Mosaic retroposon insertion patterns in placental mammals

Common tenrecs (Tenrec ecaudatus) reduce oxygen consumption in hypoxia and in hypercapnia without concordant changes to body temperature or heart rate

Common tenrecs (Tenrec ecaudatus) are fossorial mammals that use burrows during both active and hibernating seasons in Madagascar and its neighboring islands. Prevailing thought was that tenrecs hibernate for 8-9 months individually, but 13 tenrecs were removed from the same sealed burrow 1 m deep from the surface. Such group hibernation in sealed burrows presumably creates a hypoxic and/or hypercapnic environment and suggests that this placental mammal may have an increased tolerance to hypoxia and hypercapnia. Higher tolerances to hypoxia and hypercapnia have been documented for other mammals capable of hibernation and to determine if this is the case for tenrecs, we exposed them to acute hypoxia (4 h of 16 or 7% O2), progressive hypoxia (2 h of 16, 10 and 4% O2), or progressive hypercapnia (2 h of 2, 5 and 10% CO2) at cold (16 °C) or warm (28 °C) ambient temperatures (Ta). Oxygen equilibrium curves were also constructed on the whole blood of tenrecs at 10, 25, and 37 °C to determine if hemoglobin (Hb)-O2 affinity contributes to hypoxia tolerance. In animals held at 16 °C, normoxic and normocapnic levels of oxygen consumption rate (V ˙ O 2 ), body temperature (Tb), and heart rate (HR) were highly variable between individuals. This inter-individual variation was greatly reduced in animals held at 28 °C for oxygen consumption rate and body temperature. Both hypoxia (acute and progressive) and progressive hypercapnia led to decreases inV ˙ O 2 as well as the variation inV ˙ O 2 between animals held at 16 °C. The fall in oxygen consumption rate in 7% O2 independent of changes in body temperature in tenrecs held at 16 °C is unique and not consistent with the typical hypoxic metabolic response seen in other hibernating species that depends on concomitant falls in Tb. In animals held at 28 °C, exposure to O2 levels as low as 4% and CO2 levels as high as 10% had no significant effect onV ˙ O 2 , HR, or Tb, indicative of high tolerance to both hypoxia and hypercapnia. High variation in heart rate remained between individuals in all gas compositions and at all temperatures. Tenrec Hb-O2 affinity was similar to other homeothermic placental mammals and likely does not contribute to the increased hypoxia tolerance. Ultimately, our results suggest changes in Ta dictate physiological responses to hypoxia or hypercapnia in tenrecs, responses more characteristic of reptiles than of most placental mammals. Given that numerous anatomical and physiological characteristics of tenrecs suggest that they may be representative of an ancestral placental mammal, our findings suggest the typical hypoxic metabolic response evolved later in mammalian evolution.

Cis-regulatory landscapes in the evolution and development of the mammalian skull

Extensive morphological variation found in mammals reflects the wide spectrum of their ecological adaptations. The highest morphological diversity is present in the craniofacial region, where geometry is mainly dictated by the bony skull. Mammalian craniofacial development represents complex multistep processes governed by numerous conserved genes that require precise spatio-temporal control. A central question in contemporary evolutionary biology is how a defined set of conserved genes can orchestrate formation of fundamentally different structures, and therefore how morphological variability arises. In principle, differential gene expression patterns during development are the source of morphological variation. With the emergence of multicellular organisms, precise regulation of gene expression in time and space is attributed to cis-regulatory elements. These elements contribute to higher-order chromatin structure and together with trans-acting factors control transcriptional landscapes that underlie intricate morphogenetic processes. Consequently, divergence in cis-regulation is believed to rewire existing gene regulatory networks and form the core of morphological evolution. This review outlines the fundamental principles of the genetic code and genomic regulation interplay during development. Recent work that deepened our comprehension of cis-regulatory element origin, divergence and function is presented here to illustrate the state-of-the-art research that uncovered the principles of morphological novelty. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

SINEs as Credible Signs to Prove Common Ancestry in the Tree of Life: A Brief Review of Pioneering Case Studies in Retroposon Systematics

Currently, the insertions of SINEs (and other retrotransposed elements) are regarded as one of the most reliable synapomorphies in molecular systematics. The methodological mainstream of molecular systematics is the calculation of nucleotide (or amino acid) sequence divergences under a suitable substitution model. In contrast, SINE insertion analysis does not require any complex model because SINE insertions are unidirectional and irreversible. This straightforward methodology was named the “SINE method,” which resolved various taxonomic issues that could not be settled by sequence comparison alone. The SINE method has challenged several traditional hypotheses proposed based on the fossil record and anatomy, prompting constructive discussions in the Evo/Devo era. Here, we review our pioneering SINE studies on salmon, cichlids, cetaceans, Afrotherian mammals, and birds. We emphasize the power of the SINE method in detecting incomplete lineage sorting by tracing the genealogy of specific genomic loci with minimal noise. Finally, in the context of the whole-genome era, we discuss how the SINE method can be applied to further our understanding of the tree of life.

SINE-Based Phylogenomics Reveal Extensive Introgression and Incomplete Lineage Sorting in Myotis

Using presence/absence data from over 10,000 Ves SINE insertions, we reconstructed a phylogeny for 11 Myotis species. With nearly one-third of individual Ves gene trees discordant with the overall species tree, phylogenetic conflict appears to be rampant in this genus. From the observed conflict, we infer that ILS is likely a major contributor to the discordance. Much of the discordance can be attributed to the hypothesized split between the Old World and New World Myotis clades and with the first radiation of Myotis within the New World. Quartet asymmetry tests reveal signs of introgression between Old and New World taxa that may have persisted until approximately 8 MYA. Our introgression tests also revealed evidence of both historic and more recent, perhaps even contemporary, gene flow among Myotis species of the New World. Our findings suggest that hybridization likely played an important role in the evolutionary history of Myotis and may still be happening in areas of sympatry. Despite limitations arising from extreme discordance, our SINE-based phylogeny better resolved deeper relationships (particularly the positioning of M. brandtii) and was able to identify potential introgression pathways among the Myotis species sampled.

The performance of outgroup-free rooting under evolutionary radiations

Tree rooting implies a temporal dimension to phylogenies. Only after defining the position of the root node is that the ancestral-descendant relationship between branches can be fully deduced. Rooting has been usually carried out by employing evolutionarily close outgroup lineages, which is a drawback when these lineages are unavailable or unknown. Alternatively, outgroup-free rooting methods were proposed, which rely on the constancy of evolutionary rates to varying degrees. In this work we analyzed the performance of two of these methods, the midpoint rooting (MPR) and the minimal ancestor deviation (MAD), in rooting topologies evolved under challenging scenarios of fast evolutionary radiations derived from empirical data, characterized by short internal branches near the crown node. Considering all branch length combinations investigated, both methods exhibited average success rates below 50%, although MAD slightly outperformed MPR. Moreover, tree balance significantly impacted the relative performance of the methods. We found that, in four-taxa unrooted trees, the outcome of whether both methodologies will correctly root the tree can be roughly predicted by two simple dimensionless metrics: the coefficient of variation of the external branch lengths, and the ratio between the internal branch length to the total sum of branch lengths, which were employed to devise a general linear model that allowed calculating the probability of correct placing the root node for any four-taxa tree. We predicted that the performance of both outgroup-free rooting methods on loci representing the placental mammal radiation ranged between 50% and 75%.

