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opencc-zeroDec 2017View details →

Data from: Differential effect of selection against LINE retrotransposons among vertebrates inferred from whole-genome data and demographic modeling

Variation in LINE composition is one of the major determinants for the substantial size and structural differences among vertebrate genomes. In particular, the larger genomes of mammals are characterized by hundreds of thousands of copies from a single LINE clade, L1, whereas nonmammalian vertebrates possess a much greater diversity of LINEs, yet with orders of magnitude less in copy number. It has been proposed that such variation in copy number among vertebrates is due to differential effect of LINE insertions on host fitness. To investigate LINE selection, we deployed a framework of demographic modeling, coalescent simulations, and probabilistic inference against population-level whole-genome data sets for four model species: one population each of threespine stickleback, green anole, and house mouse, as well as three human populations. Specifically, we inferred a null demographic background utilizing SNP data, which was then exploited to simulate a putative null distribution of summary statistics that was compared with LINE data. Subsequently,we applied the inferred null demographic model with an additional exponential size change parameter, coupled with model selection, to test for neutrality as well as estimate the strength of either negative or positive selection. We found a robust signal for purifying selection in anole and mouse, but a lack of clear evidence for selection in stickleback and human. Overall, we demonstrated LINE insertion dynamics that are not in accordance to a mammalian versus nonmammalian dichotomy, and instead the degree of existing LINE activity together with host-specific demographic history may be the main determinants of LINE abundance.

opencc-zeroDec 2014View details →

Data from: Population genomic footprints of fine-scale differentiation between habitats in Mediterranean blue tits

Linking population genetic variation to the spatial heterogeneity of the environment is of fundamental interest to evolutionary biology and ecology, in particular when phenotypic differences between populations are observed at biologically small spatial scales. Here, we applied restriction-site associated DNA sequencing (RAD-Seq) to test whether phenotypically differentiated populations of wild blue tits (Cyanistes caeruleus) breeding in a highly heterogeneous environment exhibit genetic structure related to habitat type. Using 12106 SNPs in 197 individuals from deciduous and evergreen oak woodlands, we applied complementary population genomic analyses, which revealed that genetic variation is influenced by both geographical distance and habitat type. A fine-scale genetic differentiation supported by genome- and transcriptome-wide analyses was found within Corsica, between two adjacent habitats where blue tits exhibit marked differences in breeding time while nesting less than 6 km apart. Using redundancy analysis (RDA), we show that genomic variation remains associated with habitat type when controlling for spatial and temporal effects. Finally, our results suggest that the observed patterns of genomic differentiation were not driven by a small proportion of highly differentiated loci, but rather emerged through a process such as habitat choice, which reduces gene flow between habitats across the entire genome. The pattern of genomic isolation-by-environment closely matches differentiation observed at the phenotypic level, thereby offering significant potential for future inference of phenotype-genotype associations in a heterogeneous environment.

opencc-zeroDec 2017View details →

Data from: Genomic and geographic footprints of differential introgression between two divergent fish species (Solea spp.)

Investigating variation in gene flow across the genome between closely related species is important to understand how reproductive barriers shape genome divergence before speciation is complete. An efficient way to characterize differential gene flow is to study how the genetic interactions that take place in hybrid zones selectively filter gene exchange between species, leading to heterogeneous genome divergence. In the present study, genome-wide divergence and introgression patterns were investigated between two sole species, Solea senegalensis and Solea aegyptiaca, using restriction-associated DNA sequencing (RAD-Seq) to analyse samples taken from a transect spanning the hybrid zone. An integrative approach combining geographic and genomic clines methods with an analysis of individual locus introgression accounting for the demographic history of divergence was conducted. Our results showed that the two sole species have come into secondary contact postglacially, after experiencing a prolonged period (ca. 1.1 to 1.8 Myrs) of allopatric separation. Secondary contact resulted in the formation of a tension zone characterized by strong reproductive isolation, allowing introgression in only a minor fraction of the genome. We found multiple evidence for a preferential direction of introgression in the S. aegyptiaca genetic background, indicating a possible recent or ongoing movement of the hybrid zone. Deviant introgression signals found in the opposite direction suggested that S. senegalensis could have possibly undergone adaptive introgression that has not yet spread throughout the entire species range. Our study thus illustrates the varied outcomes of genetic interactions between divergent gene pools that recently met after a long history of divergence.

