The recent evolution of a pseudogene: diversity and divergence of a mitochondria-localized small heat shock protein in Arabidopsis thaliana

In this study we examined the evolution of the genes for three organelle-localized small heat shock proteins in Arabidopsis thaliana: the chloroplast-localized (CP) protein HSP21 and two mitochondria-localized (MT) proteins, HSP23.5 and HSP23.6. We found that the CP protein and one of the MT proteins, HSP23.6, are evolving under purifying selection to maintain function. In contrast, the gene for HSP23.5, the other MT protein, is highly variable within A. thaliana, and in some accessions or ecotypes this gene may be a pseudogene. HSP23.5 and HSP23.6 are related via a segmental duplication event, and the presence of orthologs of each gene in other species within the Brassicaceae indicates that the duplication generating HSP23.5 and HSP23.6 may have occurred as much as 20 million years ago. This is considerably longer than the 4 million year half-life of gene duplicates (functional genes as well as pseudogenes) reported by some studies. Our results are consistent with the prediction that after gene duplication one gene duplicate can be maintained for some time under relaxed selection while it accumulates random mutations. By capturing a pseudogene in the making our study provides important information on how pseudogenes are formed.     

Concerted evolution of the immunoglobulin VH gene family

With the aim of understanding the concerted evolution of the immunoglobulin VH multigene family, a phylogenetic tree for the DNA sequences of 16 mouse and five human germ line genes was constructed. This tree indicates that all genes in this family have undergone substantial evolutionary divergence. The most closely related genes so far identified in the mouse genome seem to have diverged about 6 million years (MY) ago, whereas the most distantly related genes diverged about 300 MY ago. This suggests that gene duplication caused by unequal crossing-over or gene conversion occurs very slowly in this gene family. The rate of occurrence of gene duplication in the VH gene family has been estimated to be 5 x 10(-7) per gene per year, which seems to be at least about 100 times lower than that for the rRNA gene family. This low rate of concerted evolution in the VH gene family helps retain intergenic genetic variability that in turn contributes to antibody diversity. Because of accumulation of destructive mutations, however, about one-third of the mouse and human VH genes seem to have become nonfunctional. Many of these pseudogenes have apparently originated recently, but some of them seem to have existed in the genome for more than 10 MY. The rate of nucleotide substitution for the complementarity-determining regions (CDRs) is as high as that of pseudogenes. This suggests that there is virtually no purifying selection operating in the CDRs and that germ line mutations are effectively used for generating antibody diversity.      

A genome-wide survey of human thioredoxin and glutaredoxin family pseudogenes

The thioredoxin/glutaredoxin family consists of small heat-stable proteins that have a highly conserved CXXC active site and that participate in the regulation of many redox reactions. We have searched the human genome sequence to find putative pseudogenes (non-functional copies of protein-coding genes) for all known members of this family. This survey has resulted in the identification of seven processed pseudogenes for human Trx1 and two more for human Grx1. No evidence for the presence of processed pseudogenes has been found for the remaining members of this family. In addition, we have been unable to detect any non-processed pseudogenes derived from any member of the family in the human genome. The seven thioredoxin pseudogenes can be divided into two groups: Trx1-psi2, -psi3, -psi4, -psi5 and -psi6 arose from the functional ancestor, whereas Trx1-psi1 and -psi7 originated from Trx1-psi2 and -psi6, respectively. In all cases, the pseudogenes originated after the human/rodent radiation as shown by phylogenetic analysis. This is also the case for Grx1-psi1 and Grx1-psi2, which are placed between rodent and human sequences in the phylogenetic tree. Our study provides a molecular record of the recent evolution of these two genes in the hominid lineage.     

Human U1 RNA pseudogenes may be generated by both DNA- and RNA-mediated mechanisms.

