Alu elements and hominid phylogenetics

Transposable element insertions have strongly affected human evolution

Comparison of a full collection of the transposable element (TE) sequences of vertebrates with genome sequences shows that the human genome makes 655 perfect full-length matches. The cause is that the human genome contains many active TEs that have caused TE inserts in relatively recent times. These TE inserts in the human genome are several types of young Alus (AluYa5, AluYb8, AluYc1, etc.). Work in many laboratories has shown that such inserts have many effects including changes in gene expression, increases in recombination, and unequal crossover. The time of these very effective changes in the human lineage genome extends back about 4 million years according to these data and very likely much earlier. Rapid human lineage-specific evolution, including brain size is known to have also occurred in the last few million years. Alu insertions likely underlie rapid human lineage evolution. They are known to have many effects. Examples are listed in which TE sequences have influenced human-specific genes. The proposed model is that the many TE insertions created many potentially effective changes and those selected were responsible for a part of the striking human lineage evolution. The combination of the results of these events that were selected during human lineage evolution was apparently effective in producing a successful and rapidly evolving species.

Mobile element scanning (ME-Scan) by targeted high-throughput sequencing

Background

Mobile elements (MEs) are diverse, common and dynamic inhabitants of nearly all genomes. ME transposition generates a steady stream of polymorphic genetic markers, deleterious and adaptive mutations, and substrates for further genomic rearrangements. Research on the impacts, population dynamics, and evolution of MEs is constrained by the difficulty of ascertaining rare polymorphic ME insertions that occur against a large background of pre-existing fixed elements and then genotyping them in many individuals.

Results

Here we present a novel method for identifying nearly all insertions of a ME subfamily in the whole genomes of multiple individuals and simultaneously genotyping (for presence or absence) those insertions that are variable in the population. We use ME-specific primers to construct DNA libraries that contain the junctions of all ME insertions of the subfamily, with their flanking genomic sequences, from many individuals. Individual-specific "index" sequences are designed into the oligonucleotide adapters used to construct the individual libraries. These libraries are then pooled and sequenced using a ME-specific sequencing primer. Mobile element insertion loci of the target subfamily are uniquely identified by their junction sequence, and all insertion junctions are linked to their individual libraries by the corresponding index sequence. To test this method's feasibility, we apply it to the human AluYb8 and AluYb9 subfamilies. In four individuals, we identified a total of 2,758 AluYb8 and AluYb9 insertions, including nearly all those that are present in the reference genome, as well as 487 that are not. Index counts show the sequenced products from each sample reflect the intended proportions to within 1%. At a sequencing depth of 355,000 paired reads per sample, the sensitivity and specificity of ME-Scan are both approximately 95%.

Conclusions

Mobile Element Scanning (ME-Scan) is an efficient method for quickly genotyping mobile element insertions with very high sensitivity and specificity. In light of recent improvements to high-throughput sequencing technology, it should be possible to employ ME-Scan to genotype insertions of almost any mobile element family in many individuals from any species.

Alu-repeat-induced deletions within the NCF2 gene causing p67-phox-deficient chronic granulomatous disease (CGD)

Mutations that impair expression or function of the components of the phagocyte NADPH oxidase complex cause chronic granulomatous disease (CGD), which is associated with life-threatening infections and dysregulated granulomatous inflammation. In five CGD patients from four consanguineous families of two different ethnic backgrounds, we found similar genomic homozygous deletions of 1,380 bp comprising exon 5 of NCF2, which could be traced to Alu-mediated recombination events. cDNA sequencing showed in-frame deletions of phase zero exon 5, which encodes one of the tandem repeat motifs in the tetratricopeptide (TPR4) domain of p67-phox. The resulting shortened protein (p67Delta5) had a 10-fold reduced intracellular half-life and was unable to form a functional NADPH oxidase complex. No dominant negative inhibition of oxidase activity by p67Delta5 was observed. We conclude that Alu-induced deletion of the TPR4 domain of p67-phox leads to loss of function and accelerated degradation of the protein, and thus represents a new mechanism causing p67-phox-deficient CGD.

Laboratory methods for the analysis of primate mobile elements

Mobile elements represent a unique and powerful set of tools for understanding the variation in a genome. Methods exist not only to utilize the polymorphisms among and within taxa to various ends but also to investigate the mechanism through which mobilization occurs. The number of methods to accomplish these ends is ever growing. Here, we present several protocols designed to assay mobile element-based variation within and among individual genomes.

Phylogeny of the macaques (Cercopithecidae: Macaca) based on Alu elements

Genus Macaca (Cercopithecidae: Papionini) is one of the most successful primate radiations. Despite previous studies on morphology and mitochondrial DNA analysis, a number of issues regarding the details of macaque evolution remain unsolved. Alu elements are a class of non-autonomous retroposons belonging to short interspersed elements that are specific to the primate lineage. Because retroposon insertions show very little homoplasy, and because the ancestral state (absence of the SINE) is known, Alu elements are useful genetic markers and have been utilized for analyzing primate phylogenentic relationships and human population genetic relationships. Using PCR display methodology, 298 new Alu insertions have been identified from ten species of macaques. Together with 60 loci reported previously, a total of 358 loci are used to infer the phylogenetic relationships of genus Macaca. With regard to earlier unresolved issues on the macaque evolution, the topology of our tree suggests that: 1) genus Macaca contains four monophyletic species groups; 2) within the Asian macaques, the silenus group diverged first, and members of the sinica and fascicularis groups share a common ancestor; 3) Macaca arctoides are classified in the sinica group. Our results provide a robust molecular phylogeny for genus Macaca with stronger statistical support than previous studies. The present study also illustrates that SINE-based approaches are a powerful tool in primate phylogenetic studies and can be used to successfully resolve evolutionary relationships between taxa at scales from the ordinal level to closely related species within one genus.

The impact of retrotransposons on human genome evolution

Their ability to move within genomes gives transposable elements an intrinsic propensity to affect genome evolution. Non-long terminal repeat (LTR) retrotransposons--including LINE-1, Alu and SVA elements--have proliferated over the past 80 million years of primate evolution and now account for approximately one-third of the human genome. In this Review, we focus on this major class of elements and discuss the many ways that they affect the human genome: from generating insertion mutations and genomic instability to altering gene expression and contributing to genetic innovation. Increasingly detailed analyses of human and other primate genomes are revealing the scale and complexity of the past and current contributions of non-LTR retrotransposons to genomic change in the human lineage.

