Phylogenetic analysis of six species of pacific abalone (Haliotidae) based on DNA sequences of 16s rRNA and cytochrome c oxidase subunit I mitochondrial genes

Six species of abalones (Haliotidae) are found on the Korean coasts. Identification and characterization of these abalones are usually based on morphologic characters. In this research we compared the partial sequences of the mitochondrial 16S ribosomal RNA and cytochrome c oxidase subunit I genes to identify species using molecular data and to determine their phylogenetic relationships. Sequence alignments and phylogenetic analysis revealed that the 6 species fell into 2 distinct groups which were genetically distant from each other and exhibited little internal phylogenetic resolution. One group included Haliotis discus hannai, H. discus discus, H. madaka, and H. gigantea, while the other group contained H. diversicolor supertexta and H. diversicolor diversicolor. The 16S rRNA sequences were relatively more conserved than to the COI sequences, but both gene sequences provided sufficient phylogenetic information to distinguish among the 6 species of Pacific abalone, and thus could be valuable molecular characters for species identification.     

Evolution of putative barrier loci at an intermediate stage of speciation with gene flow in campions (Silene)

Understanding the origin of new species is a central goal in evolutionary biology. Diverging lineages often evolve highly heterogeneous patterns of genetic differentiation; however, the underlying mechanisms are not well understood. We investigated evolutionary processes governing genetic differentiation between the hybridizing campions Silene dioica (L.) Clairv. and S. latifolia Poiret. Demographic modelling indicated that the two species diverged with gene flow. The best‐supported scenario with heterogeneity in both migration rate and effective population size suggested that a small proportion of the loci evolved without gene flow. Differentiation (F _ST) and sequence divergence (d _XY) were correlated and both tended to peak in the middle of most linkage groups, consistent with reduced gene flow at highly differentiated loci. Highly differentiated loci further exhibited signatures of selection. In between‐species population pairs, isolation by distance was stronger for genomic regions with low between‐species differentiation than for highly differentiated regions that may contain barrier loci. Moreover, differentiation landscapes within and between species were only weakly correlated, suggesting that linked selection due to shared recombination and gene density landscapes is not the dominant determinant of genetic differentiation in these lineages. Instead, our results suggest that divergent selection shaped the genomic landscape of differentiation between the two Silene species, consistent with predictions for speciation in the face of gene flow.     

Background selection and FST: Consequences for detecting local adaptation

Background selection is a process whereby recurrent deleterious mutations cause a decrease in the effective population size and genetic diversity at linked loci. Several authors have suggested that variation in the intensity of background selection could cause variation in F_ST across the genome, which could confound signals of local adaptation in genome scans. We performed realistic simulations of DNA sequences, using recombination maps from humans and sticklebacks, to investigate how variation in the intensity of background selection affects F_ST and other statistics of population differentiation in sexual, outcrossing species. We show that, in populations connected by gene flow, Weir and Cockerham's (1984; Evolution, 38 , 1358) estimator of F_ST is largely insensitive to locus‐to‐locus variation in the intensity of background selection. Unlike F_ST, however, d_XY is negatively correlated with background selection. Moreover, background selection does not greatly affect the false‐positive rate in F_ST outlier studies in populations connected by gene flow. Overall, our study indicates that background selection will not greatly interfere with finding the variants responsible for local adaptation.     

Genetic Barriers to Historical Gene Flow between Cryptic Species of Alpine Bumblebees Revealed by Comparative Population Genomics

Evidence is accumulating that gene flow commonly occurs between recently diverged species, despite the existence of barriers to gene flow in their genomes. However, we still know little about what regions of the genome become barriers to gene flow and how such barriers form. Here, we compare genetic differentiation across the genomes of bumblebee species living in sympatry and allopatry to reveal the potential impact of gene flow during species divergence and uncover genetic barrier loci. We first compared the genomes of the alpine bumblebee Bombus sylvicola and a previously unidentified sister species living in sympatry in the Rocky Mountains, revealing prominent islands of elevated genetic divergence in the genome that colocalize with centromeres and regions of low recombination. This same pattern is observed between the genomes of another pair of closely related species living in allopatry (B. bifarius and B. vancouverensis). Strikingly however, the genomic islands exhibit significantly elevated absolute divergence (d_XY) in the sympatric, but not the allopatric, comparison indicating that they contain loci that have acted as barriers to historical gene flow in sympatry. Our results suggest that intrinsic barriers to gene flow between species may often accumulate in regions of low recombination and near centromeres through processes such as genetic hitchhiking, and that divergence in these regions is accentuated in the presence of gene flow.     

