Molecular evolutionary genomics of birds

Evolution, Inheritance, and Strata Formation of the W Chromosome in Duck (Anas platyrhynchos)

The nonrecombining female-limited W chromosome is predicted to experience unique evolutionary processes. Difficulties in assembling W chromosome sequences have hindered the identification of duck W-linked sequences and their evolutionary footprint. To address this, we conducted three initial contig-level genome assemblies and developed a rigorous pipeline by which to successfully expand the W-linked data set, including 11 known genes and 24 newly identified genes. Our results indicate that the W chromosome expression may not be subject to female-specific selection; a significant convergent pattern of upregulation associated with increased female-specific selection was not detected. The genetic stability of the W chromosome is also reflected in the strong evolutionary correlation between it and the mitochondria; the complete consistency of the cladogram topology constructed from their gene sequences proves the shared maternal coevolution. By detecting the evolutionary trajectories of W-linked sequences, we have found that recombination suppression started in four distinct strata, of which three were conserved across Neognathae. Taken together, our results have revealed a unique evolutionary pattern and an independent stratum evolutionary pattern for sex chromosomes.

Main Factors Shaping Amino Acid Usage Across Evolution

The standard genetic code determines that in most species, including viruses, there are 20 amino acids that are coded by 61 codons, while the other three codons are stop triplets. Considering the whole proteome each species features its own amino acid frequencies, given the slow rate of change, closely related species display similar GC content and amino acids usage. In contrast, distantly related species display different amino acid frequencies. Furthermore, within certain multicellular species, as mammals, intragenomic differences in the usage of amino acids are evident. In this communication, we shall summarize some of the most prominent and well-established factors that determine the differences found in the amino acid usage, both across evolution and intragenomically.

A 14,000-year-old genome sheds light on the evolution and extinction of a Pleistocene vulture

The New World Vulture [Coragyps] occidentalis (L. Miller, 1909) is one of many species that were extinct by the end of the Pleistocene. To understand its evolutionary history we sequenced the genome of a 14,000 year old [Coragyps] occidentalis found associated with megaherbivores in the Peruvian Andes. occidentalis has been viewed as the ancestor, or possibly sister, to the extant Black Vulture Coragyps atratus, but genomic data shows occidentalis to be deeply nested within the South American clade of atratus. Coragyps atratus inhabits lowlands, but the fossil record indicates that occidentalis mostly occupied high elevations. Our results suggest that occidentalis evolved from a population of atratus in southwestern South America that colonized the High Andes 300 to 400 kya. The morphological and morphometric differences between occidentalis and atratus may thus be explained by ecological diversification following from the natural selection imposed by this new and extreme, high elevation environment. The sudden evolution of a population with significantly larger body size and different anatomical proportions than atratus thus constitutes an example of punctuated evolution.

14,000 year old DNA reveals the evolutionary dynamics and adaptations of South American vultures.

Comparative Population Genomics of Cryptic Speciation and Adaptive Divergence in Bicknell's and Gray-Cheeked Thrushes (Aves: Catharus bicknelli and Catharus minimus)

Cryptic speciation may occur when reproductive isolation is recent or the accumulation of morphological differences between sister lineages is slowed by stabilizing selection preventing phenotypic differentiation. In North America, Bicknell's Thrush (Catharus bicknelli) and its sister species, the Gray-cheeked Thrush (Catharus minimus), are parapatrically breeding migratory songbirds, distinguishable in nature only by subtle differences in song and coloration, and were recognized as distinct species only in the 1990s. Previous molecular studies have estimated that the species diverged approximately 120,000-420,000 YBP and found very low levels of introgression despite their similarity and sympatry in the spring (prebreeding) migration. To further clarify the history, genetic divergence, genomic structure, and adaptive processes in C. bicknelli and C. minimus, we sequenced and assembled high-coverage reference genomes of both species and resequenced genomes from population samples of C. bicknelli, C. minimus, and two individuals of the Swainson's Thrush (Catharus ustulatus). The genome of C. bicknelli exhibits markedly higher abundances of transposable elements compared with other Catharus and chicken. Demographic and admixture analyses confirm moderate genome-wide differentiation (Fst ≈ 0.10) and limited gene flow between C. bicknelli and C. minimus, but suggest a more recent divergence than estimates based on mtDNA. We find evidence of rapid evolution of the Z-chromosome and elevated divergence consistent with natural selection on genomic regions near genes involved with neuronal processes in C. bicknelli. These genomes are a useful resource for future investigations of speciation, migration, and adaptation in Catharus thrushes.

Phylogeny and evolutionary history of the Sombre Tit, Poecile lugubris in the western Palearctic (Aves, Paridae)

Few studies have delimited evolutionary entities within the Sombre Tit, Poecile lugubris. Here, we explored its morphological and genetic variability using 24 morphometric variables, two mitochondrial (COX1 and ND2), two nuclear (ODC and MB), and 10 microsatellite loci. Genetic and morphometric characters supported the species status of the Caspian Tit, Poecile hyrcanus with a separation from P. lugubris more than 4.5 MYA. The phylogenetic analyses uncovered three distinct clades within P. lugubris. The subspecies P. l. lugubris, with strong genetic differences from the other subspecies (4.5%), diverged at ∼1.1 MYA. Samples from Iran formed the remaining two clades. Individuals from western and northwestern Iran were placed in a single clade (anatoliae), while those from southern and southwestern Iran were in another independent clade (dubius-kirmanensis). Morphometric analyses also confirmed this pattern. The microsatellite results discriminated Iranian subspecies as discrete clusters with signs of nuclear admixture between dubius and anatoliae in the Zagros Mountains. Signs of population expansion for anatoliae and dubius-kirmanensis coincided with the late LGM. Our results shed new light on the phylogenetic relationships, evolutionary history, and past demographic processes of P. lugubris.