Comparative analysis of Alu repeats in primate genomes

Using bacteria artificial chromosome (BAC) end sequences (16.9 Mb) and high-quality alignments of genomic sequences (17.4 Mb), we performed a global assessment of the divergence distributions, phylogenies, and consensus sequences for Alu elements in primates including lemur, marmoset, macaque, baboon, and chimpanzee as compared to human. We found that in lemurs, Alu elements show a broader and more symmetric sequence divergence distribution, suggesting a steady rate of Alu retrotransposition activity among prosimians. In contrast, Alu elements in anthropoids show a skewed distribution shifted toward more ancient elements with continual declining rates in recent Alu activity along the hominoid lineage of evolution. Using an integrated approach combining mutation profile and insertion/deletion analyses, we identified nine novel lineage-specific Alu subfamilies in lemur (seven), marmoset (one), and baboon/macaque (one) containing multiple diagnostic mutations distinct from their human counterparts-Alu J, S, and Y subfamilies, respectively. Among these primates, we show that that the lemur has the lowest density of Alu repeats (55 repeats/Mb), while marmoset has the greatest abundance (188 repeats/Mb). We estimate that approximately 70% of lemur and 16% of marmoset Alu elements belong to lineage-specific subfamilies. Our analysis has provided an evolutionary framework for further classification and refinement of the Alu repeat phylogeny. The differences in the distribution and rates of Alu activity have played an important role in subtly reshaping the structure of primate genomes. The functional consequences of these changes among the diverse primate lineages over such short periods of evolutionary time are an important area of future investigation.

Mitochondrial insertions into primate nuclear genomes suggest the use of numts as a tool for phylogeny

Homoplasy-free characters are a valuable and highly desired tool for molecular systematics. Nuclear sequences of mitochondrial origin (numts) are fragments of mitochondrial DNA that have been transferred into the nuclear genome. numts are passively captured into genomes and have no transposition activity, which suggests they may have utility as phylogenetic markers. Here, five fully sequenced primate genomes (human, chimpanzee, orangutan, rhesus macaque, and marmoset) are used to reconstruct the evolutionary dynamics of recent numt accumulation in a phylogenetic context. The status of 367 numt loci is used as categorical data, and a maximum parsimony approach is used to trace numt insertions on different branches of the taxonomically undisputed primate phylogenetic tree. The presence of a given numt in related taxa implies orthologous integration, whereas the absence of a numt indicates the plesiomorphic condition prior to integration. An average rate of 5.65 numts per 1 My is estimated on the tree, but insertion rates vary significantly on different branches. Two instances in which the presence-absence pattern of numts does not agree with the phylogenetic tree were identified. These events may be the result of either lineage sorting or reversal. Using the numts reported here to reconstruct primate phylogeny produces the canonical primate tree topology with high bootstrap support. Moreover, numts identified in gorilla Supercontigs were used to test the human-chimp-gorilla trichotomy, yielding a high level of support for the sister relationship of human and chimpanzee. These analyses suggest that numts are valuable phylogenetic markers that can be used for molecular systematics. It remains to be tested whether numts are useful at deeper phylogenetic levels.

Primate phylogenomics: developing numerous nuclear non-coding, non-repetitive markers for ecological and phylogenetic applications and analysis of evolutionary rate variation

Background

Genetic analyses are often limited by the availability of appropriate molecular markers. Markers from neutrally evolving genomic regions may be particularly useful for inferring evolutionary histories because they escape the constraints of natural selection. For the majority of taxa however, obtaining such markers is challenging. Advances in genomics have the potential to alleviate the shortage of neutral markers. Here we present a method to develop numerous markers from putatively neutral regions of primate genomes.

Results

We began with the available whole genome sequences of human, chimpanzee and macaque. Using computational methods, we identified a total of 280 potential amplicons from putatively neutral, non-coding, non-repetitive regions of these genomes. Subsequently we amplified, using experimental methods, many of these amplicons from diverse primate taxa, including a ring-tailed lemur, which is distantly related to the genomic resources. Using a subset of 10 markers, we demonstrate the utility of the developed markers in phylogenetic and evolutionary rate analyses. Particularly, we uncovered substantial evolutionary rate variation among lineages, some of which are previously not reported.

Conclusion

We successfully developed numerous markers from putatively neutral regions of primate genomes using a strategy combining computational and experimental methods. Applying these markers to phylogenetic and evolutionary rate variation analyses exemplifies the utility of these markers. Diverse ecological and evolutionary analyses will benefit from these markers. Importantly, methods similar to those presented here can be applied to other taxa in the near future.

Analysis of newly identified low copy AluYj subfamily

Human specific AluY elements were investigated by comparative analysis between human chromosome 21 and chimpanzee chromosome 22. Human specific AluY element was identified on human chromosome 21q22 (accession no. AL163282), and then that was a new member of AluYj subfamily. From the bioinformatic analysis, AluYj subfamily was investigated in human whole genome using AluYj4 consensus sequence (accession no. AL163282). Thirteen members of the AluYj4 elements (4 diagnostic mutations) and eight members of the AluYj3 elements (3 diagnostic mutations) were identified with distinct diagnostic mutation from AluY consensus sequence. The results of the molecular clock calculation of non-CpG region substitution indicated that, AluYj4 elements (2.1 million years old) may be proliferated more recent time than AluYj3 elements (14.1 million years old). For the verification of recent insertion time, four of AluYj4 elements (ch2-AC017101, ch10-AC044786, ch12-AC007656 and ch21-AL163282) from human chromosomes 2, 10, 12, 21 were analyzed by PCR amplification using various human and primate DNA samples. Though, no polymorphism was detected in human population, we identified the new AluYj4 subfamily as the human specific elements.

Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals

As a consequence of recent developments in molecular phylogenomics, all extant orders of placental mammals have been grouped into 3 lineages: Afrotheria, Xenarthra, and Boreotheria, which originated in Africa, South America, and Laurasia, respectively. Despite this advancement, the order of divergence of these 3 lineages remains unresolved. Here, we performed extensive retroposon analysis with mammalian genomic data. Surprisingly, we identified a similar number of informative retroposon loci that support each of 3 possible phylogenetic hypotheses: the basal position for Afrotheria (22 loci), Xenarthra (25 loci), and Boreotheria (21 loci). This result indicates that the divergence of the placental common ancestor into the 3 lineages occurred nearly simultaneously. Thus, we examined whether these molecular data could be integrated into the geological context by incorporating recent geological data. We obtained firm evidence that complete separation of Gondwana into Africa and South America occurred 120 +/- 10 Ma. Accordingly, the previous reported time frame (division of Pangea into Gondwana and Laurasia at 148-138 Ma and division of Gondwana at 105 Ma) cannot be used to validate mammalian divergence order. Instead, we use our retroposon results and the recent geological data to propose that near-simultaneous divisions of continents leading to isolated Africa, South America, and Laurasia caused nearly concomitant divergence of the ancient placental ancestor into 3 lineages, Afrotheria, Xenarthra, and Boreotheria, approximately 120 Ma.

