A candidate sex determination locus in amphibians which evolved by structural variation between X- and Y-chromosomes.

Most vertebrates develop distinct females and males, where sex is determined by repeatedly evolved environmental or genetic triggers. Undifferentiated sex chromosomes and large genomes have caused major knowledge gaps in amphibians. Only a single master sex-determining gene, the dmrt1-paralogue (dm-w) of female-heterogametic clawed frogs (Xenopus; ZW♀/ZZ♂), is known across >8740 species of amphibians. In this study, by combining chromosome-scale female and male genomes of a non-model amphibian, the European green toad, Bufo(tes) viridis, with ddRAD- and whole genome pool-sequencing, we reveal a candidate master locus, governing a male-heterogametic system (XX♀/XY♂). Targeted sequencing across multiple taxa uncovered structural X/Y-variation in the 5'-regulatory region of the gene bod1l, where a Y-specific non-coding RNA (ncRNA-Y), only expressed in males, suggests that this locus initiates sex-specific differentiation. Developmental transcriptomes and RNA in-situ hybridization show timely and spatially relevant sex-specific ncRNA-Y and bod1l-gene expression in primordial gonads. This coincided with differential H3K4me-methylation in pre-granulosa/pre-Sertoli cells, pointing to a specific mechanism of amphibian sex determination.

Multi-genome comparisons reveal gain-and-loss evolution of the anti-Mullerian hormone receptor type 2 gene, an old master sex determining gene, in Percidae.

The Percidae family comprises many fish species of major importance for aquaculture and fisheries. Based on three new chromosome-scale assemblies in Perca fluviatilis, Perca schrenkii and Sander vitreus along with additional percid fish reference genomes, we provide an evolutionary and comparative genomic analysis of their sex-determination systems. We explored the fate of a duplicated anti-Mullerian hormone receptor type-2 gene (amhr2bY), previously suggested to be the master sex determining (MSD) gene in P. flavescens. Phylogenetically related and structurally similar amhr2 duplications (amhr2b) were found in P. schrenkii and Sander lucioperca, potentially dating this duplication event to their last common ancestor around 19-27 Mya. In P. fluviatilis and S. vitreus, this amhr2b duplicate has been lost while it was subject to amplification in S. lucioperca. Analyses of the amhr2b locus in P. schrenkii suggest that this duplication could be also male-specific as it is in P. flavescens. In P. fluviatilis, a relatively small (100 kb) non-recombinant sex-determining region (SDR) was characterized on chromosome-18 using population-genomics approaches. This SDR is characterized by many male-specific single-nucleotide variants (SNVs) and no large duplication/insertion event, suggesting that P. fluviatilis has a male heterogametic sex determination system (XX/XY), generated by allelic diversification. This SDR contains six annotated genes, including three (c18h1orf198, hsdl1, tbc1d32) with higher expression in testis than ovary. Together, our results provide a new example of the highly dynamic sex chromosome turnover in teleosts and provide new genomic resources for Percidae, including sex-genotyping tools for all three known Perca species.

Equilibrated evolution of the mixed auto-/allopolyploid haplotype-resolved genome of the invasive hexaploid Prussian carp.

Understanding genome evolution of polyploids requires dissection of their often highly similar subgenomes and haplotypes. Polyploid animal genome assemblies so far restricted homologous chromosomes to a 'collapsed' representation. Here, we sequenced the genome of the asexual Prussian carp, which is a close relative of the goldfish, and present a haplotype-resolved chromosome-scale assembly of a hexaploid animal. Genome-wide comparisons of the 150 chromosomes with those of two ancestral diploid cyprinids and the allotetraploid goldfish and common carp revealed the genomic structure, phylogeny and genome duplication history of its genome. It consists of 25 syntenic, homeologous chromosome groups and evolved by a recent autoploid addition to an allotetraploid ancestor. We show that de-polyploidization of the alloploid subgenomes on the individual gene level occurred in an equilibrated fashion. Analysis of the highly conserved actinopterygian gene set uncovered a subgenome dominance in duplicate gene loss of one ancestral chromosome set.