Molecular systematics and phylogeography of a widespread Neotropical avian lineage: evidence for cryptic speciation with protracted gene flow throughout the Late Quaternary

Here we use an integrative approach, including coalescent-based methods, isolation–migration and species distribution models, to infer population structure, divergence times and diversification in the two species of the genus Cymbilaimus (Aves, Thamnophilidae). Our results support a recent and rapid diversification with both incomplete lineage sorting and gene flow shaping the evolutionary history of Cymbilaimus. The spatio-temporal pattern of cladogenesis suggests that Cymbilaimus originated in the north/western portion of cis-Andean South America and then diversified into the Brazilian Shield and Central America after consolidation of the modern Amazonian drainage and the Andean range. This evolutionary scenario is explained by cycles of range expansion and dispersal, followed by isolation, and recurrent gene flow, during the last 1.2 Myr. Our results agree with those recently reported for other closely related suboscine lineages, whereby the window of introgression between closely related taxa remains open for up to a few million years after their original split. In Cymbilaimus, introgression was recurrent between C. lineatus and C. sanctaemariae, even after they acquired vocal and ecological differentiation, supporting the claim that at least in Neotropical suboscines, full reproductive compatibility may take millions of years to evolve and cannot be interpreted as synonymous with a lack of speciation.

The wild species genome ancestry of domestic chickens

BACKGROUND

Hybridisation and introgression play key roles in the evolutionary history of animal species. They are commonly observed within several orders in wild birds. The domestic chicken Gallus gallus domesticus is the most common livestock species. More than 65 billion chickens are raised annually to produce meat and 80 million metric tons of egg for global human consumption by the commercial sector. Unravelling the origin of its genetic diversity has major application for sustainable breeding improvement programmes.

RESULTS

In this study, we report genome-wide analyses for signatures of introgression between indigenous domestic village chicken and the four wild Gallus species. We first assess the genome-wide phylogeny and divergence time across the genus Gallus. Genome-wide sequence divergence analysis supports a sister relationship between the Grey junglefowl G. sonneratii and Ceylon junglefowl G. lafayettii. Both species form a clade that is sister to the Red junglefowl G. gallus, with the Green junglefowl G. varius the most ancient lineage within the genus. We reveal extensive bidirectional introgression between the Grey junglefowl and the domestic chicken and to a much lesser extent with the Ceylon junglefowl. We identify a single case of Green junglefowl introgression. These introgressed regions include genes with biological functions related to development and immune system.

CONCLUSIONS

Our study shows that while the Red junglefowl is the main ancestral species, introgressive hybridisation episodes have impacted the genome and contributed to the diversity of the domestic chicken, although likely at different levels across its geographic range.

Historical climate changes and hybridization shaped the evolution of Atlantic Forest spinetails (Aves: Furnariidae)

Combining phylogeographic approaches and hybrid zone inference in a single framework is a robust way to depict respectively the biogeographic history of lineages and the evolutionary processes responsible for speciation. Here, we studied the spatiotemporal patterns of diversification and characterize the hybrid zone between two Atlantic Forest spinetails (Synallaxis ruficapilla and Synallaxis cinerea) using mitochondrial DNA and nuclear (autosomal and Z-linked) genes. We consistently recovered divergence between and within the two species during the late Pliocene and Pleistocene using an isolation with migration model. Also, our results indicate distinct levels of introgression among lineages. Ecological niche models and demographic inferences, used to infer range distributions throughout the late Quaternary, were not consistent with the hypothesis of a large river as a primary barrier responsible for the divergence of the two species. Instead, a scenario of isolation and divergence followed by geographic expansion and admixture as a consequence of Quaternary climatic oscillations was supported. Paleomodels also were not consistent with the idea that the hybrid zone originated in primary differentiation and favor a secondary contact scenario. Model fitting indicated that clines of different loci spanning the hybrid zone are coincident and concordant. The narrow cline for one Z-linked locus could be indicative of some form of post-zygotic selection hindering genetic homogenization between the two species.

The niches of nuthatches affect their lineage evolution differently across latitude

Ecological niche evolution can promote or hinder the differentiation of taxa and determine their distribution. Niche-mediated evolution may differ among climatic regimes, and thus, species that occur across a wide latitudinal range offer a chance to test these heterogeneous evolutionary processes. In this study, we examine (a) how many lineages have evolved across the continent-wide range of the Eurasian nuthatch (Sitta europaea), (b) whether the lineages' niches are significantly divergent or conserved and (c) how their niche evolution explains their geographic distribution. Phylogenetic reconstruction and ecological niche models (ENMs) showed that the Eurasian nuthatch contained six parapatric lineages that diverged within 2 Myr and did not share identical climatic niches. However, the niche discrepancy between these distinct lineages was relatively conserved compared with the environmental differences between their ranges and thus was unlikely to drive lineage divergence. The ENMs of southern lineages tended to cross-predict with their neighbouring lineages whereas those of northern lineages generally matched with their abutting ranges. The coalescence-based analyses revealed more stable populations for the southern lineages than the northern ones during the last glaciation cycle. In contrast to the overlapping ENMs, the smaller parapatric distribution suggests that the southern lineages might have experienced competitive exclusion to prevent them from becoming sympatric. On the other hand, the northern lineages have expanded their ranges and their current abutting distribution might have resulted from lineages adapting to different climatic conditions in allopatry. This study suggests that niche evolution may affect lineage distribution in different ways across latitude.