Integration target site selection by a resurrected human endogenous retrovirus

At least 8% of the human genome was formed by integration of retroviral DNA sequences. Here we analyze the forces directing the accumulation of human endogenous retroviruses (HERVs) by comparing de novo HERV integration targeting with the distribution of fixed HERV elements in the human genome. All known genomic HERVs are inactive due to mutation, but we were able to study integration targeting using a reconstituted consensus HERV-K (designated HERV-K(Con)). We found that HERV-K(Con) integrated preferentially in transcription units, in gene-rich regions, and near features associated with active transcription units and associated regulatory regions. In contrast, genomic HERV-K proviruses are found preferentially outside transcription units. The minority of genomic HERVKs present inside transcription units are in opposite transcriptional orientation relative to the host gene, the orientation predicted to be minimally disruptive to host mRNA synthesis, but de novo HERV-K(Con) integration within transcription units showed no orientation bias. We also found that the youngest HERV-K elements in the human genome showed a distribution intermediate between de novo HERV-K(Con) integration sites and older fixed HERV-Ks. These findings indicate that accumulation of HERVs in the human germline is a two-step process: integration targeting biases direct initial accumulation, then purifying selection leads to loss of proviruses disrupting gene function.

Support patterns from different outgroups provide a strong phylogenetic signal

It is known that the accuracy of phylogenetic reconstruction decreases when more distant outgroups are used. We quantify this phenomenon with a novel scoring method, the outgroup score pOG. This score expresses if the support for a particular branch of a tree decreases with increasingly distant outgroups. Large-scale simulations confirmed that the outgroup support follows this expectation and that the pOG score captures this pattern. The score often identifies the correct topology even when the primary reconstruction methods fail, particularly in the presence of model violations. In simulations of problematic phylogenetic scenarios such as rate variation among lineages (which can lead to long-branch attraction artifacts) and quartet-based reconstruction, the pOG analysis outperformed the primary reconstruction methods. Because the pOG method does not make any assumptions about the evolutionary model (besides the decreasing support from increasingly distant outgroups), it can detect cases of violations not treated by a specific model or too strong to be fully corrected. When used as an optimization criterion in the construction of a tree of 23 mammals, the outgroup signal confirmed many well-accepted mammalian orders and superorders. It supports Atlantogenata, a clade of Afrotheria and Xenarthra, and suggests an Artiodactyla-Chiroptera clade.

The evolution of two partner LINE/SINE families and a full-length chromodomain-containing Ty3/Gypsy LTR element in the first reptilian genome of Anolis carolinensis

Transposable elements have been characterized in a number of vertebrates, including whole genomes of mammals, birds, and fishes. The Anolis carolinensis draft assembly provides the first opportunity to study retroposons in a reptilian genome. Here, we identified and reconstructed a number of retroposons based on database searches: Five Sauria short interspersed element (SINE) subfamilies, 5S-Sauria SINE chimeras, Anolis Bov-B long interspersed element (LINE), Anolis SINE 2, Anolis LINE 2, Anolis LINE 1, Anolis CR 1, and a chromodomain-containing Ty3/Gypsy LTR element. We focused on two SINE families (Anolis Sauria SINE and Anolis SINE 2) and their partner LINE families (Anolis Bov-B LINE and Anolis LINE 2). We demonstrate that each SINE/LINE pair is distributed similarly and predict that the retrotransposition of evolutionarily younger Sauria SINE members is via younger Bov-B LINE members while a correlation also exists between their respective evolutionarily older SINE/LINE members. The evolutionarily youngest Sauria SINE sequences evolved as part of novel rolling-circle transposons. The evolutionary time frame when Bov-B LINEs and Sauria SINEs were less active in their retrotransposition is characterized by a high retrotransposition burst of Anolis SINE 2 and Anolis LINE 2 elements. We also characterized the first full-length chromoviral LTR element in amniotes (Amn-ichi). This newly identified chromovirus is widespread in the Anolis genome and has been very well preserved, indicating that it is still active. Transposable elements in the Anolis genome account for approximately 20% of the total DNA sequence, whereas the proportion is more than double that in many mammalian genomes in which such elements have important biological functions. Nevertheless, 20% transposable element coverage is sufficient to predict that Anolis retroposons and other mobile elements also may have biologically and evolutionarily relevant functions. The new SINEs and LINEs and other ubiquitous genomic elements characterized in the Anolis genome will prove very useful for studies in comparative genomics, phylogenetics, and functional genetics.

Beyond DNA: RNA editing and steps toward Alu exonization in primates

The exaptation of transposed elements into protein-coding domains by a process called exonization is one important evolutionary pathway for generating novel variant functions of gene products. Adenosine-to-inosine (A-to-I) modification is a recently discovered, RNA-editing-mediated mechanism that contributes to the exonization of previously unprocessed mRNA introns. In the human nuclear prelamin A recognition factor gene transcript, the alternatively spliced exon 8 results from an A-to-I editing-generated 3' splice site located within an intronic Alu short interspersed element. Sequence comparisons of representatives of all primate infraorders revealed the critical evolutionary steps leading to this editing-mediated exonization. The source of exon 8 was seeded within the primary transcript about 58-40 million years ago by the head-to-head insertions of two primate-specific Alu short interspersed elements in the common ancestor of anthropoids. The latent protein-coding potential was realized 34-52 million years later in a common ancestor of gorilla, chimpanzee, and human as a result of numerous changes at the RNA and DNA level. Comparisons of 426 processed mRNA clones from various primate species with their genomic sequences identified seven different RNA-editing-mediated alternative splice variants. In total, 30 A-to-I editing sites were identified. The gorilla, chimpanzee, and human nuclear prelamin A recognition factor genes exemplify the versatile interplay of pre- and posttranscriptional modifications leading to novel genetic potential.

