Comparative Performance of Popular Methods for Hybrid Detection using Genomic Data

Incomplete lineage sorting and hybridization underlie tree discordance in Petunia and related genera (Petunieae, Solanaceae)

Despite the overarching history of species divergence, phylogenetic studies often reveal distinct topologies across regions of the genome. The sources of these gene tree discordances are variable, but incomplete lineage sorting (ILS) and hybridization are among those with the most biological importance. Petunia serves as a classic system for studying hybridization in the wild. While field studies suggest that hybridization is frequent, the extent of reticulation within Petunia and its closely related genera has never been examined from a phylogenetic perspective. In this study, we used transcriptomic data from 11 Petunia, 16 Calibrachoa, and 10 Fabiana species to illuminate the relationships between these species and investigate whether hybridization played a significant role in the diversification of the clade. We inferred that gene tree discordance within genera is linked to hybridization events along with high levels of ILS due to their rapid diversification. Moreover, network analyses estimated deeper hybridization events between Petunia and Calibrachoa, genera that have different chromosome numbers. Although these genera cannot hybridize at the present time, ancestral hybridization could have played a role in their parallel radiations, as they share the same habitat and life history.

Introgression Underlies Phylogenetic Uncertainty But Not Parallel Plumage Evolution in a Recent Songbird Radiation

Instances of parallel phenotypic evolution offer great opportunities to understand the evolutionary processes underlying phenotypic changes. However, confirming parallel phenotypic evolution and studying its causes requires a robust phylogenetic framework. One such example is the "black-and-white wagtails," a group of 5 species in the songbird genus Motacilla: 1 species, Motacilla alba, shows wide intra-specific plumage variation, while the 4r others form 2 pairs of very similar-looking species (M. aguimp + M. samveasnae and M. grandis + M. maderaspatensis, respectively). However, the 2 species in each of these pairs were not recovered as sisters in previous phylogenetic inferences. Their relationships varied depending on the markers used, suggesting that gene tree heterogeneity might have hampered accurate phylogenetic inference. Here, we use whole genome resequencing data to explore the phylogenetic relationships within this group, with a special emphasis on characterizing the extent of gene tree heterogeneity and its underlying causes. We first used multispecies coalescent methods to generate a "complete evidence" phylogenetic hypothesis based on genome-wide variants, while accounting for incomplete lineage sorting (ILS) and introgression. We then investigated the variation in phylogenetic signal across the genome to quantify the extent of discordance across genomic regions and test its underlying causes. We found that wagtail genomes are mosaics of regions supporting variable genealogies, because of ILS and inter-specific introgression. The most common topology across the genome, supporting M. alba and M. aguimp as sister species, appears to be influenced by ancient introgression. Additionally, we inferred another ancient introgression event, between M. alba and M. grandis. By combining results from multiple analyses, we propose a phylogenetic network for the black-and-white wagtails that confirms that similar phenotypes evolved in non-sister lineages, supporting parallel plumage evolution. Furthermore, the inferred reticulations do not connect species with similar plumage coloration, suggesting that introgression does not underlie parallel plumage evolution in this group. Our results demonstrate the importance of investing genome-wide patterns of gene tree heterogeneity to help understand the mechanisms underlying phenotypic evolution. [Gene tree heterogeneity; incomplete lineage sorting; introgression; parallel evolution; phylogenomics; plumage evolution; wagtails.].

Detection of Ghost Introgression Requires Exploiting Topological and Branch Length Information

In recent years, the study of hybridization and introgression has made significant progress, with ghost introgression-the transfer of genetic material from extinct or unsampled lineages to extant species-emerging as a key area for research. Accurately identifying ghost introgression, however, presents a challenge. To address this issue, we focused on simple cases involving 3 species with a known phylogenetic tree. Using mathematical analyses and simulations, we evaluated the performance of popular phylogenetic methods, including HyDe and PhyloNet/MPL, and the full-likelihood method, Bayesian Phylogenetics and Phylogeography (BPP), in detecting ghost introgression. Our findings suggest that heuristic approaches relying on site-pattern counts or gene-tree topologies struggle to differentiate ghost introgression from introgression between sampled non-sister species, frequently leading to incorrect identification of donor and recipient species. The full-likelihood method BPP uses multilocus sequence alignments directly-hence taking into account both gene-tree topologies and branch lengths, by contrast, is capable of detecting ghost introgression in phylogenomic datasets. We analyzed a real-world phylogenomic dataset of 14 species of Jaltomata (Solanaceae) to showcase the potential of full-likelihood methods for accurate inference of introgression.

