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Parallel genetic adaptation amid a background of changing effective population sizes in divergent yellow perch (Perca flavescens) populations.

Aquatic ecosystems are highly dynamic environments vulnerable to natural and anthropogenic disturbances. High-economic-value fisheries are one of many ecosystem services affected by these disturbances, and it is critical to accurately characterize the genetic diversity and effective population sizes of valuable fish stocks through time. We used genome-wide data to reconstruct the demographic histories of economically important yellow perch (Perca flavescens) populations. In two isolated and genetically divergent populations, we provide independent evidence for simultaneous increases in effective population sizes over both historic and contemporary time scales including negative genome-wide estimates of Tajima's D, 3.1 times more single nucleotide polymorphisms than adjacent populations, and contemporary effective population sizes that have increased 10- and 47-fold from their minimum, respectively. The excess of segregating sites and negative Tajima's D values probably arose from mutations accompanying historic population expansions with insufficient time for purifying selection, whereas linkage disequilibrium-based estimates of Ne also suggest contemporary increases that may have been driven by reduced fishing pressure or environmental remediation. We also identified parallel, genetic adaptation to reduced visual clarity in the same two habitats. These results suggest that the synchrony of key ecological and evolutionary processes can drive parallel demographic and evolutionary trajectories across independent populations.

Animals

Seasonal fluctuations in fitness result in severe reductions in effective population size.

Genetic evidence for fluctuating selection has begun to accumulate for different species over the past few decades, especially for the Drosophila genus where studies have reported hundreds of loci undergoing putatively adaptive oscillations across successive seasons. However, most theoretical and simulation studies of fluctuating selection have relied on abstract or weakly parameterized models, making it difficult to assess their relevance for natural populations. In this study, we simulate multilocus seasonally fluctuating selection under a recently developed model and examine its effect on the variance effective population size (Ne ) at a genome-wide scale. By recapitulating genomic, demographic, and evolutionary parameters from natural Drosophila populations in our simulations, we were able to reproduce allele frequency oscillations reported in recent studies and show that these lead to ~50% genome-wide reductions in Ne . We also demonstrate that Ne reductions are well predicted by the maximum frequency amplitude among all adaptively fluctuating loci, and that the frequency amplitudes are largely determined by the number of adaptively fluctuating loci and the strength of their epistatic interactions. Our results demonstrate that fluctuating selection can substantially reduce effective population size and underscore the importance of temporally variable selection in shaping genome-wide patterns of variation beyond classical models.

Drosophila melanogaster

Seasonal fluctuations in fitness result in severe reductions in effective population size.

Genetic evidence for fluctuating selection has begun to accumulate for different species over the past few decades, especially for the Drosophila genus where studies have reported hundreds of loci undergoing putatively adaptive oscillations across successive seasons. However, most theoretical and simulation studies of fluctuating selection have relied on abstract or weakly parameterized models, making it difficult to assess their relevance for natural populations. In this study, we simulate multilocus seasonally fluctuating selection acting on standing genetic variation under a recently developed model and examine its effect on the variance effective population size (Ne) at a genome-wide scale. By recapitulating genomic, demographic, and evolutionary parameters from natural Drosophila populations in our simulations, we were able to reproduce allele frequency oscillations reported in recent studies and show that these lead to ∼50% genome-wide reductions in Ne. We also demonstrate that Ne reductions are well predicted by the maximum frequency amplitude among all adaptively fluctuating loci, and that the frequency amplitudes are largely determined by the number of adaptively fluctuating loci and the strength of their epistatic interactions. Our results demonstrate that fluctuating selection can substantially reduce effective population size and underscore the importance of temporally variable selection in shaping genome-wide patterns of variation beyond classical models.

Drosophila melanogaster

Long-term small effective population size, inbreeding, and a recessive lethal haplotype drive premature death in the endangered Devils Hole pupfish (Cyprinodon diabolis).