Beyond linear sequence comparisons: the use of genome-level characters for phylogenetic reconstruction

The first whole genomes to be compared for phylogenetic inference were those of mitochondria, which provided the first sets of genome-level characters for phylogenetic reconstruction. Most powerful among these characters has been the comparisons of the relative arrangements of genes, which has convincingly resolved numerous branch points, including those that had remained recalcitrant even to very large molecular sequence comparisons. Now the world faces a tsunami of complete nuclear genome sequences. In addition to the tremendous amount of DNA sequence that is becoming available for comparison, there is also a potential for many more genome-level characters to be developed, including the relative positions of introns, the domain structures of proteins, gene family membership, the presence of particular biochemical pathways, aspects of DNA replication or transcription, and many others. These characters can be especially convincing owing to their low likelihood of reverting to a primitive condition or occurring independently in separate lineages, thereby reducing the occurrence of homoplasy. The comparisons of organelle genomes pioneered the way for using such features for phylogenetic reconstructions, and it is almost certainly true, as ever more genomic sequence becomes available, that further use of genome-level characters will play a big role in outlining the relationships among major animal groups.

The evolutionary history of the hominin hand since the last common ancestor of Pan and Homo

Molecular evidence indicates that the last common ancestor of the genus Pan and the hominin clade existed between 8 and 4 million years ago (Ma). The current fossil record indicates the Pan-Homo last common ancestor existed at least 5 Ma and most likely between 6 and 7 Ma. Together, the molecular and fossil evidence has important consequences for interpreting the evolutionary history of the hand within the tribe Hominini (hominins). Firstly, parsimony supports the hypothesis that the hand of the last common ancestor most likely resembled that of an extant great ape overall (Pan, Gorilla, and Pongo), and that of an African ape in particular. Second, it provides a context for interpreting the derived changes to the hand that have evolved in various hominins. For example, the Australopithecus afarensis hand is likely derived in comparison with that of the Pan-Homo last common ancestor in having shorter fingers relative to thumb length and more proximo-distally oriented joints between its capitate, second metacarpal, and trapezium. This evidence suggests that these derived features evolved prior to the intensification of stone tool-related hominin behaviors beginning around 2.5 Ma. However, a majority of primitive features most likely present in the Pan-Homo last common ancestor are retained in the hands of Australopithecus, Paranthropus/early Homo, and Homo floresiensis. This evidence suggests that further derived changes to the hands of other hominins such as modern humans and Neandertals did not evolve until after 2.5 Ma and possibly even later than 1.5 Ma, which is currently the earliest evidence of Acheulian technology. The derived hands of modern humans and Neandertals may indicate a morphological commitment to tool-related manipulative behaviors beyond that observed in other hominins, including those (e.g. H. floresiensis) which may be descended from earlier tool-making species.

Mobile DNA elements in primate and human evolution

Roughly 50% of the primate genome consists of mobile, repetitive DNA sequences such as Alu and LINE1 elements. The causes and evolutionary consequences of mobile element insertion, which have received considerable attention during the past decade, are reviewed in this article. Because of their unique mutational mechanisms, these elements are highly useful for answering phylogenetic questions. We demonstrate how they have been used to help resolve a number of questions in primate phylogeny, including the human-chimpanzee-gorilla trichotomy and New World primate phylogeny. Alu and LINE1 element insertion polymorphisms have also been analyzed in human populations to test hypotheses about human evolution and population affinities and to address forensic issues. Finally, these elements have had impacts on the genome itself. We review how they have influenced fundamental ongoing processes like nonhomologous recombination, genomic deletion, and X chromosome inactivation.

Phylogenetic analysis of Oryza rufipogon strains and their relations to Oryza sativa strains by insertion polymorphism of rice SINEs

Oryza rufipogon, the progenitor of the cultivated rice species Oryza sativa, is known by its wide intraspecific variation. In this study, we performed phylogenetic analyses of O. rufipogon strains and their relationships to O. sativa strains by using 26 newly identified p-SINE1 members from O. rufipogon strains, in addition to 23 members previously identified from O. sativa strains. A total of 103 strains of O. rufipogon and O. sativa were examined for the presence and absence of each of the p-SINE1 members at respective loci by PCR with a pair of primers that hybridize to the regions flanking each p-SINE1 member. A phylogenetic tree constructed on the basis of the insertion polymorphism of p-SINE1 members showed that O. rufipogon and O. sativa strains are classified into three groups. The first group consisted of O. rufipogon perennial strains mostly from China and O. sativa ssp. japonica strains, which included javanica strains forming a distinct subgroup. The second group consisted of almost all the O. rufipogon annual strains, a few O. rufipogon perennial strains and O. sativa ssp. indica strains. These groupings, in addition to other results, support the previous notion that annual O. rufipogon originated in the O. rufipogon perennial population, and that O. sativa originated polyphyletically in the O. rufipogon populations. The third group consisted of the other perennial strains and intermediate-type strains of O. rufipogon, in which the intermediate-type strains are most closely related to a hypothetical ancestor with no p-SINE1 members at the respective loci and to those belonging to the other rice species with the AA genome. This suggests that O. rufipogon perennial strains are likely to have originated from the O. rufipogon intermediate-ecotype population.

Waves of genomic hitchhikers shed light on the evolution of gamebirds (Aves: Galliformes)

BACKGROUND

The phylogenetic tree of Galliformes (gamebirds, including megapodes, currassows, guinea fowl, New and Old World quails, chicken, pheasants, grouse, and turkeys) has been considerably remodeled over the last decades as new data and analytical methods became available. Analyzing presence/absence patterns of retroposed elements avoids the problems of homoplastic characters inherent in other methodologies. In gamebirds, chicken repeats 1 (CR1) are the most prevalent retroposed elements, but little is known about the activity of their various subtypes over time. Ascertaining the fixation patterns of CR1 elements would help unravel the phylogeny of gamebirds and other poorly resolved avian clades.

RESULTS

We analyzed 1,978 nested CR1 elements and developed a multidimensional approach taking advantage of their transposition in transposition character (TinT) to characterize the fixation patterns of all 22 known chicken CR1 subtypes. The presence/absence patterns of those elements that were active at different periods of gamebird evolution provided evidence for a clade (Cracidae + (Numididae + (Odontophoridae + Phasianidae))) not including Megapodiidae; and for Rollulus as the sister taxon of the other analyzed Phasianidae. Genomic trace sequences of the turkey genome further demonstrated that the endangered African Congo Peafowl (Afropavo congensis) is the sister taxon of the Asian Peafowl (Pavo), rejecting other predominantly morphology-based groupings, and that phasianids are monophyletic, including the sister taxa Tetraoninae and Meleagridinae.

CONCLUSION

The TinT information concerning relative fixation times of CR1 subtypes enabled us to efficiently investigate gamebird phylogeny and to reconstruct an unambiguous tree topology. This method should provide a useful tool for investigations in other taxonomic groups as well.