Baby Genomics: Tracing the Evolutionary Changes That Gave Rise to Placentation

It has long been challenging to uncover the molecular mechanisms behind striking morphological innovations such as mammalian pregnancy. We studied the power of a robust comparative orthology pipeline based on gene synteny to address such problems. We inferred orthology relations between human genes and genes from each of 43 other vertebrate genomes, resulting in ∼18,000 orthologous pairs for each genome comparison. By identifying genes that first appear coincident with origin of the placental mammals, we hypothesized that we would define a subset of the genome enriched for genes that played a role in placental evolution. We thus pinpointed orthologs that appeared before and after the divergence of eutherian mammals from marsupials. Reinforcing previous work, we found instead that much of the genetic toolkit of mammalian pregnancy evolved through the repurposing of preexisting genes to new roles. These genes acquired regulatory controls for their novel roles from a group of regulatory genes, many of which did in fact originate at the appearance of the eutherians. Thus, orthologs appearing at the origin of the eutherians are enriched in functions such as transcriptional regulation by Krüppel-associated box-zinc-finger proteins, innate immune responses, keratinization, and the melanoma-associated antigen protein class. Because the cellular mechanisms of invasive placentae are similar to those of metastatic cancers, we then used our orthology inferences to explore the association between placenta invasion and cancer metastasis. Again echoing previous work, we find that genes that are phylogenetically older are more likely to be implicated in cancer development.

Retrotransposons spread potential cis-regulatory elements during mammary gland evolution

Acquisition of cis-elements is a major driving force for rewiring a gene regulatory network. Several kinds of transposable elements (TEs), mostly retrotransposons that propagate via a copy-and-paste mechanism, are known to possess transcription factor binding motifs and have provided source sequences for enhancers/promoters. However, it remains largely unknown whether retrotransposons have spread the binding sites of master regulators of morphogenesis and accelerated cis-regulatory expansion involved in common mammalian morphological features during evolution. Here, I demonstrate that thousands of binding sites for estrogen receptor α (ERα) and three related pioneer factors (FoxA1, GATA3 and AP2γ) that are essential regulators of mammary gland development arose from a spreading of the binding motifs by retrotransposons. The TE-derived functional elements serve primarily as distal enhancers and are enriched around genes associated with mammary gland morphogenesis. The source TEs occurred via a two-phased expansion consisting of mainly L2/MIR in a eutherian ancestor and endogenous retrovirus 1 (ERV1) in simian primates and murines. Thus the build-up of potential sources for cis-elements by retrotransposons followed by their frequent utilization by the host (co-option/exaptation) may have a general accelerating effect on both establishing and diversifying a gene regulatory network, leading to morphological innovation.

Transposable elements as genetic accelerators of evolution: contribution to genome size, gene regulatory network rewiring and morphological innovation

In the current era, as a growing number of genome sequence assemblies have been reported in animals, an in-depth analysis of transposable elements (TEs) is one of the most fundamental and essential studies for evolutionary genomics. Although TEs have, in general, been regarded as non-functional junk/selfish DNA, parasitic elements or harmful mutagens, studies have revealed that TEs have had a substantial and sometimes beneficial impact on host genomes in several ways. First, TEs are themselves diverse and thus provide lineage-specific characteristics to the genomes. Second, because TEs constitute a substantial fraction of animal genomes, they are a major contributing factor to evolutionary changes in genome size and composition. Third, host organisms have co-opted many repetitive sequences as genes, cis-regulatory elements and chromatin domain boundaries, which alter gene regulatory networks and in addition are partly involved in morphological evolution, as has been well documented in mammals. Here, I review the impact of TEs on various aspects of the genome, such as genome size and diversity in animals, as well as the evolution of gene networks and genome architecture in mammals. Given that a number of TE families probably remain to be discovered in many non-model organisms, unknown TEs may have contributed to gene networks in a much wider variety of animals than considered previously.

ILS-Aware Analysis of Low-Homoplasy Retroelement Insertions: Inference of Species Trees and Introgression Using Quartets

DNA sequence alignments have provided the majority of data for inferring phylogenetic relationships with both concatenation and coalescent methods. However, DNA sequences are susceptible to extensive homoplasy, especially for deep divergences in the Tree of Life. Retroelement insertions have emerged as a powerful alternative to sequences for deciphering evolutionary relationships because these data are nearly homoplasy-free. In addition, retroelement insertions satisfy the “no intralocus-recombination” assumption of summary coalescent methods because they are singular events and better approximate neutrality relative to DNA loci commonly sampled in phylogenomic studies. Retroelements have traditionally been analyzed with parsimony, distance, and network methods. Here, we analyze retroelement data sets for vertebrate clades (Placentalia, Laurasiatheria, Balaenopteroidea, Palaeognathae) with 2 ILS-aware methods that operate by extracting, weighting, and then assembling unrooted quartets into a species tree. The first approach constructs a species tree from retroelement bipartitions with ASTRAL, and the second method is based on split-decomposition with parsimony. We also develop a Quartet-Asymmetry test to detect hybridization using retroelements. Both ILS-aware methods recovered the same species-tree topology for each data set. The ASTRAL species trees for Laurasiatheria have consecutive short branch lengths in the anomaly zone whereas Palaeognathae is outside of this zone. For the Balaenopteroidea data set, which includes rorquals (Balaenopteridae) and gray whale (Eschrichtiidae), both ILS-aware methods resolved balaeonopterids as paraphyletic. Application of the Quartet-Asymmetry test to this data set detected 19 different quartets of species for which historical introgression may be inferred. Evidence for introgression was not detected in the other data sets.

Retrophylogenomics in rorquals indicate large ancestral population sizes and a rapid radiation

Background

Baleen whales (Mysticeti) are the largest animals on earth and their evolutionary history has been studied in detail, but some relationships still remain contentious. In particular, reconstructing the phylogenetic position of the gray whales (Eschrichtiidae) has been complicated by evolutionary processes such as gene flow and incomplete lineage sorting (ILS). Here, whole-genome sequencing data of the extant baleen whale radiation allowed us to identify transposable element (TE) insertions in order to perform phylogenomic analyses and measure germline insertion rates of TEs. Baleen whales exhibit the slowest nucleotide substitution rate among mammals, hence we additionally examined the evolutionary insertion rates of TE insertions across the genomes.

Results

In eleven whole-genome sequences representing the extant radiation of baleen whales, we identified 91,859 CHR-SINE insertions that were used to reconstruct the phylogeny with different approaches as well as perform evolutionary network analyses and a quantification of conflicting phylogenetic signals. Our results indicate that the radiation of rorquals and gray whales might not be bifurcating. The morphologically derived gray whales are placed inside the rorqual group, as the sister-species to humpback and fin whales. Detailed investigation of TE insertion rates confirm that a mutational slow down in the whale lineage is present but less pronounced for TEs than for nucleotide substitutions.

Conclusions

Whole genome sequencing based detection of TE insertions showed that the speciation processes in baleen whales represent a rapid radiation. Large genome-scale TE data sets in addition allow to understand retrotransposition rates in non-model organisms and show the potential for TE calling methods to study the evolutionary history of species.

Electronic supplementary material

The online version of this article (10.1186/s13100-018-0143-2) contains supplementary material, which is available to authorized users.

The Volcano Rabbit in the Phylogenetic Network of Lagomorphs

The order Lagomorpha unifies pikas (Ochotonidae) and the hares plus rabbits (Leporidae). Phylogenetic reconstructions of the species within Leporidae based on traditional morphological or molecular sequence data provide support for conflicting hypotheses. The retroposon presence/absence patterns analyzed in this study revealed strong support for the broadly accepted splitting of lagomorphs into ochotonids and leporids with Pronolagus as the first divergence in the leporid tree. Furthermore, the retroposon presence/absence patterns nested the rare volcano rabbit, Romerolagus diazi, within an unresolved network of deeper leporid relationships and provide the first homoplasy-free image of incomplete lineage sorting and/or ancestral hybridization/introgression in rapidly radiated Leporidae. At the same time, the strongest retroposon presence/absence signal supports the volcano rabbit as a separate branch between the Pronolagus junction and a unified cluster of the remaining leporids.