As anthropogenic habitat fragmentation and population decline accelerate globally, growing numbers of species face compounding demographic and genetic threats to long-term survival. Many populations are already forced to persist at chronically small sizes, yet the genomic and fitness consequences of this fate remain poorly understood. Here we leverage the demographic history of the Devils Hole pupfish to investigate how long-term small population size and recent bottlenecks have shaped genetic diversity, genetic load, inbreeding, and fitness through comparative population genomics, historical sequencing, and sampling embryos that died prematurely during development. We find that genetic diversity in Devils Hole pupfish is among the lowest recorded in the wild and that fixed load is high, consistent with thousands of generations of isolation at small population size. Even in the face of this low diversity and high fixed load, we show that inbreeding is still strongly associated with premature embryonic death, which affects up to 25% of offspring in the captive refuge and can be identified in advance based on a characteristic elongated heart tube and reduced heart rate. We discovered a recessive lethal haplotype segregating at ~20% frequency that accounts for 50% of embryonic deaths and contains mutations in MIB1 and MMP16, genes associated with cardiomyopathy and atrial fibrillation. Our findings link genotype, phenotype, and fitness in an iconic endangered species to provide a rare comprehensive view into the evolutionary dynamics and consequences of long-term small effective population size, demonstrating that endangered species remain vulnerable to inbreeding depression despite extremely low genetic diversity.

Journal Article

Bacterial lifestyle shapes pangenomes.

Pangenomes vary across bacteria. Some species have fluid pangenomes, with a high proportion of genes varying between individual genomes. Other species have less fluid pangenomes, with different genomes tending to contain the same genes. Two main hypotheses have been suggested to explain this variation: differences in species' bacterial lifestyle and effective population size. However, previous studies have not been able to test between these hypotheses because the different features of lifestyle and effective population size are highly correlated with each other, and phylogenetically conserved, making it hard to disentangle their relative importance. We used phylogeny-based analyses, across 126 bacterial species, to tease apart the causal role of different factors. We found that pangenome fluidity was lower in i) host-associated compared with free-living species and ii) host-associated species that are obligately dependent on a host, live inside cells, and are more pathogenic and less motile. In contrast, we found no support for the competing hypothesis that larger effective population sizes lead to more fluid pangenomes. Effective population size appears to correlate with pangenome variation because it is also driven by bacterial lifestyle, rather than because of a causal relationship.

Genome, Bacterial

Contrasting Genomic Responses of Hydrothermal Vent Animals and Their Symbionts to Population Decline After the Hunga Volcanic Eruption.

Genetic bottlenecks are evolutionary events that reduce the effective size and diversity of natural populations, often limiting a population's ability to adapt to environmental change. Given the accelerating human impact on ecosystems worldwide, understanding how populations evolve after a genetic bottleneck is becoming increasingly important for species conservation. Ash deposits from the 2022 Hunga volcanic eruption in the Southwest Pacific led to a drastic decline of animal symbioses associated with hydrothermal vents in this region, allowing insights into the effects of population bottlenecks in the deep sea. Here, we applied metagenomic sequencing to pre- and post-eruption samples of mollusc-microbial symbioses from the Lau Basin to investigate patterns of genetic variation and effective population size. Our data indicate that animal host populations currently show only small changes in genome-wide diversity but in most cases experienced a long-term decline in effective size that was likely intensified by the volcanic impact. By contrast, host-associated symbiont populations exhibited a notable decrease in genomic variation, including potential loss of certain habitat-specific strains. However, detection of environmental sequences resembling mollusc symbionts suggests that lost host-associated symbiont diversity might be recovered from the free-living symbiont pool. The differences between host and symbiont populations might be related to their contrasting genetic structures and pre-existing levels of connectivity, although the full extent of population bottlenecks in the host animals might only be recognisable after a few generations. These results add to our understanding of the evolutionary dynamics of animal-microbe populations following a natural disturbance and help assess their resilience to both natural and anthropogenic impacts.

Animals

Mutation rates, population sizes and amounts of electrophoretic variation of enzyme loci in natural populations.

A method is presented for estimating relative mutation rates or relative effective population sizes, under the hypothesis of adaptively neutral allelic variation. This method was applied to seven surveys of electrophoretic variation. It was observed that electrophoretic mutation rates so obtained follow the gamma distribution and, in Drosophila, are positively correlated with the molecular weights of the enzymes subunits. The variance in mutation rate is larger under the step-wise model of electrophoretic mutation than under the infinite-alleles model. Rates for the most variable loci may exceed rates for less variable loci by a factor of 500. For completely invariant loci, this factor may be as high as 4 X 10(4), an observation suggesting that these loci are subject to purifying selection. In contrast to mutation rates, effective population sizes may vary at the most by a factor of ten. These results support the hypothesis that differences in the amount of electrophoretic variability among polymorphic loci may reflect differences in the rate by which electrophoretically detectable variation is generated in population.