Whole-genome resequencing and bisulfite sequencing provide new insights into the feeding habit domestication in mandarin fish (Siniperca chuatsi).

Mandarin fish (Siniperca chuatsi) is one of the most economically important fish in China. However, it has the peculiar feeding habit that it feeds solely on live prey fish since first-feeding, while refuses dead prey fish or artificial diets. After the specific training procedure, partial individuals could accept dead prey fish and artificial diets. The genetic basis of individual difference in artificial diet feeding habit is still unknown. In the present study, the resequencing was performed between 10 individuals which could be domesticated to accept artificial diets and 10 individuals which could not. Through the selective sweep analysis based on heterozygosity (Hp) and population differentiation coefficient (Fst), 57 candidate windows were identified as the putative selected regions for feeding habit domestication of mandarin fish, involved in 149 genes. These genes were related to memory, vision and olfaction function, which could be potential targets of molecular marker assistant breeding of artificial diet feeding trait. Beside of the DNA sequence, we also explored the potential role of DNA methylation in feeding habit domestication in mandarin fish. Whole-genome bisulfite sequencing was performed between the individuals which could be domesticated to accept artificial diets and those could not. 5,976 differentially methylated regions were identified, referring to 3,522 genes, such as the genes involved in cAMP signaling pathway. The DNA methylation changes of these genes might contribute to the adaption of artificial diets in mandarin fish. In conclusion, the putative selected regions and the differentially methylated regions were identified in the whole genome, providing new insights into the feeding habit domestication from live prey fish to artificial diets in mandarin fish. And the involved genes were identified as the candidate genes for molecular breeding of artificial diet utilization in mandarin fish.

A brief review of vertebrate sex evolution with a pledge for integrative research: towards 'sexomics'.

Triggers and biological processes controlling male or female gonadal differentiation vary in vertebrates, with sex determination (SD) governed by environmental factors or simple to complex genetic mechanisms that evolved repeatedly and independently in various groups. Here, we review sex evolution across major clades of vertebrates with information on SD, sexual development and reproductive modes. We offer an up-to-date review of divergence times, species diversity, genomic resources, genome size, occurrence and nature of polyploids, SD systems, sex chromosomes, SD genes, dosage compensation and sex-biased gene expression. Advances in sequencing technologies now enable us to study the evolution of SD at broader evolutionary scales, and we now hope to pursue a sexomics integrative research initiative across vertebrates. The vertebrate sexome comprises interdisciplinary and integrated information on sexual differentiation, development and reproduction at all biological levels, from genomes, transcriptomes and proteomes, to the organs involved in sexual and sex-specific processes, including gonads, secondary sex organs and those with transcriptional sex-bias. The sexome also includes ontogenetic and behavioural aspects of sexual differentiation, including malfunction and impairment of SD, sexual differentiation and fertility. Starting from data generated by high-throughput approaches, we encourage others to contribute expertise to building understanding of the sexomes of many key vertebrate species. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

A 180 Myr-old female-specific genome region in sturgeon reveals the oldest known vertebrate sex determining system with undifferentiated sex chromosomes.

