The study of microbiomes across organisms and environments has become a prominent focus in molecular ecology. This perspective article explores methodological advancements, common challenges and future directions in the field. Key research areas include understanding the drivers of microbiome community assembly, linking microbiome composition to host genetics, exploring microbial functions, transience, and spatial partitioning, and disentangling non-bacterial components of the microbiome. Methodological advancements, such as quantifying absolute abundances, sequencing complete genomes, and utilizing novel statistical approaches, are also useful tools for understanding complex microbial diversity patterns. Our aims are to encourage robust practices in microbiome studies and inspire researchers to explore the next frontier of this rapidly changing field.
Introductions of invasive species to new environments often result in rapid rates of trait evolution. While in some cases these evolutionary transitions are adaptive and driven by natural selection, they can also result from patterns of genetic and phenotypic variation associated with the invasion history. Here, we examined the brown anole (Anolis sagrei), a widespread invasive lizard for which genetic data have helped trace the sources of non-native populations. We focused on the dewlap, a complex signaling trait known to be subject to multiple selective pressures. We measured dewlap reflectance, pattern, and size in 30 non-native populations across the southeastern United States. As well, we quantified environmental variables known to influence dewlap signal effectiveness, such as canopy openness. Further, we used genome-wide data to estimate genetic ancestry, perform association mapping, and test for signatures of selection. We found that among-population variation in dewlap characteristics was best explained by genetic ancestry. This result was supported by genome-wide association mapping, which identified several ancestry-specific loci associated with dewlap traits. Despite the strong imprint of this aspect of the invasion history on dewlap variation, we also detected significant relationships between dewlap traits and local environmental conditions. However, we found limited evidence that dewlap-associated genetic variants have been subject to selection. Our study emphasizes the importance of genetic ancestry and admixture in shaping phenotypes during biological invasion, while leaving the role of selection unresolved, likely due to the polygenic genetic architecture of dewlaps and selection acting on many genes of small effect.
Numerous mechanisms drive ecological speciation, including isolation by adaptation, barrier, distance, environment, hierarchy, and resistance. These promote genetic and phenotypic differentiation of local populations, formation of phylogeographic lineages, and ultimately, completed speciation via reinforcement. In contrast, it is possible that similar mechanisms might lead to lineage cohesion through stabilizing rather than diversifying ecomorphological selection and the long-term persistence of population structure within species. Processes that drive the formation and maintenance of geographic genetic diversity while facilitating high rates of migration and limiting phenotypic divergence may thereby result in population structure that is not accompanied by divergence towards reproductive isolation. We suggest that this framework can be applied more broadly to address the classic dilemma of “structure versus speciation” when evaluating phylogeographic diversity, unifying population genetics, species delimitation, and the underlying study of speciation. We demonstrate one such instance in the Seepage Salamander (Desmognathus aeneus) from the southeastern United States. Recent studies estimated up to 6.3% mitochondrial divergence and 4 phylogenomic lineages with broad admixture across geographic hybrid zones, which could potentially represent distinct species. However, while limited dispersal promotes substantial isolation by distance, extreme microhabitat specificity appears to yield stabilizing selection on ecologically mediated phenotypes. As a result, climatic cycles promote recurrent contact between lineages that are not adaptively differentiated and therefore experience repeated bouts of high migration and introgression through time. This leads to a unified, single species with deeply divergent phylogeographic lineages that nonetheless do not appear to represent incipient species.