Global Rate Variation in Bony Vertebrates

This study investigated long-term substitution rate differences using three calibration points, divergences between lobe-finned vertebrates and ray-finned fish, between mammals and sauropsids, and between holosteans (gar and bowfin) and teleost fish with amino acid sequence data of 625 genes for 25 bony vertebrates. The result showed that the substitution rate was two to three times higher in the stem branches of lobe-finned vertebrates before the mammal-sauropsid divergence than in amniotes. The rate in the stem branch of ray-finned fish before the holostean-teleost fish divergence was also a few times higher than the holostean rate, whereas it was similar to or somewhat slower than the teleost fish rate. The phylogenetic relationship of coelacanth and lungfish with tetrapod was difficult to determine because of the short interval of the divergences. Considering the high rate in the stem branches, the divergences of coelacanth and lungfish from the stem branch were estimated as 408–427 Ma and 399–414 Ma, respectively, with the interval of 9–13 Myr. With the external calibration of the mammal-sauropsid split, the estimated times for ordinal divergences within eutherian mammals tend to be smaller than those in previous studies that used the calibration points within the lineage, with deeper divergences before the Cretaceous–Paleogene boundary and shallower ones after the boundary. In contrast the estimated times within birds were larger than those of previous studies, with the divergence between Galliformes and Anseriformes ∼80 Ma and that between Galloanserae and Neoaves 110 Ma.

Lineage-independent retrotransposition of UTP14 associated with male fertility has occurred multiple times throughout mammalian evolution

In mammals, gamete production is essential for reproductive success. This is particularly true for males where large quantities of sperm are produced to fertilize a limited number of eggs released by the female. Because of this, new genes associated with increased spermatogenic efficiency have been accumulating throughout the evolution of therian mammals. Many of these new genes are testis-specific retrotransposed copies of housekeeping genes located on the X chromosome. Of particular interest are retrotransposed copies of UTP14 that are present in many distantly related eutherian mammals. Analysis of genomic data available in ENSEMBL indicates that these UTP14 retrogenes have arisen independently in the various eutherian clades. They represent an interesting aspect of evolution whereby new homologues of UTP14 have become independently fixed in multiple mammalian lineages due to the reproductive advantage that may be conferred to males. Surprisingly, these genes may also be lost, even after being present within a lineage for millions of years. This phenomenon may potentially be used to delineate evolutionary trees in closely related groups of mammals, particularly in the case of South American primates. Studying these retrogenes will yield new insights into the evolutionary history of male gamete production and the phylogeny of eutherian mammals.

Phylogenetic Conflict in Bears Identified by Automated Discovery of Transposable Element Insertions in Low-Coverage Genomes

Phylogenetic reconstruction from transposable elements (TEs) offers an additional perspective to study evolutionary processes. However, detecting phylogenetically informative TE insertions requires tedious experimental work, limiting the power of phylogenetic inference. Here, we analyzed the genomes of seven bear species using high-throughput sequencing data to detect thousands of TE insertions. The newly developed pipeline for TE detection called TeddyPi (TE detection and discovery for Phylogenetic Inference) identified 150,513 high-quality TE insertions in the genomes of ursine and tremarctine bears. By integrating different TE insertion callers and using a stringent filtering approach, the TeddyPi pipeline produced highly reliable TE insertion calls, which were confirmed by extensive in vitro validation experiments. Analysis of single nucleotide substitutions in the flanking regions of the TEs shows that these substitutions correlate with the phylogenetic signal from the TE insertions. Our phylogenomic analyses show that TEs are a major driver of genomic variation in bears and enabled phylogenetic reconstruction of a well-resolved species tree, despite strong signals for incomplete lineage sorting and introgression. The analyses show that the Asiatic black, sun, and sloth bear form a monophyletic clade, in which phylogenetic incongruence originates from incomplete lineage sorting. TeddyPi is open source and can be adapted to various TE and structural variation callers. The pipeline makes it possible to confidently extract thousands of TE insertions even from low-coverage genomes (∼10×) of nonmodel organisms. This opens new possibilities for biologists to study phylogenies and evolutionary processes as well as rates and patterns of (retro-)transposition and structural variation.

Speciation network in Laurasiatheria: retrophylogenomic signals

Rapid species radiation due to adaptive changes or occupation of new ecospaces challenges our understanding of ancestral speciation and the relationships of modern species. At the molecular level, rapid radiation with successive speciations over short time periods-too short to fix polymorphic alleles-is described as incomplete lineage sorting. Incomplete lineage sorting leads to random fixation of genetic markers and hence, random signals of relationships in phylogenetic reconstructions. The situation is further complicated when you consider that the genome is a mosaic of ancestral and modern incompletely sorted sequence blocks that leads to reconstructed affiliations to one or the other relative, depending on the fixation of their shared ancestral polymorphic alleles. The laurasiatherian relationships among Chiroptera, Perissodactyla, Cetartiodactyla, and Carnivora present a prime example for such enigmatic affiliations. We performed whole-genome screenings for phylogenetically diagnostic retrotransposon insertions involving the representatives bat (Chiroptera), horse (Perissodactyla), cow (Cetartiodactyla), and dog (Carnivora), and extracted among 162,000 preselected cases 102 virtually homoplasy-free, phylogenetically informative retroelements to draw a complete picture of the highly complex evolutionary relations within Laurasiatheria. All possible evolutionary scenarios received considerable retrotransposon support, leaving us with a network of affiliations. However, the Cetartiodactyla-Carnivora relationship as well as the basal position of Chiroptera and an ancestral laurasiatherian hybridization process did exhibit some very clear, distinct signals. The significant accordance of retrotransposon presence/absence patterns and flanking nucleotide changes suggest an important influence of mosaic genome structures in the reconstruction of species histories.

Resolving the relationships of Paleocene placental mammals

The 'Age of Mammals' began in the Paleocene epoch, the 10 million year interval immediately following the Cretaceous-Palaeogene mass extinction. The apparently rapid shift in mammalian ecomorphs from small, largely insectivorous forms to many small-to-large-bodied, diverse taxa has driven a hypothesis that the end-Cretaceous heralded an adaptive radiation in placental mammal evolution. However, the affinities of most Paleocene mammals have remained unresolved, despite significant advances in understanding the relationships of the extant orders, hindering efforts to reconstruct robustly the origin and early evolution of placental mammals. Here we present the largest cladistic analysis of Paleocene placentals to date, from a data matrix including 177 taxa (130 of which are Palaeogene) and 680 morphological characters. We improve the resolution of the relationships of several enigmatic Paleocene clades, including families of 'condylarths'. Protungulatum is resolved as a stem eutherian, meaning that no crown-placental mammal unambiguously pre-dates the Cretaceous-Palaeogene boundary. Our results support an Atlantogenata-Boreoeutheria split at the root of crown Placentalia, the presence of phenacodontids as closest relatives of Perissodactyla, the validity of Euungulata, and the placement of Arctocyonidae close to Carnivora. Periptychidae and Pantodonta are resolved as sister taxa, Leptictida and Cimolestidae are found to be stem eutherians, and Hyopsodontidae is highly polyphyletic. The inclusion of Paleocene taxa in a placental phylogeny alters interpretations of relationships and key events in mammalian evolutionary history. Paleocene mammals are an essential source of data for understanding fully the biotic dynamics associated with the end-Cretaceous mass extinction. The relationships presented here mark a critical first step towards accurate reconstruction of this important interval in the evolution of the modern fauna.