Data from: Recurrent selection explains parallel evolution of genomic regions of high relative but low absolute differentiation in a ring species

Recent technological developments allow investigation of the repeatability of evolution at the genomic level. Such investigation is particularly powerful when applied to a ring species, in which spatial variation represents changes during the evolution of two species from one. We examined genomic variation among three subspecies of the greenish warbler ring species, using genotypes at 13 013 950 nucleotide sites along a new greenish warbler consensus genome assembly. Genomic regions of low within-group variation are remarkably consistent between the three populations. These regions show high relative differentiation but low absolute differentiation between populations. Comparisons with outgroup species show the locations of these peaks of relative differentiation are not well explained by phylogenetically conserved variation in recombination rates or selection. These patterns are consistent with a model in which selection in an ancestral form has reduced variation at some parts of the genome, and those same regions experience recurrent selection that subsequently reduces variation within each subspecies. The degree of heterogeneity in nucleotide diversity is greater than explained by models of background selection, but is consistent with selective sweeps. Given the evidence that greenish warblers have had both population differentiation for a long period of time and periods of gene flow between those populations, we propose that some genomic regions underwent selective sweeps over a broad geographic area followed by within-population selection-induced reductions in variation. An important implication of this 'sweep-before-differentiation' model is that genomic regions of high relative differentiation may have moved among populations more recently than other genomic regions.

Data from: Genome-wide differentiation in closely related populations: the roles of selection and geographic isolation

Population divergence in geographic isolation is due to a combination of factors. Natural and sexual selection may be important in shaping patterns of population differentiation, a pattern referred to as 'isolation by adaptation' (IBA). IBA can be complementary to the well-known pattern of 'isolation by distance' (IBD), in which the divergence of closely related populations (via any evolutionary process) is associated with geographic isolation. The barn swallow Hirundo rustica complex comprises six closely related subspecies, where divergent sexual selection is associated with phenotypic differentiation among allopatric populations. To investigate the relative contributions of selection and geographic distance to genome-wide differentiation, we compared genotypic and phenotypic variation from 350 barn swallows sampled across eight populations (28 pairwise comparisons) from four different subspecies. We report a draft whole-genome sequence for H. rustica, to which we aligned a set of 9493 single nucleotide polymorphisms (SNPs). Using statistical approaches to control for spatial autocorrelation of phenotypic variables and geographic distance, we find that divergence in traits related to migratory behaviour and sexual signalling, as well as geographic distance, together explain over 70% of genome-wide divergence among populations. Controlling for IBD, we find 42% of genomewide divergence is attributable to IBA through pairwise differences in traits related to migratory behaviour and sexual signalling alone. By (i) combining these results with prior studies of how selection shapes morphological differentiation and (ii) accounting for spatial autocorrelation, we infer that morphological adaptation plays a large role in shaping population-level differentiation in this group of closely related populations.

opencc-zeroDec 2012View details →

Data from: Heterogeneous genomic differentiation in marine threespine sticklebacks: adaptation along an environmental gradient

