Orphan genes, ORF ans, [1] [2] or taxonomically restricted genes (TRGs) [3] are genes that lack a detectable homologue outside of a given species or lineage. [2] Most genes have known homologues. Two genes are homologous when they share an evolutionary history, and the study of groups of homologous genes allows for an understanding of their evolutionary history and divergence. Common mechanisms that have been uncovered as sources for new genes through studies of homologues include gene duplication, exon shuffling, gene fusion and fission, etc. [4] [5] Studying the origins of a gene becomes more difficult when there is no evident homologue. [6] The discovery that about 10% or more of the genes of the average microbial species is constituted by orphan genes raises questions about the evolutionary origins of different species as well as how to study and uncover the evolutionary origins of orphan genes.
In some cases, a gene can be classified as an orphan gene due to undersampling of the existing genome space. While it is possible that homologues exist for a given gene, that gene will still be classified as an orphan if the organisms harbouring homologues have not yet been discovered and had their genomes sequenced and properly annotated. For example, one study of orphan genes across 119 archaeal and bacterial genomes could identify that at least 56% were recently acquired from integrative elements (or mobile genetic elements) from non-cellular sources such as viruses and plasmids that remain to be explored and characterized, and another 7% arise through horizontal gene transfer from distant cellular sources (with an unknown proportion of the remaining 37% potentially coming from still unknown families of integrative elements). [7] In other cases, limitations in computational methods for detecting homologues may result in missed homologous sequences and thus classification of a gene as an orphan. Homology detection failure appears to account for the majority, but not all orphan genes. [8] In other cases, homology between genes may go undetected due to rapid evolution and divergence of one or both of these genes from each other to the point where they do not meet the criteria used to classify genes as evidently homologous by computational methods. One analysis suggests that divergence accounts for a third of orphan gene identifications in eukaryotes. [9] When homologous genes exist but are simply undetected, the emergence of these orphan genes can be explained by well-characterized phenomena such as genomic recombination, exon shuffling, gene duplication and divergence, etc. Orphan genes may also simply lack true homologues and in such cases have an independent origins via de novo gene birth, which tends to be a more recent event. [2] These processes may act at different rates in insects, primates, and plants. [10] Despite their relatively recent origin, orphan genes may encode functionally important proteins. [11] [12] Characteristics of orphan genes include AT richness, relatively recent origins, taxonomic restriction to a single genome, elevated evolution rates, and shorter sequences. [13]
Some approaches characterize all microbial genes as part of one of two classes of genes. One class is characterized by conservation or partial conservation across lineages, whereas the other (represented by orphan genes) is characterized by evolutionarily instantaneous rates of gene turnover/replacement with a negligible effect on fitness when such genes are either gained or lost. These orphan genes primarily derive from mobile genetic elements and tend to be 'passively selfish', often devoid of cellular functions (which is why they experience little selective pressure in their gain or loss from genomes) but persist in the biosphere due to their transient movement across genomes. [14] [15]
Orphan genes evolve more rapidly than other genes. They often originate through two primary mechanisms: de novo gene birth, where new genes emerge from non-coding sequences within the genome, and horizontal gene transfer, the acquisition of genetic material from another organism.
Biologists believe orphan genes may play a crucial role in developing species-specific traits, environmental adaptations, or responses to changing ecological niches. These functional innovations necessitate rapid evolutionary changes to optimize their efficacy within the organism's biology.
Multiple studies have supported these evolutionary theories regarding orphan genes. Domazet-Loso and Tautz [16] conducted a study focusing on orphan genes in Drosophila, revealing that these genes evolve at a faster pace compared to conserved genes. This finding suggests a potential correlation between evolutionary rate and gene novelty. Similarly, Tautz and Domazet-Loso [17] presented evidence indicating a substantial contribution of orphan genes to phenotypic diversity and adaptation across different species. Their research underscores the crucial role of orphan genes in driving evolutionary innovation and shaping biological diversity.