Mammal madness: is the mammal tree of life not yet resolved?

Most molecular phylogenetic studies place all placental mammals into four superordinal groups, Laurasiatheria (e.g. dogs, bats, whales), Euarchontoglires (e.g. humans, rodents, colugos), Xenarthra (e.g. armadillos, anteaters) and Afrotheria (e.g. elephants, sea cows, tenrecs), and estimate that these clades last shared a common ancestor 90-110 million years ago. This phylogeny has provided a framework for numerous functional and comparative studies. Despite the high level of congruence among most molecular studies, questions still remain regarding the position and divergence time of the root of placental mammals, and certain 'hard nodes' such as the Laurasiatheria polytomy and Paenungulata that seem impossible to resolve. Here, we explore recent consensus and conflict among mammalian phylogenetic studies and explore the reasons for the remaining conflicts. The question of whether the mammal tree of life is or can be ever resolved is also addressed.This article is part of the themed issue 'Dating species divergences using rocks and clocks'.

Viruses and the placenta: the essential virus first view

A virus first perspective is presented as an alternative hypothesis to explain the role of various endogenized retroviruses in the origin of the mammalian placenta. It is argued that virus-host persistence is a key determinant of host survival and the various ERVs involved have directly affected virus-host persistence.

Alu SINE analyses of 3,000-year-old human skeletal remains: a pilot study

Background

As Short Interspersed Elements (SINEs), human-specific Alu elements can be used for population genetic studies. Very recent inserts are polymorphic within and between human populations. In a sample of 30 elements originating from three different Alu subfamilies, we investigated whether they are preserved in prehistorical skeletal human remains from the Bronze Age Lichtenstein cave in Lower Saxony, Germany. In the present study, we examined a prehistoric triad of father, mother and daughter.

Results

For 26 of the 30 Alu loci investigated, definite results were obtained. We were able to demonstrate that presence/absence analyses of Alu elements can be conducted on individuals who lived 3,000 years ago. The preservation of the ancient DNA (aDNA) is good enough in two out of three ancient individuals to routinely allow the amplification of 500 bp fragments. The third individual revealed less well-preserved DNA, which results in allelic dropout or complete amplification failures. We here present an alternative molecular approach to deal with these degradation phenomena by using internal Alu subfamily specific primers producing short fragments of approximately 150 bp.

Conclusions

Our data clearly show the possibility of presence/absence analyses of Alu elements in individuals from the Lichtenstein cave. Thus, we demonstrate that our method is reliably applicable for aDNA samples with good or moderate DNA preservation. This method will be very useful for further investigations with more Alu loci and larger datasets. Human population genetic studies and other large-scale investigations would provide insight into Alu SINE-based microevolutionary processes in humans during the last few thousand years and help us comprehend the evolutionary dynamics of our genome.

Electronic supplementary material

The online version of this article (doi:10.1186/s13100-016-0063-y) contains supplementary material, which is available to authorized users.

Incomplete Lineage Sorting and Hybridization Statistics for Large-Scale Retroposon Insertion Data

Ancient retroposon insertions can be used as virtually homoplasy-free markers to reconstruct the phylogenetic history of species. Inherited, orthologous insertions in related species offer reliable signals of a common origin of the given species. One prerequisite for such a phylogenetically informative insertion is that the inserted element was fixed in the ancestral population before speciation; if not, polymorphically inserted elements may lead to random distributions of presence/absence states during speciation and possibly to apparently conflicting reconstructions of their ancestry. Fortunately, such misleading fixed cases are relatively rare but nevertheless, need to be considered. Here, we present novel, comprehensive statistical models applicable for (1) analyzing any pattern of rare genomic changes, (2) testing and differentiating conflicting phylogenetic reconstructions based on rare genomic changes caused by incomplete lineage sorting or/and ancestral hybridization, and (3) differentiating between search strategies involving genome information from one or several lineages. When the new statistics are applied, in non-conflicting cases a minimum of three elements present in both of two species and absent in a third group are considered significant support (p<0.05) for the branching of the third from the other two, if all three of the given species are screened equally for genome or experimental data. Five elements are necessary for significant support (p<0.05) if a diagnostic locus derived from only one of three species is screened, and no conflicting markers are detected. Most potentially conflicting patterns can be evaluated for their significance and ancestral hybridization can be distinguished from incomplete lineage sorting by considering symmetric or asymmetric distribution of rare genomic changes among possible tree configurations. Additionally, we provide an R-application to make the new KKSC insertion significance test available for the scientific community at http://retrogenomics.uni-muenster.de:3838/KKSC_significance_test/.

The presence/absence patterns of transposed elements, so called jumping genes, provide invaluable information about evolution. Unfortunately, there is still no clear all-encompassing analysis of the statistical significance of insertion patterns, and the single existing model of insertion data is no longer sufficient for the emerging genomic era. Here, we have provided a comprehensive statistical framework for testing the significance of support for phylogenetic hypotheses derived from genome-level presence/absence data such as retroposon insertions and for evaluating such data for different evolutionary scenarios, including polytomy, incomplete lineage sorting, and ancestral hybridization. This statistical framework is especially important for high-throughput applications of current and upcoming genome projects due to its treatment of unlimited numbers of testable markers, and is embedded in a user-friendly R-application available to the scientific community online. Finally, a reliable, adaptable calculation for the significance of support for phylogenetic trees derived from retroposon presence/absence data is now available.

The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference

Placental mammals comprise three principal clades: Afrotheria (e.g., elephants and tenrecs), Xenarthra (e.g., armadillos and sloths), and Boreoeutheria (all other placental mammals), the relationships among which are the subject of controversy and a touchstone for debate on the limits of phylogenetic inference. Previous analyses have found support for all three hypotheses, leading some to conclude that this phylogenetic problem might be impossible to resolve due to the compounded effects of incomplete lineage sorting (ILS) and a rapid radiation. Here we show, using a genome scale nucleotide data set, microRNAs, and the reanalysis of the three largest previously published amino acid data sets, that the root of Placentalia lies between Atlantogenata and Boreoeutheria. Although we found evidence for ILS in early placental evolution, we are able to reject previous conclusions that the placental root is a hard polytomy that cannot be resolved. Reanalyses of previous data sets recover Atlantogenata + Boreoeutheria and show that contradictory results are a consequence of poorly fitting evolutionary models; instead, when the evolutionary process is better-modeled, all data sets converge on Atlantogenata. Our Bayesian molecular clock analysis estimates that marsupials diverged from placentals 157-170 Ma, crown Placentalia diverged 86-100 Ma, and crown Atlantogenata diverged 84-97 Ma. Our results are compatible with placental diversification being driven by dispersal rather than vicariance mechanisms, postdating early phases in the protracted opening of the Atlantic Ocean.

The devil is in the details: Transposable element analysis of the Tasmanian devil genome

The third marsupial genome was sequenced from the Tasmanian devil (Sarcophilus harrisii), a species that currently is driven to extinction by a rare transmissible cancer. The transposable element (TE) landscape of the Tasmanian devil genome revealed that the main driver of retrotransposition the Long INterspersed Element 1 (LINE1) seem to have become inactivated during the past 12 million years. Strangely, the Short INterspersed Elements (SINE), that normally hijacks the LINE1 retrotransposition system, became inactive prior to LINE1 at around 30 million years ago. The SINE inactivation was in vitro verified in several species. Here I discuss that the apparent LINE1 inactivation might be caused by a genome assembly artifact. The repetitive fraction of any genome is highly complex to assemble and the observed problems are not unique to the Tasmanian devil genome.