Evolutionary divergence among populations occupying ecologically distinct environments can occur even in the face of on-going gene flow. However, the genetic underpinnings, as well as the scale and magnitude at which this differentiation occurs in marine habitats are not well understood. We investigated the patterns and degree of genomic heterogeneity in threespine sticklebacks (Gasterosteus aculeatus) by assessing genetic variability in 20 nongenic and 20 genic (associated with genes important for freshwater adaptation) microsatellite loci in samples collected from 38 locations spanning the entire Baltic Sea coast to the North Sea boundary. Population divergence (FST ≈ 0.026) and structuring (five genetic clusters) was significantly more pronounced in the genic as compared to nongenic markers (FST ≈ 0.008; no genetic clusters). Patterns of divergence in the genic markers—45% of which were identified as outliers—correlated with local differences in salinity. Yet, a strong positive correlation between divergence in genic and nongenic markers, and their association with environmental factors suggests that adaptive divergence is reducing gene flow across the genome. Apart from providing a clear demonstration of heterogeneous genomic patterns of differentiation in a marine species, the results are indicative of adaptive population structuring across the relatively young Baltic Sea in spite of ample opportunities for gene flow.

opencc-zeroDec 2018View details →

Data from: Characterizing neutral and adaptive genomic differentiation in a changing climate: the most northerly freshwater fish as a model

Arctic freshwater ecosystems have been profoundly affected by climate change. Given that the Arctic charr (Salvelinus alpinus) is often the only fish species inhabiting these ecosystems, it represents a valuable model for studying the impacts of climate change on species life history diversity and adaptability. Using a genotyping-by-sequencing approach, we identified 5976 neutral single nucleotide polymorphisms (SNPs) and found evidence for reduced gene flow between allopatric morphs from two high Arctic lakes, Linne´vatn (Anadromous, Normal, and Dwarf) and Ellasjøen (Littoral and Pelagic). Within each lake, the degree of genetic differentiation ranged from low (Pelagic vs. Littoral) to moderate (Anadromous and Normal vs. Dwarf). We identified 17 highly diagnostic, putative adaptive SNPs that differentiated the allopatric morphs. Although we found no evidence for adaptive differences between morphs within Ellasjøen, we found evidence for moderate (Anadromous vs. Normal) to high genetic differentiation (Anadromous and Normal vs. Dwarf) among morphs within Linne´vatn based on two adaptive loci. As these freshwater ecosystems become more productive, the frequency of sympatric morphs in Ellasjøen will likely shift based on foraging opportunities, whereas the propensity to migrate may decrease in Linne´vatn, increasing the frequency of the Normal morph. The Dwarf charr was the most genetically distinct group. Identifying the biological basis for small body size should elucidate the potential for increased growth and subsequent interbreeding with sympatric morphs. Overall, neutral and adaptive genomic differentiation between allopatric and some sympatric morphs suggests that the response of Arctic charr to climate change will be variable across freshwater ecosystems.

Data from: Level of genetic differentiation affects relative performances of EST- and genomic SSRs

Microsatellites, also called simple sequence repeats (SSRs) are markers of choice to estimate relevant parameters for conservation genetics, such as migration rates, effective population size and kinship. Cross-amplification of SSRs is the simplest way to obtain sets of markers and highly conserved SSRs have recently been developed from Expressed Sequence Tags (EST) to improve SRR cross-species utility. As EST-SSRs are located in coding regions, the higher stability of their flanking regions reduces the frequency of null alleles and improves cross-species amplification. However, EST-SSRs have generally less allelic variability than genomic SSRs, potentially leading to differences in estimates of population genetic parameters such as genetic differentiation. To assess the potential of EST-SSRs in studies of within-species genetic diversity, we compared the relative performance of EST- and genomic SSRs following a multi-species approach on passerine birds. We tested whether patterns and levels of genetic diversity within- and between-populations assessed from EST- and from genomic SSRs are congruent and we investigated how the relative efficiency of EST- and genomic SSRs is influenced by levels of differentiation. EST- and genomic SSRs ensured comparable inferences of population genetic structure in cases of strong genetic differentiation, and genomic SSRs performed slightly better than EST-SSRs when differentiation is moderate. However and interestingly, EST-SSRs had a higher power to detect weak genetic structure compared to genomic SSRs. Our study attests that EST-SRRs may be valuable molecular markers for conservation genetic studies in taxa such as birds, where the development of genomic SSRs is impeded by their low frequency.