Orphan genes were first discovered when the yeast genome-sequencing project began in 1996. [2] Orphan genes accounted for an estimated 26% of the yeast genome, but it was believed that these genes could be classified with homologues when more genomes were sequenced. [3] At the time, gene duplication was considered the only serious model of gene evolution [2] [4] [18] and there were few sequenced genomes for comparison, so a lack of detectable homologues was thought to be most likely due to a lack of sequencing data and not due to a true lack of homology. [3] However, orphan genes continued to persist as the quantity of sequenced genomes grew, [3] [19] eventually leading to the conclusion that orphan genes are ubiquitous to all genomes. [2] Estimates of the percentage of genes which are orphans varies enormously between species and between studies; 10-30% is a commonly cited figure. [3]
The study of orphan genes emerged largely after the turn of the century. In 2003, a study of Caenorhabditis briggsae and related species compared over 2000 genes. [3] They proposed that these genes must be evolving too quickly to be detected and are consequently sites of very rapid evolution. [3] In 2005, Wilson examined 122 bacterial species to try to examine whether the large number of orphan genes in many species was legitimate. [19] The study found that it was legitimate and played a role in bacterial adaptation. The definition of taxonomically-restricted genes was introduced into the literature to make orphan genes seem less "mysterious." [19]
In 2008, a yeast protein of established functionality, BSC4, was found to have evolved de novo from non-coding sequences whose homology was still detectable in sister species. [20]
In 2009, an orphan gene was discovered to regulate an internal biological network: the orphan gene, QQS, from Arabidopsis thaliana modifies plant composition. [21] The QQS orphan protein interacts with a conserved transcription factor, these data explain the compositional changes (increased protein) that are induced when QQS is engineered into diverse species. [22] In 2011, a comprehensive genome-wide study of the extent and evolutionary origins of orphan genes in plants was conducted in the model plant Arabidopsis thaliana " [23]
Genes can be tentatively classified as orphans if no orthologous proteins can be found in nearby species. [10]
One method used to estimate nucleotide or protein sequence similarity indicative of homology (i.e. similarity due to common origin) is the Basic Local Alignment Search Tool (BLAST). BLAST allows query sequences to be rapidly searched against large sequence databases. [24] [25] Simulations suggest that under certain conditions BLAST is suitable for detecting distant relatives of a gene. [26] However, genes that are short and evolve rapidly can easily be missed by BLAST. [27]
The systematic detection of homology to annotate orphan genes is called phylostratigraphy. [28] Phylostratigraphy generates a phylogenetic tree in which the homology is calculated between all genes of a focal species and the genes of other species. The earliest common ancestor for a gene determines the age, or phylostratum, of the gene. The term "orphan" is sometimes used only for the youngest phylostratum containing only a single species, but when interpreted broadly as a taxonomically-restricted gene, it can refer to all but the oldest phylostratum, with the gene orphaned within a larger clade.