Deep divergence of Red-crowned Ant Tanager (Habia rubica: Cardinalidae), a multilocus phylogenetic analysis with emphasis in Mesoamerica

Many neotropical species have a complex history of diversification as a result of the influence of geographical, ecological, climatic, and geological factors that determine the distribution of populations within a lineage. Phylogeography identifies such populations, determines their geographic distributions, and quantifies the degree of genetic divergence. In this work we explored the genetic structure of Habia rubica populations, a polytypic taxon with 17 subspecies described, in order to obtain hypotheses about their evolutionary history and processes of diversification. We undertook multilocus analyses using sequences of five molecular markers (ND2, ACOI-I9, MUSK, FGB-I5 and ODC), and sampling from across the species’ distribution range, an area encompassing from Central Mexico throughout much of South America. With these data, we obtained a robust phylogenetic hypothesis, a species delimitation analysis, and estimates of divergence times for these lineages. The phylogenetic hypothesis of concatenated molecular markers shows that H. rubica can be divided in three main clades: the first includes Mexican Pacific coast populations, the second is formed by population from east of Mexico to Panama and the third comprises the South American populations. Within these clades we recognize seven principal phylogroups whose limits have a clear correspondence with important geographical discontinuities including the Isthmus of Tehuantepec in southern Mexico, the Talamanca Cordillera, and the Isthmus of Panama in North America. In South America, we observed a marked separation of two phylogroups that include the populations that inhabit mesic forests in western and central South America (Amazon Forest) and those inhabiting the seasonal forest from the eastern and northern regions of the South America (Atlantic Forest). These areas are separated by an intervening dry vegetation “diagonal” (Chaco, Cerrado and Caatinga). The geographic and genetic structure of these phylogroups describes a history of diversification more active and complex in the northern distribution of this species, producing at least seven well-supported lineages that could be considered species.

Species delimitation and biogeography of the gnatcatchers and gnatwrens (Aves: Polioptilidae)

The New World avian family Polioptilidae (gnatcatchers and gnatwrens) is distributed from Argentina to Canada and includes 15 species and more than 60 subspecies. No study to date has evaluated phylogenetic relationships within this family and the historical pattern of diversification within the group remains unknown. Moreover, species limits, particularly in widespread taxa that show geographic variation, remain unclear. In this study, we delimited species and estimated phylogenetic relationships using multilocus data for the entire family. We then used the inferred diversity along with alternative taxonomic classification schemes to evaluate how lumping and splitting of both taxa and geographical areas influenced biogeographic inference. Species-tree analyses grouped Polioptilidae into four main clades: Microbates, Ramphocaenus, a Polioptila guianensis complex, and the remaining members of Polioptila. Ramphocaenus melanurus was sister to the clade containing M. cinereiventris and M. collaris, which formed a clade sister to all species within Polioptila. Polioptila was composed of two clades, the first of which included the P. guianensis complex; the other contained all remaining species in the genus. Using multispecies coalescent modeling, we inferred a more than 3-fold increase in species diversity, of which 87% represent currently recognized species or subspecies. Much of this diversity corresponded to subspecies that occur in the Neotropics. We identified three polyphyletic species, and delimited 4-6 previously undescribed candidate taxa. Probabilistic modeling of geographic ranges on the species tree indicated that the family likely had an ancestral origin in South America, with all three genera independently colonizing North America. Support for this hypothesis, however, was sensitive to the taxonomic classification scheme used and the number of geographical areas allowed. Our study proposes the first phylogenetic hypothesis for Polioptilidae and provides genealogical support for the reclassification of species limits. Species limits and the resolution of geographical areas that taxa inhabit influence the inferred spatial diversification history.

Molecular systematics and biogeography of lowland antpittas (Aves, Grallariidae): The role of vicariance and dispersal in the diversification of a widespread Neotropical lineage

We infer phylogenetic relationships, divergence times, and the diversification history of the avian Neotropical antpitta genera Hylopezus and Myrmothera (Grallariidae), based on sequence data (3,139 base pairs) from two mitochondrial (ND2 and ND3) and three nuclear nuclear introns (TGFB2, MUSK and FGB-I5) from 142 individuals of the 12 currently recognized species in Hylopezus and Myrmothera and 5 outgroup species. Phylogenetic analyses recovered 19 lineages clustered into two major clades, both distributed in Central and South America. Hylopezus nattereri, previously considered a subspecies of H. ochroleucus, was consistently recovered as the most divergent lineage within the Grallaricula/Hylopezus/Myrmothera clade. Ancestral range estimation suggested that modern lowland antpittas probably originated in the Amazonian Sedimentary basin during the middle Miocene, and that most lineages within the Hylopezus/Myrmothera clade appeared in the Plio-Pleistocene. However, the rate of diversification in the Hylopezus/Myrmothera clade appeared to have remained constant through time, with no major shifts over the 20 million years. Although the timing when most modern lineages of the Hylopezus/Myrmothera clade coincides with a period of intense landscape changes in the Neotropics (Plio-Pleistocene), the absence of any significant shifts in diversification rates over the last 20 million years challenges the view that there is a strict causal relationship between intensification of landscape changes and cladogenesis. The relative old age of the Hylopezus/Myrmothera clade coupled with an important role ascribed to dispersal for its diversification, favor an alternative scenario whereby long-term persistence and dispersal across an ever-changing landscape might explain constant rates of cladogenesis through time.