Several hypotheses explain the prevalence of undifferentiated sex chromosomes in poikilothermic vertebrates. Turnovers change the master sex determination gene, the sex chromosome or the sex determination system (e.g. XY to WZ). Jumping master genes stay main triggers but translocate to other chromosomes. Occasional recombination (e.g. in sex-reversed females) prevents sex chromosome degeneration. Recent research has uncovered conserved heteromorphic or even homomorphic sex chromosomes in several clades of non-avian and non-mammalian vertebrates. Sex determination in sturgeons (Acipenseridae) has been a long-standing basic biological question, linked to economical demands by the caviar-producing aquaculture. Here, we report the discovery of a sex-specific sequence from sterlet (Acipenser ruthenus). Using chromosome-scale assemblies and pool-sequencing, we first identified an approximately 16 kb female-specific region. We developed a PCR-genotyping test, yielding female-specific products in six species, spanning the entire phylogeny with the most divergent extant lineages (A. sturio, A. oxyrinchus versus A. ruthenus, Huso huso), stemming from an ancient tetraploidization. Similar results were obtained in two octoploid species (A. gueldenstaedtii, A. baerii). Conservation of a female-specific sequence for a long period, representing 180 Myr of sturgeon evolution, and across at least one polyploidization event, raises many interesting biological questions. We discuss a conserved undifferentiated sex chromosome system with a ZZ/ZW-mode of sex determination and potential alternatives. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Genome-wide analysis of European sea bass provides insights into the evolution and functions of single-exon genes.

Several studies have attempted to understand the origin and evolution of single-exon genes (SEGs) in eukaryotic organisms, including fishes, but few have examined the functional and evolutionary relationships between SEGs and multiple-exon gene (MEG) paralogs, in particular the conservation of promoter regions. Given that SEGs originate via the reverse transcription of mRNA from a "parental" MEGs, such comparisons may enable identifying evolutionarily-related SEG/MEG paralogs, which might fulfill equivalent physiological functions. Here, the relationship of SEG proportion with MEG count, gene density, intron count, and chromosome size was assessed for the genome of the European sea bass, Dicentrarchus labrax. Then, SEGs with an MEG parent were identified, and promoter sequences of SEG/MEG paralogs were compared, to identify highly conserved functional motifs. The results revealed a total count of 1,585 (8.3% of total genes) SEGs in the European sea bass genome, which was correlated with MEG count but not with gene density. The significant correlation of SEG content with the number of MEGs suggests that SEGs were continuously and independently generated over evolutionary time following species divergence through retrotranscription events, followed by tandem duplications. Functional annotation showed that the majority of SEGs are functional, as is evident from their expression in RNA-seq data used to support homology-based genome annotation. Differences in 5'UTR and 3'UTR lengths between SEG/MEG paralogs observed in this study may contribute to gene expression divergence between them and therefore lead to the emergence of new SEG functions. The comparison of nonsynonymous to synonymous changes (Ka/Ks) between SEG/MEG parents showed that 74 of them are under positive selection (Ka/Ks > 1; p = .0447). An additional fifteen SEGs with an MEG parent have a common promoter, which implies that they are under the influence of common regulatory networks.

An Unbiased Molecular Approach Using 3'-UTRs Resolves the Avian Family-Level Tree of Life.

Presumably, due to a rapid early diversification, major parts of the higher-level phylogeny of birds are still resolved controversially in different analyses or are considered unresolvable. To address this problem, we produced an avian tree of life, which includes molecular sequences of one or several species of ∼90% of the currently recognized family-level taxa (429 species, 379 genera) including all 106 family-level taxa of the nonpasserines and 115 of the passerines (Passeriformes). The unconstrained analyses of noncoding 3-prime untranslated region (3'-UTR) sequences and those of coding sequences yielded different trees. In contrast to the coding sequences, the 3'-UTR sequences resulted in a well-resolved and stable tree topology. The 3'-UTR contained, unexpectedly, transcription factor binding motifs that were specific for different higher-level taxa. In this tree, grebes and flamingos are the sister clade of all other Neoaves, which are subdivided into five major clades. All nonpasserine taxa were placed with robust statistical support including the long-time enigmatic hoatzin (Opisthocomiformes), which was found being the sister taxon of the Caprimulgiformes. The comparatively late radiation of family-level clades of the songbirds (oscine Passeriformes) contrasts with the attenuated diversification of nonpasseriform taxa since the early Miocene. This correlates with the evolution of vocal production learning, an important speciation factor, which is ancestral for songbirds and evolved convergent only in hummingbirds and parrots. As 3'-UTR-based phylotranscriptomics resolved the avian family-level tree of life, we suggest that this procedure will also resolve the all-species avian tree of life.