Homology detection failure accounts for a majority of classified orphan genes. [8] Some scientists have attempted to recover some homology by using more sensitive methods, such as remote homology detection. In one study, remote homology detection techniques were used to demonstrate that a sizable fraction of orphan genes (over 15%) still exhibited remote homology despite being missed by conventional homology detection techniques, and that their functions were often related to the functions of nearby genes at genomic loci. [29]
Orphan genes arise from multiple sources, predominantly through de novo origination, duplication and rapid divergence, and horizontal gene transfer. [2]
Novel orphan genes continually arise de novo from non-coding sequences. [30] These novel genes may be sufficiently beneficial to be swept to fixation by selection. Or, more likely, they will fade back into the non-genic background. This latter option is supported by research in Drosophila showing that young genes are more likely go extinct. [31]
De novo genes were once thought to be a near impossibility due to the complex and potentially fragile intricacies of creating and maintaining functional polypeptides, [18] but research from the past 10 years or so has found multiple examples of de novo genes, some of which are associated with important biological processes, particularly testes function in animals. De novo genes were also found in fungi and plants. [20] [32] [33] [5] [34] [35] [11] [36]
For young orphan genes, it is sometimes possible to find homologous non-coding DNA sequences in sister taxa, which is generally accepted as strong evidence of de novo origin. However, the contribution of de novo origination to taxonomically-restricted genes of older origin, particularly in relation to the traditional gene duplication theory of gene evolution, remains contested. [37] [38] Logistically, de novo origination is much easier for RNA genes than protein-coding ones and Nathan H. Lents and colleagues recently reported the existence of several young microRNA genes on human chromosome 21. [39]
The duplication and divergence model for orphan genes involves a new gene being created from some duplication or divergence event and undergoing a period of rapid evolution where all detectable similarity to the originally duplicated gene is lost. [2] While this explanation is consistent with current understandings of duplication mechanisms, [2] the number of mutations needed to lose detectable similarity is large enough as to be a rare event, [2] [26] and the evolutionary mechanism by which a gene duplicate could be sequestered and diverge so rapidly remains unclear. [2] [40]
Another explanation for how orphan genes arise is through a duplication mechanism called horizontal gene transfer, where the original duplicated gene derives from a separate, unknown lineage. [2] This explanation for the origin of orphan genes is especially relevant in bacteria and archaea, where horizontal gene transfer is common.
Orphans genes tend to be very short (~6 times shorter than mature genes), and some are weakly expressed, tissue specific and simpler in codon usage and amino acid composition. [41] Orphan genes tend to encode more intrinsically disordered proteins, [42] [43] [44] although some structure has been found in one of the best characterized orphan genes. [45] Of the tens of thousands of enzymes of primary or specialized metabolism that have been characterized to date, none are orphans, or even of restricted lineage; apparently, catalysis requires hundreds of millions of years of evolution. [41]
Orphan genes, which have no detectable homologs in other species, represent a fascinating area of study in genomics. Their evolutionary role and biological significance remain subjects of ongoing research and debate. Orphan genes are important in evolution and speciation because of the potential for the production of novel genes and functions. [46] Orphan genes are theorized to play a critical role in the evolution of species, as they allow organisms to respond to changes in their environment and develop new adaptations rapidly. [47]
Orphan genes can have diverse functions, ranging from basic metabolic functions to complex regulatory processes. For example, some orphan genes are involved in the regulation of growth and development, while others play a role in the response to the environmental stresses. [48] Their evolutionary role and biological significance remain subjects of ongoing research and debate.
Some scientists propose that many orphan genes may not play a direct evolutionary role. They argue that genomes contain non-functional open reading frames (ORFs) which might produce spurious polypeptides not maintained by natural selection. Such genes are likely to be unique to a species because they do not undergo conservation across species, hence are categorized as orphan genes. [49]
Contrary to the view that they are evolutionary noise, emerging studies have illustrated the functional importance of orphan genes:
These examples confirm the functionality of some orphan genes but also suggest their potential involvement in the emergence of novel phenotypes, thereby contributing to species-specific adaptations.
Orphan genes have garnered interest across multiple scientific disciplines such as evolutionary biology and medicine, due to their nature and potential implications. [52]
In evolutionary biology, orphan genes diverge from traditional models of gene evolution and provide valuable insights into the process of de novo gene origination and lineage-specific adaptation. The term "de novo gene" specifically denotes the emergence of a functional gene without ancestral genetic material, whether as a protein-coding gene or a functional RNA molecule. [53] This understanding of de novo genes, coupled with the study of orphan genes, enriches the traditional Charles Darwin's model of evolution, also called Darwinism or Darwinian theory, by revealing additional mechanisms through which genetic diversity and adaptation can occur. By clarifying that de novo genes can arise from non-genic sequences and contribute to lineage-specific adaptation, this research expands our understanding of the creative forces of evolution, adding depth and complexity to Darwin's foundational principles.