Identifying genetic conservation units (CUs) in threatened species is critical for the preservation of adaptive capacity and evolutionary potential in the face of climate change. However, delineating CUs in highly mobile species remains a challenge due to high rates of gene flow and genetic signatures of isolation by distance. Even when CUs are delineated in highly mobile species, the CUs often lack key biological information about what populations have the most conservation need to guide management decisions. Here we implement a framework for rigorous CU identification in the Canada Warbler (Cardellina canadensis), a highly mobile migratory bird species of conservation concern, and then integrate demographic modeling and genomic offset within a CU framework to guide conservation decisions. We find that whole-genome structure in this highly mobile species is primarily driven by putative adaptive variation. Identification of CUs across the breeding range revealed that Canada Warblers fall into two Evolutionarily Significant Units (ESU), and three putative Adaptive Units (AUs) in the South, East and Northwest. Quantification of genomic offset within each AU reveals significant spatial variation in climate vulnerability, with the Northwestern AU being identified as the most vulnerable to future climate change based on genomic offset predictions. Alternatively, quantification of past population trends within each AU revealed the steepest population declines have occurred within the Eastern AU. Overall, we illustrate that genomics-informed CUs provide a strong foundation for identifying current and potential future regional threats that can be used to manage highly mobile species in a rapidly changing world.
Invasive species are predicted to adjust their morphological, physiological, and life-history traits to adapt to their non-native environments. Although a loss of genetic variation during invasion may restrict local adaptation, introduced species often thrive in novel environments. Despite being founded by just a few individuals, the bumblebee Bombus terrestris (Hymenoptera: Apidae) has successfully spread across the island of Tasmania (Australia) in less than 30 years, becoming abundant and competitive with native pollinators. We use RADseq to investigate the neutral and adaptive genetic processes associated with environmental and morphological variation following the invasion of B. terrestris in Tasmania. Across 15 sites, we found high gene flow with low genetic diversity, significant isolation-by-distance, and spatial variation in effective migration rates. A longitudinal band of restricted migration was evident across the mid-central region of Tasmania, corresponding to sites with high elevation, pastural land, low wind speeds and low precipitation seasonality. Tajima’s D indicated a recent population expansion for central sites extending from the south to the north of the island. Significant selection signatures were found for loci in relation to precipitation, wind speed, and wing loading. Candidate loci were annotated to genes with functions related to cuticle water retention, and insect flight muscle stability. Understanding how a genetically impoverished invasive bumblebee has rapidly adapted to a novel island environment provides further understanding about the evolutionary processes that determine successful insect invasions, and the potential for invasive hymenopteran pollinators to spread globally.
While the role of selection in divergence along the speciation continuum is theoretically well understood, defining specific signatures of selection in the genomic landscape of divergence is empirically challenging. Modelling approaches can provide insight into the potential role of selection on the emergence of a heterogenous genomic landscape of divergence. Here, we extend and apply an individual-based approach that simulates the phenotypic and genotypic distributions of two populations under a variety of selection regimes, genotype-phenotype maps, modes of migration, and genotype-environment interactions. We show that genomic islands of high differentiation and genomic valleys of similarity may respectively form under divergent and parallel selection between populations. For both types of between-population selection, negative and positive frequency-dependent selection within populations generated genomic islands of higher magnitude and genomic valleys of similarity respectively. Divergence rates decreased under strong dominance with divergent selection, as well as in models including genotype-environment interactions under parallel selection. For both divergent and parallel selection models, divergence rate was higher under an intermittent migration regime between populations, in contrast to a constant level of migration across generations, despite an equal number of total migrants. We highlight that interpreting a particular evolutionary history from an observed genomic pattern must be done cautiously, as similar patterns may be obtained from different combinations of evolutionary processes. Modelling approaches such as ours provide an opportunity to narrow the potential routes that generate the genomic patterns of specific evolutionary histories.