PWD/Ph-Encoded Genetic Variants Modulate the Cellular Wnt/ß-Catenin Response to Suppress Apc Min-Triggered Intestinal Tumor Formation.

Genetic predisposition affects the penetrance of tumor-initiating mutations, such as APC mutations that stabilize ß-catenin and cause intestinal tumors in mice and humans. However, the mechanisms involved in genetically predisposed penetrance are not well understood. Here, we analyzed tumor multiplicity and gene expression in tumor-prone Apc Min/+ mice on highly variant C57BL/6J (B6) and PWD/Ph (PWD) genetic backgrounds. (B6 × PWD) F1 APC Min offspring mice were largely free of intestinal adenoma, and several chromosome substitution (consomic) strains carrying single PWD chromosomes on the B6 genetic background displayed reduced adenoma numbers. Multiple dosage-dependent modifier loci on PWD chromosome 5 each contributed to tumor suppression. Activation of ß-catenin-driven and stem cell-specific gene expression in the presence of Apc Min or following APC loss remained moderate in intestines carrying PWD chromosome 5, suggesting that PWD variants restrict adenoma initiation by controlling stem cell homeostasis. Gene expression of modifier candidates and DNA methylation on chromosome 5 were predominantly cis controlled and largely reflected parental patterns, providing a genetic basis for inheritance of tumor susceptibility. Human SNP variants of several modifier candidates were depleted in colorectal cancer genomes, suggesting that similar mechanisms may also affect the penetrance of cancer driver mutations in humans. Overall, our analysis highlights the strong impact that multiple genetic variants acting in networks can exert on tumor development. SIGNIFICANCE: These findings in mice show that, in addition to accidental mutations, cancer risk is determined by networks of individual gene variants.

Evolution of MicroRNA Biogenesis Genes in the Sterlet (Acipenser ruthenus) and Other Polyploid Vertebrates.

MicroRNAs play a crucial role in eukaryotic gene regulation. For a long time, only little was known about microRNA-based gene regulatory mechanisms in polyploid animal genomes due to difficulties of polyploid genome assembly. However, in recent years, several polyploid genomes of fish, amphibian, and even invertebrate species have been sequenced and assembled. Here we investigated several key microRNA-associated genes in the recently sequenced sterlet (Acipenser ruthenus) genome, whose lineage has undergone a whole genome duplication around 180 MYA. We show that two paralogs of drosha, dgcr8, xpo1, and xpo5 as well as most ago genes have been retained after the acipenserid-specific whole genome duplication, while ago1 and ago3 genes have lost one paralog. While most diploid vertebrates possess only a single copy of dicer1, we strikingly found four paralogs of this gene in the sterlet genome, derived from a tandem segmental duplication that occurred prior to the last whole genome duplication. ago1,3,4 and exportins1,5 look to be prone to additional segment duplications producing up to four-five paralog copies in ray-finned fishes. We demonstrate for the first time exon microsatellite amplification in the acipenserid drosha2 gene, resulting in a highly variable protein product, which may indicate sub- or neofunctionalization. Paralogous copies of most microRNA metabolism genes exhibit different expression profiles in various tissues and remain functional despite the rediploidization process. Subfunctionalization of microRNA processing gene paralogs may be beneficial for different pathways of microRNA metabolism. Genetic variability of microRNA processing genes may represent a substrate for natural selection, and, by increasing genetic plasticity, could facilitate adaptations to changing environments.

The mole genome reveals regulatory rearrangements associated with adaptive intersexuality.