The biogeographic and evolutionary history of an endemic clade of Middle American sparrows: Melozone and Aimophila (Aves: Passerellidae)

The large number of endemic species in Middle America is frequently attributed to the interplay of geographical barriers and historical climatic changes in the region. This process promotes genetic divergence between populations, and given enough time, may yield new species. Animals that inhabit mid-elevation or highland habitats may be disproportionately affected in this way. Genetic analyses of animals in this region allow us to better understand how historical patterns of isolation have influenced the generation of new species in this biodiversity hotspot. We studied the biogeography and systematics of two closely related genera of sparrows (Passerellidae): Melozone and Aimophila. Collectively, this group is distributed from the southwestern United States and southward as far as central Costa Rica. We sampled 81 individuals of 8 Melozone and 2 Aimophila species, from 19 localities distributed throughout their ranges. We reconstructed phylogenetic relationships and time-calibrated species trees using multilocus sequence data comprised of one mitochondrial gene and five nuclear genes. We conducted an ancestral area reconstruction analysis to determine the probability of ancestral range at each divergent event. Despite analyzing six loci, we were unable to obtain a fully resolved phylogenetic tree. We recovered four main lineages: lineage 1 includes four Melozone species distributed north of Isthmus of Tehuantepec (M. albicollis, M. crissalis, M. aberti, M. fusca); lineage 2 includes three Melozone species distributed south of the Isthmus of Tehuantepec (M. biarcuata, M. cabanisi, M. leucotis); lineage 3 lineage consists of a single species endemic to the Pacific coast of Mexico (M. kieneri); and lineage 4 includes the more widely distributed sparrows in the genus Aimophila. Our analyses suggest that these genera probably originated during the late Miocene in the Madrean Highlands of southern Mexico. We identified dispersal as the prevalent cause of speciation in this clade with most lineages dispersing to their current distributions from southern Mexico either to the north following a developing and expanding Madro-Tertiary flora, or to the south across the Isthmus of Tehuantepec. A similar pattern of dispersal from this biogeographic region has been reported in other taxa including fishes, reptiles, and birds. Our results reveal that the four lineages identified represent geographically coherent and ecologically similar assemblages of taxa. Finally, when our genetic results are considered, along with apparent differences in morphology and song, the allopatric forms M. b. cabanisi and M. l. occipitalis warrant recognition as biological species.

Inferring Individual Inbreeding and Demographic History from Segments of Identity by Descent in Ficedula Flycatcher Genome Sequences

Individual inbreeding and historical demography can be estimated by analyzing runs of homozygosity (ROH), which are indicative of chromosomal segments of identity by descent (IBD). Such analyses have so far been rare in natural populations due to limited genomic resources. We analyzed ROH in whole genome sequences from 287 Ficedula flycatchers representing four species, with the objectives of evaluating the causes of genome-wide variation in the abundance of ROH and inferring historical demography. ROH were clearly more abundant in genomic regions with low recombination rate. However, this pattern was substantially weaker when ROH were mapped using genetic rather than physical single nucleotide polymorphism (SNP) coordinates in the genome. Empirical results and simulations suggest that high ROH abundance in regions of low recombination was partly caused by increased power to detect the very long IBD segments typical of regions with a low recombination rate. Simulations also showed that hard selective sweeps (but not soft sweeps or background selection) likely contributed to variation in the abundance of ROH across the genome. Comparisons of the abundance of ROH among several study populations indicated that the Spanish pied flycatcher population had the smallest historical effective population size (N_e) for this species, and that a putatively recently founded island (Baltic) population had the smallest historical N_e among the collared flycatchers. Analysis of pairwise IBD in Baltic collared flycatchers indicated that this population was founded <60 generations ago. This study provides a rare genomic glimpse into demographic history and the mechanisms underlying the genome-wide distribution of ROH.

Is Continued Genetic Improvement of Livestock Sustainable?

Large genetic improvements in the quantitative traits of growth, production, and efficiency of farmed livestock have been made over recent decades, and by introduction of genomic technology these are being enhanced. Such continued improvement requires that there be available variation to utilize. The evidence is that little variation has been lost and such rates are indeed sustainable in the future.

Matching loci surveyed to questions asked in phylogeography

Although mitochondrial DNA (mtDNA) has long been used for assessing genetic variation within and between populations, its workhorse role in phylogeography has been criticized owing to its single-locus nature. The only choice for testing mtDNA results is to survey nuclear loci, which brings into contrast the difference in locus effective size and coalescence times. Thus, it remains unclear how erroneous mtDNA-based estimates of species history might be, especially for evolutionary events in the recent past. To test the robustness of mtDNA and nuclear sequences in phylogeography, we provide one of the largest paired comparisons of summary statistics and demographic parameters estimated from mitochondrial, five Z-linked and 10 autosomal genes of 30 avian species co-distributed in the Caucasus and Europe. The results suggest that mtDNA is robust in estimating inter-population divergence but not in intra-population diversity, which is sensitive to population size change. Here, we provide empirical evidence showing that mtDNA was more likely to detect population divergence than any other single locus owing to its smaller Ne and thus faster coalescent time. Therefore, at least in birds, numerous studies that have based their inferences of phylogeographic patterns solely on mtDNA should not be readily dismissed.

Region-wide and ecotype-specific differences in demographic histories of threespine stickleback populations, estimated from whole genome sequences

We analysed 81 whole genome sequences of threespine sticklebacks from Pacific North America, Greenland and Northern Europe, representing 16 populations. Principal component analysis of nuclear SNPs grouped populations according to geographical location, with Pacific populations being more divergent from each other relative to European and Greenlandic populations. Analysis of mitogenome sequences showed Northern European populations to represent a single phylogeographical lineage, whereas Greenlandic and particularly Pacific populations showed admixture between lineages. We estimated demographic history using a genomewide coalescence with recombination approach. The Pacific populations showed gradual population expansion starting >100 Kya, possibly reflecting persistence in cryptic refuges near the present distributional range, although we do not rule out possible influence of ancient admixture. Sharp population declines ca. 14-15 Kya were suggested to reflect founding of freshwater populations by marine ancestors. In Greenland and Northern Europe, demographic expansion started ca. 20-25 Kya coinciding with the end of the Last Glacial Maximum. In both regions, marine and freshwater populations started to show different demographic trajectories ca. 8-9 Kya, suggesting that this was the time of recolonization. In Northern Europe, this estimate was surprisingly late, but found support in subfossil evidence for presence of several freshwater fish species but not sticklebacks 12 Kya. The results demonstrate distinctly different demographic histories across geographical regions with potential consequences for adaptive processes. They also provide empirical support for previous assumptions about freshwater populations being founded independently from large, coherent marine populations, a key element in the Transporter Hypothesis invoked to explain the widespread occurrence of parallel evolution across freshwater stickleback populations.

