The origin of the Jingwei gene and the complex modular structure of its parental gene, yellow emperor, in Drosophila melanogaster

Functional dissection and assembly of a small, newly evolved, W chromosome-specific genomic region of the African clawed frog Xenopus laevis

Genetic triggers for sex determination are frequently co-inherited with other linked genes that may also influence one or more sex-specific phenotypes. To better understand how sex-limited regions evolve and function, we studied a small W chromosome-specific region of the frog Xenopus laevis that contains only three genes (dm-w, scan-w, ccdc69-w) and that drives female differentiation. Using gene editing, we found that the sex-determining function of this region requires dm-w but that scan-w and ccdc69-w are not essential for viability, female development, or fertility. Analysis of mesonephros+gonad transcriptomes during sexual differentiation illustrates masculinization of the dm-w knockout transcriptome, and identifies mostly non-overlapping sets of differentially expressed genes in separate knockout lines for each of these three W-specific gene compared to wildtype sisters. Capture sequencing of almost all Xenopus species and PCR surveys indicate that the female-determining function of dm-w is present in only a subset of species that carry this gene. These findings map out a dynamic evolutionary history of a newly evolved W chromosome-specific genomic region, whose components have distinctive functions that frequently degraded during Xenopus diversification, and evidence the evolutionary consequences of recombination suppression.

Evolution and function of developmentally dynamic pseudogenes in mammals

BACKGROUND

Pseudogenes are excellent markers for genome evolution, which are emerging as crucial regulators of development and disease, especially cancer. However, systematic functional characterization and evolution of pseudogenes remain largely unexplored.

RESULTS

To systematically characterize pseudogenes, we date the origin of human and mouse pseudogenes across vertebrates and observe a burst of pseudogene gain in these two lineages. Based on a hybrid sequencing dataset combining full-length PacBio sequencing, sample-matched Illumina sequencing, and public time-course transcriptome data, we observe that abundant mammalian pseudogenes could be transcribed, which contribute to the establishment of organ identity. Our analyses reveal that developmentally dynamic pseudogenes are evolutionarily conserved and show an increasing weight during development. Besides, they are involved in complex transcriptional and post-transcriptional modulation, exhibiting the signatures of functional enrichment. Coding potential evaluation suggests that 19% of human pseudogenes could be translated, thus serving as a new way for protein innovation. Moreover, pseudogenes carry disease-associated SNPs and conduce to cancer transcriptome perturbation.

CONCLUSIONS

Our discovery reveals an unexpectedly high abundance of mammalian pseudogenes that can be transcribed and translated, and these pseudogenes represent a novel regulatory layer. Our study also prioritizes developmentally dynamic pseudogenes with signatures of functional enrichment and provides a hybrid sequencing dataset for further unraveling their biological mechanisms in organ development and carcinogenesis in the future.

Improved RAD51 binders through motif shuffling based on the modularity of BRC repeats

Exchanges of protein sequence modules support leaps in function unavailable through point mutations during evolution. Here we study the role of the two RAD51-interacting modules within the eight binding BRC repeats of BRCA2. We created 64 chimeric repeats by shuffling these modules and measured their binding to RAD51. We found that certain shuffled module combinations were stronger binders than any of the module combinations in the natural repeats. Surprisingly, the contribution from the two modules was poorly correlated with affinities of natural repeats, with a weak BRC8 repeat containing the most effective N-terminal module. The binding of the strongest chimera, BRC8-2, to RAD51 was improved by -2.4 kCal/mol compared to the strongest natural repeat, BRC4. A crystal structure of RAD51:BRC8-2 complex shows an improved interface fit and an extended β-hairpin in this repeat. BRC8-2 was shown to function in human cells, preventing the formation of nuclear RAD51 foci after ionizing radiation.

Hidden genomic evolution in a morphospecies-The landscape of rapidly evolving genes in Tetrahymena

A morphospecies is defined as a taxonomic species based wholly on morphology, but often morphospecies consist of clusters of cryptic species that can be identified genetically or molecularly. The nature of the evolutionary novelty that accompanies speciation in a morphospecies is an intriguing question. Morphospecies are particularly common among ciliates, a group of unicellular eukaryotes that separates 2 kinds of nuclei—the silenced germline nucleus (micronucleus [MIC]) and the actively expressed somatic nucleus (macronucleus [MAC])—within a common cytoplasm. Because of their very similar morphologies, members of the Tetrahymena genus are considered a morphospecies. We explored the hidden genomic evolution within this genus by performing a comprehensive comparative analysis of the somatic genomes of 10 species and the germline genomes of 2 species of Tetrahymena. These species show high genetic divergence; phylogenomic analysis suggests that the genus originated about 300 million years ago (Mya). Seven universal protein domains are preferentially included among the species-specific (i.e., the youngest) Tetrahymena genes. In particular, leucine-rich repeat (LRR) genes make the largest contribution to the high level of genome divergence of the 10 species. LRR genes can be sorted into 3 different age groups. Parallel evolutionary trajectories have independently occurred among LRR genes in the different Tetrahymena species. Thousands of young LRR genes contain tandem arrays of exactly 90-bp exons. The introns separating these exons show a unique, extreme phase 2 bias, suggesting a clonal origin and successive expansions of 90-bp–exon LRR genes. Identifying LRR gene age groups allowed us to document a Tetrahymena intron length cycle. The youngest 90-bp exon LRR genes in T. thermophila are concentrated in pericentromeric and subtelomeric regions of the 5 micronuclear chromosomes, suggesting that these regions act as genome innovation centers. Copies of a Tetrahymena Long interspersed element (LINE)-like retrotransposon are very frequently found physically adjacent to 90-bp exon/intron repeat units of the youngest LRR genes. We propose that Tetrahymena species have used a massive exon-shuffling mechanism, involving unequal crossing over possibly in concert with retrotransposition, to create the unique 90-bp exon array LRR genes.

Genomic comparison of ten morphologically very similar species of ciliate from the genus Tetrahymena reveals how parallel microevolutionary processes have shaped their genomes and created unique genes through retrotransposition.

Evolution of a novel chimeric maltotriose transporter in Saccharomyces eubayanus from parent proteins unable to perform this function

At the molecular level, the evolution of new traits can be broadly divided between changes in gene expression and changes in protein-coding sequence. For proteins, the evolution of novel functions is generally thought to proceed through sequential point mutations or recombination of whole functional units. In Saccharomyces, the uptake of the sugar maltotriose into the cell is the primary limiting factor in its utilization, but maltotriose transporters are relatively rare, except in brewing strains. No known wild strains of Saccharomyces eubayanus, the cold-tolerant parent of hybrid lager-brewing yeasts (Saccharomyces cerevisiae x S. eubayanus), are able to consume maltotriose, which limits their ability to fully ferment malt extract. In one strain of S. eubayanus, we found a gene closely related to a known maltotriose transporter and were able to confer maltotriose consumption by overexpressing this gene or by passaging the strain on maltose. Even so, most wild strains of S. eubayanus lack native maltotriose transporters. To determine how this rare trait could evolve in naive genetic backgrounds, we performed an adaptive evolution experiment for maltotriose consumption, which yielded a single strain of S. eubayanus able to grow on maltotriose. We mapped the causative locus to a gene encoding a novel chimeric transporter that was formed by an ectopic recombination event between two genes encoding transporters that are unable to import maltotriose. In contrast to classic models of the evolution of novel protein functions, the recombination breakpoints occurred within a single functional domain. Thus, the ability of the new protein to carry maltotriose was likely acquired through epistatic interactions between independently evolved substitutions. By acquiring multiple mutations at once, the transporter rapidly gained a novel function, while bypassing potentially deleterious intermediate steps. This study provides an illuminating example of how recombination between paralogs can establish novel interactions among substitutions to create adaptive functions.