Animals

Genetic structure and selection signatures of Beijing-You chicken populations provide insight into breed conservation.

Preserving genetic diversity and maintaining population viability are critical yet challenging goals that demand rigorous evaluation of conservation strategies. Beijing-You chicken, as the sole indigenous chicken breed originating from Beijing, China, is currently maintained as four independent populations under distinct conservation programs. How different conservation regimes have shaped its genomic architecture remains largely unknown, limiting evidence-based evaluation. Here, we generated whole-genome resequencing data from 240 individuals representing four Beijing-You chicken populations to assess population structure, genetic diversity, and signatures of selection over decades of conservation. All four populations formed distinct clusters, reflecting measurable differentiation after decades of separate conservation. The differences in genetic diversity were broadly consistent with the variation in effective population size estimates. Runs of homozygosity and linkage disequilibrium decay patterns further characterized each population, with extended values indicating reduced effective population size and increased inbreeding under long-term conservation. We applied the fixation index (FST) and pairwise diversity ratio (θπ) methods to identify selection signatures. A total of 171 genes were identified as candidates. These genes were enriched in pathways related to reproduction, growth regulation, and environmental adaptation. These findings highlight patterns of reduced diversity and skewed relatedness, which could arise from management-related factors such as breeding preferences or mating strategies. Still, they are also compatible with neutral processes, including drift and founder effects. Regardless of the underlying cause, integrating scientifically informed conservation strategies with routine genomic monitoring across generations is essential for sustaining genetic diversity in Beijing-You chicken and other indigenous breeds.

Beijing-You chicken

Genetic Diversity Analysis of Red Fox Populations (Vulpes vulpes L., 1758) in Natural and Anthropogenic Isolation.

This study presents a comparative analysis of the genetic structure and diversity of three red fox (Vulpes vulpes L.) populations representing different microevolutionary scenarios: panmixia (free-ranging Belarusian foxes), geographic isolation (free-ranging Scottish foxes), and anthropogenic selection (farm-bred foxes). Using a validated set of STR markers, multivariate statistical analysis was conducted to assess the genetic structure and the degree of genetic erosion across the studied groups. The wild red fox population in Belarus has been shown to maintain a state close to panmixia (PHWE = 0.090), characterized by a high effective population size (Ne = 694) and high allelic diversity. The island population from Scotland exhibits moderate gene pool depletion (Ne = 75.9) and a pronounced heterozygote deficiency (FIS = 0.18). Critical genetic erosion, which was characterized by a minimal effective population size (Ne = 60.2) and allelic fixation, was detected in the farm-bred group. The genetic distance between farm-bred and wild foxes (FST = 0.279; p = 0.001) reflects both the phylogeographic divergence between the Nearctic ancestors of farmed lineages and Palearctic wild populations, and the consequences of prolonged anthropogenic isolation, genetic drift, and selective breeding. These data indicate that artificial isolation and the impacts of genetic drift and targeted selection lead to a substantial depletion of the species' adaptive potential.

Animals

Stepwise mutation model and distribution of allelic frequencies in a finite population.

A mathematical theory is developed that enables us to derive a formula for the equilibrium distribution of allelic frequencies in a finite population when selectively neutral alleles are produced in stepwise fashion (stepwise mutation model). It is shown that the stepwise mutation model has a remarkable property that distinguishes it from the conventional infinite allele model (Kimura-Crow model): as the population size increases indefinitely while the product of the effective population size and the mutation rate is kept at a fixed value, the mean number of different alleles contained in the population rapidly reaches a plateau which is not much larger than the effective number of alleles (reciprocal of homozygosity).

Alleles

The distribution of fitness effects of nonsynonymous mutations varies phylogenetically across animals.