SINEs of speciation: tracking lineages with retroposons

The value of short interspersed elements (SINEs) for diagnosing common ancestry is being expanded to examine the differential sorting of lineages through the course of speciation events. Because most SINEs are neutral markers of identical descent, are not precisely excised from the genome and have a known ancestral condition, they are advantageous for reconciling gene trees and species trees with minimal phylogenetic error. A population perspective on SINE evolution combined with coalescence theory provides a context for investigating the phenomenon of ancestral polymorphism and its role in producing incongruent SINE insertion patterns among multiple loci. Studies of human Alu repeats demonstrate the value of young polymorphic SINEs for assessing human genomic diversity and tracking ancient demographics of human populations, whereas incongruent insertion patterns revealed by older fixed SINE loci, such as those in African cichlid fishes, contain information that might help identify ancient radiations that are otherwise obscured by accumulated mutations in sequence data. Here, we review the utility of retroposons for inferring common ancestry, discuss limits to the method, and clarify confusion by providing examples from the literature that illustrate how discordant multi-locus insertion patterns of retroelements can indicate lineage-sorting events that should not be misinterpreted as phylogenetic noise.

SINEs of a nearly perfect character

Mobile elements have been recognized as powerful tools for phylogenetic and population-level analyses. However, issues regarding potential sources of homoplasy and other misleading events have been raised. We have collected available data for all phylogenetic and population level studies of primates utilizing Alu insertion data and examined them for potentially homoplasious and other misleading events. Very low levels of each potential confounding factor in a phylogenetic or population analysis (i.e., lineage sorting, parallel insertions, and precise excision) were found. Although taxa known to be subject to high levels of these types of events may indeed be subject to problems when using SINE analysis, we propose that most taxa will respond as the order Primates has--by the resolution of several long-standing problems observed using sequence-based methods.

Automated scanning for phylogenetically informative transposed elements in rodents

Transposed elements constitute an attractive, useful source of phylogenetic markers to elucidate the evolutionary history of their hosts. Frequent and successive amplifications over evolutionary time are important requirements for utilizing their presence or absence as landmarks of evolution. Although transposed elements are well distributed in rodent taxa, the generally high degree of genomic sequence divergence among species complicates our access to presence/absence data. With this in mind we developed a novel, high-throughput computational strategy, called CPAL (Conserved Presence/Absence Locus-finder), to identify genome-wide distributed, phylogenetically informative transposed elements flanked by highly conserved regions. From a total of 232 extracted chromosomal mouse loci we randomly selected 14 of these plus 2 others from previous test screens and attempted to amplify them via PCR in representative rodent species. All loci were amplifiable and ultimately contributed 31 phylogenetically informative markers distributed throughout the major groups of Rodentia.

A mobile element-based evolutionary history of guenons (tribe Cercopithecini)

BACKGROUND

Guenons (tribe Cercopithecini) are a species-rich group of primates that have attracted considerable attention from both primatologists and evolutionary biologists. The complex speciation pattern has made the elucidation of their relationships a challenging task, and many questions remain unanswered. SINEs are a class of non-autonomous mobile elements and are essentially homoplasy-free characters with known ancestral states, making them useful genetic markers for phylogenetic studies.

RESULTS

We identified 151 novel Alu insertion loci from 11 species of tribe Cercopithecini, and used these insertions and 17 previously reported loci to infer a phylogenetic tree of the tribe Cercopithecini. Our results robustly supported the following relationships: (i) Allenopithecus is the basal lineage within the tribe; (ii) Cercopithecus lhoesti (L'Hoest's monkey) forms a clade with Chlorocebus aethiops (African green monkey) and Erythrocebus patas (patas monkey), supporting a single arboreal to terrestrial transition within the tribe; (iii) all of the Cercopithecus except C. lhoesti form a monophyletic group; and (iv) contrary to the common belief that Miopithecus is one of the most basal lineages in the tribe, M. talapoin (talapoin) forms a clade with arboreal members of Cercopithecus, and the terrestrial group (C. lhoesti, Chlorocebus aethiops and E. patas) diverged from this clade after the divergence of Allenopithecus. Some incongruent loci were found among the relationships within the arboreal Cercopithecus group. Several factors, including incomplete lineage sorting, concurrent polymorphism and hybridization between species may have contributed to the incongruence.

CONCLUSION

This study presents one of the most robust phylogenetic hypotheses for the tribe Cercopithecini and demonstrates the advantages of SINE insertions for phylogenetic studies.

Extensive Morphological Convergence and Rapid Radiation in the Evolutionary History of the Family Geoemydidae (Old World Pond Turtles) Revealed by SINE Insertion Analysis

The family Geoemydidae is one of three in the superfamily Testudinoidea and is the most diversified family of extant turtle species. The phylogenetic relationships in this family and among related families have been vigorously investigated from both morphological and molecular viewpoints. The evolutionary history of Geoemydidae, however, remains controversial. Therefore, to elucidate the phylogenetic relationships of Geoemydidae and related species, we applied the SINE insertion method to investigate 49 informative SINE loci in 28 species. We detected four major evolutionary lineages (Testudinidae, Batagur group, Siebenrockiella group, and Geoemyda group) in the clade Testuguria (a clade of Geoemydidae + Testudinidae). All five specimens of Testudinidae form a monophyletic clade. The Batagur group comprises five batagurines. The Siebenrockiella group has one species, Siebenrockiella crassicollis. The Geoemyda group comprises 15 geoemydines (including three former batagurines, Mauremys reevesii, Mauremys sinensis, and Heosemys annandalii). Among these four groups, the SINE insertion patterns were inconsistent at four loci, suggesting that an ancestral species of Testuguria radiated and rapidly diverged into the four lineages during the initial stage of its evolution. Furthermore, within the Geoemyda group we identified three evolutionary lineages, namely Mauremys, Cuora, and Heosemys. The Heosemys lineage comprises Heosemys, Sacalia, Notochelys, and Melanochelys species, and its monophyly is a novel assemblage in Geoemydidae. Our SINE phylogenetic tree demonstrates extensive convergent morphological evolution between the Batagur group and the three species of the Geoemyda group, M. reevesii, M. sinensis, and H. annandalii.

SINEs of progress: Mobile element applications to molecular ecology

Mobile elements represent a unique and under-utilized set of tools for molecular ecologists. They are essentially homoplasy-free characters with the ability to be genotyped in a simple and efficient manner. Interpretation of the data generated using mobile elements can be simple compared to other genetic markers. They exist in a wide variety of taxa and are useful over a wide selection of temporal ranges within those taxa. Furthermore, their mode of evolution instills them with another advantage over other types of multilocus genotype data: the ability to determine loci applicable to a range of time spans in the history of a taxon. In this review, I discuss the application of mobile element markers, especially short interspersed elements (SINEs), to phylogenetic and population data, with an emphasis on potential applications to molecular ecology.