Hybrids of the yeasts Saccharomyces cerevisiae and Saccharomyces eubayanus (lager-brewing yeasts) dominate the modern brewing industry. S. cerevisiae, also known as baker’s yeast, is well-known for its role in industry and scientific research. Less well recognized is S. eubayanus, which was only discovered as a pure species in 2011. While most lager-brewing yeasts rapidly and completely utilize the important brewing sugar maltotriose, no strain of S. eubayanus isolated to date is known to do so. Despite being unable to consume maltotriose, we identified one strain of S. eubayanus carrying a gene for a functional maltotriose transporter, although most strains lack this gene. During an adaptive evolution experiment, a strain of S. eubayanus without native maltotriose transporters evolved the ability to grow on maltotriose. Maltotriose consumption in the evolved strain resulted from a chimeric transporter that arose by shuffling genes encoding parent proteins that were unable to transport maltotriose. Traditionally, functional chimeric proteins are thought to evolve by shuffling discrete functional domains or modules, but the breakpoints in the chimera studied here occurred within the single functional module of the protein. These results support the less well-recognized role of shuffling duplicate gene sequences to generate novel proteins with adaptive functions.

The Genomic Impact of Gene Retrocopies: What Have We Learned from Comparative Genomics, Population Genomics, and Transcriptomic Analyses?

Gene duplication is a major driver of organismal evolution. Gene retroposition is a mechanism of gene duplication whereby a gene's transcript is used as a template to generate retroposed gene copies, or retrocopies. Intriguingly, the formation of retrocopies depends upon the enzymatic machinery encoded by retrotransposable elements, genomic parasites occurring in the majority of eukaryotes. Most retrocopies are depleted of the regulatory regions found upstream of their parental genes; therefore, they were initially considered transcriptionally incompetent gene copies, or retropseudogenes. However, examples of functional retrocopies, or retrogenes, have accumulated since the 1980s. Here, we review what we have learned about retrocopies in animals, plants and other eukaryotic organisms, with a particular emphasis on comparative and population genomic analyses complemented with transcriptomic datasets. In addition, these data have provided information about the dynamics of the different "life cycle" stages of retrocopies (i.e., polymorphic retrocopy number variants, fixed retropseudogenes and retrogenes) and have provided key insights into the retroduplication mechanisms, the patterns and evolutionary forces at work during the fixation process and the biological function of retrogenes. Functional genomic and transcriptomic data have also revealed that many retropseudogenes are transcriptionally active and a biological role has been experimentally determined for many. Finally, we have learned that not only non-long terminal repeat retroelements but also long terminal repeat retroelements play a role in the emergence of retrocopies across eukaryotes. This body of work has shown that mRNA-mediated duplication represents a widespread phenomenon that produces an array of new genes that contribute to organismal diversity and adaptation.

Multi-step formation, evolution, and functionalization of new cytoplasmic male sterility genes in the plant mitochondrial genomes

New gene origination is a major source of genomic innovations that confer phenotypic changes and biological diversity. Generation of new mitochondrial genes in plants may cause cytoplasmic male sterility (CMS), which can promote outcrossing and increase fitness. However, how mitochondrial genes originate and evolve in structure and function remains unclear. The rice Wild Abortive type of CMS is conferred by the mitochondrial gene WA352c (previously named WA352) and has been widely exploited in hybrid rice breeding. Here, we reconstruct the evolutionary trajectory of WA352c by the identification and analyses of 11 mitochondrial genomic recombinant structures related to WA352c in wild and cultivated rice. We deduce that these structures arose through multiple rearrangements among conserved mitochondrial sequences in the mitochondrial genome of the wild rice Oryza rufipogon, coupled with substoichiometric shifting and sequence variation. We identify two expressed but nonfunctional protogenes among these structures, and show that they could evolve into functional CMS genes via sequence variations that could relieve the self-inhibitory potential of the proteins. These sequence changes would endow the proteins the ability to interact with the nucleus-encoded mitochondrial protein COX11, resulting in premature programmed cell death in the anther tapetum and male sterility. Furthermore, we show that the sequences that encode the COX11-interaction domains in these WA352c-related genes have experienced purifying selection during evolution. We propose a model for the formation and evolution of new CMS genes via a "multi-recombination/protogene formation/functionalization" mechanism involving gradual variations in the structure, sequence, copy number, and function.

Maize opaque10 Encodes a Cereal-Specific Protein That Is Essential for the Proper Distribution of Zeins in Endosperm Protein Bodies

Cereal storage proteins are major nitrogen sources for humans and livestock. Prolamins are the most abundant storage protein in most cereals. They are deposited into protein bodies (PBs) in seed endosperm. The inner structure and the storage mechanism for prolamin PBs is poorly understood. Maize opaque10 (o10) is a classic opaque endosperm mutant with misshapen PBs. Through positional cloning, we found that O10 encodes a novel cereal-specific PB protein. Its middle domain contains a seven-repeat sequence that is responsible for its dimerization. Its C terminus contains a transmembrane motif that is required for its ER localization and PB deposition. A cellular fractionation assay indicated that O10 is initially synthesized in the cytoplasm and then anchored to the ER and eventually deposited in the PB. O10 can interact with 19-kD and 22-kD α-zeins and 16-kD and 50-kD γ-zeins through its N-terminal domain. An immunolocalization assay indicated that O10 co-localizes with 16-kD γ-zein and 22-kD α-zein in PBs, forming a ring-shaped structure at the interface between the α-zein-rich core and the γ-zein-rich peripheral region. The loss of O10 function disrupts this ring-shaped distribution of 22-kD and 16-kD zeins, resulting in misshapen PBs. These results showed that O10, as a newly evolved PB protein, is essential for the ring-shaped distribution of 22-kD and 16-kD zeins and controls PB morphology in maize endosperm.

Through the positional cloning of the maize classic endosperm mutant opaque10 (o10), we identified a novel protein critical for PB morphology. O10 is a fast-evolving cereal-specific gene with recent origin. A thorough characterization of its three functional domains revealed its important functions for storage protein deposition and organization in PBs. O10 determines a ring-shaped layer in PBs through direct interaction with two major storage proteins (22-kD and 16-kD zeins). This newly characterized PB layer maintains a stable spherical morphology for PB. This study advanced our understanding of PB structure and function. Furthermore, this study demonstrated the origin of a new functional gene and the functional evolution of a storage organelle that is highly valuable to humans.

Correlated expression of retrocopies and parental genes in zebrafish

Previous studies of the function and evolution of retrocopies in plants, Drosophila and non-mammalian chordates provided new insights into the origin of novel genes. However, little is known about retrocopies and their parental genes in teleosts, and it remains obscure whether there is any correlation between them. The present study aimed to characterize the spatial and temporal expression profiles of retrogenes and their parental genes based on RNA-Seq data from Danio rerio embryos and tissues from adult. Using a modified pipeline, 306 retrocopies were identified in the zebrafish genome, most of which exhibited ancient retroposition, and 76 of these showed a Ks < 2.0. Expression of a retrocopy is generally expected to present no correlation with its parental gene, as regulatory regions are not part of the retroposition event. Here, this assumption was tested based on RNA-Seq data from eight stages and thirteen tissue types of zebrafish. However, the result suggested that retrocopies displayed correlated expression with their parental genes. The level of correlation was found to decrease during embryogenesis, but to increase slightly within a tissue using Ks as the proxy for the divergence time. Tissue specificity was also observed: retrocopies were found to be expressed at a more specific level compared with their parental genes. Unlike Drosophila, which has sex chromosomes, zebrafish do not show testis-biased expression. Our study elaborated temporal and spatial patterns of expression of retrocopies in zebrafish, examined the correlation between retrocopies and parental genes and analyzed potential source of regulated elements of retrocopies, which lay a foundation for further functional study of retrocopies.