Individual assignment tests proved genetic boundaries in a species complex of Pacific abalone (genus <Emphasis Type="Italic">Haliotis</Emphasis>)

We conducted this study to find genetic evidence to distinguish the members of Pacific abalone species complex (Haliotis discus hannai, H. discus discus, H. madaka, and H. gigantea) based on microsatellite DNA markers, illustrating the potential of microsatellites for species-assignment. First, we addressed the transferability of H. discus hannai microsatellites to the three other members of Pacific abalone and five additional species (H. diversicolor aquatilis, H. midae, H. corrugata, H. fulgens, and H. rubra). Second, using the microsatellites we applied two types of individual assignment testing (the distance-based assignment and Bayesian model-based clustering) to individuals from the Pacific abalone species. A total of 24 microsatellites were subjected to PCR trials for nine Haliotis species, and the cross-species amplification performance of these markers turned out to drop precipitously even for less divergent congeners. Within the Pacific abalone species complex, four of the 24 markers were not transferable to H. gigantea, suggesting a solid genetic boundary between H. gigantea and H. discus hannai, H. discus discus, and H. madaka. Among the three latter abalones, both assignment tests achieved approximately 90% or more success rate of assignment. The feasibility of the microsatellite markers to classify species sheds light on the genetic management of the Pacific abalone species complex. Electronic Supplementary Material  Supplementary material is available in the online version of this article at and is accessible for authorized users.

The genomic bases of morphological divergence and reproductive isolation driven by ecological speciation in Senecio (Asteraceae)

Ecological speciation, driven by adaptation to contrasting environments, provides an attractive opportunity to study the formation of distinct species, and the role of selection and genomic divergence in this process. Here, we focus on a particularly clear‐cut case of ecological speciation to reveal the genomic bases of reproductive isolation and morphological differences between closely related Senecio species, whose recent divergence within the last ~200 000 years was likely driven by the uplift of Mt. Etna (Sicily). These species form a hybrid zone, yet remain morphologically and ecologically distinct, despite active gene exchange. Here, we report a high‐density genetic map of the Senecio genome and map hybrid breakdown to one large and several small quantitative trait loci (QTL). Loci under diversifying selection cluster in three 5 cM regions which are characterized by a significant increase in relative (F_ST), but not absolute (d_XY), interspecific differentiation. They also correspond to some of the regions of greatest marker density, possibly corresponding to ‘cold‐spots’ of recombination, such as centromeres or chromosomal inversions. Morphological QTL for leaf and floral traits overlap these clusters. We also detected three genomic regions with significant transmission ratio distortion (TRD), possibly indicating accumulation of intrinsic genetic incompatibilities between these recently diverged species. One of the TRD regions overlapped with a cluster of high species differentiation, and another overlaps the large QTL for hybrid breakdown, indicating that divergence of these species may have occurred due to a complex interplay of ecological divergence and accumulation of intrinsic genetic incompatibilities.     

Genomic islands of divergence linked to ecotypic variation in sockeye salmon

Regions of the genome displaying elevated differentiation (genomic islands of divergence) are thought to play an important role in local adaptation, especially in populations experiencing high gene flow. However, the characteristics of these islands as well as the functional significance of genes located within them remain largely unknown. Here, we used data from thousands of SNPs aligned to a linkage map to investigate genomic islands of divergence in three ecotypes of sockeye salmon (Oncorhynchus nerka) from a single drainage in southwestern Alaska. We found ten islands displaying high differentiation among ecotypes. Conversely, neutral structure observed throughout the rest of the genome was low and not partitioned by ecotype. One island on linkage group So13 was particularly large and contained six SNPs with F_ST > 0.14 (average F_ST of neutral SNPs = 0.01). Functional annotation revealed that the peak of this island contained a nonsynonymous mutation in a gene involved in growth in other species (TULP4). The islands that we discovered were relatively small (80–402 Kb), loci found in islands did not show reduced levels of diversity, and loci in islands displayed slightly elevated linkage disequilibrium. These attributes suggest that the islands discovered here were likely generated by divergence hitchhiking; however, we cannot rule out the possibility that other mechanisms may have produced them. Our results suggest that islands of divergence serve an important role in local adaptation with gene flow and represent a significant advance towards understanding the genetic basis of ecotypic differentiation.     