opencc-zeroDec 2013View details →

Data from: Genomic atolls of differentiation in coral reef fishes (Hypoplectrus spp, Serranidae)

Because the vast majority of species are well-diverged, relatively little is known about the genomic architecture of speciation during the early stages of divergence. Species within recent evolutionary radiations are often minimally diverged from a genomic perspective, and therefore provide rare opportunities to address this question. Here, we leverage the hamlet radiation (Hypoplectrus spp, brightly colored reef fishes from the tropical western Atlantic) to characterize genomic divergence during the early stages of speciation. Transect surveys and spawning observations in Belize, Honduras, and Panama confirm that sympatric barred (H. puella), black (H. nigricans) and butter (H. unicolor) hamlets are phenotypically distinct and reproductively isolated, although hybrid spawnings and individuals with intermediate phenotypes are seen on rare occasions. A survey of approximately 100,000 restriction-site associated SNPs in 126 samples from the three species across the three replicate populations reveals extremely slight genome-wide divergence among species (Fst=0.0038), indicating that ecomorphological differences and functional reproductive isolation are maintained in sympatry in a backdrop of extraordinary genomic similarity. Nonetheless, a very small proportion of SNPs (0.05% on average) are identified as Fst outliers among sympatric species. Remarkably, a single SNP is identified as an outlier in repeated populations for the same species pair. A mini-contig assembled de novo around this SNP falls into the genomic region containing the HoxCa10 and HoxCa11 genes in 10 teleost species, suggesting an important role for Hox gene evolution in this radiation. This finding, if confirmed, would provide a better understanding of the links between micro- and macroevolutionary processes.

Single-cell RNA sequencing of CNS-infiltrating HSC-derived phagocytes of Ms4a3Ai14, BM chimeric mice (CD45.2 Csf2rb-/-: CD45.1 Csf2rb+/+ and CD45.2 Ifngr1-/-: CD45.1 Ifngr1+/+) using 10X Genomics platform. IFN-γ and GM-CSF control complementary differentiation programs in the monocyte to phagocyte transition during neuroinflammation.

Single-cell RNA sequencing of CNS-infiltrating HSC-derived phagocytes of Ms4a3Ai14 at onset and peak EAE,  BM chimeric mice (CD45.2 Csf2rb-/-: CD45.1 Csf2rb+/+ and CD45.2 Ifngr1-/-: CD45.1 Ifngr1+/+) using 10X Genomics platform. The sorted cells were loaded into 10x Genomics Chromium in parallel. Libraries were prepared as per the manufacturer's protocol (Chromium Next GEM Single Cell 3ʹ Reagent Kits v3.1 protocol) and sequenced on an Illumina NovaSeq sequencer according to 10X Genomics recommendations (paired-end reads, R1=28, i7=8, R2=91) to a depth of around 50,000 reads per cell. Initial processing was done using Cell Ranger (v3.1.0) mkfastq and count (reads were aligned to GENCODE reference build GRCm38.p6 Release M23 with added tdTomato sequence for the dataset from Ms4a3Ai14 mouse and collapse UMIs). Starting from the filtered gene-cell count matrix produced by CellRranger's in-built cell calling algorithms, we proceeded with Seurat v4 workflow.

Single-cell RNA sequencing of Lymph node-infiltrating HSC-derived phagocytes of Ms4a3Ai14 using 10X Genomics platform. IFN-γ and GM-CSF control complementary differentiation programs in the monocyte to phagocyte transition during neuroinflammation.

