (7 families), at least one gene duplication event had occurred before the divergence between vertebrate and Drosophila, indicating a possible misleading of the 4:1 rule (member gene ratio between vertebrates and invertebrates). Our results show that Ohno's 2R conjecture is valid as a working hypothesis for providing a most parsimonious explanation. Although for some gene families, additional gene duplication is needed, the credibility of the third genome duplication (3R) remains to be investigated. Received: 13 December 1999 / Accepted: 7 April 2000     

Duplicate gene evolution and expression in the wake of vertebrate allopolyploidization

Background  

The mechanism by which duplicate genes originate – whether by duplication of a whole genome or of a genomic segment – influences their genetic fates. To study events that trigger duplicate gene persistence after whole genome duplication in vertebrates, we have analyzed molecular evolution and expression of hundreds of persistent duplicate gene pairs in allopolyploid clawed frogs (Xenopus and Silurana). We collected comparative data that allowed us to tease apart the molecular events that occurred soon after duplication from those that occurred later on. We also quantified expression profile divergence of hundreds of paralogs during development and in different tissues.     

Probabilistic cross-species inference of orthologous genomic regions created by whole-genome duplication in yeast

Identification of orthologous genes across species becomes challenging in the presence of a whole-genome duplication (WGD). We present a probabilistic method for identifying orthologs that considers all possible orthology/paralogy assignments for a set of genomes with a shared WGD (here five yeast species). This approach allows us to estimate how confident we can be in the orthology assignments in each genomic region. Two inferences produced by this model are indicative of purifying selection acting to prevent duplicate gene loss. First, our model suggests that there are significant differences (up to a factor of seven) in duplicate gene half-life. Second, we observe differences between the genes that the model infers to have been lost soon after WGD and those lost more recently. Gene losses soon after WGD appear uncorrelated with gene expression level and knockout fitness defect. However, later losses are biased toward genes whose paralogs have high expression and large knockout fitness defects, as well as showing biases toward certain functional groups such as ribosomal proteins. We suggest that while duplicate copies of some genes may be lost neutrally after WGD, another set of genes may be initially preserved in duplicate by natural selection for reasons including dosage.     

Tetraodon genome analysis provides further evidence for whole-genome duplication in the ray-finned fish lineage

A whole-genome duplication in the ray-finned fish lineage has been supported by the analyses of the genome sequence of the Japanese pufferfish, Fugu rubripes. Recently, genome sequence of a second teleost fish, the freshwater pufferfish, Tetraodon nigroviridis, was completed. Comparisons of long-range synteny between the Tetraodon and human genomes provided additional evidence for the whole-genome duplication in the ray-finned fish lineage. In the present study, we conducted phylogenetic analysis of the Tetraodon and human proteins to identify ray-finned fish lineage-specific (‘fish-specific’) duplicate genes in the Tetraodon genome. Our analyses provide evidence for 1087 well defined fish-specific duplicate genes in Tetraodon. We also analyzed the Fugu proteome that was predicted in the recent Fugu genome assembly, and identified 346 duplicate genes in addition to the 425 duplicates previously identified. We estimated the ages of duplicate genes using the molecular clock. The ages of duplicate genes in the two pufferfishes independently support a large-scale gene duplication around 380–400 Myr ago. In addition, a burst of recent gene duplications was evident in the Tetraodon lineage. These findings provide further evidence for a whole-genome duplication early in the evolution of ray-finned fishes, and suggest that independent gene duplications have occurred recently in the Tetraodon lineage.     