Gene flow and selection interact to promote adaptive divergence in regions of low recombination

Adaptation to new environments often occurs in the face of gene flow. Under these conditions, gene flow and recombination can impede adaptation by breaking down linkage disequilibrium between locally adapted alleles. Theory predicts that this decay can be halted or slowed if adaptive alleles are tightly linked in regions of low recombination, potentially favouring divergence and adaptive evolution in these regions over others. Here, we compiled a global genomic data set of over 1,300 individual threespine stickleback from 52 populations and compared the tendency for adaptive alleles to occur in regions of low recombination between populations that diverged with or without gene flow. In support of theory, we found that putatively adaptive alleles (F_ST and d_XY outliers) tend to occur more often in regions of low recombination in populations where divergent selection and gene flow have jointly occurred. This result remained significant when we employed different genomic window sizes, controlled for the effects of mutation rate and gene density, controlled for overall genetic differentiation, varied the genetic map used to estimate recombination and used a continuous (rather than discrete) measure of geographic distance as proxy for gene flow/shared ancestry. We argue that our study provides the first statistical evidence that the interaction of gene flow and selection biases divergence toward regions of low recombination.     

A New Method to Scan Genomes for Introgression in a Secondary Contact Model

Secondary contact between divergent populations or incipient species may result in the exchange and introgression of genomic material. We develop a simple DNA sequence measure, called G _min, which is designed to identify genomic regions experiencing introgression in a secondary contact model. G _min is defined as the ratio of the minimum between-population number of nucleotide differences in a genomic window to the average number of between-population differences. Although it is conceptually simple, one advantage of G _min is that it is computationally inexpensive relative to model-based methods for detecting gene flow and it scales easily to the level of whole-genome analysis. We compare the sensitivity and specificity of G _min to those of the widely used index of population differentiation, F _ST, and suggest a simple statistical test for identifying genomic outliers. Extensive computer simulations demonstrate that G _min has both greater sensitivity and specificity for detecting recent introgression than does F _ST. Furthermore, we find that the sensitivity of G _min is robust with respect to both the population mutation and recombination rates. Finally, a scan of G _min across the X chromosome of Drosophila melanogaster identifies candidate regions of introgression between sub-Saharan African and cosmopolitan populations that were previously missed by other methods. These results show that G _min is a biologically straightforward, yet powerful, alternative to F _ST, as well as to more computationally intensive model-based methods for detecting gene flow.     

SPECIATION IN THE EUROPEAN RABBIT (ORYCTOLAGUS CUNICULUS): ISLANDS OF DIFFERENTIATION ON THE X CHROMOSOME AND AUTOSOMES

Studies of gene flow between closely related taxa can provide insight into the genetic basis of speciation. To evaluate the importance of the X chromosome in reproductive isolation between subspecies of the European rabbit and to study the genomic scale over which islands of differentiation extend, we resequenced a total of 34 loci distributed along the X chromosome and chromosome 14. Previous studies based on few markers suggested that loci in centromeric regions were highly differentiated between rabbit subspecies, whereas loci in telomeric regions were less differentiated. Here, we confirmed this finding but also discovered remarkable variation in levels of differentiation among loci, with F_ST values from nearly 0 to 1. Analyses using isolation‐with‐migration models suggest that this range appears to be largely explained by differential levels of gene flow among loci. The X chromosome was significantly more differentiated than the autosomes. On chromosome 14, differentiation decayed very rapidly at increasing distances from the centromere, but on the X chromosome distinct islands of differentiation encompassing several megabases were observed both at the centromeric region and along the chromosome arms. These findings support the idea that the X chromosome plays an important role in reproductive isolation between rabbit subspecies. These results also demonstrate the mosaic nature of the genome at species boundaries.     