Evolutionary fate and implications of retrocopies in the African coelacanth genome

BACKGROUND

The coelacanth is known as a "living fossil" because of its morphological resemblance to its fossil ancestors. Thus, it serves as a useful model that provides insight into the fish that first walked on land. Retrocopies are a type of novel genetic element that are likely to contribute to genome or phenotype innovations. Thus, investigating retrocopies in the coelacanth genome can determine the role of retrocopies in coelacanth genome innovations and perhaps even water-to-land adaptations.

RESULTS

We determined the dS values, dN/dS ratios, expression patterns, and enrichment of functional categories for 472 retrocopies in the African coelacanth genome. Of the retrocopies, 85-355 were shown to be potentially functional (i.e., retrogenes). The distribution of retrocopies based on their dS values revealed a burst pattern of young retrocopies in the genome. The retrocopy birth pattern was shown to be more similar to that in tetrapods than ray-finned fish, which indicates a genomic transformation that accompanied vertebrate evolution from water to land. Among these retrocopies, retrogenes were more prevalent in old than young retrocopies, which indicates that most retrocopies may have been eliminated during evolution, even though some retrocopies survived, attained biological function as retrogenes, and became old. Transcriptome data revealed that many retrocopies showed a biased expression pattern in the testis, although the expression was not specifically associated with a particular retrocopy age range. We identified 225 Ensembl genes that overlapped with the coelacanth genome retrocopies. GO enrichment analysis revealed different overrepresented GO (gene ontology) terms between these "retrocopy-overlapped genes" and the retrocopy parent genes, which indicates potential genomic functional organization produced by retrotranspositions. Among the 225 retrocopy-overlapped genes, we also identified 46 that were coelacanth-specific, which could represent a potential molecular basis for coelacanth evolution.

CONCLUSIONS

Our study identified 472 retrocopies in the coelacanth genome. Sequence analysis of these retrocopies and their parent genes, transcriptome data, and GO annotation information revealed novel insight about the potential role of genomic retrocopies in coelacanth evolution and vertebrate adaptations during the evolutionary transition from water to land.

Independent birth of a novel TRIMCyp in Tupaia belangeri with a divergent function from its paralog TRIM5

The origin of novel genes and their evolutionary fates are long-standing questions in evolutionary biology. These questions become more complicated for genes conserved across various lineages, such as TRIM5, an antiretroviral restriction factor and a retrovirus capsid sensor in immune signaling. TRIM5 has been subjected to numerous pathogenic challenges and undergone dynamic evolution, making it an excellent example for studying gene diversification. Previous studies among several species showed that TRIM5 gained genetic and functional novelty in a lineage-specific manner, either through gene duplication or a cyclophilin A retrotransposing into the TRIM5 locus, creating the gene fusion known as TRIM5-Cyclophilin A (TRIMCyp). To date, the general pattern of TRIM5 across the mammalian lineage remains elusive. In this study, we surveyed 36 mammalian genomes to verify a potentially novel TRIM5 pattern that uniquely seems to have occurred in tree shrews (Tupaia belangeri), and found that both gene duplication and retrotransposition worked jointly to form a specific TRIM5/TRIMCyp cluster not found among other mammals. Evolutionary analyses showed that tree shrew TRIMCyp (tsTRIMCyp) originated independently in comparison with previously reported TRIMCyps and underwent strong positive selection, whereas no signal of positive selection was detected for other tree shrew TRIM5 (tsTRIM5) genes. Functional assay results suggest a functional divergence between tsTRIMCyp and its closest paralog TRIM5-4, likely reflecting different fates under diverse evolutionary forces. These findings present a rare example of novel gene origination resulting from a combination of gene duplication, retrotransposition, and exon shuffling processes, providing a new paradigm to study genetic innovations and evolutionary fates of duplicated genes.

Relocation facilitates the acquisition of short cis-regulatory regions that drive the expression of retrogenes during spermatogenesis in Drosophila

Retrogenes are functional processed copies of genes that originate via the retrotranscription of an mRNA intermediate and often exhibit testis-specific expression. Although this expression pattern appears to be favored by selection, the origin of such expression bias remains unexplained. Here, we study the regulation of two young testis-specific Drosophila retrogenes, Dntf-2r and Pros28.1A, using genetic transformation and the enhanced green fluorescent protein reporter gene in Drosophila melanogaster. We show that two different short (<24 bp) regions upstream of the transcription start sites (TSSs) act as testis-specific regulatory motifs in these genes. The Dntf-2r regulatory region is similar to the known β2 tubulin 14-bp testis motif (β2-tubulin gene upstream element 1 [β2-UE1]). Comparative sequence analyses reveal that this motif was already present before the Dntf-2r insertion and was likely driving the transcription of a noncoding RNA. We also show that the β2-UE1 occurs in the regulatory regions of other testis-specific retrogenes, and is functional in either orientation. In contrast, the Pros28.1A testes regulatory region in D. melanogaster appears to be novel. Only Pros28.1B, an older paralog of the Pros28.1 gene family, seems to carry a similar regulatory sequence. It is unclear how the Pros28.1A regulatory region was acquired in D. melanogaster, but it might have evolved de novo from within a region that may have been preprimed for testes expression. We conclude that relocation is critical for the evolutionary origin of male germline-specific cis-regulatory regions of retrogenes because expression depends on either the site of the retrogene insertion or the sequence changes close to the TSS thereafter. As a consequence we infer that positive selection will play a role in the evolution of these regulatory regions and can often act from the moment of the retrocopy insertion.

New genes as drivers of phenotypic evolution

During the course of evolution, genomes acquire novel genetic elements as sources of functional and phenotypic diversity, including new genes that originated in recent evolution. In the past few years, substantial progress has been made in understanding the evolution and phenotypic effects of new genes. In particular, an emerging picture is that new genes, despite being present in the genomes of only a subset of species, can rapidly evolve indispensable roles in fundamental biological processes, including development, reproduction, brain function and behaviour. The molecular underpinnings of how new genes can develop these roles are starting to be characterized. These recent discoveries yield fresh insights into our broad understanding of biological diversity at refined resolution.