Shotgun Mitogenomics Provides a Reference Phylogenetic Framework and Timescale for Living Xenarthrans

Xenarthra (armadillos, sloths, and anteaters) constitutes one of the four major clades of placental mammals. Despite their phylogenetic distinctiveness in mammals, a reference phylogeny is still lacking for the 31 described species. Here we used Illumina shotgun sequencing to assemble 33 new complete mitochondrial genomes, establishing Xenarthra as the first major placental clade to be fully sequenced at the species level for mitogenomes. The resulting data set allowed the reconstruction of a robust phylogenetic framework and timescale that are consistent with previous studies conducted at the genus level using nuclear genes. Incorporating the full species diversity of extant xenarthrans points to a number of inconsistencies in xenarthran systematics and species definition. We propose to split armadillos into two distinct families Dasypodidae (dasypodines) and Chlamyphoridae (euphractines, chlamyphorines, and tolypeutines) to better reflect their ancient divergence, estimated around 42 Ma. Species delimitation within long-nosed armadillos (genus Dasypus) appeared more complex than anticipated, with the discovery of a divergent lineage in French Guiana. Diversification analyses showed Xenarthra to be an ancient clade with a constant diversification rate through time with a species turnover driven by high but constant extinction. We also detected a significant negative correlation between speciation rate and past temperature fluctuations with an increase in speciation rate corresponding to the general cooling observed during the last 15 My. Biogeographic reconstructions identified the tropical rainforest biome of Amazonia and the Guiana Shield as the cradle of xenarthran evolutionary history with subsequent dispersions into more open and dry habitats.

Determining the Position of Storks on the Phylogenetic Tree of Waterbirds by Retroposon Insertion Analysis

Despite many studies on avian phylogenetics in recent decades that used morphology, mitochondrial genomes, and/or nuclear genes, the phylogenetic positions of several birds (e.g., storks) remain unsettled. In addition to the aforementioned approaches, analysis of retroposon insertions, which are nearly homoplasy-free phylogenetic markers, has also been used in avian phylogenetics. However, the first step in the analysis of retroposon insertions, that is, isolation of retroposons from genomic libraries, is a costly and time-consuming procedure. Therefore, we developed a high-throughput and cost-effective protocol to collect retroposon insertion information based on next-generation sequencing technology, which we call here the STRONG (Screening of Transposons Obtained by Next Generation Sequencing) method, and applied it to 3 waterbird species, for which we identified 35,470 loci containing chicken repeat 1 retroposons (CR1). Our analysis of the presence/absence of 30 CR1 insertions demonstrated the intra- and interordinal phylogenetic relationships in the waterbird assemblage, namely 1) Loons diverged first among the waterbirds, 2) penguins (Sphenisciformes) and petrels (Procellariiformes) diverged next, and 3) among the remaining families of waterbirds traditionally classified in Ciconiiformes/Pelecaniformes, storks (Ciconiidae) diverged first. Furthermore, our genome-scale, in silico retroposon analysis based on published genome data uncovered a complex divergence history among pelican, heron, and ibis lineages, presumably involving ancient interspecies hybridization between the heron and ibis lineages. Thus, our retroposon-based waterbird phylogeny and the established phylogenetic position of storks will help to understand the evolutionary processes of aquatic adaptation and related morphological convergent evolution.

Exploring Massive Incomplete Lineage Sorting in Arctoids (Laurasiatheria, Carnivora)

Freed from the competition of large raptors, Paleocene carnivores could expand their newly acquired habitats in search of prey. Such changing conditions might have led to their successful distribution and rapid radiation. Today, molecular evolutionary biologists are faced, however, with the consequences of such accelerated adaptive radiations, because they led to sequential speciation more rapidly than phylogenetic markers could be fixed. The repercussions being that current genealogies based on such markers are incongruent with species trees.Our aim was to explore such conflicting phylogenetic zones of evolution during the early arctoid radiation, especially to distinguish diagnostic from misleading phylogenetic signals, and to examine other carnivore-related speciation events. We applied a combination of high-throughput computational strategies to screen carnivore and related genomes in silico for randomly inserted retroposed elements that we then used to identify inconsistent phylogenetic patterns in the Arctoidea group, which is well known for phylogenetic discordances.Our combined retrophylogenomic and in vitro wet lab approach detected hundreds of carnivore-specific insertions, many of them confirming well-established splits or identifying and solving conflicting species distributions. Our systematic genome-wide screens for Long INterspersed Elements detected homoplasy-free markers with insertion-specific truncation points that we used to distinguish phylogenetically informative markers from conflicting signals. The results were independently confirmed by phylogenetic diagnostic Short INterspersed Elements. As statistical analysis ruled out ancestral hybridization, these doubly verified but still conflicting patterns were statistically determined to be genomic remnants from a time of ancestral incomplete lineage sorting that especially accompanied large parts of Arctoidea evolution.

Disentangling the relationship of the Australian marsupial orders using retrotransposon and evolutionary network analyses

The ancestors to the Australian marsupials entered Australia around 60 (54-72) Ma from Antarctica, and radiated into the four living orders Peramelemorphia, Dasyuromorphia, Diprotodontia, and Notoryctemorphia. The relationship between the four Australian marsupial orders has been a long-standing question, because different phylogenetic studies have not been able to consistently reconstruct the same topology. Initial in silico analysis of the Tasmanian devil genome and experimental screening in the seven marsupial orders revealed 20 informative transposable element insertions for resolving the inter- and intraordinal relationships of Australian and South American orders. However, the retrotransposon insertions support three conflicting topologies regarding Peramelemorphia, Dasyuromorphia, and Notoryctemorphia, indicating that the split between the three orders may be best understood as a network. This finding is supported by a phylogenetic reanalysis of nuclear gene sequences, using a consensus network approach that allows depicting hidden phylogenetic conflict, otherwise lost when forcing the data into a bifurcating tree. The consensus network analysis agrees with the transposable element analysis in that all possible topologies regarding Peramelemorphia, Dasyuromorphia, and Notoryctemorphia in a rooted four-taxon topology are equally well supported. In addition, retrotransposon insertion data support the South American order Didelphimorphia being the sistergroup to all other living marsupial orders. The four Australian orders originated within 3 Myr at the Cretaceous-Paleogene boundary. The rapid divergences left conflicting phylogenetic information in the genome possibly generated by incomplete lineage sorting or introgressive hybridization, leaving the relationship among Australian marsupial orders unresolvable as a bifurcating process millions of years later.

Dispersal of an ancient retroposon in the TP53 promoter of Bovidae: phylogeny, novel mechanisms, and potential implications for cow milk persistency

BACKGROUND

In recent years, the perception of transposable genetic elements has changed from "junk DNA" to a focus of interest when appearing near or inside genes. Bov-A2 is a short interspersed nuclear element (SINE) that was first found in Bovidae and later in other ruminants. This retroposon is mostly used as a marker for phylogenetic analysis.

RESULTS

We describe insertions of Bov-A2 in the promoter region of TP53, a key tumor suppressor gene that is indispensable for diverse developmental processes, in Antilopinae and Tragelaphini (belonging to the Bovinae subfamily). In Tragelaphini two Bov-A2 elements were inserted sequentially, whereas in 5 tribes of Antilopinae only one Bov-A2 element was inserted, in a different site and reverse orientation. The entrance site in both cases employed short palindromes that can form hairpin secondary structures. Interestingly, mutations that create or disrupt base pairing in the palindrome sequence dictated the presence or absence of Bov-A2, such as in the domestic cow and buffalo, which lack Bov-A2. Transcription factor binding site analysis revealed unique binding sites for STAT3 and NFκB within the Bov-A2 sequence, which together with TP53 itself are known to play a crucial role in mammary involution.