Hybrids as mirrors of the past: genomic footprints reveal spatio-temporal dynamics and extinction risk of alpine extremophytes in the mountains of Central Asia

Hybridization is one of the key processes shaping lineage diversification, particularly in regions that experienced strong climate oscillations. The alpine biome with its rich history of glacial-interglacial cycles and complex patterns of species distribution shifts offers an excellent system to investigate the impact of gene flow on population dynamics and speciation, important issues for evolutionary biology and biodiversity conservation. In this study, we combined genomic data (DArTseq), chloroplast markers, and morphology to examine phylogenetic relationships and the permeability of species boundaries and their evolutionary outcomes among the alpine extremophilic species of Puccinellia (Poaceae) in the Pamir Mountains, a part of the Mountains of Central Asia biodiversity hotspot. We determined the occurrence of interspecific hybrids between P. himalaica and P. pamirica, which demonstrated almost symmetric ancestry from their parental species and did not show signals of introgression. According to our integrative revision, the natural hybrids between P. himalaica and P. pamirica should be classified as Puccinellia ×vachanica (pro species). Using approximate Bayesian computation for population history inference, we uncovered that P. himalaica hybridized with P. pamirica independently in multiple localities over the Holocene. Hybrids inherited the fine-scale genetic structure from their parental species, which developed these patterns earlier, during the Late Pleistocene. Hybridization had different consequences for the involved parental lineages, likely playing an important role in a continuing decline of P. himalaica in the Pamir Mountains over the Holocene. Our results show that P. himalaica should be considered a critically endangered species in the Pamir Mountains and could also be retreating across its entire range of distribution in High Mountain Asia. Using a comparative phylogeographic framework, we revealed the risk of extinction of a cold-adapted alpine species in a global biodiversity hotspot. This study highlights that genomics could unravel diversity trends under climate change and provides valuable evidence for conservation management.

A global test of hybrid ancestry from genome-scale data

Methods based on the multi-species coalescent have been widely used in phylogenetic tree estimation using genome-scale DNA sequence data to understand the underlying evolutionary relationship between the sampled species. Evolutionary processes such as hybridization, which creates new species through interbreeding between two different species, necessitate inferring a species network instead of a species tree. A species tree is strictly bifurcating and thus fails to incorporate hybridization events which require an internal node of degree three. Hence, it is crucial to decide whether a tree or network analysis should be performed given a DNA sequence data set, a decision that is based on the presence of hybrid species in the sampled species. Although many methods have been proposed for hybridization detection, it is rare to find a technique that does so globally while considering a data generation mechanism that allows both hybridization and incomplete lineage sorting. In this paper, we consider hybridization and coalescence in a unified framework and propose a new test that can detect whether there are any hybrid species in a set of species of arbitrary size. Based on this global test of hybridization, one can decide whether a tree or network analysis is appropriate for a given data set.

Summary Tests of Introgression Are Highly Sensitive to Rate Variation Across Lineages

The evolutionary implications and frequency of hybridization and introgression are increasingly being recognized across the tree of life. To detect hybridization from multi-locus and genome-wide sequence data, a popular class of methods are based on summary statistics from subsets of 3 or 4 taxa. However, these methods often carry the assumption of a constant substitution rate across lineages and genes, which is commonly violated in many groups. In this work, we quantify the effects of rate variation on the D test (also known as ABBA-BABA test), the D3 test, and HyDe. All 3 tests are used widely across a range of taxonomic groups, in part because they are very fast to compute. We consider rate variation across species lineages, across genes, their lineage-by-gene interaction, and rate variation across gene-tree edges. We simulated species networks according to a birth-death-hybridization process, so as to capture a range of realistic species phylogenies. For all 3 methods tested, we found a marked increase in the false discovery of reticulation (type-1 error rate) when there is rate variation across species lineages. The D3 test was the most sensitive, with around 80% type-1 error, such that D3 appears to more sensitive to a departure from the clock than to the presence of reticulation. For all 3 tests, the power to detect hybridization events decreased as the number of hybridization events increased, indicating that multiple hybridization events can obscure one another if they occur within a small subset of taxa. Our study highlights the need to consider rate variation when using site-based summary statistics, and points to the advantages of methods that do not require assumptions on evolutionary rates across lineages or across genes.