The Astyanax mexicanus complex include two different morphs, a surface and a cave adapted ecotype, found at three mountain ranges in Northeastern Mexico: Sierra de El Abra, Sierra de Guatemala, and Sierra de la Colmena (Micos). Since their discovery, multiple studies have attempted to characterize the timing and the number of events that gave rise to the evolution of these cave-adapted ecotypes. Here, using RAD-seq and genome-wide sequencing, we assessed the phylogenetic relationships, genetic structure, and gene flow events between the cave and surface Astyanax mexicanus populations, to estimate the time and mode of evolution of the cave-adapted ecotypes. We also evaluated the body shape evolution across different cave lineages using geometric morphometrics to examine the role of phylogenetic signal vs. environmental pressures. We found strong evidence of parallel evolution of cave-adapted ecotypes derived from two separate lineages of surface fish and hypothesize that there may be up to four independent invasions of caves from surface fish. Moreover, a strong congruence between the genetic structure and geographic distribution was observed across the cave populations, with the Sierra de Guatemala the region exhibiting most genetic drift among the cave populations analyzed. Interestingly, we found no evidence of phylogenetic signal in body shape evolution, but we found support for parallel evolution in body shape across independent cave lineages, with cavefish from the Sierra de El Abra reflected the most divergent morphology relative to surface and other cavefish populations.
Identifying and understanding patterns of biological diversity is crucial at a time when even the most remote and pristine marine ecosystems are threatened by resource exploitation such as deep-seabed mining. Metabarcoding provides the means through which to perform comprehensive investigations of diversity by examining entire assemblages simultaneously. Nematodes commonly represent the most abundant infaunal metazoan group in marine soft sediments. In this meta-analysis, we compiled all publicly available metabarcoding datasets targeting the 18S rRNA v1-v2 region from sediment samples to conduct a global-scale examination of nematode Amplicon Sequence Variant (ASV) alpha diversity patterns, evolutionary distinctiveness (ED) and phylogenetic community structure at different depths and environments. We found that nematode ASV richness followed a parabolic trend, increasing from the intertidal to the shelf, reaching a maximum in the bathyal and decreasing in the abyssal zone. No depth- or environment-specific assemblages were identified as a large fraction of genera were shared. Contrastingly, the vast majority of ASVs were unique to each environment and/or depth zone; genetic diversity was thus highly localised. The abyssal samples had the highest ED values, indicating that a relatively stable yet very old environment can produce highly diversified assemblages. Overwhelmingly, nematode ASVs in all environments exhibited phylogenetic clustering, pointing to environmental filtering as the primary force defining community assembly rather than competitive interactions. This finding stresses the importance of habitat preservation for the maintenance of marine nematode diversity.
The origin of new genes has long been a central interest of evolutionary biologists. However, their novelty evades reconstruction by the classical tools of evolutionary modeling. This evasion of insight from deep ancestral investigation necessitates intensive study of model species within well-sampled, recently diversified, clades. One such clade is the model genus Neurospora, members of which lack recent gene duplications, yet harbor clusters of lineage-specific genes (LSGs) adjacent to the telomeres. Several Neurospora species are comprehensively characterized organisms apt for studying the evolution of LSGs. Using gene synteny, we documented that 78% of Neurospora LSGs clusters are located in chromosomal regions featuring extensive tracts of non-coding DNA and duplicated genes. Here we report several instances of LSGs that are likely from regional rearrangements and potentially from gene rebirth. To broadly investigate functions of LSGs, we assembled transcriptomics data from 68 experimental data points and identified co-regulatory modules using Weighted Gene Correlation Network Analysis, revealing that LSGs are widely but peripherally involved in known regulatory machinery for diverse functions. The ancestral status of mas-1 and its neighbors was investigated in detail, suggesting that it arose from an ancient lysophospholipase precursor that is ubiquitous in lineages of the Sordariomycetes; mas-1 plays a role in cell-wall integrity and cellular sensitivity to antifungal toxins. Our discoveries illuminate a “rummage region” in the N. crassa genome that enables formation of new genes and functions to arise via gene duplication and relocation, followed by fast mutation and recombination facilitated by tandem repeats and deconstrained non-coding sequences.