Single-cell RNA sequencing of Lymph node-infiltrating HSC-derived phagocytes of Ms4a3Ai14 at onset and peak EAE using 10X Genomics platform. The sorted cells were loaded into 10x Genomics Chromium in parallel. Libraries were prepared as per the manufacturer's protocol (Chromium Next GEM Single Cell 3ʹ Reagent Kits v3.1 protocol) and sequenced on an Illumina NovaSeq sequencer according to 10X Genomics recommendations (paired-end reads, R1=28, i7=8, R2=91) to a depth of around 50,000 reads per cell. Initial processing was done using Cell Ranger (v3.1.0) mkfastq and count (reads were aligned to GENCODE reference build GRCm38.p6 Release M23 with added tdTomato sequence for the dataset from Ms4a3Ai14 mouse and collapse UMIs). Starting from the filtered gene-cell count matrix produced by CellRranger's in-built cell calling algorithms, we proceeded with Seurat v4 workflow.

Single-cell RNA sequencing of Bone Marrow-infiltrating HSC-derived phagocytes of Ms4a3Ai14 using 10X Genomics platform. IFN-γ and GM-CSF control complementary differentiation programs in the monocyte to phagocyte transition during neuroinflammation.

Single-cell RNA sequencing of Bone Marrow-infiltrating HSC-derived phagocytes of Ms4a3Ai14 at onset and peak EAE using 10X Genomics platform. The sorted cells were loaded into 10x Genomics Chromium in parallel. Libraries were prepared as per the manufacturer's protocol (Chromium Next GEM Single Cell 3ʹ Reagent Kits v3.1 protocol) and sequenced on an Illumina NovaSeq sequencer according to 10X Genomics recommendations (paired-end reads, R1=28, i7=8, R2=91) to a depth of around 50,000 reads per cell. Initial processing was done using Cell Ranger (v3.1.0) mkfastq and count (reads were aligned to GENCODE reference build GRCm38.p6 Release M23 with added tdTomato sequence for the dataset from Ms4a3Ai14 mouse and collapse UMIs). Starting from the filtered gene-cell count matrix produced by CellRranger's in-built cell calling algorithms, we proceeded with Seurat v4 workflow.

Single-cell RNA sequencing of Blood-infiltrating HSC-derived phagocytes of Ms4a3Ai14 using 10X Genomics platform. IFN-γ and GM-CSF control complementary differentiation programs in the monocyte to phagocyte transition during neuroinflammation.

Single-cell RNA sequencing of Blood-infiltrating HSC-derived phagocytes of Ms4a3Ai14 at onset and peak EAE using 10X Genomics platform. The sorted cells were loaded into 10x Genomics Chromium in parallel. Libraries were prepared as per the manufacturer's protocol (Chromium Next GEM Single Cell 3ʹ Reagent Kits v3.1 protocol) and sequenced on an Illumina NovaSeq sequencer according to 10X Genomics recommendations (paired-end reads, R1=28, i7=8, R2=91) to a depth of around 50,000 reads per cell. Initial processing was done using Cell Ranger (v3.1.0) mkfastq and count (reads were aligned to GENCODE reference build GRCm38.p6 Release M23 with added tdTomato sequence for the dataset from Ms4a3Ai14 mouse and collapse UMIs). Starting from the filtered gene-cell count matrix produced by CellRranger's in-built cell calling algorithms, we proceeded with Seurat v4 workflow.

opencc-zeroJul 2022View details →

Data from: Sustained plumage divergence despite weak genomic differentiation and broad sympatry in sister species of Australian woodswallows (Artamus spp.)

Plumage divergence can function as a strong premating barrier when species come into secondary contact. When it fails to do so, the results are often genome homogenization and phenotypic hybrids at the zone of contact. This is not the case in the largely sympatric masked woodswallow and white-browed woodswallow species (Passeriformes: Artamidae: Artamus spp) complex in Australia where phenotypic integrity is sustained despite no discernible mitochondrial structure in earlier work. This lack of structure may suggest recent divergence, ongoing gene flow or both, and phenotypic hybrids are reported albeit rarely. Here, we further assessed the population structure and differentiation across the species' nuclear genomes using ddRAD-seq. As found in the mitochondrial genome, no structure or divergence within or between the two species was detected in the nuclear genome. This coarse sampling of the genome nonetheless revealed peaks of differentiation around the genes SOX5 and Axin1. Both are involved in the Wnt/β-catenin signaling pathway, which regulates feather development. Reconstruction of demographic history and estimation of parameters supports a scenario of secondary contact. Our study informs how divergent plumage morphs may arise and be sustained despite whole-genome homogenization and reveals new candidate genes potentially involved in plumage divergence.