High occurrence of functional new chimeric genes in survey of rice chromosome 3 short arm genome sequences

In an effort to identify newly evolved genes in rice, we searched the genomes of Asian-cultivated rice Oryza sativa ssp. japonica and its wild progenitors, looking for lineage-specific genes. Using genome pairwise comparison of approximately 20-Mb DNA sequences from the chromosome 3 short arm (Chr3s) in six rice species, O. sativa, O. nivara, O. rufipogon, O. glaberrima, O. barthii, and O. punctata, combined with synonymous substitution rate tests and other evidence, we were able to identify potential recently duplicated genes, which evolved within the last 1 Myr. We identified 28 functional O. sativa genes, which likely originated after O. sativa diverged from O. glaberrima. These genes account for around 1% (28/3,176) of all annotated genes on O. sativa's Chr3s. Among the 28 new genes, two recently duplicated segments contained eight genes. Fourteen of the 28 new genes consist of chimeric gene structure derived from one or multiple parental genes and flanking targeting sequences. Although the majority of these 28 new genes were formed by single or segmental DNA-based gene duplication and recombination, we found two genes that were likely originated partially through exon shuffling. Sequence divergence tests between new genes and their putative progenitors indicated that new genes were most likely evolving under natural selection. We showed all 28 new genes appeared to be functional, as suggested by Ka/Ks analysis and the presence of RNA-seq, cDNA, expressed sequence tag, massively parallel signature sequencing, and/or small RNA data. The high rate of new gene origination and of chimeric gene formation in rice may demonstrate rice's broad diversification, domestication, its environmental adaptation, and the role of new genes in rice speciation.

New gene evolution: little did we know

Genes are perpetually added to and deleted from genomes during evolution. Thus, it is important to understand how new genes are formed and how they evolve to be critical components of the genetic systems that determine the biological diversity of life. Two decades of effort have shed light on the process of new gene origination and have contributed to an emerging comprehensive picture of how new genes are added to genomes, ranging from the mechanisms that generate new gene structures to the presence of new genes in different organisms to the rates and patterns of new gene origination and the roles of new genes in phenotypic evolution. We review each of these aspects of new gene evolution, summarizing the main evidence for the origination and importance of new genes in evolution. We highlight findings showing that new genes rapidly change existing genetic systems that govern various molecular, cellular, and phenotypic functions.

Pseudogenes: pseudo or real functional elements?

Pseudogenes are genomic remnants of ancient protein-coding genes which have lost their coding potentials through evolution. Although broadly existed, pseudogenes used to be considered as junk or relics of genomes which have not drawn enough attentions of biologists until recent years. With the broad applications of high-throughput experimental techniques, growing lines of evidence have strongly suggested that some pseudogenes possess special functions, including regulating parental gene expression and participating in the regulation of many biological processes. In this review, we summarize some basic features of pseudogenes and their functions in regulating development and diseases. All of these observations indicate that pseudogenes are not purely dead fossils of genomes, but warrant further exploration in their distribution, expression regulation and functions. A new nomenclature is desirable for the currently called 'pseudogenes' to better describe their functions.

Asymmetric functional divergence of young, dispersed gene duplicates in Arabidopsis thaliana

One prediction of the classic Ohno model of gene duplication predicts that new genes form from the asymmetric functional divergence of a newly arisen, redundant duplicate locus. In order to understand the mechanisms which give rise to functional divergence of newly formed dispersed duplicates, we assessed the expression and molecular evolutionary divergence of a suite of 19 highly similar dispersed duplicates in Arabidopsis thaliana. These duplicates have a K sil equal to or less than 5 % and are specific to the A. thaliana lineage; thus, they predictably represent some of the youngest duplicates in the A. thaliana genome. We found that the majority of young duplicate loci exhibit asymmetric expression patterns, with the daughter locus exhibiting reduced expression across all tissues analyzed relative to the progenitor locus or simply not expressed. Furthermore, daughter loci, on the whole, have significantly more nonsynonymous substitutions than the progenitor loci. We also identified four pairs of loci which exhibit significant (P < 0.05) evolutionary rate asymmetry, three of which exhibit elevated dN/dS in the duplicate copy. We suggest, based on these data, that functional diversification initially takes the form of asymmetric regulatory divergence that can be a direct consequence of the mode of duplication. The reduced and/or absence of expression in the daughter copy relaxes functional constraint on its protein coding sequence leading to the asymmetric accumulation of nonsynonymous mutations. Thus, our data both affirm Ohno's prediction while explaining the mechanism by which functional divergence initially occurs following duplication for dispersed gene duplicates.

Sex and speciation: Drosophila reproductive tract proteins- twenty five years later

The protein electrophoresis revolution, nearly fifty years ago, provided the first glimpse into the nature of molecular genetic variation within and between species and showed that the amount of genetic differences between newly arisen species was minimal. Twenty years later, 2D electrophoresis showed that, in contrast to general gene-enzyme variation, reproductive tract proteins were less polymorphic within species but highly diverged between species. The 2D results were interesting and revolutionary, but somewhat uninterpretable because, at the time, rapid evolution and selective sweeps were not yet part of the common vocabulary of evolutionary biologists. Since then, genomic studies of sex and reproduction-related (SRR) genes have grown rapidly into a large area of research in evolutionary biology and are shedding light on a number of phenomena. Here we review some of the major and current fields of research that have greatly contributed to our understanding of the evolutionary dynamics and importance of SRR genes and genetic systems in understanding reproductive biology and speciation.

In with the old, in with the new: the promiscuity of the duplication process engenders diverse pathways for novel gene creation

The gene duplication process has exhibited far greater promiscuity in the creation of paralogs with novel exon-intron structures than anticipated even by Ohno. In this paper I explore the history of the field, from the neo-Darwinian synthesis through Ohno's formulation of the canonical model for the evolution of gene duplicates and culminating in the present genomic era. I delineate the major tenets of Ohno's model and discuss its failure to encapsulate the full complexity of the duplication process as revealed in the era of genomics. I discuss the diverse classes of paralogs originating from both DNA- and RNA-mediated duplication events and their evolutionary potential for assuming radically altered functions, as well as the degree to which they can function unconstrained from the pressure of gene conversion. Lastly, I explore theoretical population-genetic considerations of how the effective population size (N(e)) of a species may influence the probability of emergence of genes with radically altered functions.

Novel genes from formation to function

The study of the evolution of novel genes generally focuses on the formation of new coding sequences. However, equally important in the evolution of novel functional genes are the formation of regulatory regions that allow the expression of the genes and the effects of the new genes in the organism as well. Herein, we discuss the current knowledge on the evolution of novel functional genes, and we examine in more detail the youngest genes discovered. We examine the existing data on a very recent and rapidly evolving cluster of duplicated genes, the Sdic gene cluster. This cluster of genes is an excellent model for the evolution of novel genes, as it is very recent and may still be in the process of evolving.

Evolution at the subgene level: domain rearrangements in the Drosophila phylogeny

Although the possibility of gene evolution by domain rearrangements has long been appreciated, current methods for reconstructing and systematically analyzing gene family evolution are limited to events such as duplication, loss, and sometimes, horizontal transfer. However, within the Drosophila clade, we find domain rearrangements occur in 35.9% of gene families, and thus, any comprehensive study of gene evolution in these species will need to account for such events. Here, we present a new computational model and algorithm for reconstructing gene evolution at the domain level. We develop a method for detecting homologous domains between genes and present a phylogenetic algorithm for reconstructing maximum parsimony evolutionary histories that include domain generation, duplication, loss, merge (fusion), and split (fission) events. Using this method, we find that genes involved in fusion and fission are enriched in signaling and development, suggesting that domain rearrangements and reuse may be crucial in these processes. We also find that fusion is more abundant than fission, and that fusion and fission events occur predominantly alongside duplication, with 92.5% and 34.3% of fusion and fission events retaining ancestral architectures in the duplicated copies. We provide a catalog of ∼9,000 genes that undergo domain rearrangement across nine sequenced species, along with possible mechanisms for their formation. These results dramatically expand on evolution at the subgene level and offer several insights into how new genes and functions arise between species.