Untangling the responses of protistan communities associated with soil and plant compartments and their associations with bacterial and fungal communities to pathogen invasion are critical for understanding the ecological processes governing plant microbiome assembly. Here we examined the protistan communities across the soil–plant continuum of healthy chili peppers and those with Fusarium wilt disease (FWD) and integrated the bacterial and fungal microbiome data from our previous investigation in China. We found that FWD was associated with a significant enrichment of phagotrophic protists in roots and an increase in the proportion and connectivity of these phagotrophic protists in intra- and interkingdom networks. Specifically, FWD increased the negative correlations between phagotrophic protists (especially Cercozoa and Ciliophora) and several members of Actinobacteria, Alphaproteobacteria, and Gammaproteobacteria in the interkingdom networks. Furthermore, the microbiomes of diseased plants not only exhibit a higher relative abundance of functional genes related to bacterial anti-predator responses compared to healthy plants, but also contained a greater abundance of metagenome-assembled genomes possessing functional traits involved in this response. The increased microbial interkingdom correlations among bacteria, fungi, and protists, coupled with the enhanced effects of protists on bacteria and fungi, as well as the notable bacterial anti-predator feedback in the diseased plant microbiome, all suggest that FWD catalyzes the associations between different groups of microbiomes. These findings highlight the role of predatory protists in shaping microbial assembly and functionality through top-down forces during pathogenic stress, potentially contributing to co-evolution within these soil and plant microbiomes.
Host-associated microbiota can be affected by factors related to environmental change, such as urbanization and invasive species. For example, urban areas often affect food availability for animals, which can change their gut microbiota. Invasive parasites can also influence microbiota through either competition or indirectly through a change in the host immune response. These interacting factors can have complex effects on host fitness, but few studies have disentangled the relationship between urbanization and parasitism on an organism’s gut microbial composition. To address this gap in knowledge, we investigated the effects of urbanization and parasitism by the invasive avian vampire fly (Philornis downsi) on the gut microbiota of nestling small ground finches (Geospiza fuliginosa) on San Cristóbal Island, Galápagos. We conducted a factorial study in which we experimentally manipulated parasite presence in an urban and non-urban area. Feces were then collected when nestlings to characterize the gut microbiota (i.e., alpha and beta diversity, community composition). Although we did not find an interactive effect of urbanization and parasitism on the microbiota, we did find main effects of each variable. Urban and parasitized nestlings had lower bacterial diversity and differences in relative abundance of bacterial phyla and genera compared to non-urban non-parasitized nestlings, respectively. Overall, this study advances our understanding of the complex effects of anthropogenic stressors on the gut microbiota of birds.
The commercially important Atlantic bluefin tuna (Thunnus thynnus), a large migratory fish, has experienced notable recovery aided by accurate resource assessment and effective fisheries management efforts. Traditionally, this species has been perceived as consisting of eastern and western populations, spawning respectively in the Mediterranean Sea and the Gulf of Mexico, with mixing occurring throughout the Atlantic. However, recent studies have emerged challenging this assumption by revealing weak genetic differentiation and identifying a previously unknown spawning ground in the Slope Sea used by Atlantic bluefin tuna of uncertain origin. To further understand the current and past population structure and connectivity of Atlantic bluefin tuna, we have assembled a unique dataset including thousands of genome-wide Single Nucleotide Polymorphisms (SNPs) from five hundred larvae, young of the year and spawning adult samples covering the three spawning grounds and including individuals of other Thunnus species. Our analyses support two weakly differentiated but demographically connected ancestral populations that interbreed in the Slope Sea. Moreover, we also identified signatures of introgression from albacore into the Atlantic bluefin tuna genome, exhibiting varied frequencies across spawning areas, indicating strong gene flow from the Mediterranean Sea towards the Slope Sea. We hypothesize that the observed genetic differentiation may be attributed to increased gene flow caused by a recent intensification of westward migration by the eastern population, which could have implications for the genetic diversity and conservation of western populations. Future conservation efforts should consider these findings to address potential genetic homogenization in the species.