opencc-by-4.0Jul 2022View details →

Spruce giga-genomes: structurally similar yet distinctive with differentially expanding gene families and rapidly evolving genes - orthogroups dataset

Orthogroups clustering and analysis of pines and spruces, as reported in Gagalova et al., 2022

opencc-zeroAug 2022View details →

Contrasting population genomic differentiation in two sympatric Triplophysa loaches on the Qinghai-Tibet Plateau

Under global climate change, the Qinghai-Tibet Plateau (QTP) has experienced dramatic environmental changes, including salinity changes of water bodies, which may threaten the biodiversity of aquatic organisms on the 'roof of the world'. The Tibetan loaches (the genus Triplophysa) are the largest component of the QTP ichthyofauna. Here we compared the population structure and adaptive mechanisms to salinity of two sympatric Tibetan loach species, T. stewarti and T. stenura, using population genomics methods. Using both the 'common' and 'individual' SNP datasets of seven populations from five localities of the two species, we found out that the two species showed entirely different patterns of population differentiation, with T. stewarti populations deeply diverged and T. stenura populations mixed. We identified a catalogue of candidate genes possibly involved in salinity acclimatation of the two species using both unsupervised and supervised population differentiation methods. In this section, a new approach - linkage disequilibrium (LD) graph learning - was developed and utilized to identify clusters of loci showing similar genetic differentiation patterns. However, we found limited parallel adaptive signals to salinity of the two species using these methods. Our findings broaden our understandings of the population characteristics and adaptive mechanisms of these previously underexplored Tibetan loach species, and will play a role in the biodiversity protection of Triplophysa species on the QTP.

opencc-by-4.0Aug 2024View details →

Protocol for Genome-scale differential flux analysis (GS-FFA)

Here, we introduce a genome-scale differential flux analysis (GS-DFA) protocol to interrogate the metabolic variances derived from condition-specific gene expression data. It includes scripts for normalizing the gene expression data and integrating it into the HumanGEM framework to create condition-specific models. Moreover, the protocol includes the script for analyzing genome-scale differential flux across the biochemical network, employing condition-specific models. This analytical phase bridges RNA-Seq data with cellular insights, elucidating altered metabolic activities across diverse conditions. The insights can offer valuable perspectives into the intricate metabolic disturbances that play a pivotal role in cellular homeostasis.

opencc-zeroOct 2019View details →

Widespread intersex differentiation across the stickleback genome – the signature of sexually antagonistic selection?

Females and males within a species commonly have distinct reproductive roles, and the associated traits may be under perpetual divergent natural selection between the sexes if their sex-specific control has not yet evolved. We here explore whether such sexually antagonistic selection can be detected based on the magnitude of differentiation between the sexes across genome-wide genetic polymorphisms by whole-genome sequencing of large pools of female and male threespine stickleback fish. We find numerous autosomal genome regions exhibiting intersex allele frequency differences beyond the range plausible under pure sampling stochasticity. Alternative sequence alignment strategies rule out that these high-differentiation regions represent sex chromosome segments misassembled into the autosomes. Instead, comparing allele frequencies and sequence read depth between the sexes reveals that regions of high intersex differentiation arise because autosomal chromosome segments got copied into the male-specific sex chromosome (Y), where they acquired new mutations. Because the Y chromosome is missing in the stickleback reference genome, sequence reads from derived DNA copies on the Y chromosome still align to the original homologous regions on the autosomes. We argue that this phenomenon hampers the identification of sexually antagonistic selection within a genome, and can lead to spurious conclusions from population genomic analyses when the underlying samples differ in sex ratios. Because the hemizygous sex chromosome sequence (Y or W) is not represented in most reference genomes, these problems may apply broadly.

