Dead-End Hybridization in Walnut Trees Revealed by Large-Scale Genomic Sequence Data

A model of hybrid speciation process drawn from three new poplar species originating from distant hybridization between sections

Although numerous studies have been conducted on hybrid speciation, our understanding of this process remains limited. Through an 18-year systematic investigation of all taxa of Populus on the Qinghai-Tibet Plateau, we discovered three new taxa with clear characteristics of sect. Leucoides. Further evidence was gathered from morphology, whole-genome bioinformatics, biogeography, and breeding to demonstrate synthetically that they all originated from distant hybridization between sect. Leucoides and sect. Tacamahaca. P. gonggaensis originated from the hybridization of P. lasiocarpa with P. cathayana, P. butuoensis from the hybridization of P. wilsonii with P. szechuanica, and P. dafengensis from the hybridization of P. lasiocarpa with P. szechuanica. Due to heterosis, the three hybrid taxa possess greater ecological adaptability than their ancestral species. We propose a hybrid speciation process model that incorporates orthogonal, reverse, and backcrossing events. This model can adequately explain some crucial evolutionary concerns, such as the nuclear-cytoplasmic conflict on phylogeny and the extinction of ancestral species within the distribution range of hybrid species.

A Multiplex PCR System of Novel Microsatellite Loci for Population Genetic Application in Walnuts

Multiplex polymerase chain reaction (PCR) of microsatellite loci allows for simultaneous amplification of two or more pairs of primers in a single PCR reaction; hence, it is cost and time effective. However, very few attempts have been reported in non-model species. In this study, by combining a genome-based de novo development and cross-species application approach, a multiplex PCR system comprising 5 PCR reactions of 33 microsatellites consisting of 26 novel genomic and 7 literature-sourced loci was tested for polymorphisms, cross-species transferability, and the ability to assess genetic diversity and population structure of three walnut species (Juglans spp.). We found that the genome-based approach is more efficient than other methods. An allelic ladder was developed for each locus to enhance consistent genotyping among laboratories. The population genetic analysis results showed that all 33 loci were successfully transferred across the three species, showing high polymorphism and a strong genetic structure. Hence, the multiplex PCR system is highly applicable in walnut species. Furthermore, we propose an efficient pipeline to characterize and genotype polymorphic microsatellite loci. The novel toolbox developed here will aid future ecology and evolution studies in walnut and could serve as a model for other plant species.

Telomere-to-telomere and haplotype-resolved genome assembly of the Chinese cork oak (Quercus variabilis)

The Quercus variabilis, a deciduous broadleaved tree species, holds significant ecological and economical value. While a chromosome-level genome for this species has been made available, it remains riddled with unanchored sequences and gaps. In this study, we present a nearly complete comprehensive telomere-to-telomere (T2T) and haplotype-resolved reference genome for Q. variabilis. This was achieved through the integration of ONT ultra-long reads, PacBio HiFi long reads, and Hi-C data. The resultant two haplotype genomes measure 789 Mb and 768 Mb in length, with a contig N50 of 65 Mb and 56 Mb, and were anchored to 12 allelic chromosomes. Within this T2T haplotype-resolved assembly, we predicted 36,830 and 36,370 protein-coding genes, with 95.9% and 96.0% functional annotation for each haplotype genome. The availability of the T2T and haplotype-resolved reference genome lays a solid foundation, not only for illustrating genome structure and functional genomics studies but also to inform and facilitate genetic breeding and improvement of cultivated Quercus species.

Adaptive divergence and genetic vulnerability of relict species under climate change: a case study of Pterocarya macroptera

BACKGROUND AND AIMS

Understanding adaptive genetic variation and whether it can keep pace with predicted future climate change is critical in assessing the genetic vulnerability of species and developing conservation management strategies. The lack of information on adaptive genetic variation in relict species carrying abundant genetic resources hinders the assessment of genetic vulnerability. Using a landscape genomics approach, this study aimed to determine how adaptive genetic variation shapes population divergence and to predict the adaptive potential of Pterocarya macroptera (a vulnerable relict species in China) under future climate scenarios.

METHODS

We applied restriction site-associated DNA sequencing (RAD-seq) to obtain 8244 single-nucleotide polymorphisms (SNPs) from 160 individuals across 28 populations. We examined the pattern of genetic diversity and divergence, and then identified outliers by genetic differentiation (FST) and genotype-environment association (GEA) methods. We further dissected the effect of geographical/environmental gradients on genetic variation. Finally, we predicted genetic vulnerability and adaptive risk under future climate scenarios.