Analysis of cloned human genomic loci homologous to the small nuclear RNA U1 established that such sequences are abundant and dispersed in the human genome and that only a fraction represent bona fide genes. The majority of genomic loci bear defective gene copies, or pseudogenes, which contain scattered base mismatches and in some cases lack the sequence corresponding to the 3' end of U1 RNA. Although all of the U1 genes examined to date are flanked by essentially identical sequences and therefore appear to comprise a single multigene family, we present evidence for the existence of at least three structurally distinct classes of U1 pseudogenes. Class I pseudogenes had considerable flanking sequence homology with the U1 gene family and were probably derived from it by a DNA-mediated event such as gene duplication. In contrast, the U1 sequence in class II and III U1 pseudogenes was flanked by single-copy genomic sequences completely unrelated to those flanking the U1 gene family; in addition, short direct repeats flanked the class III but not the class II pseudogenes. We therefore propose that both class II and III U1 pseudogenes were generated by an RNA-mediated mechanism involving the insertion of U1 sequence information into a new chromosomal locus. We also noted that two other types of repetitive DNA sequences in eucaryotes, the Alu family in vertebrates and the ribosomal DNA insertions in Drosophila, bore a striking structural resemblance to the classes of U1 pseudogenes described here and may have been created by an RNA-mediated insertion event.     

Evolutionary origins of the transforming growth factor-beta gene family.

A molecular phylogeny for the transforming growth factor-beta (TGF-beta) gene family based on a comparison of nucleotide sequences is proposed. A phylogenetic tree constructed from these sequences shows that the family evolved from a common ancestral gene that came into existence at about the time of arthropod and chordate divergence. This model suggests that the present day TGF-beta gene family consists of four members: TGF-beta 1 (= TGF-beta 4), TGF-beta 2, TGF-beta 3, and TGF-beta 5. The molecular phylogeny and Southern hybridization data also suggest that the proteins for mammalian TGF-beta 1 and chicken TGF-beta 4 are the products of homologous rather than duplicated genes. If the gene duplication event that produced the ancestral gene for TGF-beta 1 occurred before the divergence of birds and mammals, then sufficient time would have elapsed to generate these quite distinct avian and mammalian TGF-beta 1 proteins. Therefore, the TGF-beta family contains four distinct proteins, TGF-beta 1, 2, 3, and 5.     

Evolution of trace amine associated receptor (TAAR) gene family in vertebrates: lineage-specific expansions and degradations of a second class of vertebrate chemosensory receptors expressed in the olfactory epithelium

The trace amine-associated receptors (TAARs) form a specific family of G protein-coupled receptors in vertebrates. TAARs were initially considered neurotransmitter receptors, but recent study showed that mouse TAARs function as chemosensory receptors in the olfactory epithelium. To clarify the evolutionary dynamics of the TAAR gene family in vertebrates, near-complete repertoires of TAAR genes and pseudogenes were identified from the genomic assemblies of 4 teleost fishes (zebrafish, fugu, stickleback, and medaka), western clawed frogs, chickens, 3 mammals (humans, mice, and opossum), and sea lampreys. Database searches revealed that fishes had many putatively functional TAAR genes (13-109 genes), whereas relatively small numbers of TAAR genes (3-22 genes) were identified in tetrapods. Phylogenetic analysis of these genes indicated that the TAAR gene family was subdivided into 5 subfamilies that diverged before the divergence of ray-finned fishes and tetrapods. In tetrapods, virtually all TAAR genes were located in 1 specific region of their genomes as a gene cluster; however, in fishes, TAAR genes were scattered throughout more than 2 genomic locations. This possibly reflects a whole-genome duplication that occurred in the common ancestor of ray-finned fishes. Expression analysis of zebrafish and stickleback TAAR genes revealed that many TAARs in these fishes were expressed in the olfactory organ, suggesting the relatively high importance of TAARs as chemosensory receptors in fishes. A possible evolutionary history of the vertebrate TAAR gene family was inferred from the phylogenetic and comparative genomic analyses.     