Phylogenomics Reveals High Levels of Incomplete Lineage Sorting at the Ancestral Nodes of the Macaque Radiation

The genus Macaca includes 23 species assigned into 4 to 7 groups. It exhibits the largest geographic range and represents the most successful example of adaptive radiation of nonhuman primates. However, intrageneric phylogenetic relationships among species remain controversial and have not been resolved so far. In this study, we conducted a phylogenomic analysis on 16 newly generated and 8 published macaque genomes. We found strong evidence supporting the division of this genus into 7 species groups. Incomplete lineage sorting (ILS) was the primary factor contributing to the discordance observed among gene trees; however, we also found evidence of hybridization events, specifically between the ancestral arctoides/sinica and silenus/nigra lineages that resulted in the hybrid formation of the fascicularis/mulatta group. Combined with fossil data, our phylogenomic data were used to establish a scenario for macaque radiation. These findings provide insights into ILS and potential ancient introgression events that were involved in the radiation of macaques, which will lead to a better understanding of the rapid speciation occurring in nonhuman primates.

Extensive introgression among strongylocentrotid sea urchins revealed by phylogenomics

Gametic isolation is thought to play an important role in the evolution of reproductive isolation in broadcast-spawning marine invertebrates. However, it is unclear whether gametic isolation commonly evolves early in the speciation process or only accumulates after other reproductive barriers are already in place. It is also unknown whether gametic isolation is an effective barrier to introgression following speciation. Here, we used whole-genome sequencing data and multiple complementary phylogenomic approaches to test whether the well-documented gametic incompatibilities among the strongylocentrotid sea urchins have limited introgression. We quantified phylogenetic discordance, inferred reticulate phylogenetic networks, and applied the Δ statistic using gene tree topologies reconstructed from multiple sequence alignments of protein-coding single-copy orthologs. In addition, we conducted ABBA-BABA tests on genome-wide single nucleotide variants and reconstructed a phylogeny of mitochondrial genomes. Our results revealed strong mito-nuclear discordance and considerable nonrandom gene tree discordance that cannot be explained by incomplete lineage sorting alone. Eight of the nine species examined demonstrated a history of introgression with at least one other species or ancestral lineage, indicating that introgression was common during the diversification of the strongylocentrotid urchins. There was strong support for introgression between four extant species pairs (Strongylocentrotus pallidus ⇔ S. droebachiensis, S. intermedius ⇔ S. pallidus, S. purpuratus ⇔ S. fragilis, and Mesocentrotus franciscanus ⇔ Pseudocentrotus depressus) and additional evidence for introgression on internal branches of the phylogeny. Our results suggest that the existing gametic incompatibilities among the strongylocentrotid urchin species have not been a complete barrier to hybridization and introgression following speciation. Their continued divergence in the face of widespread introgression indicates that other reproductive isolating barriers likely exist and may have been more critical in establishing reproductive isolation early in speciation.

Genomic evidence for homoploid hybrid speciation in a marine mammal apex predator

Hybridization is widespread and constitutes an important source of genetic variability and evolution. In animals, its role in generating novel and independent lineages (hybrid speciation) has been strongly debated, with only a few cases supported by genomic data. The South American fur seal (SAfs) Arctocephalus australis is a marine apex predator of Pacific and Atlantic waters, with a disjunct set of populations in Peru and Northern Chile [Peruvian fur seal (Pfs)] with controversial taxonomic status. We demonstrate, using complete genome and reduced representation sequencing, that the Pfs is a genetically distinct species with an admixed genome that originated from hybridization between the SAfs and the Galapagos fur seal (Arctocephalus galapagoensis) ~400,000 years ago. Our results strongly support the origin of Pfs by homoploid hybrid speciation over alternative introgression scenarios. This study highlights the role of hybridization in promoting species-level biodiversity in large vertebrates.

When One's Not Enough: Colony Pool-Seq Outperforms Individual-Based Methods for Assessing Introgression in Apis mellifera mellifera

The human management of honey bees (Apis mellifera) has resulted in the widespread introduction of subspecies outside of their native ranges. One well known example of this is Apis mellifera mellifera, native to Northern Europe, which has now been significantly introgressed by the introduction of C lineage honey bees. Introgression has consequences for species in terms of future adaptive potential and long-term viability. However, estimating introgression in colony-living haplodiploid species is challenging. Previous studies have estimated introgression using individual workers, individual drones, multiple drones, and pooled workers. Here, we compare introgression estimates via three genetic approaches: SNP array, individual RAD-seq, and pooled colony RAD-seq. We also compare two statistical approaches: a maximum likelihood cluster program (ADMIXTURE) and an incomplete lineage sorting model (ABBA BABA). Overall, individual approaches resulted in lower introgression estimates than pooled colonies when using ADMIXTURE. However, the pooled colony ABBA BABA approach resulted in generally lower introgression estimates than all three ADMIXTURE estimates. These results highlight that sometimes one individual is not enough to assess colony-level introgression, and future studies that do use colony pools should not be solely dependent on clustering programs for introgression estimates.