Epigenetic mechanisms, such as DNA methylation, can influence gene regulation and affect phenotypic variation, raising the possibility that they contribute to ecological adaptation. To being to address this issue requires high-resolution sequencing studies of natural populations to pinpoint epigenetic regions of potential ecological and evolutionary significance. However, such studies are still relatively uncommon, especially in insects, and are mainly restricted to a few model organisms. Here, we characterize patterns of DNA methylation for natural populations of Timema cristinae adapted to two host plant species (i.e., ecotypes). By integrating results from sequencing of whole transcriptomes, genomes, and methylomes, we investigate whether environmental, host, and genetic differences of these stick insects are associated with methylation levels of cytosine nucleotides in CpG context. We report an overall genome-wide methylation level for T. cristinae of ~14%, being enriched in gene bodies and impoverished in repetitive elements. Genome-wide DNA methylation variation was strongly positively correlated with genetic distance (relatedness), but also exhibited significant host-plant effects. Using methylome-environment association analysis, we pinpointed specific genomic regions that are differentially methylated between ecotypes, with these regions being enriched for genes with functions in membrane processes. The observed association between methylation variation with genetic relatedness and the ecologically-important variable of host plant suggest a potential role for epigenetic modification in T. cristinae adaptation. To substantiate such adaptive significance, future studies could test if methylation has a heritable component and the extent to which it responds to experimental manipulation in field and laboratory studies.
Understanding the geographic linkages among populations across the annual cycle is an essential component for understanding the ecology and evolution of migratory species and for facilitating their effective conservation. While genetic markers have been widely applied to describe migratory connections, the rapid development of new sequencing methods, such as low-coverage whole genome sequencing (lcWGS), provides new opportunities for improved estimates of migratory connectivity. Here, we use lcWGS to identify fine-scale population structure in a widespread songbird, the American Redstart (Setophaga ruticilla), and accurately assign individuals to genetically distinct breeding populations. Assignment of individuals from the nonbreeding range reveals population-specific patterns of varying migratory connectivity. By combining migratory connectivity results with demographic analysis of population abundance and trends, we consider full annual cycle conservation strategies for preserving numbers of individuals and genetic diversity. Notably, we highlight the importance of the Northern Temperate-Greater Antilles migratory population as containing the largest proportion of individuals in the species. Finally, we highlight valuable considerations for other population assignment studies aimed at using lcWGS. Our results have broad implications for improving our understanding of the ecology and evolution of migratory species through conservation genomics approaches.
Chromosomal inversions have been identified in many natural populations and can be responsible for novel traits and fast adaptation. In zebra finch, an inversion in the Z chromosome has a pleiotropic effect on multiple traits but especially on sperm. Males heterokaryotypic for the inversion have sperm with a longer midpiece and higher velocity. They have higher fertility and siring success than homokaryotypic males. To understand the effect the Z inversion on these traits, we examined young males at different stages of testis development using two homokaryotypic lines and heterokaryotypes. We sampled both testis and liver tissue to identify gene expression differences. In testis, 520 differentially expressed genes were found and most of them were located on chromosome Z. They were functionally enriched for sperm-related traits. In liver, 420 differentially expressed genes were not enriched for any specific functions but showed also an overrepresentation on chromosome Z. In both tissues, this overrepresentation was located near the distal end of Z but also in the middle of the chromosome. For the heterokaryotype, we observed several genes with expression patterns more similar to one or the other karyotype, based on similarity to homokaryotype invidividuals. This was confirmed with SNPs for three genes, and interestingly one gene, dmgdh, had allele-specific expression originating from only one inversion-allele in the testis, yet both inversion alleles were expressed in the liver. This allele-specific difference in tissue-specific expression demonstrates a pleiotropic effect of the inversion and thus suggests a mechanism for divergent phenotypic effects resulting from an inversion.