Phylogenetic and Evolutionary Analyses of the Frizzled Gene Family in Common Carp (Cyprinus carpio) Provide Insights into Gene Expansion from Whole-Genome Duplications

In humans, the frizzled (FZD) gene family encodes 10 homologous proteins that commonly localize to the plasma membrane. Besides being associated with three main signaling pathways for cell development, most FZDs have different physiological effects and are major determinants in the development process of vertebrates and. Here, we identified and annotated the FZD genes in the whole-genome of common carp (Cyprinus carpio), a teleost fish, and determined their phylogenetic relationships to FZDs in other vertebrates. Our analyses revealed extensive gene duplications in the common carp that have led to the 26 FZD genes that we detected in the common carp genome. All 26 FZD genes were assigned orthology to the 10 FZD genes of on-land vertebrates, with none of genes being specific to the fish lineage. We postulated that the expansion of the FZD gene family in common carp was the result of an additional whole genome duplication event and that the FZD gene family in other teleosts has been lost in their evolution history with the reason that the functions of genes are redundant and conservation. Through the expression profiling of FZD genes in common carp, we speculate that the ancestral gene was likely capable of performing all functions and was expressed broadly, while some descendant duplicate genes only performed partial functions and were specifically expressed at certain stages of development.     

Phylogeny and molecular dating of the cerato-platanin-encoding genes

The cerato-platanin family consists of proteins that can induce immune responses, cause necrosis, change chemotaxis and locomotion and may be related to the growth and development of various fungi. In this work, we analyzed the phylogenetic relationships among genes encoding members of the cerato-platanin family and computed the divergence times of the genes and corresponding fungi. The results showed that cerato-platanin-encoding genes could be classified into 10 groups but did not cluster according to fungal classes or their functions. The genes transferred horizontally and showed duplication. Molecular dating and adaptive evolution analyses indicated that the cerato-platanin gene originated with the appearance of saprophytes and that the gene was under positive selection. This finding suggests that cerato-platanin-encoding genes evolved with the development of fungal parasitic characteristics.     

Duplication of phospholipase C-delta gene family in fish genomes

Fishes possess more genes than other vertebrates, possibly because of a genome duplication event during the evolution of the teleost (ray-finned) fish lineage. To further explore this idea, we cloned five genes encoding phosphoinositide-specific phospholipase C-delta (PLC-delta), designated respectively PoPLC-deltas, from olive flounder (Paralichthys olivaceus), and we performed phylogenetic analysis and sequence comparison to compare our putative gene products (PoPLC-deltas) with the sequences of known human PLC isoforms. The deduced amino acid sequences shared high sequence identity with human PLC-delta1, -delta3, and -delta4 isozymes and exhibited similar primary structures. In phylogenetic analysis of PoPLC-deltas with PLC-deltas of five teleost fishes (zebrafish, stickleback, medaka, Tetraodon, and Takifugu), three tetrapods (human, chicken, and frog), and two tunicates (sea squirt and pacific sea squirt), whose putative sequences of PLC-delta are available in Ensembl genome browser, the result also indicated that the two paralogous genes corresponding to each PLC-delta isoform originated from fish-specific genome duplication prior to the divergence of teleost fish. Our analyses suggest that an ancestral PLC-delta gene underwent three rounds of genome duplication during the evolution of vertebrates, leading to the six genes of three PLC-delta isoforms in teleost fish.     

Comprehensive analysis of keratin gene clusters in humans and rodents

Here, we present the comparative analysis of the two keratin (K) gene clusters in the genomes of man, mouse and rat. Overall, there is a remarkable but not perfect synteny among the clusters of the three mammalian species. The human type I keratin gene cluster consists of 27 genes and 4 pseudogenes, all in the same orientation. It is interrupted by a domain of multiple genes encoding keratin-associated proteins (KAPs). Cytokeratin, hair and inner root sheath keratin genes are grouped together in small subclusters, indicating that evolution occurred by duplication events. At the end of the rodent type I gene cluster, a novel gene related to K14 and K17 was identified, which is converted to a pseudogene in humans. The human type II cluster consists of 27 genes and 5 pseudogenes, most of which are arranged in the same orientation. Of the 26 type II murine keratin genes now known, the expression of two new genes was identified by RT-PCR. Kb20, the first gene in the cluster, was detected in lung tissue. Kb39, a new ortholog of K1, is expressed in certain stratified epithelia. It represents a candidate gene for those hyperkeratotic skin syndromes in which no K1 mutations were identified so far. Most remarkably, the human K3 gene which causes Meesmann's corneal dystrophy when mutated, lacks a counterpart in the mouse genome. While the human genome has 138 pseudogenes related to K8 and K18, the mouse and rat genomes contain only 4 and 6 such pseudogenes. Our results also provide the basis for a unified keratin nomenclature and for future functional studies.     