In medicine, orphan genes represent a rich yet relatively unexplored resource that holds promise for understanding human health and addressing disease. These genes, which lack detectable homologs in other lineages, offer unique opportunities for biomedical research. [52] By elucidating the functions and regulatory mechanisms of orphan genes, researchers can gain insights into various aspects of human health. Orphan genes may play crucial roles in diseases that are poorly understood or have unknown genetic origins. Studying these genes can uncover novel disease mechanisms and therapeutic targets, paving the way for the development of innovative treatment strategies. To name a few, the orphan gene Gpr49, identified in humans, presents itself as a potential novel therapeutic target in combating hepatocellular carcinoma, the predominant form of liver cancer. [52] Furthermore, the gene C19orf12 is implicated in the manifestation of a particular clinical subtype of neurodegeneration characterized by brain iron accumulation. [52] An excerpt from a table listing various orphan genes across diverse species along with their respective functions is shown. [52]
Orphan genes have the potential to serve as biomarkers for disease diagnosis, prognosis, and treatment response. Their lineage-specific nature and expression patterns may provide valuable information for personalized medicine approaches, enabling more accurate and targeted interventions for individuals affected by various diseases. Thus, harnessing the potential of orphan genes in understanding human health has significant implications for advancing biomedical research and improving clinical outcomes.
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosis, or meiosis or other types of damage to DNA, which then may undergo error-prone repair, cause an error during other forms of repair, or cause an error during replication. Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements.
Evolutionary developmental biology is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.
Molecular evolution describes how inherited DNA and/or RNA change over evolutionary time, and the consequences of this for proteins and other components of cells and organisms. Molecular evolution is the basis of phylogenetic approaches to describing the tree of life. Molecular evolution overlaps with population genetics, especially on shorter timescales. Topics in molecular evolution include the origins of new genes, the genetic nature of complex traits, the genetic basis of adaptation and speciation, the evolution of development, and patterns and processes underlying genomic changes during evolution.
Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by gene duplication or indirectly by reverse transcription of an mRNA transcript. Pseudogenes are usually identified when genome sequence analysis finds gene-like sequences that lack regulatory sequences needed for transcription or translation, or whose coding sequences are obviously defective due to frameshifts or premature stop codons. Pseudogenes are a type of junk DNA.
A protein family is a group of evolutionarily related proteins. In many cases, a protein family has a corresponding gene family, in which each gene encodes a corresponding protein with a 1:1 relationship. The term "protein family" should not be confused with family as it is used in taxonomy.
Gene duplication is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. Gene duplications can arise as products of several types of errors in DNA replication and repair machinery as well as through fortuitous capture by selfish genetic elements. Common sources of gene duplications include ectopic recombination, retrotransposition event, aneuploidy, polyploidy, and replication slippage.
A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions. One such family are the genes for human hemoglobin subunits; the ten genes are in two clusters on different chromosomes, called the α-globin and β-globin loci. These two gene clusters are thought to have arisen as a result of a precursor gene being duplicated approximately 500 million years ago.
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a speciation event (orthologs), or a duplication event (paralogs), or else a horizontal gene transfer event (xenologs).
Paleopolyploidy is the result of genome duplications which occurred at least several million years ago (MYA). Such an event could either double the genome of a single species (autopolyploidy) or combine those of two species (allopolyploidy). Because of functional redundancy, genes are rapidly silenced or lost from the duplicated genomes. Most paleopolyploids, through evolutionary time, have lost their polyploid status through a process called diploidization, and are currently considered diploids, e.g., baker's yeast, Arabidopsis thaliana, and perhaps humans.
Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the correct structures form in the correct places of the body. For example, Hox genes in insects specify which appendages form on a segment, and Hox genes in vertebrates specify the types and shape of vertebrae that will form. In segmented animals, Hox proteins thus confer segmental or positional identity, but do not form the actual segments themselves.