The distribution of fitness effects (DFE) describes the selection coefficients of newly arising mutations and fundamentally influences population genetic processes. However, the extent and mechanisms of differences in the DFE for non-synonymous mutations have not been systematically investigated across species with divergent phylogenetic histories and ecologies. Here, we inferred the DFE in natural populations of 11 animal (sub)species, including humans, mice, fin whales, vaquitas, wolves, collared flycatchers, pied flycatchers, halictid bees, Drosophila, and mosquitoes. We found that mammals have a higher proportion of strongly deleterious mutations (defined as s≤-0.01; 22% to 47% in mammals; 0.0% to 5.4% in insects and birds) and a lower proportion of weakly deleterious mutations than insects and birds. Further, the DFE co-varies with phylogeny, such that the mean mutation effects are more similar in closely related species (Pagel's λ = 0.84, P = 0.01). Next, we investigated whether various summary statistics of the DFE were related to variation in life-history traits across these organisms. We found some support for genome size, body mass, and long-term effective population size being correlated with the DFE. Overall, our findings are consistent with predictions derived independently from the Fisher's Geometric Model (FGM), which defines organismal complexity as the number of phenotypes under selection. FGM predicts that mutations are more deleterious in complex organisms, while strongly deleterious mutations occur more frequently in smaller populations. Our study demonstrates strong phylogenetic signal in the evolution of a fundamental population genetics parameter, and proposes that, through mechanisms of epistasis, long-term population size and organismal complexity could be underlying variation in the DFE across animals.

Journal Article

Transition from Conventional to Genomic Selection (ssGBLUP) led to improve in accuracy gains and selection decisions in Sahiwal Cattle.

By using genome-wide markers to predict an individual's genetic potential, the introduction of Genomic Selection (GS) has transformed animal breeding. This greatly accelerated selection for complex traits by lowering reliance on drawn-out field trials, allowing for faster genetic gains in livestock. However, there is little research on the effects of genomic selection on Sahiwal cattle in India, and comparing it to the current culling or selection process is even more uncommon, particularly in nations with fewer genotyped animals. This study is an initial effort to address the aforementioned gaps in knowledge. Genomic selection was implemented in Sahiwal cattle for the 305 days milk yield using univariate animal model and the single-step Genomic Best Linear Unbiased Prediction (ssGBLUP) method. The Effective Population size (Ne) of the Sahiwal herd was calculated using genomic data and was reported for the previous generation to be 71.927. The heritability of 305 days milk yield was estimated as 0.177 ± 0.068. Genomic estimated breeding values (GEBVs) were predicted for each individual using ssGBLUP, yielding a mean prediction accuracy of 43.11%, compared with 40.88% obtained using conventional pedigree-based BLUP. Cross-validation further demonstrated superior predictive performance of ssGBLUP, with accuracies of 76.82% and 70.50% for ssGBLUP and PBLUP, respectively. To further check the effectiveness of the genomic selection methodology, we also compared the GEBVs obtained and compared it with the Expected Progeny Difference (EPD) which is being applied in our farm for culling decisions. It was seen that GEBVs obtained from ssGBLUP methodology were also in line with the conventionally used method of EPD. The use of genomic selection enables genetic studies with limited pedigree information. Additionally, the ssGBLUP methodology allows to check for pedigree errors, where family relationships are incorrectly recorded. The EPD and GEBVs were consistent with one another, indicating that genomic selection may also be utilised to support culling and selection decisions in a farm. Thus, in a conventional animal breeding program with constraint resources and an incomplete pedigree, we recommend employing the ssGBLUP model for regular genomic assessment and identification of suitable candidates to effectively carry out a genomic selection program.

Animals

Genomic diversity and selection signatures in Asian Zebu Cattle: insights into adaptation and genetic erosion.