Linking genomic variation to phenotypical traits remains a major challenge in evolutionary genetics. In this study, we use phylogenomic strategies to investigate a distinctive trait among mammals: the development of masculinizing ovotestes in female moles. By combining a chromosome-scale genome assembly of the Iberian mole, Talpa occidentalis, with transcriptomic, epigenetic, and chromatin interaction datasets, we identify rearrangements altering the regulatory landscape of genes with distinct gonadal expression patterns. These include a tandem triplication involving CYP17A1, a gene controlling androgen synthesis, and an intrachromosomal inversion involving the pro-testicular growth factor gene FGF9, which is heterochronically expressed in mole ovotestes. Transgenic mice with a knock-in mole CYP17A1 enhancer or overexpressing FGF9 showed phenotypes recapitulating mole sexual features. Our results highlight how integrative genomic approaches can reveal the phenotypic impact of noncoding sequence changes.

Mandarin fish (Sinipercidae) genomes provide insights into innate predatory feeding.

Mandarin fishes (Sinipercidae) are piscivores that feed solely on live fry. Unlike higher vertebrates, teleosts exhibit feeding behavior driven mainly by genetic responses, with no modification by learning from parents. Mandarin fishes could serve as excellent model organisms for studying feeding behavior. We report a long-read, chromosomal-scale genome assembly for Siniperca chuatsi and genome assemblies for Siniperca kneri, Siniperca scherzeri and Coreoperca whiteheadi. Positive selection analysis revealed rapid adaptive evolution of genes related to predatory feeding/aggression, growth, pyloric caeca and euryhalinity. Very few gill rakers are observed in mandarin fishes; analogously, we found that zebrafish deficient in edar had a gill raker loss phenotype and a more predatory habit, with reduced intake of zooplankton but increased intake of prey fish. Higher expression of bmp4, which could inhibit edar expression and gill raker development through binding of a Xvent-1 site upstream of edar, may cause predatory feeding in Siniperca.

CSA: A high-throughput chromosome-scale assembly pipeline for vertebrate genomes.

BACKGROUND: Easy-to-use and fast bioinformatics pipelines for long-read assembly that go beyond the contig level to generate highly continuous chromosome-scale genomes from raw data remain scarce. RESULT: Chromosome-Scale Assembler (CSA) is a novel computationally highly efficient bioinformatics pipeline that fills this gap. CSA integrates information from scaffolded assemblies (e.g., Hi-C or 10X Genomics) or even from diverged reference genomes into the assembly process. As CSA performs automated assembly of chromosome-sized scaffolds, we benchmark its performance against state-of-the-art reference genomes, i.e., conventionally built in a laborious fashion using multiple separate assembly tools and manual curation. CSA increases the contig lengths using scaffolding, local re-assembly, and gap closing. On certain datasets, initial contig N50 may be increased up to 4.5-fold. For smaller vertebrate genomes, chromosome-scale assemblies can be achieved within 12 h using low-cost, high-end desktop computers. Mammalian genomes can be processed within 16 h on compute-servers. Using diverged reference genomes for fish, birds, and mammals, we demonstrate that CSA calculates chromosome-scale assemblies from long-read data and genome comparisons alone. Even contig-level draft assemblies of diverged genomes are helpful for reconstructing chromosome-scale sequences. CSA is also capable of assembling ultra-long reads. CONCLUSIONS: CSA can speed up and simplify chromosome-level assembly and significantly lower costs of large-scale family-level vertebrate genome projects.

The sterlet sturgeon genome sequence and the mechanisms of segmental rediploidization.

Sturgeons seem to be frozen in time. The archaic characteristics of this ancient fish lineage place it in a key phylogenetic position at the base of the ~30,000 modern teleost fish species. Moreover, sturgeons are notoriously polyploid, providing unique opportunities to investigate the evolution of polyploid genomes. We assembled a high-quality chromosome-level reference genome for the sterlet, Acipenser ruthenus. Our analysis revealed a very low protein evolution rate that is at least as slow as in other deep branches of the vertebrate tree, such as that of the coelacanth. We uncovered a whole-genome duplication that occurred in the Jurassic, early in the evolution of the entire sturgeon lineage. Following this polyploidization, the rediploidization of the genome included the loss of whole chromosomes in a segmental deduplication process. While known adaptive processes helped conserve a high degree of structural and functional tetraploidy over more than 180 million years, the reduction of redundancy of the polyploid genome seems to have been remarkably random.