Genomics reveals broad hybridization in deeply divergent Palearctic grass and water snakes (Natrix spp.)

Understanding speciation is one of the cornerstones of biological diversity research. Currently, speciation is often understood as a continuous process of divergence that continues until genetic or other incompatibilities minimize or prevent interbreeding. The Palearctic snake genus Natrix is an ideal group to study speciation, as it comprises taxa representing distinct stages of the speciation process, ranging from widely interbreeding parapatric taxa through parapatric species with very limited gene flow in narrow hybrid zones to widely sympatric species. To understand the evolution of reproductive isolation through time, we have sequenced the genomes of all five species within this genus and two additional subspecies. We used both long-read and short-read methods to sequence and de-novo-assemble two high-quality genomes (Natrix h. helvetica, Natrix n. natrix) to their 1.7 Gb length with a contig N50 of 4.6 Mbp and 1.5 Mbp, respectively, and used these as references to assemble the remaining short-read-based genomes. Our phylogenomic analyses yielded a well-supported dated phylogeny and evidence for a surprisingly complex history of interspecific gene flow, including between widely sympatric species. Furthermore, evidence for gene flow was also found for currently allopatric species pairs. Genetic exchange among these well-defined, distinct, and several million-year-old reptile species emphasizes that speciation and maintenance of species distinctness can occur despite continued genetic exchange.

Testing hypotheses of hybrid taxon formation in the shrubby beardtongues (Penstemon subgenus Dasanthera)

PREMISE

Hybridization is increasingly being identified in the genomes of species across the tree of life, leading to a general recognition that hybridization plays an important role in the generation of species diversity. While hybridization may increase species diversity directly via the formation of new taxa through hybrid speciation, it may also act indirectly via the exchange of phenotypic and genetic variance between species, which may in turn stimulate future speciation events.

METHODS

Using high-throughput sequence data, we resolved phylogenetic relationships and investigated the role of hybridization as a diversification mechanism in the shrubby beardtongues (Penstemon subgenus Dasanthera), a group of North American wildflowers that has undergone a recent and rapid adaptive radiation. Specifically, we tested four hypotheses of hybrid taxon formation resulting from hybridization between P. davidsonii and P. fruticosus.

RESULTS

Species tree inference supports the monophyly of subgenus Dasanthera and elucidates relationships between taxa distributed in the Cascades and Sierra Nevada Mountains. Results also provide evidence of gene flow between P. davidsonii and P. fruticosus and support at least one hybrid origin hypothesis (P. davidsonii var. menziesii) in a region of contemporary distributional overlap. Hybridization may have also been facilitated by historical overlap in geographic distribution caused by species' responses to climatic changes during the Pleistocene.

CONCLUSIONS

Our results support a history of hybridization between focal taxa in a rapidly radiating clade of plants and more broadly contribute to our growing understanding of the role of hybridization as a diversification mechanism in plants.

Variable hybridization between two Lake Tanganyikan cichlid species in recent secondary contact

Closely related taxa frequently exist in sympatry before the evolution of robust reproductive barriers, which can lead to substantial gene flow. Post-divergence gene flow can promote several disparate trajectories of divergence ranging from the erosion of distinctiveness and eventual collapse of the taxa to the strengthening of reproductive isolation. Among many relevant factors, understanding the demographic history of divergence (e.g. divergence time, extent of historical gene flow) can be particularly informative when examining contemporary gene flow between closely related taxa because this history can influence gene flow's prevalence and consequences. Here, we used genotyping-by-sequencing data to investigate speciation and contemporary hybridization in two closely related and sympatrically distributed Lake Tanganyikan cichlid species in the genus Petrochromis. Demographic modeling supported a speciation scenario involving divergence in isolation followed by secondary contact with bidirectional gene flow. Further investigation of this recent gene flow found evidence of ongoing hybridization between the species that varied in extent between different co-occurring populations. Relationships between abundance and the degree of admixture across populations suggest that the availability of conspecific mates may influence patterns of hybridization. These results, together with the observation that sets of recently diverged cichlid taxa are generally geographically separated in the lake, suggest that ongoing speciation in Lake Tanganyikan cichlids relies on initial spatial isolation. Additionally, the spatially heterogeneous patterns of admixture between the Petrochromis species illustrates the complexities of hybridization when species are in recent secondary contact.