Transposable elements and early evolution of sex chromosomes in fish

In many organisms, the sex chromosome pair can be recognized due to heteromorphy; the Y and W chromosomes have often lost many genes due to the absence of recombination during meiosis and are frequently heterochromatic. Repetitive sequences are found at a high proportion on such heterochromatic sex chromosomes and the evolution and emergence of sex chromosomes has been connected to the dynamics of repeats and transposable elements. With an amazing plasticity of sex determination mechanisms and numerous instances of independent emergence of novel sex chromosomes, fish represent an excellent lineage to investigate the early stages of sex chromosome differentiation, where sex chromosomes often are homomorphic and not heterochromatic. We have analyzed the composition, distribution, and relative age of TEs from available sex chromosome sequences of seven teleost fish. We observed recent bursts of TEs and simple repeat accumulations around young sex determination loci. More strikingly, we detected transposable element (TE) amplifications not only on the sex determination regions of the Y and W sex chromosomes, but also on the corresponding regions of the X and Z chromosomes. In one species, we also clearly demonstrated that the observed TE-rich sex determination locus originated from a TE-poor genomic region, strengthening the link between TE accumulation and emergence of the sex determination locus. Altogether, our results highlight the role of TEs in the initial steps of differentiation and evolution of sex chromosomes.

Multi-locus phylogenetic inference among New World Vultures (Aves: Cathartidae)

New World Vultures are large-bodied carrion feeding birds in the family Cathartidae, currently consisting of seven species from five genera with geographic distributions in North and South America. No study to date has included all cathartid species in a single phylogenetic analysis. In this study, we investigated the phylogenetic relationships among all cathartid species using five nuclear (nuc; 4060bp) and two mitochondrial (mt; 2165bp) DNA loci with fossil calibrated gene tree (27 outgroup taxa) and coalescent-based species tree (2 outgroup taxa) analyses. We also included an additional four nuclear loci (2578bp) for the species tree analysis to explore changes in nodal support values. Although the stem lineage is inferred to have originated ∼69 million years ago (Ma; 74.5-64.9 credible interval), a more recent basal split within Cathartidae was recovered at ∼14Ma (17.1-11.1 credible interval). Two primary clades were identified: (1) Black Vulture (Coragyps atratus) together with the three Cathartes species (Lesser C. burrovianus and Greater C. melambrotus Yellow-headed Vultures, and Turkey Vulture C. aura), and (2) King Vulture (Sarcoramphus papa), California (Gymnogyps californianus) and Andean (Vultur gryphus) Condors. Support for taxon relationships within the two basal clades were inconsistent between analyses with the exception of Black Vulture sister to a monophyletic Cathartes clade. Increased support for a yellow-headed vulture clade was recovered in the species tree analysis using the four additional nuclear loci. Overall, these results are in agreement with cathartid life history (e.g. olfaction ability and behavior) and contrasting habitat affinities among sister taxa with overlapping geographic distributions. More research is needed using additional molecular loci to further resolve the phylogenetic relationships within the two basal cathartid clades, as speciation appeared to have occurred in a relatively short period of time.

Whole-genome resequencing of extreme phenotypes in collared flycatchers highlights the difficulty of detecting quantitative trait loci in natural populations

Dissecting the genetic basis of phenotypic variation in natural populations is a long-standing goal in evolutionary biology. One open question is whether quantitative traits are determined only by large numbers of genes with small effects, or whether variation also exists in large-effect loci. We conducted genomewide association analyses of forehead patch size (a sexually selected trait) on 81 whole-genome-resequenced male collared flycatchers with extreme phenotypes, and on 415 males sampled independent of patch size and genotyped with a 50K SNP chip. No SNPs were genomewide statistically significantly associated with patch size. Simulation-based power analyses suggest that the power to detect large-effect loci responsible for 10% of phenotypic variance was <0.5 in the genome resequencing analysis, and <0.1 in the SNP chip analysis. Reducing the recombination by two-thirds relative to collared flycatchers modestly increased power. Tripling sample size increased power to >0.8 for resequencing of extreme phenotypes (N = 243), but power remained <0.2 for the 50K SNP chip analysis (N = 1245). At least 1 million SNPs were necessary to achieve power >0.8 when analysing 415 randomly sampled phenotypes. However, power of the 50K SNP chip to detect large-effect loci was nearly 0.8 in simulations with a small effective population size of 1500. These results suggest that reliably detecting large-effect trait loci in large natural populations will often require thousands of individuals and near complete sampling of the genome. Encouragingly, far fewer individuals and loci will often be sufficient to reliably detect large-effect loci in small populations with widespread strong linkage disequilibrium.