CONCLUSIONS

This report demonstrates how short palindromes serve as hot spots for Bov-A2 retroposon insertion into the mammalian genome. The strict correlation between point mutation in the palindromes and the presence/absence of Bov-A2 retroposon insertions, questions the use of singular insertion events as valid phylogenetic markers inside families. Bov-A2 insertion into the TP53 promoter in Antilopinae and Tragelaphini may not only provide a genetic network that regulates mammary involution, but can also answer the need for rapid mammary involution in Savanna antelopes after weaning, partially in response to predation stress. The absence of Bov-A2 in domestic bovids may constitute the molecular background for greater lactation persistency.

Whole-genome analyses resolve early branches in the tree of life of modern birds

To better determine the history of modern birds, we performed a genome-scale phylogenetic analysis of 48 species representing all orders of Neoaves using phylogenomic methods created to handle genome-scale data. We recovered a highly resolved tree that confirms previously controversial sister or close relationships. We identified the first divergence in Neoaves, two groups we named Passerea and Columbea, representing independent lineages of diverse and convergently evolved land and water bird species. Among Passerea, we infer the common ancestor of core landbirds to have been an apex predator and confirm independent gains of vocal learning. Among Columbea, we identify pigeons and flamingoes as belonging to sister clades. Even with whole genomes, some of the earliest branches in Neoaves proved challenging to resolve, which was best explained by massive protein-coding sequence convergence and high levels of incomplete lineage sorting that occurred during a rapid radiation after the Cretaceous-Paleogene mass extinction event about 66 million years ago.

Multiple lineages of ancient CR1 retroposons shaped the early genome evolution of amniotes

Chicken repeat 1 (CR1) retroposons are long interspersed elements (LINEs) that are ubiquitous within amniote genomes and constitute the most abundant family of transposed elements in birds, crocodilians, turtles, and snakes. They are also present in mammalian genomes, where they reside as numerous relics of ancient retroposition events. Yet, despite their relevance for understanding amniote genome evolution, the diversity and evolution of CR1 elements has never been studied on an amniote-wide level. We reconstruct the temporal and quantitative activity of CR1 subfamilies via presence/absence analyses across crocodilian phylogeny and comparative analyses of 12 crocodilian genomes, revealing relative genomic stasis of retroposition during genome evolution of extant Crocodylia. Our large-scale phylogenetic analysis of amniote CR1 subfamilies suggests the presence of at least seven ancient CR1 lineages in the amniote ancestor; and amniote-wide analyses of CR1 successions and quantities reveal differential retention (presence of ancient relics or recent activity) of these CR1 lineages across amniote genome evolution. Interestingly, birds and lepidosaurs retained the fewest ancient CR1 lineages among amniotes and also exhibit smaller genome sizes. Our study is the first to analyze CR1 evolution in a genome-wide and amniote-wide context and the data strongly suggest that the ancestral amniote genome contained myriad CR1 elements from multiple ancient lineages, and remnants of these are still detectable in the relatively stable genomes of crocodilians and turtles. Early mammalian genome evolution was thus characterized by a drastic shift from CR1 prevalence to dominance and hyperactivity of L2 LINEs in monotremes and L1 LINEs in therians.

The dynamic proliferation of CanSINEs mirrors the complex evolution of Feliforms

BACKGROUND

Repetitive short interspersed elements (SINEs) are retrotransposons ubiquitous in mammalian genomes and are highly informative markers to identify species and phylogenetic associations. Of these, SINEs unique to the order Carnivora (CanSINEs) yield novel insights on genome evolution in domestic dogs and cats, but less is known about their role in related carnivores. In particular, genome-wide assessment of CanSINE evolution has yet to be completed across the Feliformia (cat-like) suborder of Carnivora. Within Feliformia, the cat family Felidae is composed of 37 species and numerous subspecies organized into eight monophyletic lineages that likely arose 10 million years ago. Using the Felidae family as a reference phylogeny, along with representative taxa from other families of Feliformia, the origin, proliferation and evolution of CanSINEs within the suborder were assessed.

RESULTS

We identified 93 novel intergenic CanSINE loci in Feliformia. Sequence analyses separated Feliform CanSINEs into two subfamilies, each characterized by distinct RNA polymerase binding motifs and phylogenetic associations. Subfamily I CanSINEs arose early within Feliformia but are no longer under active proliferation. Subfamily II loci are more recent, exclusive to Felidae and show evidence for adaptation to extant RNA polymerase activity. Further, presence/absence distributions of CanSINE loci are largely congruent with taxonomic expectations within Feliformia and the less resolved nodes in the Felidae reference phylogeny present equally ambiguous CanSINE data. SINEs are thought to be nearly impervious to excision from the genome. However, we observed a nearly complete excision of a CanSINEs locus in puma (Puma concolor). In addition, we found that CanSINE proliferation in Felidae frequently targeted existing CanSINE loci for insertion sites, resulting in tandem arrays.

CONCLUSIONS

We demonstrate the existence of at least two SINE families within the Feliformia suborder, one of which is actively involved in insertional mutagenesis. We find SINEs are powerful markers of speciation and conclude that the few inconsistencies with expected patterns of speciation likely represent incomplete lineage sorting, species hybridization and SINE-mediated genome rearrangement.

Retrophylogenomics place tarsiers on the evolutionary branch of anthropoids

One of the most disputed issues in primate evolution and thus of our own primate roots, is the phylogenetic position of the Southeast Asian tarsier. While much molecular data indicate a basal place in the primate tree shared with strepsirrhines (prosimian monophyly hypothesis), data also exist supporting either an earlier divergence in primates (tarsier-first hypothesis) or a close relationship with anthropoid primates (Haplorrhini hypothesis). The use of retroposon insertions embedded in the Tarsius genome afforded us the unique opportunity to directly test all three hypotheses via three pairwise genome alignments. From millions of retroposons, we found 104 perfect orthologous insertions in both tarsiers and anthropoids to the exclusion of strepsirrhines, providing conflict-free evidence for the Haplorrhini hypothesis, and none supporting either of the other two positions. Thus, tarsiers are clearly the sister group to anthropoids in the clade Haplorrhini.

Rates and patterns of great ape retrotransposition

We analyzed 83 fully sequenced great ape genomes for mobile element insertions, predicting a total of 49,452 fixed and polymorphic Alu and long interspersed element 1 (L1) insertions not present in the human reference assembly and assigning each retrotransposition event to a different time point during great ape evolution. We used these homoplasy-free markers to construct a mobile element insertions-based phylogeny of humans and great apes and demonstrate their differential power to discern ape subspecies and populations. Within this context, we find a good correlation between L1 diversity and single-nucleotide polymorphism heterozygosity (r(2) = 0.65) in contrast to Alu repeats, which show little correlation (r(2) = 0.07). We estimate that the "rate" of Alu retrotransposition has differed by a factor of 15-fold in these lineages. Humans, chimpanzees, and bonobos show the highest rates of Alu accumulation--the latter two since divergence 1.5 Mya. The L1 insertion rate, in contrast, has remained relatively constant, with rates differing by less than a factor of three. We conclude that Alu retrotransposition has been the most variable form of genetic variation during recent human-great ape evolution, with increases and decreases occurring over very short periods of evolutionary time.