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protein-coding genes and non-coding genes.
Genomic phylostratigraphy is a novel genetic statistical method developed in order to date the origin of specific genes by looking at its homologs across species. It was first developed by Ruđer Bošković Institute in Zagreb, Croatia. The system links genes to their founder gene, allowing us to then determine their age. This could help us better understand many evolutionary processes such as patterns of gene birth throughout evolution, or the relationship between the age of a transcriptome throughout embryonic development. Bioinformatic tools like GenEra have been developed to calculate relative gene ages based on genomic phylostratigraphy.
Plant evolution is the subset of evolutionary phenomena that concern plants. Evolutionary phenomena are characteristics of populations that are described by averages, medians, distributions, and other statistical methods. This distinguishes plant evolution from plant development, a branch of developmental biology which concerns the changes that individuals go through in their lives. The study of plant evolution attempts to explain how the present diversity of plants arose over geologic time. It includes the study of genetic change and the consequent variation that often results in speciation, one of the most important types of radiation into taxonomic groups called clades. A description of radiation is called a phylogeny and is often represented by type of diagram called a phylogenetic tree.
Gene redundancy is the existence of multiple genes in the genome of an organism that perform the same function. Gene redundancy can result from gene duplication. Such duplication events are responsible for many sets of paralogous genes. When an individual gene in such a set is disrupted by mutation or targeted knockout, there can be little effect on phenotype as a result of gene redundancy, whereas the effect is large for the knockout of a gene with only one copy. Gene knockout is a method utilized in some studies aiming to characterize the maintenance and fitness effects functional overlap.
Genome evolution is the process by which a genome changes in structure (sequence) or size over time. The study of genome evolution involves multiple fields such as structural analysis of the genome, the study of genomic parasites, gene and ancient genome duplications, polyploidy, and comparative genomics. Genome evolution is a constantly changing and evolving field due to the steadily growing number of sequenced genomes, both prokaryotic and eukaryotic, available to the scientific community and the public at large.
Zerknüllt is a gene in the Antennapedia complex of Drosophila and other insects, where it operates very differently from the canonical Hox genes in the same gene cluster. Comparison of Hox genes between species showed that the Zerknüllt gene evolved from one of the standard Hox genes in insects through accumulating many amino acid changes, changing expression pattern, losing ancestral function and gaining a new function.
An overlapping gene is a gene whose expressible nucleotide sequence partially overlaps with the expressible nucleotide sequence of another gene. In this way, a nucleotide sequence may make a contribution to the function of one or more gene products. Overlapping genes are present in and a fundamental feature of both cellular and viral genomes. The current definition of an overlapping gene varies significantly between eukaryotes, prokaryotes, and viruses. In prokaryotes and viruses overlap must be between coding sequences but not mRNA transcripts, and is defined when these coding sequences share a nucleotide on either the same or opposite strands. In eukaryotes, gene overlap is almost always defined as mRNA transcript overlap. Specifically, a gene overlap in eukaryotes is defined when at least one nucleotide is shared between the boundaries of the primary mRNA transcripts of two or more genes, such that a DNA base mutation at any point of the overlapping region would affect the transcripts of all genes involved. This definition includes 5′ and 3′ untranslated regions (UTRs) along with introns.
Diethard Tautz is a German biologist and geneticist, who is primarily concerned with the molecular basis of the evolution of mammals. Since 2006 he is director at the Max Planck Institute for Evolutionary Biology in Plön.
De novo gene birth is the process by which new genes evolve from non-coding DNA. De novo genes represent a subset of novel genes, and may be protein-coding or instead act as RNA genes. The processes that govern de novo gene birth are not well understood, although several models exist that describe possible mechanisms by which de novo gene birth may occur.
Erich Bornberg-Bauer is an Austrian biochemist, theoretical biologist and bioinformatician.