Morphological and genomic comparisons of Hawaiian and Japanese Black-footed Albatrosses (Phoebastria nigripes) using double digest RADseq: implications for conservation

Evaluating the genetic and demographic independence of populations of threatened species is important for determining appropriate conservation measures, but different technologies can yield different conclusions. Despite multiple studies, the taxonomic status and extent of gene flow between the main breeding populations of Black-footed Albatross (Phoebastria nigripes), a Near-Threatened philopatric seabird, are still controversial. Here, we employ double digest RADseq to quantify the extent of genomewide divergence and gene flow in this species. Our genomewide data set of 9760 loci containing 3455 single nucleotide polymorphisms yielded estimates of genetic diversity and gene flow that were generally robust across seven different filtering and sampling protocols and suggest a low level of genomic variation (θ per site = ∼0.00002-0.00028), with estimates of effective population size (N e = ∼500-15 881) falling far below current census size. Genetic differentiation was small but detectable between Japan and Hawaii (F ST ≈ 0.038-0.049), with no F ST outliers. Additionally, using museum specimens, we found that effect sizes of morphological differences by sex or population rarely exceeded 4%. These patterns suggest that the Hawaiian and Japanese populations exhibit small but significant differences and should be considered separate management units, although the evolutionary and adaptive consequences of this differentiation remain to be identified.

Molecular phylogenetics suggests a New Guinean origin and frequent episodes of founder-event speciation in the nectarivorous lories and lorikeets (Aves: Psittaciformes)

The lories and lorikeets (Aves: Loriinae: Loriini) are a readily recognizable, discrete group of nectarivorous parrots confined to the Indo-Pacific region between Wallace's Line and the Pitcairn Island group in the central-east Pacific Ocean. We present the first phylogenetic analysis of all currently recognized genera in the group using two mitochondrial and five nuclear loci. Our analyses suggest a New Guinean origin for the group at about 10million years ago (95% HPD 4.8-14.8) but this origin must be interpreted within the context of that island's complicated, recent geological history. That is, the origin and early diversification of the group may have taken place as New Guinea's Central Cordillera arose and the final constituent terranes that form present-day New Guinea were accreted. The latter activity may have promoted dispersal as a key element in the group's history. We have detected several instances of dispersal out of New Guinea that we argue constitute instances of founder-event speciation. Some phenotypically cohesive genera are affirmed as monophyletic but other genera are clearly in need of taxonomic dismantlement and reclassification. We recognize Parvipsitta Mathews, 1916 for two species usually placed in Glossopsitta and we advocate transfer of Chalcopsitta cardinalis into Pseudeos Peters, 1935. Other non-monophyletic genera such as Charmosyna, Psitteuteles and, probably, Trichoglossus, require improved taxon sampling and further phylogenetic analysis before their systematics can be resolved. Cursory examination of trait mapping across the group suggests that many traits are ancestral and of little use in determining genus-level systematics.

Disentangling the complex evolutionary history of the Western Palearctic blue tits (Cyanistes spp.) - phylogenomic analyses suggest radiation by multiple colonization events and subsequent isolation

Isolated islands and their often unique biota continue to play key roles for understanding the importance of drift, genetic variation and adaptation in the process of population differentiation and speciation. One island system that has inspired and intrigued evolutionary biologists is the blue tit complex (Cyanistes spp.) in Europe and Africa, in particular the complex evolutionary history of the multiple genetically distinct taxa of the Canary Islands. Understanding Afrocanarian colonization events is of particular importance because of recent unconventional suggestions that these island populations acted as source of the widespread population in mainland Africa. We investigated the relationship between mainland and island blue tits using a combination of Sanger sequencing at a population level (20 loci; 12 500 nucleotides) and next-generation sequencing of single population representatives (>3 200 000 nucleotides), analysed in coalescence and phylogenetic frameworks. We found (i) that Afrocanarian blue tits are monophyletic and represent four major clades, (ii) that the blue tit complex has a continental origin and that the Canary Islands were colonized three times, (iii) that all island populations have low genetic variation, indicating low long-term effective population sizes and (iv) that populations on La Palma and in Libya represent relicts of an ancestral North African population. Further, demographic reconstructions revealed (v) that the Canary Islands, conforming to traditional views, hold sink populations, which have not served as source for back colonization of the African mainland. Our study demonstrates the importance of complete taxon sampling and an extensive multimarker study design to obtain robust phylogeographical inferences.

Differential introgression and effective size of marker type influence phylogenetic inference of a recently divergent avian group (Phasianidae: Tympanuchus)

Life history strategies can influence the effective population size (Ne) of loci differently based on their mode of inheritance. Recognizing how this may affect the rate of lineage sorting among marker types is important for studies focused on resolving phylogenetic relationships among recently divergent taxa. In this study, we use gene tree, coalescent-based species tree, and isolation-with-migration analyses to explore the differences between marker types (autosomal, Z-linked, and mitochondrial) in resolving phylogenetic relationships among North American prairie grouse (Tympanuchus). We found that Z-linked loci were more likely to identify monophyletic relationships among prairie grouse species compared to autosomal and mtDNA loci in both species and gene tree analyses, with species tree analyses outperforming gene trees. These results were further supported with isolation-with-migration analyses, where Z-linked loci largely followed a strict isolation model while autosomal loci were more likely to fit a model with gene flow between species following population divergence. While accounting for differences in inheritance pattern (or Ne) for marker type, results suggest that additional factors, such as strong sexual selection and sex-biased introgression (i.e., male-biased postzygotic hybrid behavioral isolation or "unsexy son"), may further explain the decreased diversity levels and increased rate of lineage sorting observed with the Z-linked loci relative to autosomal and mtDNA loci. In fact, to our knowledge no hybrid male prairie grouse have been observed breeding in the wild, yet hybrid females along with backcross females are known to produce viable offspring. Overall, this study highlights that more work is needed to determine how complex models of gene flow (i.e., sex biased introgression) and differences in the effective size among marker types based on differing life history strategies influence divergence date estimation and species delimitation.