Less is more in mammalian phylogenomics: AT-rich genes minimize tree conflicts and unravel the root of placental mammals

Despite the rapid increase of size in phylogenomic data sets, a number of important nodes on animal phylogeny are still unresolved. Among these, the rooting of the placental mammal tree is still a controversial issue. One difficulty lies in the pervasive phylogenetic conflicts among genes, with each one telling its own story, which may be reliable or not. Here, we identified a simple criterion, that is, the GC content, which substantially helps in determining which gene trees best reflect the species tree. We assessed the ability of 13,111 coding sequence alignments to correctly reconstruct the placental phylogeny. We found that GC-rich genes induced a higher amount of conflict among gene trees and performed worse than AT-rich genes in retrieving well-supported, consensual nodes on the placental tree. We interpret this GC effect mainly as a consequence of genome-wide variations in recombination rate. Indeed, recombination is known to drive GC-content evolution through GC-biased gene conversion and might be problematic for phylogenetic reconstruction, for instance, in an incomplete lineage sorting context. When we focused on the AT-richest fraction of the data set, the resolution level of the placental phylogeny was greatly increased, and a strong support was obtained in favor of an Afrotheria rooting, that is, Afrotheria as the sister group of all other placentals. We show that in mammals most conflicts among gene trees, which have so far hampered the resolution of the placental tree, are concentrated in the GC-rich regions of the genome. We argue that the GC content-because it is a reliable indicator of the long-term recombination rate-is an informative criterion that could help in identifying the most reliable molecular markers for species tree inference.

Identifying localized biases in large datasets: a case study using the avian tree of life

Large-scale multi-locus studies have become common in molecular phylogenetics, with new studies continually adding to previous datasets in an effort to fully resolve the tree of life. Total evidence analyses that combine existing data with newly collected data are expected to increase the power of phylogenetic analyses to resolve difficult relationships. However, they might be subject to localized biases, with one or a few loci having a strong and potentially misleading influence upon the results. To examine this possibility we combined a newly collected 31-locus dataset that includes representatives of all major avian lineages with a published dataset of 19 loci that has a comparable number of sites (Hackett et al., 2008. Science 320, 1763-1768). This allowed us to explore the advantages of conducting total evidence analyses, and to determine whether it was also important to analyze new datasets independent of published ones. The total evidence analysis yielded results very similar to the published results, with only slightly increased support at a few nodes. However, analyzing the 31- and 19-locus datasets separately highlighted several differences. Two clades received strong support in the published dataset and total evidence analysis, but the support appeared to reflect bias at a single locus (β-fibrinogen [FGB]). The signal in FGB that supported these relationships was sufficient to result in their recovery with bootstrap support, even when combined with 49 loci lacking that signal. FGB did not appear to have a substantial impact upon the results of species tree methods, but another locus (brain-derived neurotrophic factor [BDNF]) did have an impact upon those analyses. These results demonstrated that localized biases can influence large-scale phylogenetic analyses but they also indicated that considering independent evidence and exploring multiple analytical approaches could reveal them.

Coalescent-based genome analyses resolve the early branches of the euarchontoglires

Despite numerous large-scale phylogenomic studies, certain parts of the mammalian tree are extraordinarily difficult to resolve. We used the coding regions from 19 completely sequenced genomes to study the relationships within the super-clade Euarchontoglires (Primates, Rodentia, Lagomorpha, Dermoptera and Scandentia) because the placement of Scandentia within this clade is controversial. The difficulty in resolving this issue is due to the short time spans between the early divergences of Euarchontoglires, which may cause incongruent gene trees. The conflict in the data can be depicted by network analyses and the contentious relationships are best reconstructed by coalescent-based analyses. This method is expected to be superior to analyses of concatenated data in reconstructing a species tree from numerous gene trees. The total concatenated dataset used to study the relationships in this group comprises 5,875 protein-coding genes (9,799,170 nucleotides) from all orders except Dermoptera (flying lemurs). Reconstruction of the species tree from 1,006 gene trees using coalescent models placed Scandentia as sister group to the primates, which is in agreement with maximum likelihood analyses of concatenated nucleotide sequence data. Additionally, both analytical approaches favoured the Tarsier to be sister taxon to Anthropoidea, thus belonging to the Haplorrhine clade. When divergence times are short such as in radiations over periods of a few million years, even genome scale analyses struggle to resolve phylogenetic relationships. On these short branches processes such as incomplete lineage sorting and possibly hybridization occur and make it preferable to base phylogenomic analyses on coalescent methods.

Ancestry of the Australian termitivorous numbat

The Australian numbat, Myrmecobius fasciatus, is the only marsupial that feeds almost exclusively on termites and that has a life following the diurnally restricted and dynamic geographical distribution of termites. The millions of years of this adaptation led to unique morphological and anatomical features, especially basicranial and dental characteristics, that make it difficult to identify a clear phylogenetic affiliation to other marsupials. From DNA sequence analyses, the family Myrmecobiidae is placed within the dasyuromorph marsupials, but the exact position varies from study to study, and support values are mostly rather modest. Here, we report the recovery and analysis of approximately 110,000 quasifossilized traces of mobile element insertions into the genome of a dasyurid marsupial (Tasmanian devil), 25 of which are phylogenetically informative for early dasyuromorphial evolution. Fourteen of these ancient retroposon insertions are shared by the 16 Dasyuromorphia species analyzed, including the numbat, but are absent in the outgroups. An additional 11 other insertions are present in all Dasyuridae but are absent in the numbat. These findings place numbats as the sister group to all living Dasyuridae and show that the investigated Dasyuromorphia, including the Myrmecobiidae, constitutes a monophyletic group that is separated from Peramelemorphia, Notoryctemorphia, and other marsupials.

Genesis and regulatory wiring of retroelement-derived domesticated genes: a phylogenomic perspective

Molecular domestications of transposable elements have occurred repeatedly during the evolution of eukaryotes. Vertebrates, especially mammals, possess numerous single copy domesticated genes (DGs) that have originated from the intronless multicopy transposable elements. However, the origin and evolution of the retroelement-derived DGs (RDDGs) that originated from Metaviridae has been only partially elucidated, due to absence of genome data or to limited analysis of a single family of DGs. We traced the genesis and regulatory wiring of the Metaviridae-derived DGs through phylogenomic analysis, using whole-genome information from more than 90 chordate genomes. Phylogenomic analysis of these DGs in chordate genomes provided direct evidence that major diversification has occurred in the ancestor of placental mammals. Mammalian RDDGs have been shown to originate in several steps by independent domestication events and to diversify later by gene duplications. Analysis of syntenic loci has shown that diverse RDDGs and their chromosomal positions were fully established in the ancestor of placental mammals. By analysis of active Metaviridae lineages in amniotes, we have demonstrated that RDDGs originated from retroelement remains. The chromosomal gene movements of RDDGs were highly dynamic only in the ancestor of placental mammals. During the domestication process, de novo acquisition of regulatory regions is shown to be a prerequisite for the survival of the DGs. The origin and evolution of de novo acquired promoters and untranslated regions in diverse mammalian RDDGs have been explained by comparative analysis of orthologous gene loci. The origin of placental mammal-specific innovations and adaptations, such as placenta and newly evolved brain functions, was most probably connected to the regulatory wiring of DGs and their rapid fixation in the ancestor of placental mammals.

The chromosomes of Afrotheria and their bearing on mammalian genome evolution

Afrotheria is the clade of placental mammals that, together with Xenarthra, Euarchontoglires and Laurasiatheria, represents 1 of the 4 main recognized supraordinal eutherian clades. It reunites 6 orders of African origin: Proboscidea, Sirenia, Hyracoidea, Macroscelidea, Afrosoricida and Tubulidentata. The apparently unlikely relationship among such disparate morphological taxa and their possible basal position at the base of the eutherian phylogenetic tree led to a great deal of attention and research on the group. The use of biomolecular data was pivotal in Afrotheria studies, as they were the basis for the recognition of this clade. Although morphological evidence is still scarce, a plethora of molecular data firmly attests to the phylogenetic relationship among these mammals of African origin. Modern cytogenetic techniques also gave a significant contribution to the study of Afrotheria, revealing chromosome signatures for the group as a whole, as well as for some of its internal relationships. The associations of human chromosomes HSA1/19 and 5/21 were found to be chromosome signatures for the group and provided further support for Afrotheria. Additional chromosome synapomorphies were also identified linking elephants and manatees in Tethytheria (the associations HSA2/3, 3/13, 8/22, 18/19 and the lack of HSA4/8) and elephant shrews with the aardvark (HSA2/8, 3/20 and 10/17). Herein, we review the current knowledge on Afrotheria chromosomes and genome evolution. The already available data on the group suggests that further work on this apparently bizarre assemblage of mammals will provide important data to a better understanding on mammalian genome evolution.