Genomic footprints of speciation in Atlantic eels (Anguilla anguilla and A. rostrata)

The importance of speciation‐with‐geneflow scenarios is increasingly appreciated. However, the specific processes and the resulting genomic footprints of selection are subject to much discussion. We studied the genomics of speciation between the two panmictic, sympatrically spawning sister species; European (Anguilla anguilla) and American eel (A. rostrata). Divergence is assumed to have initiated more than 3 Ma, and although low gene flow still occurs, strong postzygotic barriers are present. Restriction‐site‐associated DNA (RAD) sequencing identified 328 300 SNPs for subsequent analysis. However, despite the presence of 3757 strongly differentiated SNPs (F_ST > 0.8), sliding window analyses of F_ST showed no larger genomic regions (i.e. hundreds of thousands to millions of bases) of elevated differentiation. Overall F_ST was 0.041, and linkage disequilibrium was virtually absent for SNPs separated by more than 1000 bp. We suggest this to reflect a case of genomic hitchhiking, where multiple regions are under directional selection between the species. However, low but biologically significant gene flow and high effective population sizes leading to very low genetic drift preclude accumulation of strong background differentiation. Genes containing candidate SNPs for positive selection showed significant enrichment for gene ontology (GO) terms relating to developmental processes and phosphorylation, which seems consistent with assumptions that differences in larval phase duration and migratory distances underlie speciation. Most SNPs under putative selection were found outside coding regions, lending support to emerging views that noncoding regions may be more functionally important than previously assumed. In total, the results demonstrate the necessity of interpreting genomic footprints of selection in the context of demographic parameters and life‐history features of the studied species.     

Investigating the extent of parallelism in morphological and genomic divergence among lake trout ecotypes in Lake Superior

Understanding the emergence of species through the process of ecological speciation is a central question in evolutionary biology which also has implications for conservation and management. Lake trout (Salvelinus namaycush) is renowned for the occurrence of different ecotypes linked to resource and habitat use throughout North America. We aimed to unravel the fine genetic structure of the four lake trout ecotypes in Lake Superior. A total of 486 individuals from four sites were genotyped at 6822 filtered SNPs using RADseq technology. Our results revealed different extent of morphological and genetic differentiation within the different sites. Overall, genetic differentiation was weak but significant and was on average three times higher between sites (mean F_ST = 0.016) than between ecotypes within sites (mean F_ST = 0.005) indicating higher level of gene flow or a more recent shared ancestor between ecotypes within each site than between populations of the same ecotype. Evidence of divergent selection was also found between ecotypes and/or in association with morphological variation. Outlier loci found in genes related to lipid metabolism and visual acuity were of particular interest in this context of ecotypic divergence. However, we did not find clear indication of parallelism at the genomic level, despite the presence of phenotypic parallelism among some ecotypes from different sampling sites. Overall, the occurrence of different levels of both genomic and phenotypic differentiation between ecotypes within each site with several differentiated loci linked to relevant biological functions supports the presence of a continuum of divergence in lake trout.     

Genomic divergence in sympatry indicates strong reproductive barriers and cryptic species within Eucalyptus salubris

Genetic studies are increasingly detecting cryptic taxa that likely represent a significant component of global biodiversity. However, cryptic taxa are often criticized because they are typically detected serendipitously and may not receive the follow‐up study required to verify their geographic or evolutionary limits. Here, we follow‐up a study of Eucalyptus salubris that unexpectedly detected two divergent lineages but was not sampled sufficiently to make clear interpretations. We undertook comprehensive sampling for an independent genomic analysis (3,605 SNPs) to investigate whether the two purported lineages remain discrete genetic entities or if they intergrade throughout the species’ range. We also assessed morphological and ecological traits, and sequenced chloroplast DNA. SNP results showed strong genome‐wide divergence (F _ST = 0.252) between two discrete lineages: one dominated the north and one the southern regions of the species’ range. Within lineages, gene flow was high, with low differentiation (mean F _ST = 0.056) spanning hundreds of kilometers. In the central region, the lineages were interspersed but maintained their genomic distinctiveness: an indirect demonstration of reproductive isolation. Populations of the southern lineage exhibited significantly lower specific leaf area and occurred on soils with lower phosphorus relative to the northern lineage. Finally, two major chloroplast haplotypes were associated with each lineage but were shared between lineages in the central distribution. Together, these results suggest that these lineages have non‐contemporary origins and that ecotypic adaptive processes strengthened their divergence more recently. We conclude that these lineages warrant taxonomic recognition as separate species and provide fascinating insight into eucalypt speciation.     