Amplification biases and consistent recovery of loci in a double-digest RAD-seq protocol

A growing variety of "genotype-by-sequencing" (GBS) methods use restriction enzymes and high throughput DNA sequencing to generate data for a subset of genomic loci, allowing the simultaneous discovery and genotyping of thousands of polymorphisms in a set of multiplexed samples. We evaluated a "double-digest" restriction-site associated DNA sequencing (ddRAD-seq) protocol by 1) comparing results for a zebra finch (Taeniopygia guttata) sample with in silico predictions from the zebra finch reference genome; 2) assessing data quality for a population sample of indigobirds (Vidua spp.); and 3) testing for consistent recovery of loci across multiple samples and sequencing runs. Comparison with in silico predictions revealed that 1) over 90% of predicted, single-copy loci in our targeted size range (178-328 bp) were recovered; 2) short restriction fragments (38-178 bp) were carried through the size selection step and sequenced at appreciable depth, generating unexpected but nonetheless useful data; 3) amplification bias favored shorter, GC-rich fragments, contributing to among locus variation in sequencing depth that was strongly correlated across samples; 4) our use of restriction enzymes with a GC-rich recognition sequence resulted in an up to four-fold overrepresentation of GC-rich portions of the genome; and 5) star activity (i.e., non-specific cutting) resulted in thousands of "extra" loci sequenced at low depth. Results for three species of indigobirds show that a common set of thousands of loci can be consistently recovered across both individual samples and sequencing runs. In a run with 46 samples, we genotyped 5,996 loci in all individuals and 9,833 loci in 42 or more individuals, resulting in <1% missing data for the larger data set. We compare our approach to similar methods and discuss the range of factors (fragment library preparation, natural genetic variation, bioinformatics) influencing the recovery of a consistent set of loci among samples.

Australia's arid-adapted butcherbirds experienced range expansions during Pleistocene glacial maxima

A model of range expansions during glacial maxima (GM) for cold-adapted species is generally accepted for the Northern Hemisphere. Given that GM in Australia largely resulted in the expansion of arid zones, rather than glaciation, it could be expected that arid-adapted species might have had expanded ranges at GM, as cold-adapted species did in the Northern Hemisphere. For Australian biota, however, it remains paradigmatic that arid-adapted species contracted to refugia at GM. Here we use multilocus data and ecological niche models (ENMs) to test alternative GM models for butcherbirds. ENMs, mtDNA and estimates of nuclear introgression and past population sizes support a model of GM expansion in the arid-tolerant Grey Butcherbird that resulted in secondary contact with its close relative--the savanna-inhabiting Silver-backed Butcherbird--whose contemporary distribution is widely separated. Together, these data reject the universal use of a GM contraction model for Australia's dry woodland and arid biota.

Anolis sex chromosomes are derived from a single ancestral pair

To explain the frequency and distribution of heteromorphic sex chromosomes in the lizard genus Anolis, we compared the relative roles of sex chromosome conservation versus turnover of sex-determining mechanisms. We used model-based comparative methods to reconstruct karyotype evolution and the presence of heteromorphic sex chromosomes onto a newly generated Anolis phylogeny. We found that heteromorphic sex chromosomes evolved multiple times in the genus. Fluorescent in situ hybridization (FISH) of repetitive DNA showed variable rates of Y chromosome degeneration among Anolis species and identified previously undetected, homomorphic sex chromosomes in two species. We confirmed homology of sex chromosomes in the genus by performing FISH of an X-linked bacterial artificial chromosome (BAC) and quantitative PCR of X-linked genes in multiple Anolis species sampled across the phylogeny. Taken together, these results are consistent with long-term conservation of sex chromosomes in the group. Our results pave the way to address additional questions related to Anolis sex chromosome evolution and describe a conceptual framework that can be used to evaluate the origins and evolution of heteromorphic sex chromosomes in other clades.

On estimation and identifiability issues of sex-linked inheritance with a case study of pigmentation in Swiss barn owl (Tyto alba)

Genetic evaluation using animal models or pedigree-based models generally assume only autosomal inheritance. Bayesian animal models provide a flexible framework for genetic evaluation, and we show how the model readily can accommodate situations where the trait of interest is influenced by both autosomal and sex-linked inheritance. This allows for simultaneous calculation of autosomal and sex-chromosomal additive genetic effects. Inferences were performed using integrated nested Laplace approximations (INLA), a nonsampling-based Bayesian inference methodology. We provide a detailed description of how to calculate the inverse of the X- or Z-chromosomal additive genetic relationship matrix, needed for inference. The case study of eumelanic spot diameter in a Swiss barn owl (Tyto alba) population shows that this trait is substantially influenced by variation in genes on the Z-chromosome ( and ). Further, a simulation study for this study system shows that the animal model accounting for both autosomal and sex-chromosome-linked inheritance is identifiable, that is, the two effects can be distinguished, and provides accurate inference on the variance components.

Distinguishing the effects of selection from demographic history in the genetic variation of two sister passerines based on mitochondrial-nuclear comparison

Determining the mechanisms responsible for the distribution of genetic diversity in natural populations has occupied a central role in molecular evolution. Our study was motivated by the unprecedented observation that a widespread Eurasian flycatcher, Ficedula albicilla, exhibited no variation at the mitochondrial DNA (mtDNA) ND2 gene in 75 individuals sampled over a 5000-km distance. In contrast, its sister species, F. parva, had low but considerably higher levels of mtDNA variation. We assessed whether natural selection or demographic factors could explain the absence of mtDNA variation in F. albicilla. Eighteen nuclear genes were sequenced to estimate the two species' phylogeographic histories, and for comparison to the mtDNA data. Multilocus coalescence analyses suggested that F. albicilla experienced a population expansion perhaps following a population bottleneck. Simulations based on this demographic history, however, did not replicate the extremely low level of mtDNA variation. Historical range changes based on ecological niche models also failed to explain the observed mtDNA patterns. Neutrality tests (DHEW and ML-HKA) suggested a non-neutral pattern in the mtDNA of F. albicilla. We found a transmembrane-skewed distribution of nonsynonymous substitutions between the two species, three of which caused functional change; the results implied that positive selection could have targeted mtDNA. Several lines of evidence support selection rather than demographic history as the main force influencing the patterns of mtDNA variation. Despite the influence of natural selection, many of the phylogeographic inferences derived from mtDNA were robust, including species limits and a high level of gene flow among populations within species.