Genomic evidence for homoploid hybrid speciation between ancestors of two different genera

Homoploid hybrid speciation (HHS) has been increasingly recognized as occurring widely during species diversification of both plants and animals. However, previous studies on HHS have mostly focused on closely-related species while it has been rarely reported or tested between ancestors of different genera. Here, we explore the likely HHS origin of Carpinus sect. Distegocarpus between sect. Carpinus and Ostrya in the family Betulaceae. We generate a chromosome-level reference genome for C. viminea of sect. Carpinus and re-sequence genomes of 44 individuals from the genera Carpinus and Ostrya. Our integrated analyses of all genomic data suggest that sect. Distegocarpus, which has three species, likely originates through HHS during the early divergence between Carpinus and Ostrya. Our study highlights the likelihood of an HHS event between ancestors of the extant genera during their initial divergences, which may have led to reticulate phylogenies at higher taxonomic levels.

The dynamics of introgression across an avian radiation

Hybridization and resulting introgression can play both a destructive and a creative role in the evolution of diversity. Thus, characterizing when and where introgression is most likely to occur can help us understand the causes of diversification dynamics. Here, we examine the prevalence of and variation in introgression using phylogenomic data from a large (1300+ species), geographically widespread avian group, the suboscine birds. We first examine patterns of gene tree discordance across the geographic distribution of the entire clade. We then evaluate the signal of introgression in a subset of 206 species triads using Patterson's D‐statistic and test for associations between introgression signal and evolutionary, geographic, and environmental variables. We find that gene tree discordance varies across lineages and geographic regions. The signal of introgression is highest in cases where species occur in close geographic proximity and in regions with more dynamic climates since the Pleistocene. Our results highlight the potential of phylogenomic datasets for examining broad patterns of hybridization and suggest that the degree of introgression between diverging lineages might be predictable based on the setting in which they occur.

Hybridization Dynamics and Extensive Introgression in the Daphnia longispina Species Complex: New Insights from a High-Quality Daphnia galeata Reference Genome

Hybridization and introgression are recognized as an important source of variation that influence adaptive processes; both phenomena are frequent in the genus Daphnia, a keystone zooplankton taxon in freshwater ecosystems that comprises several species complexes. To investigate genome-wide consequences of introgression between species, we provide here the first high-quality genome assembly for a member of the Daphnia longispina species complex, Daphnia galeata. We further resequenced 49 whole genomes of three species of the complex and their interspecific hybrids both from genotypes sampled in the water column and from single resting eggs extracted from sediment cores. Populations from habitats with diverse ecological conditions offered an opportunity to study the dynamics of hybridization linked to ecological changes and revealed a high prevalence of hybrids. Using phylogenetic and population genomic approaches, we provide first insights into the intra- and interspecific genome-wide variability in this species complex and identify regions of high divergence. Finally, we assess the length of ancestry tracts in hybrids to characterize introgression patterns across the genome. Our analyses uncover a complex history of hybridization and introgression reflecting multiple generations of hybridization and backcrossing in the Daphnia longispina species complex. Overall, this study and the new resources presented here pave the way for a better understanding of ancient and contemporary gene flow in the species complex and facilitate future studies on resting egg banks accumulating in lake sediment.

Phylogenomic approaches to detecting and characterizing introgression

Phylogenomics has revealed the remarkable frequency with which introgression occurs across the tree of life. These discoveries have been enabled by the rapid growth of methods designed to detect and characterize introgression from whole-genome sequencing data. A large class of phylogenomic methods makes use of data across species to infer and characterize introgression based on expectations from the multispecies coalescent. These methods range from simple tests, such as the D-statistic, to model-based approaches for inferring phylogenetic networks. Here, we provide a detailed overview of the various signals that different modes of introgression are expected leave in the genome, and how current methods are designed to detect them. We discuss the strengths and pitfalls of these approaches and identify areas for future development, highlighting the different signals of introgression, and the power of each method to detect them. We conclude with a discussion of current challenges in inferring introgression and how they could potentially be addressed.

Deep ancestral introgression shapes evolutionary history of dragonflies and damselflies

Introgression is an important biological process affecting at least 10% of the extant species in the animal kingdom. Introgression significantly impacts inference of phylogenetic species relationships where a strictly binary tree model cannot adequately explain reticulate net-like species relationships. Here we use phylogenomic approaches to understand patterns of introgression along the evolutionary history of a unique, non-model insect system: dragonflies and damselflies (Odonata). We demonstrate that introgression is a pervasive evolutionary force across various taxonomic levels within Odonata. In particular, we show that the morphologically "intermediate" species of Anisozygoptera (one of the three primary suborders within Odonata besides Zygoptera and Anisoptera), which retain phenotypic characteristics of the other two suborders, experienced high levels of introgression likely coming from zygopteran genomes. Additionally, we find evidence for multiple cases of deep inter-superfamilial ancestral introgression.