Expansion and functional diversification of a leucyl aminopeptidase family that encodes the major protein constituents of Drosophila sperm

Background

The evolutionary diversification of gene families through gene creation (and loss) is a dynamic process believed to be critical to the evolution of functional novelty. Previous identification of a closely related family of eight annotated metalloprotease genes of the M17 Merops family in the Drosophila sperm proteome (termed, Sperm-LeucylAminoPeptidases, S-LAPs 1-8) led us to hypothesize that this gene family may have experienced such a diversification during insect evolution.

Results

To assess putative functional activities of S-LAPs, we (i) demonstrated that all S-LAPs are specifically expressed in the testis, (ii) confirmed their presence in sperm by two-dimensional gel electrophoresis and mass spectrometry, (iii) determined that they represent a major portion of the total protein in sperm and (iv) identified aminopeptidase enzymatic activity in sperm extracts using LAP-specific substrates. Functionally significant divergence at the canonical M17 active site indicates that the largest phylogenetic group of S-LAPs lost catalytic activity and likely acquired novel, as yet undetermined, functions in sperm prior to the expansion of the gene family.

Conclusions

Comparative genomic and phylogenetic analyses revealed the dramatic expansion of the S-LAP gene family during Drosophila evolution and copy number heterogeneity in the genomes of related insects. This finding, in conjunction with the loss of catalytic activity and potential neofunctionalization amongst some family members, extends empirical support for pervasive "revolving door" turnover in the evolution of reproductive gene family composition and function.

Faster evolution of Z-linked duplicate genes in chicken

It has been shown that duplicate genes on the X chromosome evolve much faster than duplicate genes on autosomes in Drosophila melanogaster. However, whether this phenomenon is general and can be applied to other species is not known. Here we examined this issue in chicken that have heterogametic females (females have ZW sex chromosome). We compared sequence divergence of duplicate genes on the Z chromosome with those on autosomes. We found that duplications on the Z chromosome indeed evolved faster than those on autosomes and show distinct patterns of molecular evolution from autosomal duplications. Examination of the expression of duplicate genes revealed an enrichment of duplications on the Z chromosome having male-biased expression and an enrichment of duplications on the autosomes having female-biased expression. These results suggest an evolutionary trend of the recruitment of duplicate genes towards reproduction-specific function. The faster evolution of duplications on Z than on the autosomes is most likely contributed by the selective forces driving the fixation of adaptive mutations on Z. Therefore, the common phenomena observed in both flies and chicken suggest that duplicate genes on sex chromosomes have distinct dynamics and are more influenced by natural selection than autosomal duplications, regardless of the kind of sex determination systems.

Evolution of enzymatic activities of testis-specific short-chain dehydrogenase/reductase in Drosophila

The testis-specific gene Jingwei (jgw) is a newly evolved short-chain dehydrogenase/reductase in Drosophila. Preliminary substrate screening indicated that JGW prefers long-chain primary alcohols as substrates, including several exotic alcohols such as farnesol and geraniol. Using steady-state kinetics analyses and molecular docking, we not only confirmed JGW's substrate specificity, but also demonstrated that the new enzymatic activities of JGW evolved extensively after exon-shuffling from a preexisting enzyme. Analysis of JGW orthologs in sister species shows that subsequent evolutionary changes following the birth of JGW altered substrate specificities and enzyme stabilities. Our results lend support to a general mechanism for the evolution of a new enzyme, in which catalytic chemistry evolves first followed by diversification of substrate utilization.

Adaptive impact of the chimeric gene Quetzalcoatl in Drosophila melanogaster

Chimeric genes, which form through the genomic fusion of two protein-coding genes, are a significant source of evolutionary novelty in Drosophila melanogaster. However, the propensity of chimeric genes to produce adaptive phenotypic changes is not fully understood. Here, we describe the chimeric gene Quetzalcoatl (Qtzl; CG31864), which formed in the recent past and swept to fixation in D. melanogaster. Qtzl arose through a duplication on chromosome 2L that united a portion of the mitochondrially targeted peptide CG12264 with a segment of the polycomb gene escl. The 3' segment of the gene, which is derived from escl, is inherited out of frame, producing a unique peptide sequence. Nucleotide diversity is drastically reduced and site frequency spectra are significantly skewed surrounding the duplicated region, a finding consistent with a selective sweep on the duplicate region containing Qtzl. Qtzl has an expression profile that largely resembles that of escl, with expression in early pupae, adult females, and male testes. However, expression patterns appear to have been decoupled from both parental genes during later embryonic development and in head tissues of adult males, indicating that Qtzl has developed a distinct regulatory profile through the rearrangement of different 5' and 3' regulatory domains. Furthermore, misexpression of Qtzl suppresses defects in the formation of the neuromuscular junction in larvae, demonstrating that Qtzl can produce phenotypic effects in cells. Together, these results show that chimeric genes can produce structural and regulatory changes in a single mutational step and may be a major factor in adaptive evolution.

A human-specific de novo protein-coding gene associated with human brain functions

To understand whether any human-specific new genes may be associated with human brain functions, we computationally screened the genetic vulnerable factors identified through Genome-Wide Association Studies and linkage analyses of nicotine addiction and found one human-specific de novo protein-coding gene, FLJ33706 (alternative gene symbol C20orf203). Cross-species analysis revealed interesting evolutionary paths of how this gene had originated from noncoding DNA sequences: insertion of repeat elements especially Alu contributed to the formation of the first coding exon and six standard splice junctions on the branch leading to humans and chimpanzees, and two subsequent substitutions in the human lineage escaped two stop codons and created an open reading frame of 194 amino acids. We experimentally verified FLJ33706's mRNA and protein expression in the brain. Real-Time PCR in multiple tissues demonstrated that FLJ33706 was most abundantly expressed in brain. Human polymorphism data suggested that FLJ33706 encodes a protein under purifying selection. A specifically designed antibody detected its protein expression across human cortex, cerebellum and midbrain. Immunohistochemistry study in normal human brain cortex revealed the localization of FLJ33706 protein in neurons. Elevated expressions of FLJ33706 were detected in Alzheimer's brain samples, suggesting the role of this novel gene in human-specific pathogenesis of Alzheimer's disease. FLJ33706 provided the strongest evidence so far that human-specific de novo genes can have protein-coding potential and differential protein expression, and be involved in human brain functions.

Applying mobile genetic elements for genome analysis and evolution

Transposable elements (TEs) are ubiquitous components of all living organisms, and in the course of their coexistence with their respective host genomes, these parasitc DNAs have played important roles in the evolution of complex genetic networks. The interaction between mobile DNAs and their host genomes are quite diverse, ranging from modifications of gene structure and regulation to alterations in general genome architecture. Thus during evolutionary time these elements can be regarded as natural molecular tools in shaping the organization, structure, and function of eukaryotic genes and genomes. Based on their intrinsic properties and features, mobile DNAs are widely applied at present as a technical "toolbox," essential for studying a diverse spectrum of biological questions. In this review, we aim to summarize both the evolutionary impact of TEs on genome evolution and their valuable and diverse methodological applications as molecular tools.