Indigenous cattle breeds in Asia are highly adapted to their local environments providing essential commodities such as meat, milk and draught power while also playing a key role in traditional ceremonies, and sports. Despite ongoing efforts to characterize and conserve these breeds, the increasing trend of indiscriminate crossbreeding of Zebu cattle with high-yielding taurine breeds, threatens their genetic diversity. This study investigates the population structure, inbreeding levels, effective population size, gene flow and identification of selection footprints of Asian Zebu (Bos indicus) cattle. Using an Axiom 60 K SNP chip, we analyzed genotypes from 1303 cattle across 36 populations in nine countries, including seven taurine outgroups and 29 Zebu populations from Bangladesh, Cambodia, India, Myanmar, Pakistan, and Sri Lanka. Zebu populations demonstrated moderate genetic diversity, with heterozygosity levels averaging 0.356, inbreeding coefficients ranging from 0.026 to 0.074 and genetic differentiation (FST) varied between 0.01 and 0.11. Breed clusters aligned closely with their geographic locations except for Achai (Pakistan) and Baru Harak (Sri Lanka) breeds that appeared in both Zebu and taurine clusters indicating evidence of taurine admixture. Genomic analyses identified regions under selection using extended haplotype homozygosity (EHH) and fixation index (FST) methods. Candidate genes associated with key biological functions related to environmental responsiveness, including heat tolerance (HSP90AA1), immunity (RIPK3), metabolism and fertility (REC8, CLIC4, TSSK4), were identified, reflecting adaptive traits critical for Zebu survival and utility across diverse environments. These findings provide valuable insights for conservation and management strategies aimed at preserving the unique genetic diversity of Asian Bos indicus breeds.

Animals

Pleistocene island connectivity did not enhance dispersal or impact population size change in Galápagos geckos.

Patterns of biodiversity on remote archipelagos are largely shaped by intra-archipelago colonization followed by in situ diversification. Pleistocene sea-level fluctuations purportedly enhanced gene flow among terrestrial organisms by increasing connectivity during periods of lower sea level. Furthermore, changes in sea-level are hypothesized to impact population sizes as a result of fluctuations in island sizes. Here, we used genomic data to test the role of Pleistocene island connectivity on the diversification and demographics of leaf-toed geckos (Phyllodactylus) endemic to the Galápagos. Consistent with previous studies, we found that present diversity of Galápagos Phyllodactylus stems from three independent dispersal events. Contrary to the hypothesis of Pleistocene-driven diversification, we found no correspondence between lineage divergence and island connectivity. Furthermore, we found no evidence of introgression; demographic modelling indicated that all species increased rapidly in effective population size in the period 20-150 ka, and these inferred demographic expansions were largely asynchronous and apparently unassociated with species or island age. Collectively, these results indicate that more complex abiotic and/or biotic factors may better explain the recent demographic history of Phyllodactylus and underscore the need for additional population genomic studies of terrestrial taxa to understand the impact of past climate cycles on Galápagos island communities.

Animals

Temporal reconstruction of a Salmonella Enteritidis ST11 outbreak in New Zealand.

Outbreaks caused by Salmonella Enteritidis are commonly linked to eggs and poultry meat internationally, but this serovar had never been detected in Aotearoa New Zealand (NZ) poultry prior to 2021. Locally designated genomic cluster Salmonella Enteritidis_2019_C_01, was implicated in a 2019 outbreak associated with a restaurant in Auckland. Four Enteritidis_2019_C_01 sub-clusters have since been identified, two retrospectively, in the Auckland region. Authorities initiated a formal outbreak investigation after genomically indistinguishable S. Enteritidis was isolated from the NZ poultry production environment. This study analysed 231 S. Enteritidis genomes obtained from the outbreak using Bayesian phylodynamic tools to gain insight into the outbreak's dynamics and origin. We used Bayesian integrated coalescent epoch plots to estimate the change of the Enteritidis ST11 population size over time and marginal structured coalescent approximation to estimate transmission between poultry producers. We investigated human and poultry isolates to elucidate the time and location of the most recent common ancestor of the outbreak and transmission pathways. The median most recent common ancestor was estimated to be February 2019. We found evidence of amplification and spread of strain Enteritidis_2019_C_01 within the poultry industry, as well as transmission events throughout the production chain. The intervention by the public health and food safety authorities coincided with a drop in the effective population size of the S. Enteritidis ST11 as well as notified human cases. This information is crucial for understanding and preventing the transmission of S. Enteritidis in NZ poultry to ensure poultry meat and eggs are safe for consumption.

Salmonella enteritidis

Bayesian Inference of Pathogen Phylogeography using the Structured Coalescent Model.