Bushes in the tree of life

A discussion of how homoplasy (the frequency of independently evolved characters) and the spacing of cladogenetic events limit our ability to reconstruct the tree of life using existing phylogenetic methods.

Different evolutionary fates of recently integrated human and chimpanzee LINE-1 retrotransposons

The long interspersed element-1 (LINE-1 or L1) is a highly successful retrotransposon in mammals. L1 elements have continued to actively propagate subsequent to the human-chimpanzee divergence, approximately 6 million years ago, resulting in species-specific inserts. Here, we report a detailed characterization of chimpanzee-specific L1 subfamily diversity and a comparison with their human-specific counterparts. Our results indicate that L1 elements have experienced different evolutionary fates in humans and chimpanzees within the past approximately 6 million years. Although the species-specific L1 copy numbers are on the same order in both species (1200-2000 copies), the number of retrotransposition-competent elements appears to be much higher in the human genome than in the chimpanzee genome. Also, while human L1 subfamilies belong to the same lineage, we identified two lineages of recently integrated L1 subfamilies in the chimpanzee genome. The two lineages seem to have coexisted for several million years, but only one shows evidence of expansion within the past three million years. These differential evolutionary paths may be the result of random variation, or the product of competition between L1 subfamily lineages. Our results suggest that the coexistence of several L1 subfamily lineages within a species may be resolved in a very short evolutionary period of time, perhaps in just a few million years. Therefore, the chimpanzee genome constitutes an excellent model in which to analyze the evolutionary dynamics of L1 retrotransposons.

Identification and characterization of novel polymorphic LINE-1 insertions through comparison of two human genome sequence assemblies

Mobile elements represent a relatively new class of markers for the study of human evolution. Long interspersed elements (LINEs) belong to a group of retrotransposons comprising approximately 21% of the human genome. Young LINE-1 (L1) elements that have integrated recently into the human genome can be polymorphic for insertion presence/absence in different human populations at particular chromosomal locations. To identify putative novel L1 insertion polymorphisms, we computationally compared two draft assemblies of the whole human genome (Public and Celera Human Genome assemblies). We identified a total of 148 potential polymorphic L1 insertion loci, among which 73 were candidates for novel polymorphic loci. Based on additional analyses we selected 34 loci for further experimental studies. PCR-based assays and DNA sequence analysis were performed for these 34 loci in 80 unrelated individuals from four diverse human populations: African-American, Asian, Caucasian, and South American. All but two of the selected loci were confirmed as polymorphic in our human population panel. Approximately 47% of the analyzed loci integrated into other repetitive elements, most commonly older L1s. One of the insertions was accompanied by a BC200 sequence. Collectively, these mobile elements represent a valuable source of genomic polymorphism for the study of human population genetics. Our results also suggest that the exhaustive identification of L1 insertion polymorphisms is far from complete, and new whole genome sequences are valuable sources for finding novel retrotransposon insertion polymorphisms.

A SINE-based dichotomous key for primate identification

For DNA samples or 'divorced' tissues, identifying the organism from which they were taken generally requires some type of analytical method. The ideal approach would be robust even in the hands of a novice, requiring minimal equipment, time, and effort. Genotyping SINEs (Short INterspersed Elements) is such an approach as it requires only PCR-related equipment, and the analysis consists solely of interpreting fragment sizes in agarose gels. Modern primate genomes are known to contain lineage-specific insertions of Alu elements (a primate-specific SINE); thus, to demonstrate the utility of this approach, we used members of the Alu family to identify DNA samples from evolutionarily divergent primate species. For each node of a combined phylogenetic tree (56 species; n=8 [Hominids]; 11 [New World monkeys]; 21 [Old World monkeys]; 2 [Tarsiformes]; and, 14 [Strepsirrhines]), we tested loci (>400 in total) from prior phylogenetic studies as well as newly identified elements for their ability to amplify in all 56 species. Ultimately, 195 loci were selected for inclusion in this Alu-based key for primate identification. This dichotomous SINE-based key is best used through hierarchical amplification, with the starting point determined by the level of initial uncertainty regarding sample origin. With newly emerging genome databases, finding informative retrotransposon insertions is becoming much more rapid; thus, the general principle of using SINEs to identify organisms is broadly applicable.

Natural hybridization in primates: one evolutionary mechanism

The role and importance of natural hybridization in the evolutionary histories of animal taxa is still debated. This results largely from a history of zoological investigations that assumed, rather than documented, a limited evolutionary role for this process. However, it is now becoming apparent that, just as for plants, the creative effects of reticulate evolution are widespread in animal taxa as well. This conclusion is supported by the documentation of numerous instances of the formation of new taxa and the genetic enrichment through introgressive hybridization. In the present review, we use primates as a paradigm for how natural hybridization can affect the evolution of species complexes and remains a footprint on genomes. Findings for a number of groups, including basal (e.g. lemurs) and derived (e.g. Old World apes) lineages, demonstrate that introgression and hybrid speciation have caused a reticulate pattern that is still detectable in the, often mosaic, genomes of primates. For example, results from genetic analyses of our own species demonstrate the process of past introgressive hybridization with the progenitors of our sister taxa (i.e. chimpanzees and gorillas) and most likely also our extinct, close relatives in the hominid lineage.

Toothed whale monophyly reassessed by SINE insertion analysis: the absence of lineage sorting effects suggests a small population of a common ancestral species

Morphological data have indicated that toothed whales form a monophyletic group. However, research published in the last several years has made the issue of the monophyly or paraphyly of toothed whales a subject of debate. Our group previously characterized three independent loci in which SINE insertions were shared among dolphins and sperm whales, thus supporting the traditional, morphologically based hypothesis of toothed whale monophyly. Although in recent years a few additional molecular works proposed this topology, there is still skepticism over this monophyly from the view point of molecular systematics. When the phylogeny of rapidly radiated taxa is examined using the SINE method, it is important to consider the ascertainment bias that arises when choosing a particular taxon for SINE loci screening. To overcome this methodological problem specific to the SINE method, we examined all possible topologies among sperm whales, dolphins and baleen whales by extensively screening SINE loci from species of all three lineages. We characterized nine independent SINE loci from the genomes of sperm whales and dolphins, all of which cluster sperm whales and dolphins but exclude baleen whales. Furthermore, we characterized ten independent loci from baleen whales, all of which were amplified in a common ancestor of these whales. From these observations, we conclude that toothed whales form a monophyletic group and that no ancestral SINE polymorphisms hinder their phylogenetic assignment despite the short divergence times of the major lineages of extant whales during evolution. These results suggest that a small population of common ancestors of all toothed whales ultimately diverged into the lineages of sperm whales and dolphins.