Genetic variability,local selection and demographic history: genomic evidence of evolving towards allopatric speciation in Asian seabass

Genomewide analysis of genetic divergence is critically important in understanding the genetic processes of allopatric speciation. We sequenced RAD tags of 131 Asian seabass individuals of six populations from South‐East Asia and Australia/Papua New Guinea. Using 32 433 SNPs, we examined the genetic diversity and patterns of population differentiation across all the populations. We found significant evidence of genetic heterogeneity between South‐East Asian and Australian/Papua New Guinean populations. The Australian/Papua New Guinean populations showed a rather lower level of genetic diversity. F_ST and principal components analysis revealed striking divergence between South‐East Asian and Australian/Papua New Guinean populations. Interestingly, no evidence of contemporary gene flow was observed. The demographic history was further tested based on the folded joint site frequency spectrum. The scenario of ancient migration with historical population size changes was suggested to be the best fit model to explain the genetic divergence of Asian seabass between South‐East Asia and Australia/Papua New Guinea. This scenario also revealed that Australian/Papua New Guinean populations were founded by ancestors from South‐East Asia during mid‐Pleistocene and were completely isolated from the ancestral population after the last glacial retreat. We also detected footprints of local selection, which might be related to differential ecological adaptation. The ancient gene flow was examined and deemed likely insufficient to counteract the genetic differentiation caused by genetic drift. The observed genomic pattern of divergence conflicted with the ‘genomic islands’ scenario. Altogether, Asian seabass have likely been evolving towards allopatric speciation since the split from the ancestral population during mid‐Pleistocene.     

Contrasting patterns of morphological and neutral genetic divergence among geographically proximate populations of sockeye salmon Oncorhynchus nerka in Lake Aleknagik,Alaska

Levels of differentiation in morphological traits (age at maturity, body length at age, egg mass and body depth) and spawning time were examined in sockeye salmon Oncorhynchus nerka from three geographically proximate but physically distinct creeks in Lake Aleknagik, Alaska. Happy Creek fish had significantly greater values for most measured morphological traits, and Eagle Creek fish spawned significantly later than fish in the other creeks. Phenotypic differentiation between creeks, measured as P_ST, was then compared with microsatellite marker differentiation between creeks, measured as F_ST. No correlations were apparent between P_ST and F_ST values, and P_ST values were generally significantly larger than zero (P_ST= 0·0018–0·31) whereas F_ST values were not (F_ST=?0·0004 to 0·0016). The insignificant pair‐wise F_ST values between creek samples indicated that gene flow occurs between creeks, assuming the creek populations have reached migration–drift equilibrium. However, the strong homing behaviour of sockeye salmon precludes a scenario in which fish from the three creeks constitute a single population that segregates by body size. Rather, significant phenotypic differentiation suggests that strong divergent selection occurs on the phenotypic traits despite the homogenizing effects of gene flow.     

Genetic differentiation in Elaeocarpus photiniifolia (Elaeocarpaceae) associated with geographic distribution and habitat variation in the Bonin (Ogasawara) Islands

Gene flow between populations in different environmental conditions can be limited due to divergent natural selection, thus promoting genetic differentiation. Elaeocarpus photiniifolia, an endemic tree species in the Bonin Islands, is distributed in two types of habitats, dry scrubs and mesic forests. We aim to elucidate the genetic differentiation in E. photiniifolia within and between islands and between the habitat types. We investigated genotypes of 639 individuals from 19 populations of E. photiniifolia and its closely-related E. sylvestris at 24 microsatellite loci derived from expressed sequence tags. The data revealed genetic differentiation (1) between E. photiniifolia and E. sylvestris (0.307 ≤ F _ST ≤ 0.470), (2) between the E. photiniifolia populations of the Chichijima and Hahajima Island Groups in the Bonin Islands (0.033 ≤ F _ST ≤ 0.121) and (3) between E. photiniifolia populations associated with dry scrubs and mesic forests in the Chichijima Island Group (0.005 ≤ F _ST ≤ 0.071). Principal coordinate analysis and Bayesian clustering analysis also showed that genetically distinct groups were associated with the habitat types, and isolation by distance was not responsible for the genetic differentiation. These findings suggest that E. photiniifolia is divided into genetically differentiated groups associated with different environmental conditions in the Bonin Islands.     