TCRs with segment TRAV9-2 or a CDR3 histidine are overrepresented among nickel-specific CD4+ T cells.

BACKGROUND: Nickel is the most frequent cause of T cell-mediated allergic contact dermatitis worldwide. In vitro, CD4+ T cells from all donors respond to nickel but the involved αß T cell receptor (TCR) repertoire has not been comprehensively analyzed. METHODS: We introduce CD154 (CD40L) upregulation as a fast, unbiased, and quantitative method to detect nickel-specific CD4+ T cells ex vivo in blood of clinically characterized allergic and non allergic donors. Naïve (CCR7+ CD45RA+) and memory (not naïve) CD154+ CD4+ T cells were analyzed by flow cytometry after 5 hours of stimulation with 200 µmol/L NiSO4 ., TCR α- and ß-chains of sorted nickel-specific and control cells were studied by high-throughput sequencing. RESULTS: Stimulation of PBMCs with NiSO4 induced CD154 expression on ~0.1% (mean) of naïve and memory CD4+ T cells. In allergic donors with recent positive patch test, memory frequencies further increased ~13-fold and were associated with markers of in vivo activation. CD154 expression was TCR-mediated since single clones could be specifically restimulated. Among nickel-specific CD4+ T cells of allergic and non allergic donors, TCRs expressing the α-chain segment TRAV9-2 or a histidine in their α- or ß-chain complementarity determining region 3 (CDR3) were highly overrepresented. CONCLUSIONS: Induced CD154 expression represents a reliable method to study nickel-specific CD4+ T cells. TCRs with particular features respond in all donors, while strongly increased blood frequencies indicate nickel allergy for some donors. Our approach may be extended to other contact allergens for the further development of diagnostic and predictive in vitro tests.

Genes acting in synapses and neuron projections are early targets of selection during urban colonization.

When a species colonizes an urban habitat, differences in the environment can create novel selection pressures. Successful colonization will further lead to demographic perturbations and genetic drift, which can interfere with selection. Here, we test for consistent urban selection signals in multiple populations of the burrowing owl (Athene cunicularia), a species that colonized South American cities just a few decades ago. We sequenced 213 owls from three urban-rural population pairs and performed a genome-wide comparison of urban against rural birds. We further studied genome-wide associations with flight initiation distance, a measure of harm avoidance in which urban and rural birds are known to differ. Based on four samples taken over nine years from one of the urban populations, we investigated temporal allele frequency changes. The genomic data were also used to identify urban-specific signatures of selective sweeps. Single genomic sites did not reach genome-wide significance for any association. However, a gene-set analysis on the strongest signals from these four selection scans suggests a significant enrichment of genes with known functions related to synapses and neuron projections. We identified 98 genes predominantly expressed in the brain, of which many may play a role in the modulation of brain connectivity and consequently in cognitive function and motivational behaviour during urbanization. Furthermore, polymorphisms in the promoter region of the synaptic SERT gene - one of the two candidates known to correlate with urban colonization in birds - associated with the habitat in which individuals lived (urban vs. rural).

Multi-Tissue Transcriptomes Yield Information on High-Altitude Adaptation and Sex-Determination in Scutiger cf. sikimmensis.