Pegasoferae, an unexpected mammalian clade revealed by tracking ancient retroposon insertions

Despite the recent large-scale efforts dedicated to comprehensive phylogenetic analyses using mitochondrial and nuclear DNA sequences, several relationships among mammalian orders remain controversial. Here, we present an extensive application of retroposon (L1) insertion analysis to the phylogenetic relationships among almost all mammalian orders. In addition to demonstrating the validity of Glires, Euarchontoglires, Laurasiatheria, and Boreoeutheria, we demonstrate an interordinal clade that links Chiroptera, Carnivora, and Perissodactyla within Laurasiatheria. Re-examination of a large DNA sequence data set yielded results consistent with our conclusion. We propose a superordinal name "Pegasoferae" for this clade of Chiroptera + Perissodactyla + Carnivora + Pholidota. The presence of a single incongruent L1 locus generates a tree in which the group of Carnivora + Perissodactyla associates with Cetartiodactyla but not with Chiroptera. This result suggests that incomplete lineage sorting of an ancestral dimorphism occurred with regard to the presence or absence of retroposon alleles in a common ancestor of Scrotifera (Pegasoferae + Cetartiodactyla), which was followed by rapid divergence into the extant orders over an evolutionarily short period. Accordingly, Euungulata (Cetartiodactyla + Perissodactyla) and Fereuungulata (Carnivora + Pholidota + Perissodactyla + Cetartiodactyla) cannot be validated as natural groups. The interordinal mammalian relationships presented here provide a cornerstone for future studies in the reconstruction of mammalian classifications, including extinct species, on evolution of large genomic sequences and structure, and in developmental analysis of morphological diversification.

Sequencing and haplotype analysis of the Activator of CREM in the Testis (ACT) gene in populations of fertile and infertile males

cAMP-responsive element modulator (CREM) is a key transcription factor in the differentiation of round spermatids into mature spermatozoa. During spermiogenesis, CREM is regulated in part by activator of CREM in the testis (ACT), which activates CREM in a phosphorylation-independent fashion. We hypothesized that the ACT gene, which is expressed exclusively in the testis, could be involved in male factor infertility in patients with idiopathic-impaired spermatogenesis. To test this hypothesis, we sequenced the coding regions and flanking intronic regions of the ACT gene in 96 azoo- or oligospermic patients and 69 fertile controls. A total of 12 single-nucleotide polymorphisms (SNPs) was identified, and four of them leading to amino acid substitutions. An association study was performed based on calculated haplotype frequencies, and statistically significant differences were found between the patient and control populations for some haplotypes. To help establish the evolutionary relationships between the haplotypes, the coding regions of both the chimpanzee and the gorilla ACT gene were sequenced and evaluated. To test whether the different haplotypes conferred a functional change to the ACT protein, a yeast two-hybrid assay was designed to test the interaction between the two most divergent ACT haplotypes and their known binding partners, CREM and KIF17b. We identified one ACT haplotype that had a 45% reduction in its interaction with CREM. Our results suggest that different haplotypes within the ACT gene may contribute to male factor subfertility.

Retroposed elements as archives for the evolutionary history of placental mammals

Reconstruction of the placental mammalian (eutherian) evolutionary tree has undergone diverse revisions, and numerous aspects remain hotly debated. Initial hierarchical divisions based on morphology contained many misgroupings due to features that evolved independently by similar selection processes. Molecular analyses corrected many of these misgroupings and the superordinal hierarchy of placental mammals was recently assembled into four clades. However, long or rapid evolutionary periods, as well as directional mutation pressure, can produce molecular homoplasies, similar characteristics lacking common ancestors. Retroposed elements, by contrast, integrate randomly into genomes with negligible probabilities of the same element integrating independently into orthologous positions in different species. Thus, presence/absence analyses of these elements are a superior strategy for molecular systematics. By computationally scanning more than 160,000 chromosomal loci and judiciously selecting from only phylogenetically informative retroposons for experimental high-throughput PCR applications, we recovered 28 clear, independent monophyly markers that conclusively verify the earliest divergences in placental mammalian evolution. Using tests that take into account ancestral polymorphisms, multiple long interspersed elements and long terminal repeat element insertions provide highly significant evidence for the monophyletic clades Boreotheria (synonymous with Boreoeutheria), Supraprimates (synonymous with Euarchontoglires), and Laurasiatheria. More importantly, two retropositions provide new support for a prior scenario of early mammalian evolution that places the basal placental divergence between Xenarthra and Epitheria, the latter comprising all remaining placentals. Due to its virtually homoplasy-free nature, the analysis of retroposon presence/absence patterns avoids the pitfalls of other molecular methodologies and provides a rapid, unequivocal means for revealing the evolutionary history of organisms.

The authors identified and sequenced retroposons in mammalian genomes. The presence and absence of these retroposons provided evolutionary markers from which the authors reconstructed the phylogenetic history of placental mammals.

The rise and fall of the CR1 subfamily in the lineage leading to penguins

The evolution of penguins has been investigated extensively, although inconclusively, by morphologists, biogeographers and molecular phylogeneticists. We investigated this issue using retroposon analysis of insertions of CR1, which is a member of the LINE (long interspersed element) family, in the genomes of penguins and penguin relatives. The retroposon method is a powerful tool for identifying monophyletic groups. Because retroposons often show different relative frequencies of retroposition during evolution, it is first necessary to identify a certain subgroup that was specifically active during the period when the species in question diverged. Hence, we systematically analyzed many CR1 members isolated from penguin and penguin-related genomes. These CR1s are divided into at least three distinct subgroups that share diagnostic nucleotide insertions and/or deletions, namely, penguin CR1 Sph I, Sph II type A and Sph II type B. The analysis of the inserted retroposons by PCR revealed that different CR1 subfamilies or types had amplified at different rates among different periods during penguin evolution. Namely, the penguin CR1 Sph I subfamily had higher rates of retroposition in a common ancestor of all orders examined in this study or at least in a common ancestor of all extant penguins, and the subfamily Sph II type A also had the same tendency. Therefore, these CR1 members can be used to elucidate the phylogenetic relationships of Sphenisciformes (penguins) among different avian orders. In contrast, the penguin CR1 Sph II type B subfamily had higher rates of retroposition just before and after the emergence of the extant genera in Spheniscidae, suggesting that they are useful for elucidating the intra-relationships among extant penguins. This is the first report for the characterization among the members of CR1 family in avian genomes excluding those of chickens. Hence, this work will be a cornerstone for elucidating the phylogenetic relationships in penguin evolution using the retroposon method.