Structure and organization of the bovine beta-globin genes

Genomic clones spanning the entire cow beta-globin gene locus have been isolated and characterized. These clones demonstrate that the linkage of embryonic-like (epsilon) genes and pseudogenes (psi) to the previously described fetal (gamma) and adult (beta) genes is as follows: 5'-epsilon 3-epsilon 4-psi 3-beta-epsilon 1-epsilon 2-psi 1- psi 2-gamma-3'. Present data indicate that, like that of the goat, the fetal and adult genes arose via block duplication of an ancestral four- gene set: epsilon-epsilon-psi-beta. This duplication event preceded the divergence of cows and goats, which occurred greater than or equal to 18-20 Myr ago. However, cows do not have the additional four-gene block containing a preadult/stress globin gene (beta C). Furthermore, the cow fetal cluster contains an extra beta-like pseudogene, which apparently arose by a small-scale duplication. The fixation of this duplication may indicate a possible evolutionary role for pseudogenes.      

Evolution of mammalian chitinase(-like) members of family 18 glycosyl hydrolases

Family 18 of glycosyl hydrolases encompasses chitinases and so-called chi-lectins lacking enzymatic activity due to amino acid substitutions in their active site. Both types of proteins widely occur in mammals although these organisms lack endogenous chitin. Their physiological function(s) as well as evolutionary relationships are still largely enigmatic. An overview of all family members is presented and their relationships are described. Molecular phylogenetic analyses suggest that both active chitinases (chitotriosidase and AMCase) result from an early gene duplication event. Further duplication events, followed by mutations leading to loss of chitinase activity, allowed evolution of the chi-lectins. The homologous genes encoding chitinase(-like) proteins are clustered in two distinct loci that display a high degree of synteny among mammals. Despite the shared chromosomal location and high homology, individual genes have evolved independently. Orthologs are more closely related than paralogues, and calculated substitution rate ratios indicate that protein-coding sequences underwent purifying selection. Substantial gene specialization has occurred in time, allowing for tissue-specific expression of pH optimized chitinases and chi-lectins. Finally, several family 18 chitinase-like proteins are present only in certain lineages of mammals, exemplifying recent evolutionary events in the chitinase protein family.     

Evolution of immunoglobulin kappa chain variable region genes in vertebrates

The major source of immunoglobulin diversity is variation in DNA sequence among multiple copies of variable region (V) genes of the heavy- and light-chain multigene families. In order to clarify the evolutionary pattern of the multigene family of immunoglobulin light kappa chain V region (V kappa) genes, phylogenetic analyses of V kappa genes from humans and other vertebrate species were conducted. The results obtained indicate that the V kappa genes so far sequenced can be grouped into three major monophyletic clusters, the cartilaginous fish, bony fish and amphibian, and mammalian clusters, and that the cartilaginous fish cluster first separated from the rest of the V kappa genes and then the remaining two clusters diverged. The mammalian V kappa genes can further be divided into 10 V kappa groups, 7 of which are present in the human genome. Human and mouse V kappa genes from different V kappa groups are intermingled rather than clustered on the chromosome, and there are a large number of pseudogenes scattered on the chromosome. This indicates that the chromosomal locations of V kappa genes have been shuffled many times by gene duplication, deletion, and transposition in the evolutionary process and that many genes have become nonfunctional during this process. This mode of evolution is consistent with the model of birth-and-death evolution rather than with the model of concerted evolution. An analysis of duplicate V kappa functional genes and pseudogenes in the human genome has indicated that pseudogenes evolve faster than functional genes but that the rate of nonsynonymous nucleotide substitution in the complementarity-determining regions of V kappa genes has been enhanced by positive Darwinian selection.      