opencc-zeroJul 2019View details →

Data from: Genomic differentiation tracks earth-historic isolation in an Indo-Australasian archipelagic pitta (Pittidae; Aves) complex

Background: Allopatric speciation has played a particularly important role in archipelagic settings where populations evolve in isolation after colonizing different islands. The Indo-Australasian island realm is an unparalleled natural laboratory of biotic diversification. Here we explore how the level of earth-historic isolation has influenced genetic differentiation across the region by investigating phylogeographic patterns in the Pitta sordida species complex. Results: We generated a de novo genome and compared population genomics of 29 individuals of Pitta sordida from the entire distributional range and we reconstructed phylogenetic relationship using mitogenomes, a multi-nuclear gene dataset and single nucleotide polymorphisms (SNPs). We found deep divergence between an eastern and a western group of taxa across Indo-Australasia. Within both groups we have identified major lineages that are geographically separated into Philippines, Borneo, western Sundaland, and New Guinea, respectively. Although these lineages are genetically well-differentiated, suggesting a long-term isolation, there are signatures of extensive gene flow within each lineage throughout the Pleistocene, despite the wide geographic range occupied by some of them. We found little evidence of hybridization or introgression among the studied taxa, but forsteni from Sulawesi makes an exception. This individual, belonging to the eastern clade, is genetically admixed between the western and eastern clades. Geographically this makes sense as Sulawesi is not far from Borneo that houses a population of hooded pittas that belongs to the western clade. Conclusions: We found that geological vicariance events cannot explain the current genetic differentiation in the Pitta sordida species complex. Instead, the glacial-interglacial cycles may have played a major role therein. During glacials the sea level could be up to 120 m lower than today and land bridges formed within both the Sunda Shelf and the Sahul Shelf permitting dispersal of floral and faunal elements. The geographic distribution of hooded pittas shows the importance of overwater, "stepping-stone" dispersals not only to deep-sea islands, but also from one shelf to the other. The most parsimonious hypothesis is an Asian ancestral home of the Pitta sordida species complex and a colonization from west to east, probably via Wallacea.

Data from: Adaptive divergence despite strong genetic drift: genomic analysis of the evolutionary mechanisms causing genetic differentiation in the island fox (Urocyon littoralis)

The evolutionary mechanisms generating the tremendous biodiversity of islands have long fascinated evolutionary biologists. Genetic drift and divergent selection are predicted to be strong on islands and both could drive population divergence and speciation. Alternatively, strong genetic drift may preclude adaptation. We conducted a genomic analysis to test the roles of genetic drift and divergent selection in causing genetic differentiation among populations of the island fox (Urocyon littoralis). This species consists of six subspecies, each of which occupies a different California Channel Island. Analysis of 5293 SNP loci generated using Restriction-site Associated DNA (RAD) sequencing found support for genetic drift as the dominant evolutionary mechanism driving population divergence among island fox populations. In particular, populations had exceptionally low genetic variation, small Ne (range = 2.1–89.7; median = 19.4), and significant genetic signatures of bottlenecks. Moreover, islands with the lowest genetic variation (and, by inference, the strongest historical genetic drift) were most genetically differentiated from mainland grey foxes, and vice versa, indicating genetic drift drives genome-wide divergence. Nonetheless, outlier tests identified 3.6–6.6% of loci as high FST outliers, suggesting that despite strong genetic drift, divergent selection contributes to population divergence. Patterns of similarity among populations based on high FST outliers mirrored patterns based on morphology, providing additional evidence that outliers reflect adaptive divergence. Extremely low genetic variation and small Ne in some island fox populations, particularly on San Nicolas Island, suggest that they may be vulnerable to fixation of deleterious alleles, decreased fitness and reduced adaptive potential.