Gene duplication in the evolution of sexual dimorphism

Males and females share most of the same genes, so selection in one sex will typically produce a correlated response in the other sex. Yet, the sexes have evolved to differ in a multitude of behavioral, morphological, and physiological traits. How did this sexual dimorphism evolve despite the presence of a common underlying genome? We investigated the potential role of gene duplication in the evolution of sexual dimorphism. Because duplication events provide extra genetic material, the sexes each might use this redundancy to facilitate sex‐specific gene expression, permitting the evolution of dimorphism. We investigated this hypothesis at the genome‐wide level in Drosophila melanogaster, using the presence of sex‐biased expression as a proxy for the sex‐specific specialization of gene function. We expected that if sexually antagonistic selection is a potent force acting upon individual genes, duplication will result in paralog families whose members differ in sex‐biased expression. Gene members of the same duplicate family can have different expression patterns in males versus females. In particular, duplicate pairs containing a male‐biased gene are found more frequently than expected, in agreement with previous studies. Furthermore, when the singleton ortholog is unbiased, duplication appears to allow one of the paralog copies to acquire male‐biased expression. Conversely, female‐biased expression is not common among duplicates; fewer duplicate genes are expressed in the female‐soma and ovaries than in the male‐soma and testes. Expression divergence exists more in older than in younger duplicates pairs, but expression divergence does not correlate with protein sequence divergence. Finally, genomic proximity may have an effect on whether paralogs differ in sex‐biased expression. We conclude that the data are consistent with a role of gene duplication in fostering male‐biased, but not female‐biased, gene expression, thereby aiding the evolution of sexual dimorphism.     

Asymmetric functional divergence of duplicate genes in yeast

Most duplicate genes are eliminated from a genome shortly after duplication, but those that remain are an important source of biochemical diversity. Here, I present evidence from genome-scale protein-protein interaction data, microarray expression data, and large-scale gene knockout data that this diversification is often asymmetrical: one duplicate usually shows significantly more molecular or genetic interactions than the other. I propose a model that can explain this divergence pattern if asymmetrically diverging duplicate gene pairs show increased robustness to deleterious mutations.     

Copy correction and concerted evolution in the conservation of yeast genes

The yeast Saccharomyces cerevisiae and other members of the genus Saccharomyces are descendants of an ancient whole-genome duplication event. Although most of the duplicate genes have since been deleted, many remain, and so there are many pairs of related genes. We have found that poorly expressed genes diverge rapidly from their paralog, while highly expressed genes diverge little, if at all. This lack of divergence of highly expressed paralogous gene pairs seems to involve gene correction: one member of the pair “corrects” the sequence of its twin, and so the gene pair evolves as a unit. This correction presumably involves gene conversion and could occur via a reverse-transcribed cDNA intermediate. Such correction events may also occur in other organisms. These results support the idea that copies of poorly expressed genes are preserved when they diverge to take on new functions, while copies of highly expressed genes are preserved when they are needed to provide additional gene product for the original function.     

Analysing gene function after duplication

After gene duplication, mutations cause the gene copies to diverge. The classical model predicts that these mutations will generally lead to the loss of function of one gene copy; rarely, new functions will be created and both duplicate genes are conserved. In contrast, under the subfunctionalization model both duplicates are preserved due to the partition of different functions between the duplicates. A recent study provides support for the subfunctionalization model, identifying several expressed gene duplicates common to humans and mice that contain regions conserved in one duplicate but variable in the other (and vice versa). We discuss both the methodology used in this study and also how gene phylogeny may lead to additional evidence for the importance of subfunctionalization in the evolution of new genes.     

Pervasive indels and their evolutionary dynamics after the fish-specific genome duplication

Insertions and deletions (indels) in protein-coding genes are important sources of genetic variation. Their role in creating new proteins may be especially important after gene duplication. However, little is known about how indels affect the divergence of duplicate genes. We here study thousands of duplicate genes in five fish (teleost) species with completely sequenced genomes. The ancestor of these species has been subject to a fish-specific genome duplication (FSGD) event that occurred approximately 350 Ma. We find that duplicate genes contain at least 25% more indels than single-copy genes. These indels accumulated preferentially in the first 40 my after the FSGD. A lack of widespread asymmetric indel accumulation indicates that both members of a duplicate gene pair typically experience relaxed selection. Strikingly, we observe a 30-80% excess of deletions over insertions that is consistent for indels of various lengths and across the five genomes. We also find that indels preferentially accumulate inside loop regions of protein secondary structure and in regions where amino acids are exposed to solvent. We show that duplicate genes with high indel density also show high DNA sequence divergence. Indel density, but not amino acid divergence, can explain a large proportion of the tertiary structure divergence between proteins encoded by duplicate genes. Our observations are consistent across all five fish species. Taken together, they suggest a general pattern of duplicate gene evolution in which indels are important driving forces of evolutionary change.     