An integrated approach for the comparative analysis of a multigene family: The nicotianamine synthase genes of barley

Recent genomic projects reveal that about half of the gene repertoire in plant genomes is made up by multigene families. In this paper, a set of structural and phylogenetic analyses have been applied to compare the differently sized nicotianamine synthase (NAS) gene families in barley and rice. Nicotianamine acts as a chelator of iron and other heavy metals and plays a key role in uptake, phloem transport and cytoplasmic distribution of iron, challenging efforts for the breeding of iron-efficient crop plants. Nine barley NAS genes have been mapped, and co-linearity of flanking genes in barley and rice was determined. The combined analyses reveal that the NAS multigene family members in barley originated through at least one duplication event that occurred before the divergence of rice and barley. Additional duplications appear to have occurred within each of the species. Although we detected no evidence for positive selection of recently duplicated genes within species, codon-based tests revealed evidence for positive selection having contributed to the divergence of some amino acids. The integrated comparative and phylogenetic analysis improved our current view of NAS gene family evolution, might facilitate the functional characterization of individual members and is applicable to other multigene families. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Molecular evolution of Aldolase A pseudogenes in mice: multiple origins, subsequent duplications, and heterogeneity of evolutionary rates

The Aldolase multigene family comprises three functional genes (A, B, and C) with tissue-specific expression regulated during ontogeny. DGGE analysis and nucleotide sequencing reveal a family of retropseudogenes of type A in species of MUS: Significant variation in rates of evolution of Aldolase A retropseudogenes is apparent. Our analyses demonstrate that (1) multiple events of retrotransposition are needed to account for the diversity of Aldolase A processed pseudogenes found in mice; (2) some of these sequences have undergone further duplication subsequent to the original retrotransposition event; (3) the patterns of nucleotide substitution are broadly comparable with previous estimates; and (4) estimates of rates of divergence for this array of sequences are up to four times higher than those reported in the literature.     

New genes originated via multiple recombinational pathways in the beta-globin gene family of rodents

Species differences in the size or membership composition of multigene families can be attributed to lineage-specific additions of new genes via duplication, losses of genes via deletion or inactivation, and the creation of chimeric genes via domain shuffling or gene fusion. In principle, it should be possible to infer the recombinational pathways responsible for each of these different types of genomic change by conducting detailed comparative analyses of genomic sequence data. Here, we report an attempt to unravel the complex evolutionary history of the beta-globin gene family in a taxonomically diverse set of rodent species. The main objectives were: 1) to characterize the genomic structure of the beta-globin gene cluster of rodents; 2) to assign orthologous and paralogous relationships among duplicate copies of beta-like globin genes; and 3) to infer the specific recombinational pathways responsible for gene duplications, gene deletions, and the creation of chimeric fusion genes. Results of our comparative genomic analyses revealed that variation in gene family size among rodent species is mainly attributable to the differential gain and loss of later expressed beta-globin genes via unequal crossing-over. However, two distinct recombinational mechanisms were implicated in the creation of chimeric fusion genes. In muroid rodents, a chimeric gamma/epsilon fusion gene was created by unequal crossing-over between the embryonic epsilon- and gamma-globin genes. Interestingly, this gamma/epsilon fusion gene was generated in the same fashion as the "anti-Lepore" 5'-delta-(beta/delta)-beta-3' duplication mutant in humans (the reciprocal exchange product of the pathological hemoglobin Lepore deletion mutant). By contrast, in the house mouse, Mus musculus, a chimeric beta/delta fusion pseudogene was created by a beta-globin --> delta-globin gene conversion event. Although the gamma/epsilon and beta/delta fusion genes share a similar chimeric gene structure, they originated via completely different recombinational pathways.     

The protein kinase ERK3 is encoded by a single functional gene: genomic analysis of the ERK3 gene family

Extracellular signal-regulated kinase 3 (ERK3) is a distantly related member of the mitogen-activated protein (MAP) kinase family of serine/threonine kinases. Here, we report the characterization of the genomic loci encoding ERK3 in mice and humans. The mouse ERK3 gene (Mapk6) spans more than 20 kb and is split into six exons. Its structure is similar to that of the human MAPK6 gene, which extends over 40 kb. We also identified and characterized a mouse Mapk6 processed pseudogene. In humans, database analysis has revealed the presence of six MAPK6 processed pseudogenes localized on four different chromosomes. We further show that the structure of MAPK6 is closely related to that of the gene encoding the homologous protein kinase p63(MAPK) (MAPK4), suggesting that the two genes arose by duplication. Our analysis demonstrates that the ERK3 subfamily of MAP kinase genes is composed of two functional genes, MAPK6 and MAPK4, and several pseudogenes.