Genomic deletions and precise removal of transposable elements mediated by short identical DNA segments in primates

Insertion of transposable elements is a major cause of genomic expansion in eukaryotes. Less is understood, however, about mechanisms underlying contraction of genomes. In this study, we show that retroelements can, in rare cases, be precisely deleted from primate genomes, most likely via recombination between 10- to 20-bp target site duplications (TSDs) flanking the retroelement. The deleted loci are indistinguishable from pre-integration sites, effectively reversing the insertion. Through human-chimpanzee-Rhesus monkey genomic comparisons, we estimate that 0.5%-1% of apparent retroelement "insertions" distinguishing humans and chimpanzees actually represent deletions. Furthermore, we demonstrate that 19% of genomic deletions of 200-500 bp that have occurred since the human-chimpanzee divergence are associated with flanking identical repeats of at least 10 bp. A large number of deletions internal to Alu elements were also found flanked by homologies. These results suggest that illegitimate recombination between short direct repeats has played a significant role in human genome evolution. Moreover, this study lends perspective to the view that insertions of retroelements represent unidirectional genetic events.

Evolutionary implications of multiple SINE insertions in an intronic region from diverse mammals

An analysis of the nuclear beta-fibrinogen intron 7 locus from 30 taxa representing 12 placental orders of mammals reveals the enriched occurrences of short interspersed element (SINE) insertion events. Mammalian-wide interspersed repeats (MIRs) are present at orthologous sites of all examined species except those in the order Rodentia. The higher substitution rate in mouse and a rare MIR deletion from rat account for the absence of MIR in the rodents. A minimum of five lineage-specific SINE sequences are also found to have independently inserted into this intron in Carnivora, Artiodactyla and Lagomorpha. In the case of Carnivora, the unique amplification pattern of order-specific CAN SINE provides important evidence for the "pan-carnivore" hypothesis of this repeat element and reveals that the CAN SINE family may still be active today. Particularly interesting is the finding that all identified lineage-specific SINE elements show a strong tendency to insert within or in very close proximity to the preexisting MIRs for their efficient integrations, suggesting that the MIR element is a hot spot for successive insertions of other SINEs. The unexpected MIR excision as a result of a random deletion in the rat intron locus and the non-random site targeting detected by this study indicate that SINEs actually have a greater insertional flexibility and regional specificity than had previously been recognized. Implications for SINE sequence evolution upon and following integration, as well as the fascinating interactions between retroposons and the host genomes are discussed.

Tracking Alu evolution in New World primates

Background

Alu elements are Short INterspersed Elements (SINEs) in primate genomes that have proven useful as markers for studying genome evolution, population biology and phylogenetics. Most of these applications, however, have been limited to humans and their nearest relatives, chimpanzees. In an effort to expand our understanding of Alu sequence evolution and to increase the applicability of these markers to non-human primate biology, we have analyzed available Alu sequences for loci specific to platyrrhine (New World) primates.

Results

Branching patterns along an Alu sequence phylogeny indicate three major classes of platyrrhine-specific Alu sequences. Sequence comparisons further reveal at least three New World monkey-specific subfamilies; AluTa7, AluTa10, and AluTa15. Two of these subfamilies appear to be derived from a gene conversion event that has produced a recently active fusion of AluSc- and AluSp-type elements. This is a novel mode of origin for new Alu subfamilies.

Conclusion

The use of Alu elements as genetic markers in studies of genome evolution, phylogenetics, and population biology has been very productive when applied to humans. The characterization of these three new Alu subfamilies not only increases our understanding of Alu sequence evolution in primates, but also opens the door to the application of these genetic markers outside the hominid lineage.

Chompy: an infestation of MITE-like repetitive elements in the crocodilian genome

Interspersed repeats are a major component of most eukaryotic genomes and have an impact on genome size and stability, but the repetitive element landscape of crocodilian genomes has not yet been fully investigated. In this report, we provide the first detailed characterization of an interspersed repeat element in any crocodilian genome. Chompy is a putative miniature inverted-repeat transposable element (MITE) family initially recovered from the genome of Alligator mississippiensis (American alligator) but also present in the genomes of Crocodylus moreletii (Morelet's crocodile) and Gavialis gangeticus (Indian gharial). The element has all of the hallmarks of MITEs including terminal inverted repeats, possible target site duplications, and a tendency to form secondary structures. We estimate the copy number in the alligator genome to be approximately 46,000 copies. As a result of their size and unique properties, Chompy elements may provide a useful source of genomic variation for crocodilian comparative genomics.

From the margins of the genome: mobile elements shape primate evolution

As is the case with mammals in general, primate genomes are inundated with repetitive sequence. Although much of this repetitive content consists of "molecular fossils" inherited from early mammalian ancestors, a significant portion of this material comprises active mobile element lineages. Despite indications that these elements played a major role in shaping the architecture of the genome, there remain many unanswered questions surrounding the nature of the host-element relationship. Here we review advances in our understanding of the host-mobile element dynamic and its overall impact on primate evolution.

Moving primate genomics beyond the chimpanzee genome

The comparative DNA sequence data that already exist on individual genomic loci depict the phylogenetic relationships of nearly all extant primate genera. Such a phylogenetic representation of the primates, validated by many sequenced primate genomes, and encompassing the full adaptive diversity of the order, is a prerequisite for identifying the genetic basis of humankind, and for testing the proposed human uniqueness of these traits. Some of these traits have been discovered recently, particularly in genes encoding proteins that are important for brain function.

Straightening out the LINEs: LINE-1 orthologous loci

The L1Hs preTa subfamily of long interspersed elements (LINEs) originated after the divergence of human and chimpanzee and is therefore found only in the human genome. Thirty-three of the 254 L1Hs preTa elements are polymorphic for the absence/presence of the insertion, making them useful markers for studying human population genetics. The problem of homoplasy, however, can diminish the value of LINEs as phylogenetic and population genetic markers. We examined anomalous orthologous sites in a range of nonhuman primates. Only two cases of other mobile elements inserting near the preintegration sites of L1Hs preTa elements were observed: an AluY insertion in Chlorocebus and an L1PA8 insertion in Aotus. Sequence analysis showed that both elements were clearly distinguishable from their human counterparts. We conclude that L1 elements can continue to be regarded as essentially homoplasy-free genetic characters.