Transposable elements: from DNA parasites to architects of metazoan evolution

One of the most unexpected insights that followed from the completion of the human genome a decade ago was that more than half of our DNA is derived from transposable elements (TEs). Due to advances in high throughput sequencing technologies it is now clear that TEs comprise the largest molecular class within most metazoan genomes. TEs, once categorised as "junk DNA", are now known to influence genomic structure and function by increasing the coding and non-coding genetic repertoire of the host. In this way TEs are key elements that stimulate the evolution of metazoan genomes. This review highlights several lines of TE research including the horizontal transfer of TEs through host-parasite interactions, the vertical maintenance of TEs over long periods of evolutionary time, and the direct role that TEs have played in generating morphological novelty.

DNA-methylation profiling of fetal tissues reveals marked epigenetic differences between chorionic and amniotic samples

Epigenetic mechanisms including DNA methylation are supposed to play a key role in fetal development. Here we have investigated fetal DNA-methylation levels of 27,578 CpG loci in 47 chorionic villi (CVS) and 16 amniotic cell (AC) samples. Methylation levels differed significantly between karyotypically normal AC and CVS for 2,014 genes. AC showed more extreme DNA-methylation levels of these genes than CVS and the differentially methylated genes are significantly enriched for processes characteristic for the different cell types sampled. Furthermore, we identified 404 genes differentially methylated in CVS with trisomy 21. These genes were significantly enriched for high CG dinucleotid (CpG) content and developmental processes associated with Down syndrome. Our study points to major tissue-specific differences of fetal DNA-methylation and gives rise to the hypothesis that part of the Down syndrome phenotype is epigenetically programmed in the first trimester of pregnancy.

Chromosome evolution in Xenarthra: new insights from an ancient group

The Magnaorder Xenarthra is one of the four main supraordinal eutherian clades, together with Afrotheria, Euarchontoglires and Laurasiatheria. Xenarthra is an eminently Central and South American group of special interest in phylogenetic studies due to its possible position at the base of the eutherian tree. The use of modern cytogenetic techniques in some species of Xenarthra has provided important insights into the karyotypic evolution of mammals. Nevertheless, chromosome analyses in the group are still restricted, with only a few individuals of each species studied and karyotype descriptions mostly without banding patterns. In addition, it is likely that still unknown species exist and that the chromosome variability in the group is underestimated. We present a review of the currently available data on Xenarthra chromosomes and genomes and on the impact that their study has had in the understanding of mammalian genome evolution. It is clear that further cytogenetic analyses in Xenarthra, including banding patterns and molecular approaches, are likely to help in the identification of new species, reveal still undetected chromosome variations, provide information to support conservation strategies planning, and greatly contribute to a better understanding of mammalian genome evolution.

An Alu-based phylogeny of gibbons (hylobatidae)

Gibbons (Hylobatidae) are small, arboreal apes indigenous to Southeast Asia that diverged from other apes ∼15-18 Ma. Extant lineages radiated rapidly 6-10 Ma and are organized into four genera (Hylobates, Hoolock, Symphalangus, and Nomascus) consisting of 12-19 species. The use of short interspersed elements (SINEs) as phylogenetic markers has seen recent popularity due to several desirable characteristics: the ancestral state of a locus is known to be the absence of an element, rare potentially homoplasious events are relatively easy to resolve, and samples can be quickly and inexpensively genotyped. During radiation of primates, one particular family of SINEs, the Alu family, has proliferated in primate genomes. Nomascus leucogenys (northern white-cheeked gibbon) sequences were analyzed for repetitive content with RepeatMasker using a custom library. The sequences containing Alu elements identified as members of a gibbon-specific subfamily were then compared with orthologous positions in other primate genomes. A primate phylogenetic panel consisting of 18 primate species, including 13 gibbon species representing all four extant genera, was assayed for all loci, and a total of 125 gibbon-specific Alu insertions were identified. The resulting amplification patterns were used to generate a phylogenetic tree. We demonstrate significant support for Symphalangus as the most basal lineage within the family. Our findings also place Nomascus as a derived lineage, sister to Hoolock, with the Nomascus-Hoolock clade sister to Hylobates. Further, our analysis groups N. leucogenys and Nomascus siki as sister taxa to the exclusion of the other Nomascus species assayed. This study represents the first use of SINEs to determine the genus level phylogenetic relationships within the family Hylobatidae. These relationships have been resolved with robust support at most internal nodes, demonstrating the utility of SINE-based phylogenetic analysis. We postulate that hybridization and rapid radiation may have contributed to the complex and contradictory findings of the previous studies. Our findings will aid in the conservation of these threatened primates and inform future studies of the biogeographical history and distribution of modern gibbon species.

CR1 retroposons provide a new insight into the phylogeny of Phasianidae species (Aves: Galliformes)

Chicken repeat 1 (CR1) elements, a class of retroposons belonging to non-long-terminal repeats, have been recognized as powerful tools for phylogenetic studies. Here we examine the phylogenetic relationships of 11 Phasianidae species based on CR1 retroposons. Together with 19 loci reported previously, a total of 99 CR1 loci were identified from chicken genome and turkey BAC clone sequences. 75 insertion events were used to address the branching order of 11 species in Phasianidae. The topology of our tree suggests that: 1) Gallus gallus possessed a basal phylogenetic position within Phasianidae and was related to Bambusicola thoracica (BSP=100%); 2) After the split of G. gallus and B. thoracica, Arborophila rufipectus diverged from Phasianidae (BSP=100%). Nine unambiguous insertion events supported a phylogenetic position of A. rufipectus different to previous mitochondrial data suggesting a hybrid origin or an ancient introgression of A. rufipectus; and 3) 22 CR1 insertion events strongly supported the eight phasianids under investigation sharing a common ancestor. Our study has revisited the phylogenetic position of G. gallus and A. rufipectus and provided a new insight into the phylogeny of Phasianidae birds. It showed that a CR1-based methodology has a great potential to be informative within Phasianidae in resolving relationships of closely related species whose radiation and speciation have occurred very recently.

Retroposon insertion patterns of neoavian birds: strong evidence for an extensive incomplete lineage sorting era

More than 150 Ma, the avian lineage separated from that of other dinosaurs and later diversified into the more than 10,000 species extant today. The early neoavian bird radiations most likely occurred in the late Cretaceous (more than 65 Ma) but left behind few if any molecular signals of their archaic evolutionary past. Retroposed elements, once established in an ancestral population, are highly valuable, virtually homoplasy-free markers of species evolution; after applying stringent orthology criteria, their phylogenetically informative presence/absence patterns are free of random noise and independent of evolutionary rate or nucleotide composition effects. We screened for early neoavian orthologous retroposon insertions and identified six markers with conflicting presence/absence patterns, whereas six additional retroposons established before or after the presumed major neoavian radiation show consistent phylogenetic patterns. The exceptionally frequent conflicting retroposon presence/absence patterns of neoavian orders are strong indicators of an extensive incomplete lineage sorting era, potentially induced by an early rapid successive speciation of ancestral Neoaves.