The Himalayas are one of earth's hotspots of biodiversity. Among its many cryptic and undiscovered organisms, including vertebrates, this complex high-mountain ecosystem is expected to harbour many species with adaptations to life in high altitudes. However, modern evolutionary genomic studies in Himalayan vertebrates are still at the beginning. Moreover, in organisms, like most amphibians with relatively high DNA content, whole genome sequencing remains bioinformatically challenging and no complete nuclear genomes are available for Himalayan amphibians. Here, we present the first well-annotated multi-tissue transcriptome of a Greater Himalayan species, the lazy toad Scutiger cf. sikimmensis (Anura: Megophryidae). Applying Illumina NextSeq 500 RNAseq to six tissues, we obtained 41.32 Gb of sequences, assembled to ~111,000 unigenes, translating into 54362 known genes as annotated in seven functional databases. We tested 19 genes, known to play roles in anuran and reptile adaptation to high elevations, and potentially detected diversifying selection for two (TGS1, SENP5) in Scutiger. Of a list of 37 genes, we also identify 27 candidate genes for sex determination or sexual development, all of which providing the first such data for this non-model megophryid species. These transcriptomes will serve as a valuable resource for further studies on amphibian evolution in the Greater Himalaya as a biodiversity hotspot.

Ice-Age Climate Adaptations Trap the Alpine Marmot in a State of Low Genetic Diversity.

Some species responded successfully to prehistoric changes in climate [1, 2], while others failed to adapt and became extinct [3]. The factors that determine successful climate adaptation remain poorly understood. We constructed a reference genome and studied physiological adaptations in the Alpine marmot (Marmota marmota), a large ground-dwelling squirrel exquisitely adapted to the "ice-age" climate of the Pleistocene steppe [4, 5]. Since the disappearance of this habitat, the rodent persists in large numbers in the high-altitude Alpine meadow [6, 7]. Genome and metabolome showed evidence of adaptation consistent with cold climate, affecting white adipose tissue. Conversely, however, we found that the Alpine marmot has levels of genetic variation that are among the lowest for mammals, such that deleterious mutations are less effectively purged. Our data rule out typical explanations for low diversity, such as high levels of consanguineous mating, or a very recent bottleneck. Instead, ancient demographic reconstruction revealed that genetic diversity was lost during the climate shifts of the Pleistocene and has not recovered, despite the current high population size. We attribute this slow recovery to the marmot's adaptive life history. The case of the Alpine marmot reveals a complicated relationship between climatic changes, genetic diversity, and conservation status. It shows that species of extremely low genetic diversity can be very successful and persist over thousands of years, but also that climate-adapted life history can trap a species in a persistent state of low genetic diversity.

Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria.

Xenoturbella and the acoelomorph worms (Xenacoelomorpha) are simple marine animals with controversial affinities. They have been placed as the sister group of all other bilaterian animals (Nephrozoa hypothesis), implying their simplicity is an ancient characteristic [1, 2]; alternatively, they have been linked to the complex Ambulacraria (echinoderms and hemichordates) in a clade called the Xenambulacraria [3-5], suggesting their simplicity evolved by reduction from a complex ancestor. The difficulty resolving this problem implies the phylogenetic signal supporting the correct solution is weak and affected by inadequate modeling, creating a misleading non-phylogenetic signal. The idea that the Nephrozoa hypothesis might be an artifact is prompted by the faster molecular evolutionary rate observed within the Acoelomorpha. Unequal rates of evolution are known to result in the systematic artifact of long branch attraction, which would be predicted to result in an attraction between long-branch acoelomorphs and the outgroup, pulling them toward the root [6]. Other biases inadequately accommodated by the models used can also have strong effects, exacerbated in the context of short internal branches and long terminal branches [7]. We have assembled a large and informative dataset to address this problem. Analyses designed to reduce or to emphasize misleading signals show the Nephrozoa hypothesis is supported under conditions expected to exacerbate errors, and the Xenambulacraria hypothesis is preferred in conditions designed to reduce errors. Our reanalyses of two other recently published datasets [1, 2] produce the same result. We conclude that the Xenacoelomorpha are simplified relatives of the Ambulacraria.