Duplicate Gene Evolution Toward Multiple Fates at the <Emphasis Type="Italic">Drosophila melanogaster HIP</Emphasis>/<Emphasis Type="Italic">HIP-Replacement</Emphasis> Locus

Hsc/Hsp70-interacting protein (HIP) is a rapidly evolving Hsp70 cofactor. Analyses of multiple Drosophila species indicate that the HIP gene is duplicated only in D. melanogaster. The HIP region, in fact, contains seven distinctly evolving duplicated genes. The regional duplication occurred in two steps, fixed rapidly, and illustrates multiple modes of duplicate gene evolution. HIP and its duplicate HIP-R are adaptively evolving in a manner unique to the region: they exhibit elevated divergence from other drosophilids and low polymorphism within D. melanogaster. HIP and HIP-R are virtually identical, share polymorphisms, and are subject to gene conversion. In contrast, two other duplicate genes in the region, CG33221 and GP-CG32779, are pseudogenes, and the chimeric gene Crg1 is subject to balancing selection. HIP and HIP-R are evolving rapidly and adaptively; however, positive selection is not sufficient to explain the molecular evolution of the region as a whole. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular evolution of theSaccharomyces cerevisiae histone gene loci

Summary The core histone genes ofSaccharomyces cerevisiae are arranged as duplicate nonallelic sets of specifically paired genes. The identity of structural organization between the duplicated gene pairs would have its simplest evolutionary origin in the duplication of a complete locus in a single event. In such a case, the time since the duplication of one of the genes should be identical to that since duplication of the gene adjacent to it on the chromosome. A calculation of the evolutionary distances between the coding DNA sequences of the histone genes leads to a duplication paradox: The extents of sequence divergence in the silent component of third-base positions for adjacent pairs of genes are not identical. Estimates of the evolutionary distance between the two H3-H4 noncoding intergene DNA sequences are large; the divergence between the two separate sequences is indistinguishable from the divergence between either of the regions and a randomly generated permutation of itself. These results suggest that the duplication event may have occurred much earlier than previously estimated. The potential age of the duplication, and the attractive simplicity of the duplication of both the H3-H4 and the H2A-H2B gene pairs having taken place in a single event, leads to the hypothesis that modern haploidS. cerevisiae may have evolved by diploidization or fusion of two ancient fungi.     

Expression diversity and evolutionary dynamics of rice duplicate genes

Duplicate genes are believed to be a major source of new gene functions over evolutionary time. In order to evaluate the evolutionary dynamics of rice duplicate genes, formed principally by paleoployploidization prior to the speciation of the Poaceae family, we have employed a public microarray dataset including 155 gene expression omnibus sample plates and bioinformatics tools. At least 57.4% of old ~70 million years ago (MYA) duplicate gene pairs exhibit divergences in expression over the given experimental set, whereas at least 50.9% of young ~7.7-MYA duplicate gene pairs were shown to be divergent. When grouping the rice duplicate genes according to functional categories, we noted a striking and significant enrichment of divergent duplicate metabolism-associated genes, as compared to that observed in non-divergent duplicate genes. While both non-synonymous substitution (Ka) and synonymous substitution (Ks) values between non- and divergent duplicate gene pairs evidenced significant differences, the Ka/Ks values between them exhibited no significant differences. Interestingly, the average numbers of conserved motifs of the duplicate gene pairs revealed a pattern of decline along with an increase in expression diversity, partially supporting the subfunctionalization model with degenerative complementation in regulatory motifs. Duplicate gene pairs with high local similarity (HLS) segments, which might be formed via conversion between rice paleologs, evidenced higher expression correlations than were observed in the gene pairs without the HLS segments; this probably resulted in an increased likelihood of gene conversion in promoters of the gene pairs harboring HLS segments. More than 60% of the rice gene families exhibited similar high expression diversity between members as compared to that of randomly selected gene pairs. These findings are likely reflective of the evolutionary dynamics of rice duplicate genes for gene retention. An erratum to this article can be found at