The joint effects of selection and dominance on the QST - FST contrast

Q_ST measures the differentiation of quantitative traits between populations. It is often compared to F_ST, which measures population differentiation at neutral marker loci due to drift, migration, and mutation. When Q_ST is different from F_ST, it is usually taken as evidence that selection has either restrained or accelerated the differentiation of the quantitative trait relative to neutral markers. However, a number of other factors such as inbreeding, dominance, and epistasis may also affect the Q_ST − F_ST contrast. In this study, we examine the effects of dominance, selection, and inbreeding on Q_ST − F_ST. We compare Q_ST with F_ST at selected and neutral loci for populations at equilibrium between selection, drift, mutation, and migration using both analytic and simulation approaches. Interestingly, when divergent selection is acting on a locus, inbreeding and dominance generally inflate Q_ST relative to F_ST when they are both measured at the quantitative locus at equilibrium. As a consequence, dominance is unlikely to hide the signature of divergent selection on the Q_ST − F_ST contrast. However, although in theory dominance and inbreeding affect the expectation for Q_ST − F_ST, of most concern is the very large variance in both Q_ST and F_ST, suggesting that we should be cautious in attributing small differences between Q_ST and F_ST to selection.     

Geographic Influence on Genetic Structure in the Widespread Neotropical Tree Simarouba amara (Simaroubaceae)

This study aimed to assess the population genetic structure of a widespread Neotropical tree species, Simarouba amara, at local, regional and continental spatial scales. We used five microsatellite loci to examine genetic variation in 14 natural populations (N?=?478 individuals) of this vertebrate-dispersed rain forest tree species in Panama, Ecuador, and French Guiana. Estimates of genetic differentiation (F _st and R _st) were significant among all but one population pair and global differentiation was moderate (F _st?=?0.25, R _st?=?0.33) with 94% of genetic variation ascribed to differences among three main geographic regions (Central America, Western Ecuador, Amazon basin). There was no evidence of isolation by distance within regions. Allele-size mutations contributed significantly (R _ST > F _ST) to the divergences between cis- and trans-Andean populations, highlighting the role of the northern Andean cordilleras as an important geographic barrier for this species.     

Population Genomic Analyses Based on 1 Million SNPs in Commercial Egg Layers

Identifying signatures of selection can provide valuable insight about the genes or genomic regions that are or have been under selective pressure, which can lead to a better understanding of genotype-phenotype relationships. A common strategy for selection signature detection is to compare samples from several populations and search for genomic regions with outstanding genetic differentiation. Wright''s fixation index, F_ST, is a useful index for evaluation of genetic differentiation between populations. The aim of this study was to detect selective signatures between different chicken groups based on SNP-wise F_ST calculation. A total of 96 individuals of three commercial layer breeds and 14 non-commercial fancy breeds were genotyped with three different 600K SNP-chips. After filtering a total of 1 million SNPs were available for F_ST calculation. Averages of F_ST values were calculated for overlapping windows. Comparisons of these were then conducted between commercial egg layers and non-commercial fancy breeds, as well as between white egg layers and brown egg layers. Comparing non-commercial and commercial breeds resulted in the detection of 630 selective signatures, while 656 selective signatures were detected in the comparison between the commercial egg-layer breeds. Annotation of selection signature regions revealed various genes corresponding to productions traits, for which layer breeds were selected. Among them were NCOA1, SREBF2 and RALGAPA1 associated with reproductive traits, broodiness and egg production. Furthermore, several of the detected genes were associated with growth and carcass traits, including POMC, PRKAB2, SPP1, IGF2, CAPN1, TGFb2 and IGFBP2. Our approach demonstrates that including different populations with a specific breeding history can provide a unique opportunity for a better understanding of farm animal selection.