Validation of rearrangement break points identified by paired-end sequencing in natural populations of Drosophila melanogaster

Several recent studies have focused on the evolution of recently duplicated genes in Drosophila. Currently, however, little is known about the evolutionary forces acting upon duplications that are segregating in natural populations. We used a high-throughput, paired-end sequencing platform (Illumina) to identify structural variants in a population sample of African D. melanogaster. Polymerase chain reaction and sequencing confirmation of duplications detected by multiple, independent paired-ends showed that paired-end sequencing reliably uncovered the break points of structural rearrangements and allowed us to identify a number of tandem duplications segregating within a natural population. Our confirmation experiments show that rates of confirmation are very high, even at modest coverage. Our results also compare well with previous studies using microarrays (Emerson J, Cardoso-Moreira M, Borevitz JO, Long M. 2008. Natural selection shapes genome wide patterns of copy-number polymorphism in Drosophila melanogaster. Science. 320:1629-1631. and Dopman EB, Hartl DL. 2007. A portrait of copy-number polymorphism in Drosophila melanogaster. Proc Natl Acad Sci U S A. 104:19920-19925.), which both gives us confidence in the results of this study as well as confirms previous microarray results.We were also able to identify whole-gene duplications, such as a novel duplication of Or22a, an olfactory receptor, and identify copy-number differences in genes previously known to be under positive selection, like Cyp6g1, which confers resistance to dichlorodiphenyltrichloroethane. Several "hot spots" of duplications were detected in this study, which indicate that particular regions of the genome may be more prone to generating duplications. Finally, population frequency analysis of confirmed events also showed an excess of rare variants in our population, which indicates that duplications segregating in the population may be deleterious and ultimately destined to be lost from the population.

Origin and evolution of ubiquitin-conjugating enzymes from Guillardia theta nucleomorph to hominoid

The origin of eukaryotic ubiquitin-conjugating enzymes (E2s) can be traced back to the Guillardia theta nucleomorph about 2500 million years ago (Mya). E2s are largely vertically inherited over eukaryotic evolution [Lespinet, O., Wolf, Y.I., Koonin, E.V., Aravind, L., 2002. The role of lineage-specific gene family expansion in the evolution of eukaryotes. Genome Res. 1048-1059], while mammal E2s experienced evolution of multigene families by gene duplications which have been accompanied by the increase in the species complexity. Because of alternatively splicing, primate-specific expansions of E2s happened once again at a transcriptional level. Both of them resulted in increasing genomic complexity and diversity of primate E2 proteomic function. The evolutionary processes of human E2 gene structure during expansions were accompanied by exon duplication and exonization of intronic sequences. Exonizations of Transposable Elements (TEs) in UBE2D3, UBE2L3 and UBE2V1 genes from primates indicate that exaptation of TEs also plays important roles in the structural innovation of primate-specific E2s and may create alternative splicing isoforms at a transcriptional level. Estimates for the ratio of dN/dS suggest that a strong purifying selection had acted upon protein-coding sequences of their orthologous UBE2D2, UBE2A, UBE2N, UBE2I and Rbx1 genes from animals, plants and fungi. The similar rates of synonymous substitutions are in accordance with the neutral mutation-random drift hypothesis of molecular evolution. Systematic detection of the origin and evolution of E2s, analyzing the evolution of E2 multigene families by gene duplications and the evolutionary processes of E2s during expansions, and testing its evolutionary force using E2s from distant phylogenetic lineages may advance our distinguishing of ancestral E2s from created E2s, and reveal previously unknown relationships between E2s and metazoan complexity. Analysis of these conserved proteins provides strong support for a close relationship between social amoeba and eukaryote, choanoflagellate and metazoan, and for the central roles of social amoeba and choanoflagellate in the origin and evolution of eukaryote and metazoan. Retracing the different stages of primate E2 exonization by monitoring genomic events over 63 Myr of primate evolution will advance our understanding of how TEs dynamically modified primate transcriptome and proteome in the past, and continue to do so.

Gene duplication and evolutionary novelty in plants

Duplication is a prominent feature of plant genomic architecture. This has led many researchers to speculate that gene duplication may have played an important role in the evolution of phenotypic novelty within plants. Until recently, however, it was difficult to make this connection. We are now beginning to understand how duplication has contributed to adaptive evolution in plants. In this review we introduce the sources of gene duplication and predictions of the various fates of duplicates. We also highlight several recent and pertinent examples from the literature. These examples demonstrate the importance of the functional characteristics of genes and the source of duplication in influencing evolutionary outcome.

Complex rearrangements lead to novel chimeric gene fusion polymorphisms at the Arabidopsis thaliana MAF2-5 flowering time gene cluster

Tandem gene clusters of multigene families are rearrangement hotspots and may be a major source of novel gene formation. Here, we report on a molecular population genetic analysis of the MAF2-5 gene cluster of the model plant species, Arabidopsis thaliana. The MAF2-5 genes are a MADS-box multigene family cluster spanning approximately 24 kbp on chromosome 5. We find heterogeneous evolutionary dynamics among these genes, all of which are closely related to the floral repressor, FLC, and are believed to play a role in the control of flowering time in A. thaliana. Low levels of nonsynonymous single nucleotide polymorphism (SNP) observed for MAF4 and MAF5 suggest purifying selection and conservation of function. In contrast, high levels of nonsynonymous SNPs, insertion-deletion, and rearrangements are observed for MAF2 and MAF3, including novel gene fusions that persist as a moderate-frequency polymorphism in A. thaliana. These fused genes, involving MAF2 and portions of MAF3, are expressed, resulting in the production of chimeric, alternatively spliced transcripts of MAF2. Association studies support a correlation between the described MAF2-MAF3 gene rearrangements and flowering time variation in the species. The finding that complex rearrangements within gene clusters, such as those observed for MAF2, might play a role in the generation of ecologically important phenotypic variation, emphasize the need for emerging high throughput genotyping and sequencing techniques to correctly reconstruct gene chimeras and other complex polymorphisms.

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 approximately 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 approximately 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.

Male-biased genes are overrepresented among novel Drosophila pseudoobscura sex-biased genes

Background

The origin of functional innovation is among the key questions in biology. Recently, it has been shown that new genes could arise from non-coding DNA and that such novel genes are often involved in male reproduction.

Results

With the aim of identifying novel genes, we used the technique "generation of longer cDNA fragments from serial analysis of gene expression (SAGE) tags for gene identification (GLGI)" to extend 84 sex-biased 3'end SAGE tags that previously could not be mapped to the D. pseudoobscura transcriptome. Eleven male-biased and 33 female-biased GLGI fragments were obtained, of which 5 male-biased and 3 female-biased tags corresponded to putatively novel genes. This excess of novel genes with a male-biased gene expression pattern is consistent with previous results, which found novel genes to be primarily expressed in male reproductive tissues. 5' RACE analysis indicated that these novel transcripts are very short in length and could contain introns. Interspecies comparisons revealed that most novel transcripts show evidence for purifying selection.

Conclusion

Overall, our data indicate that among sex-biased genes a considerable number of novel genes (~2–4%) exist in D. pseudoobscura, which could not be predicted based on D. melanogaster gene models.

On the origin of new genes in Drosophila

Several mechanisms have been proposed to account for the origination of new genes. Despite extensive case studies, the general principles governing this fundamental process are still unclear at the whole-genome level. Here, we unveil genome-wide patterns for the mutational mechanisms leading to new genes and their subsequent lineage-specific evolution at different time nodes in the Drosophila melanogaster species subgroup. We find that (1) tandem gene duplication has generated approximately 80% of the nascent duplicates that are limited to single species (D. melanogaster or Drosophila yakuba); (2) the most abundant new genes shared by multiple species (44.1%) are dispersed duplicates, and are more likely to be retained and be functional; (3) de novo gene origination from noncoding sequences plays an unexpectedly important role during the origin of new genes, and is responsible for 11.9% of the new genes; (4) retroposition is also an important mechanism, and had generated approximately 10% of the new genes; (5) approximately 30% of the new genes in the D. melanogaster species complex recruited various genomic sequences and formed chimeric gene structures, suggesting structure innovation as an important way to help fixation of new genes; and (6) the rate of the origin of new functional genes is estimated to be five to 11 genes per million years in the D. melanogaster subgroup. Finally, we survey gene frequencies among 19 globally derived strains for D. melanogaster-specific new genes and reveal that 44.4% of them show copy number polymorphisms within a population. In conclusion, we provide a panoramic picture for the origin of new genes in Drosophila species.