The ecological and geographic context of morphological and genetic divergence in an understorey-dwelling bird

Advances in understanding the process of species formation require an integrated perspective that includes the evaluation of spatial, ecological and genetic components. One approach is to focus on multiple stages of divergence within the same species. Species that comprise phenotypically different populations segregated in apparently distinct habitats, in which range is presently continuous but was putatively geographically isolated provide an interesting system to study the mechanisms of population divergence. Here, we attempt to elucidate the role of ecology and geography in explaining observed morphological and genetic variation in an understorey-dwelling bird endemic to southeastern Africa, where two subspecies are recognized according to phenotype and habitat affinity. We carried out a range-wide analysis of climatic requirements, morphological and genetic variation across southeast Africa to test the hypothesis that the extent of gene flow among populations of the brown scrub-robin are influenced by their distinct climatic niches. We recovered two distinct trends depending on whether our analyses were hierarchically structured at the subspecies or at the within subspecies level. Between subspecies we found pronounced morphological differentiation associated with strong reproductive isolation (no gene flow) between populations occupying divergent climatic niches characterized by changes in the temperature of the warmest and wettest month. In contrast, within subspecies, we recovered continuous morphological variation with extensive gene flow among populations inhabiting the temperate and sub-tropical forests of southern Africa, despite divergence along the climate axis that is mainly determined by minimum temperature and precipitation of the coldest months. Our results highlight the role of niche divergence as a diversifying force that can promote reproductive isolation in vertebrates.

Robust estimates of divergence times and selection with a poisson random field model: a case study of comparative phylogeographic data

Mutation frequencies can be modeled as a Poisson random field (PRF) to estimate speciation times and the degree of selection on newly arisen mutations. This approach provides a quantitative theory for comparing intraspecific polymorphism with interspecific divergence in the presence of selection and can be used to estimate population genetic parameters. Although the original PRF model has been extended to more general biological settings to make statistical inference about selection and divergence among model organisms, it has not been incorporated into phylogeographic studies that focus on estimating population genetic parameters for nonmodel organisms. Here, we modified a recently developed time-dependent PRF model to independently estimate genetic parameters from a nuclear and mitochondrial DNA data set of 22 sister pairs of birds that have diverged across a biogeographic barrier. We found that species that inhabit humid habitats had more recent divergence times and larger effective population sizes than those that inhabit drier habitats, and divergence time estimated from the PRF model were similar to estimates from a coalescent species-tree approach. Selection coefficients were higher in sister pairs that inhabited drier habitats than in those in humid habitats, but overall the mitochondrial DNA was under weak selection. Our study indicates that PRF models are useful for estimating various population genetic parameters and serve as a framework for incorporating estimates of selection into comparative phylogeographic studies.

Demographic divergence history of pied flycatcher and collared flycatcher inferred from whole-genome re-sequencing data

Profound knowledge of demographic history is a prerequisite for the understanding and inference of processes involved in the evolution of population differentiation and speciation. Together with new coalescent-based methods, the recent availability of genome-wide data enables investigation of differentiation and divergence processes at unprecedented depth. We combined two powerful approaches, full Approximate Bayesian Computation analysis (ABC) and pairwise sequentially Markovian coalescent modeling (PSMC), to reconstruct the demographic history of the split between two avian speciation model species, the pied flycatcher and collared flycatcher. Using whole-genome re-sequencing data from 20 individuals, we investigated 15 demographic models including different levels and patterns of gene flow, and changes in effective population size over time. ABC provided high support for recent (mode 0.3 my, range <0.7 my) species divergence, declines in effective population size of both species since their initial divergence, and unidirectional recent gene flow from pied flycatcher into collared flycatcher. The estimated divergence time and population size changes, supported by PSMC results, suggest that the ancestral species persisted through one of the glacial periods of middle Pleistocene and then split into two large populations that first increased in size before going through severe bottlenecks and expanding into their current ranges. Secondary contact appears to have been established after the last glacial maximum. The severity of the bottlenecks at the last glacial maximum is indicated by the discrepancy between current effective population sizes (20,000-80,000) and census sizes (5-50 million birds) of the two species. The recent divergence time challenges the supposition that avian speciation is a relatively slow process with extended times for intrinsic postzygotic reproductive barriers to evolve. Our study emphasizes the importance of using genome-wide data to unravel tangled demographic histories. Moreover, it constitutes one of the first examples of the inference of divergence history from genome-wide data in non-model species.

Neurexophilin 1 gene polymorphism in chickens and its variation among species

Neurexophilin 1 (nxph1) has been considered a potential candidate marker for sperm storage in chicken sperm storage tubules. In this work, one mutation of chicken nxph1 was detected. We analyzed 18 nxph1 gene sequences from 18 species. The coding sequence length of the zebra fish nxph1 gene is 819 bp; that of the other species is 816 bp. Amino acid alignment analysis revealed that the gene product is a conserved protein, especially in mammals. The sequences of mammals are highly conserved. We found 202 conserved amino acids (70-271), and there were only eight mutations in the remaining 69 amino acids. That level of conservation could be due to the nxph1 gene having been subjected to substantial constraints or strong purifying selection during millions of years of evolution.