Geographic isolation and chromosome evolution are two of the major drivers of diversification in eukaryotes in general, and specifically, in plants. On one hand, range shifts induced by Pleistocene glacial oscillations deeply shaped the evolutionary trajectories of species in the Northern Hemisphere. On the other hand, karyotype variability within species or species complexes may have adaptive potential as different karyotypes may represent different recombination rates and linkage groups that may be associated with locally adapted genes or supergenes. Organisms with holocentric chromosomes are ideal to study the link between local adaptation and chromosome evolution, due to their high cytogenetic variability, especially when it seems to be related to environmental variation. Here, we integrate the study of the phylogeography, chromosomal evolution and ecological requirements of a plant species complex distributed in the Western Euro-Mediterranean region (Carex gr. laevigata, Cyperaceae). We aim to clarify the relative influence of these factors on population differentiation and ultimately on speciation. We obtained a well-resolved RADseq phylogeny that sheds light on the phylogeographic patterns of molecular and chromosome number variation, which are compatible with south-to-north postglacial migration. In addition, landscape genomics analyses identified candidate loci for local adaptation, and also strong significant associations between the karyotype and the environment. We conclude that karyotype distribution in C. gr. laevigata has been constrained by both range shift dynamics and local adaptation. Our study demonstrates that chromosome evolution may be responsible, at least partially, for microevolutionary patterns of population differentiation and adaptation in Carex.
Telomeres are chromosome protectors that shorten during cell replication and in stressful conditions. Developing individuals are susceptible to telomere erosion when their growth is fast and resources limited. This is critical because the rate of telomere attrition in early life is linked to health and life span of adults. The metabolic telomere attrition hypothesis (MeTA) suggests that telomere dynamics can respond to biochemical signals conveying information about the organism’s energetic state. Among these signals are glucocorticoids (hormones that promote catabolic processes, potentially impairing costly telomere maintenance) and nucleotides, which activate anabolic pathways though the cellular enzyme target of rapamycin (TOR) preventing telomere attrition. During the energetically demanding growth phase, the regulation of telomeres in response to two contrasting signals—one promoting telomere maintenance and the other inducing attrition—provides an ideal experimental setting to test MeTa. We studied nestlings of a rapidly developing free-living passerine, the great tit (Parus major), that either received glucocorticoids (Cort-chicks), nucleotides (Nuc-chicks), or a combination of both (NucCort-chicks) all compared with controls (Cnt-chicks). Contrary to Cort-chicks, which showed telomere attrition, NucCort-chicks, did not. NucCort-chicks was the only group showing increased gene expression of telo2 (proxy for TOR activation), of mitochondrial enzymes linked to ATP production (atp5f1a-atp5f1b-cox6a1-cox4) and a higher efficiency in aerobically producing ATP. NucCort-chicks had also a higher expression of telomere maintenance genes (trf2) and of enzymatic antioxidant genes (gpx4-sod1). The findings show that nucleotides availability is crucial for preventing telomere erosion during fast growth in stressful environments.
Local adaptation is often facilitated by loci clustered in relatively few regions of the genome, termed genomic islands of divergence. However, the mechanisms that create, mold, and maintain these islands are poorly understood. Here, we use sockeye salmon as a model species to investigate the mechanisms responsible for creating islands of divergence linked to adaptive variation. Previous research suggests that multiple islands are involved in adaptive radiation of sockeye salmon. However, these studies were based on low-density genomic methods that genotyped tens to thousands of loci, making it difficult to elucidate the mechanisms responsible for islands. We used whole genome resequencing to genotype millions of loci to investigate these mechanisms. We discovered 64 islands, 16 of which were shared between two isolated populations; these 16 islands were clustered in four genomic regions. Characterization of the shared regions suggested that three of four were likely created by chromosomal inversions, while the other was created by processes not involving structural variation. Additionally, all four regions were relatively small (< 600 kb), suggesting inversions and other low recombination regions do not have to span megabases to be important for adaptive divergence. In sum, our study demonstrates that heterogeneous selection can lead to a mosaic of islands created by different mechanisms within the same genome. Future studies should continue to investigate how gene flow, selection, and the architecture of genetic traits interact to influence the genomic landscape of adaptive divergence.