Repetitive element-mediated recombination as a mechanism for new gene origination in Drosophila

Previous studies of repetitive elements (REs) have implicated a mechanistic role in generating new chimerical genes. Such examples are consistent with the classic model for exon shuffling, which relies on non-homologous recombination. However, recent data for chromosomal aberrations in model organisms suggest that ectopic homology-dependent recombination may also be important. Lack of a dataset comprising experimentally verified young duplicates has hampered an effective examination of these models as well as an investigation of sequence features that mediate the rearrangements. Here we use ∼7,000 cDNA probes (∼112,000 primary images) to screen eight species within the Drosophila melanogaster subgroup and identify 17 duplicates that were generated through ectopic recombination within the last 12 mys. Most of these are functional and have evolved divergent expression patterns and novel chimeric structures. Examination of their flanking sequences revealed an excess of repetitive sequences, with the majority belonging to the transposable element DNAREP1 family, associated with the new genes. Our dataset strongly suggests an important role for REs in the generation of chimeric genes within these species.

In numerous organisms, many new genes have been found to originate through dispersed gene duplication and exon/domain shuffling. What recombination mechanisms were involved in the duplication and the shuffling processes? Lack of the intermediate products of recombination that share adequate sequence identity between homologous sequences, or the parental sequences from which the new genes were derived, often makes answering these questions difficult. We identified a number of young genes that originated in recently diverged branches in the evolutionary tree of the eight Drosophila melanogaster subgroup species, by using fluorescence in situ hybridization with polytene chromosomes. We analyzed the genomic regions surrounding 17 new dispersed duplicate genes and observed that most of these genes are flanked by repetitive elements (REs), including a large and diverged transposable element family, DNAREP1. Several copies of these REs are kept in both new and parental gene regions, and their degeneration is correlated with the increasing ages of the identified new genes. These data suggest that REs mediate the recombination responsible for the new gene origination.

A novel recently evolved gene C19orf24 encodes a non-classical secreted protein

Secreted proteins play important roles in many crucial biological processes, and can be new agents or targets for drug therapies. Here, we report on the isolation and characterization of a novel human non-classical secreted protein which is encoded by the hypothetical gene C19orf24 (chromosome 19 open reading frame 24). It has no signal peptide, but can still secrete extracellularly despite the presence of the inhibitor brefeldin A (BFA), proving its non-classical secreted protein status. Via subcellular localization using C19orf24 in vivo and transfected pEYFP-Golgi plasmid in Hela cells, C19orf24 was shown not to co-localize in the Golgi apparatus, which suggested that it secretes via a new and unknown pathway. Deglycosylation analysis with PNGase F verified that it has no N-glycosylation modification sites. Via the reverse transcription-PCR method, it was found to be expressed only in the human liver, and preferentially in normal tissue. In addition, C19orf24 was shown to be a recently evolved gene, found only in Homo sapiens and Pan troglodytes. By calculating its synonymous and non-synonymous substitution rate (d_S/d_N), we found that it experienced a purifying selection, which suggests that C19orf24 may have a special, irreplaceable biological function in the human organism.

The neutral coalescent process for recent gene duplications and copy-number variants

I describe a method for simulating samples from gene families of size two under a neutral coalescent process, for the case where the duplicate gene either has fixed recently in the population or is still segregating. When a duplicate locus has recently fixed by genetic drift, diversity in the new gene is expected to be reduced, and an excess of rare alleles is expected, relative to the predictions of the standard coalescent model. The expected patterns of polymorphism in segregating duplicates ("copy-number variants") depend both on the frequency of the duplicate in the sample and on the rate of crossing over between the two loci. When the crossover rate between the ancestral gene and the copy-number variant is low, the expected pattern of variability in the ancestral gene will be similar to the predictions of models of either balancing or positive selection, if the frequency of the duplicate in the sample is intermediate or high, respectively. Simulations are used to investigate the effect of crossing over between loci, and gene conversion between the duplicate loci, on levels of variability and the site-frequency spectrum.

Domain architecture comparison for multidomain homology identification

Homology identification is the first step for many genomic studies. Current methods, based on sequence comparison, can result in a substantial number of mis-assignments due to the similarity of homologous domains in otherwise unrelated sequences. Here we propose methods to detect homologs through explicit comparison of protein domain content. We developed several schemes for scoring the homology of a pair of protein sequences based on methods used in the field of information retrieval. We evaluate the proposed methods and methods used in the literature using a benchmark of fifteen sequence families of known evolutionary history. The results of these studies demonstrate the effectiveness of comparing domain architectures using these similarity measures. We also demonstrate the importance of both weighting promiscuous domains and of compensating for the statistical effect of having a large number of domains in a protein. Using logistic regression, we demonstrate the benefit of combining similarity measures based on domain content with sequence similarity measures.

Molecular characterization of of alpha-gliadin genes from wild emmer wheat (Triticum dicoccoides)

According to the two distal and conserved regions of known alpha-gliadin genes, gene-specific primers for alpha-gliadin were designed, and the coding regions of four gliadin genes (i.e. GliTd-1, GliTd-2, GliTd-3 and GliTd-4) with the length of about 800 bp were isolated from the genomic DNA of wild emmer wheat (Triticum dicoccoides). No introns were observed. Sequence comparison indicated that these genes should be classified as alpha-gliadins. GliTd-3 (GenBank accession No.DQ140351) and GliTd-4 (DQ140352) were potentially functional, whereas GliTd-1 (DQ140349) and GliTd-2 (DQ140350) were both pseudogenes by the definition of in-frame stop codons and frameshifts. Six conserved cysteine residues were observed. Sequence analysis suggested that the motif units of repetitive domain for the four newly detected genes were different from the known genes, and the QQQP sequence before the position 60 was more toxic to coeliac patients. Codons for proline were strongly biased. Codons (CAG and CAA) for glutamine were clustered into the specific regions, and the high percentage of pseudogenes resulted from the mutation of CAG --> TAG.

Generation of novel high quality HMW-GS genes in two introgression lines of Triticum aestivum/Agropyron elongatum

Background

High molecular weight glutenin subunits (HMW-GS) have been proved to be mostly correlated with the processing quality of common wheat (Triticum aestivum). But wheat cultivars have limited number of high quality HMW-GS. However, novel HMW-GS were found to be present in many wheat asymmetric somatic hybrid introgression lines of common wheat/Agropyron elongatum.

Results

To exploit how these new subunits were generated, we isolated HMW-GS genes from two sib hybrid lines (II-12 and 11-4-6) and compared them with those from their parents. The result shows that two genes of hybrid (H11-3-3 and H11-4-3) are directly introgressed from the donor parent Agropyron elongatum; one hybrid gene (H1Dx5) comes from point mutation of a parental wheat gene (1Dx2.1); two other hybrid genes (H1By8 and H1By16) are likely resulting from unequal crossover or slippage of a parental wheat gene (1By9.1); and the sixth novel hybrid gene (H1Dy12) may come from recombination between two parental genes.