Over the past decade, pathogen genome sequencing has become well established as a powerful approach to study infectious disease epidemiology. In particular, when multiple genomes are available from several geographical locations, comparing them is informative about the relative size of the local pathogen populations as well as past migration rates and events between locations. The structured coalescent model has a long history of being used as the underlying process for such phylogeographic analysis. However, the computational cost of using this model does not scale well to the large number of genomes frequently analysed in pathogen genomic epidemiology studies. Several approximations of the structured coalescent model have been proposed, but their effects are difficult to predict. Here we show how the exact structured coalescent model can be used to analyse a precomputed dated phylogeny, in order to perform Bayesian inference on the past migration history, the effective population sizes in each location, and the directed migration rates from any location to another. We describe an efficient reversible jump Markov Chain Monte Carlo scheme which is implemented in a new R package StructCoalescent. We use simulations to demonstrate the scalability and correctness of our method and to compare it with existing software. We also applied our new method to several state-of-the-art datasets on the population structure of real pathogens to showcase the relevance of our method to current data scales and research questions.

Bayes Theorem

Repeated evolution on oceanic islands: comparative genomics reveals species-specific processes in birds.

Understanding the interplay between genetic drift, natural selection, gene flow, and demographic history in driving phenotypic and genomic differentiation of insular populations can help us gain insight into the speciation process. Comparing patterns across different insular taxa subjected to similar selective pressures upon colonizing oceanic islands provides the opportunity to study repeated evolution and identify shared patterns in their genomic landscapes of differentiation. We selected four species of passerine birds (Common Chaffinch Fringilla coelebs/canariensis, Red-billed Chough Pyrrhocorax pyrrhocorax, House Finch  Haemorhous mexicanus and Dark-eyed/island Junco Junco hyemalis/insularis) that have both mainland and insular populations. Changes in body size between island and mainland populations were consistent with the island rule. For each species, we sequenced whole genomes from mainland and insular individuals to infer their demographic history, characterize their genomic differentiation, and identify the factors shaping them. We estimated the relative (Fst) and absolute (dxy) differentiation, nucleotide diversity (π), Tajima's D, gene density and recombination rate. We also searched for selective sweeps and chromosomal inversions along the genome. All species shared a marked reduction in effective population size (Ne) upon island colonization. We found diverse patterns of differentiated genomic regions relative to the genome average in all four species, suggesting the role of selection in island-mainland differentiation, yet the lack of congruence in the location of these regions indicates that each species evolved differently in insular environments. Our results suggest that the genomic mechanisms involved in the divergence upon island colonization-such as chromosomal inversions, and historical factors like recurrent selection-differ in each species, despite the highly conserved structure of avian genomes and the similar selective factors involved. These differences are likely influenced by factors such as genetic drift, the polygenic nature of fitness traits and the action of case-specific selective pressures.

Animals

Larval Genomics as a Viable, Fisheries-Independent Tool for Investigating Population Structure in Tropical Pacific Tunas.

Understanding how dispersal, life history, and environmental variability shape genetic connectivity in the open ocean remains a central challenge in evolutionary biology. Highly migratory marine predators like tunas have traditionally been considered genetically homogeneous across ocean basins, yet emerging genomic evidence suggests that cryptic population structure can persist even in species with high gene flow and large effective population sizes. We used 2bRAD sequencing of 348 larval and subadult skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares), and bigeye tuna (T. obesus) collected from the central Pacific across 7 years of sampling to examine species boundaries, population genetic information, genetic structure, and connectivity. Larval sampling revealed consistent spawning by all three species and enabled unbiased detection of genetic patterns prior to recruitment bottlenecks. We found strong divergence amongst species, no evidence of structuring within skipjack or bigeye, and a divergent yellowfin population detected in 2 consecutive sampling years north of American Samoa. Comparisons between larvae and subadults suggest that sampling early life history stages can be a valuable tool for assessing population genetic information before recruitment bottlenecks, selective harvest by fisheries, adult dispersal, and selective pressures acting on adult populations, thereby contributing novel insights to the research and effective management of these species. These results highlight how larval genomics can complement traditional population genomic studies of adult tunas and reveal fine-scale structure in highly vagile species, providing new perspectives on connectivity in the open ocean.

Animals