Conclusion

Therefore, we demonstrate that novel HMW-GS genes can be rapidly created through asymmetric somatic hybridization in a manner similar with the evolution mechanism of these genes supposed before. We also described gene shuffling as a new mechanism of novel HMW-GS gene formation in hybrids. The results suggest that asymmetric somatic hybridization is an important approach for widening HMW-GS genebank of wheat quality improvement.

Novel genes derived from noncoding DNA in Drosophila melanogaster are frequently X-linked and exhibit testis-biased expression

Descriptions of recently evolved genes suggest several mechanisms of origin including exon shuffling, gene fission/fusion, retrotransposition, duplication-divergence, and lateral gene transfer, all of which involve recruitment of preexisting genes or genetic elements into new function. The importance of noncoding DNA in the origin of novel genes remains an open question. We used the well annotated genome of the genetic model system Drosophila melanogaster and genome sequences of related species to carry out a whole-genome search for new D. melanogaster genes that are derived from noncoding DNA. Here, we describe five such genes, four of which are X-linked. Our RT-PCR experiments show that all five putative novel genes are expressed predominantly in testes. These data support the idea that these novel genes are derived from ancestral noncoding sequence and that new, favored genes are likely to invade populations under selective pressures relating to male reproduction.

The evolution of the novel Sdic gene cluster in Drosophila melanogaster

The origin of new genes and of new functions for existing genes are fundamental processes in molecular evolution. Sdic is a newly evolved gene that arose recently in the D. melanogaster lineage. The gene encodes a novel sperm motility protein. It is a chimeric gene formed by duplication of two other genes followed by multiple deletions and other sequence rearrangements. The Sdic gene exists in several copies in the X chromosome, and is presumed to have undergone several duplications to form a tandemly arrayed gene cluster. Given the very recent origin of the gene and the gene cluster, the analysis of the composition of this gene cluster represents an excellent opportunity to study the origin and evolution of new gene functions and the fate of gene duplications. We have analyzed the nucleotide sequence of this region and reconstructed the evolutionary history of this gene cluster. We found that the cluster is composed by four tandem copies of Sdic; these duplicates are very similar but can be distinguished by the unique pattern of insertions, deletions, and point mutations in each copy. The oldest gene copy in the array has a 3' exon that has undergone accelerated diversification, and also shows divergent regulatory sequences. Moreover, there is evidence that this might be the only gene copy in the tandem array that is transcribed at a significant level, expressing a novel sperm-specific protein. There is also a retrotransposon located at the 3' end of each Sdic gene copy. We argue that this gene cluster was formed in the last two million years by at least three tandem duplications and one retrotransposition event.

A novel chimeric gene, siren, with retroposed promoter sequence in the Drosophila bipectinata complex

Retrotransposons often produce a copy of host genes by their reverse transcriptase activity operating on host gene transcripts. Since transcripts normally do not contain promoter, a retroposed gene copy usually becomes a retropseudogene. However, in Drosophila bipectinata and a closely related species we found a new chimeric gene, whose promoter was likely produced by retroposition. This chimeric gene, named siren, consists of a tandem duplicate of Adh and a retroposed fragment of CG11779 containing the promoter and a partial intron in addition to the first exon. We found that this unusual structure of a retroposed fragment was obtained by retroposition of nanos, which overlaps with CG11779 on the complementary strand. The potential of retroposition to produce a copy of promoter and intron sequences in the context of gene overlapping was demonstrated.

Echoes from the past--are we still in an RNP world?

Availability of the human genome sequence and those of other species is unmeasured in their value for a comprehensive understanding of the architecture, function and evolution of genomes and cells. Various mechanisms keep genomes in flux and generate intra- and interspecies variation. The conversion of RNA modules into DNA and their more or less random integration into chromosomes (retroposition) is in many lineages including our own the most pervasive and perhaps the most enigmatic. The proclivity of such events in extant multicellular eukaryotes, even in more recent evolutionary times, gives the impression that the transition period from the RNP (ribonucleoprotein) world to the emergence of modern cells, where DNA became the predominant carrier of genetic information, has lasted billions of years and is an endlessly drawn-out process rather than the punctuated event one might expect. Apart from the impact of such RNA-mediated processes as retroposition, the role of RNA in a wide variety of cellular functions has only recently become more widely appreciated.

Translational effects of differential codon usage among intragenic domains of new genes in Drosophila

Evolved codon usages often pose a technical challenge over the expressing of eukaryotic genes in microbial systems because of changed translational machinery. In the present study, we investigated the translational effects of intragenic differential codon usage on the expression of the new Drosophila gene, jingwei (jgw), a chimera derived from two unrelated parental genes: Ymp and Adh. We found that jgw possesses a strong intragenic differential usage of synonymous codons, i.e. the Adh-derived C-domain has a significantly higher codon bias than that of the Ymp-derived N-domain (P=0.0023 by t-test). Additional evolutionary analysis revealed the heterogeneous distribution of rare codons, implicating its role in gene regulation and protein translation. The in vitro expression of jgw further demonstrated that the heterogeneous distribution of rare codons has played a role in regulating gene expression, particularly, affecting the quality of protein translation.

Origin and neofunctionalization of a Drosophila paternal effect gene essential for zygote viability

BACKGROUND

Although evolutionary novelty by gene duplication is well established, the origin and maintenance of essential genes that provide entirely new functions (neofunctionalization) is still largely unknown. Drosophila is a good model for the search of genes that are young enough to allow deciphering the molecular details of their evolutionary history. Recent years have seen increased interest in genes specifically required for male fertility because they often evolve rapidly. A special class of genes affecting male fertility, the paternal effect genes, have also become a focus of study to geneticists and reproductive biologists interested in fertilization and sperm-egg interactions.

RESULTS

Using molecular genetics and the annotated Drosophila melanogaster genome, we identified CG14251 as the Drosophila paternal effect gene, ms(3)K81 (K81). This assignment was subsequently confirmed by P-element rescue of K81. A search for orthologous K81 sequences revealed that the distribution of K81 is surprisingly restricted to the 9 species comprising the melanogaster subgroup. Phylogenetic analyses indicate that K81 arose through duplication, most likely retroposition, of a ubiquitously expressed gene before the radiation of the melanogaster subgroup, followed by a period of rapid divergence and acquisition of a critical male germline-specific function. Interestingly, K81 has adopted the expression profile of a flanking gene suggesting that transcriptional coregulation may have been important in the neofunctionalization of K81.

CONCLUSION

We present a detailed case history of the origin and evolution of a new essential gene and, in so doing, provide the first molecular identification of a Drosophila paternal effect gene, ms(3)K81 (K81).

Origin and evolution of a chimeric fusion gene in Drosophila subobscura, D. madeirensis and D. guanche

An understanding of the mutational and evolutionary mechanisms underlying the emergence of novel genes is critical to studies of phenotypic and genomic evolution. Here we describe a new example of a recently formed chimeric fusion gene that occurs in Drosophila guanche, D. madeirensis, and D. subobscura. This new gene, which we name Adh-Twain, resulted from an Adh mRNA that retrotransposed into the Gapdh-like gene, CG9010. Adh-Twain is transcribed; its 5' promoters and transcription patterns appear similar to those of CG9010. Population genetic and phylogenetic analyses suggest that the amino acid sequence of Adh-Twain evolved rapidly via directional selection shortly after it arose. Its more recent history, however, is characterized by slower evolution consistent with increasing functional constraints. We present a model for the origin of this new gene and discuss genetic and evolutionary factors affecting the evolution of new genes and functions.