KEY RESULTS

We identified three genetic lineages within P. macroptera: the Qinling-Daba-Tianmu Mountains (QDT), Western Sichuan (WS) and Northwest Yunnan (NWY) lineages, which showed significant signals of isolation by distance (IBD) and isolation by environment (IBE). IBD and IBE explained 3.7-5.7 and 8.6-12.8 % of the genetic structure, respectively. The identified GEA SNP-related genes were involved in chemical defence and gene regulation and may exhibit higher genetic variation to adapt to the environment. Gradient forest analysis revealed that the genetic variation was mainly shaped by temperature-related variables, indicating its adaptation to local thermal environments. A limited adaptive potential was suggested by the high levels of genetic vulnerability in marginal populations.

CONCLUSIONS

Environmental gradient mainly shaped the population differentiation of P. macroptera. Marginal populations may be at high risk of extinction, and thus proactive management measures, such as assisted gene flow, are required to ensure the survival of these populations.

Demography-driven and adaptive introgression in a hybrid zone of the Armeria syngameon

Syngameons represent networks of otherwise distinct species connected by limited gene exchange. Although most studies have focused on how species maintain their cohesiveness despite gene flow, there are additional relevant questions regarding the evolutionary dynamics of syngameons and their drivers, as well as the success of their members and the network as a whole. Using a ddRADseq approach, we analysed the genetic structure, genomic clines and demographic history of a coastal hybrid zone involving two species of the Armeria (Plumbaginaceae) syngameon in southern Spain. We inferred that a peripheral population of the sand dune-adapted A. pungens diverged from the rest of the conspecific populations and subsequently hybridized with a locally more abundant pinewood congener, A. macrophylla. Both species display extensive plastid DNA haplotype sharing. Genomic cline analysis identified bidirectional introgression, but more outlier loci with excess A. pungens than A. macrophylla ancestry, suggesting the possibility of selection for A. pungens alleles. This is consistent with the finding that the A. pungens phenotype is selected for in open habitats, and with the strong correlation found between ancestry and phenotype. Taken together, our analyses suggest an intriguing scenario in which bidirectional introgression may, on the one hand, help to avoid reduced levels of genetic diversity due to the small size and isolated location of the A. pungens range-edge population, thereby minimizing demographic risks of stochastic extinction. On the other hand, the data also suggest that introgression into A. macrophylla may allow individuals to grow in open, highly irradiated, deep sandy, salt-exposed habitats.

No wonder, it is a hybrid. Natural hybridization between Jacobaea vulgaris and J. erucifolia revealed by molecular marker systems and its potential ecological impact

Progressive changes in the environment are related to modifications of the habitat. Introducing exotic species, and interbreeding between species can lead to processes that in the case of rare species or small populations threatens their integrity. Given the declining trends of many populations due to increased hybridization, early recognition of hybrids becomes important in conservation management. Natural hybridization is prevalent in Jacobaea. There are many naturally occurring interspecific hybrids in this genus, including those between Jacobaea vulgaris and its relatives. Although Jacobaea erucifolia and J. vulgaris often co-occur and are considered closely related, apart from the few reports of German botanists on the existence of such hybrids, there is no information on research confirming hybridization between them. Morphologically intermediate individuals, found in the sympatric distributions of J. vulgaris and J. erucifolia, were hypothesized to be their hybrids. Two molecular marker systems (nuclear and chloroplast DNA markers) were employed to test this hypothesis and characterize putative hybrids. Nuclear and chloroplast DNA sequencing results and taxon-specific amplified fragment length polymorphism (AFLP) fragment distribution analysis confirmed the hybrid nature of all 25 putative hybrids. The AFLP patterns of most hybrids demonstrated a closer relationship to J. erucifolia, suggesting frequent backcrossing. Moreover, they showed that several individuals previously described as pure were probably also of hybrid origin, backcrosses to J. erucifolia and J. vulgaris. This study provides the first molecular confirmation that natural hybrids between J. vulgaris and J. erucifolia occur in Poland. Hybridization appeared to be bidirectional but asymmetrical with J. vulgaris as the usual maternal parent.

Genomic analyses reveal dead-end hybridization between two deeply divergent kiwifruit species rather than homoploid hybrid speciation

Despite the importance of hybridization in evolution, the evolutionary consequence of homoploid hybridizations in plants remains poorly understood. Specially, homoploid hybridization events have been rarely documented due to a lack of genomic resources and methodological limitations. Actinidia zhejiangensis was suspected to have arisen from hybridization of Actinidia eriantha and Actinidia hemsleyana or Actinidia rufa. However, this species was very rare in nature and exhibited sympatric distribution with its potential parent species, which implied it might be a spontaneous hybrid of ongoing homoploid hybridization. Here, we illustrate the dead-end homoploid hybridization and genomic basis of isolating barriers between A. eriantha and A. hemsleyana through whole genome sequencing and population genomic analyses. Chromosome-scale genome assemblies of A. zhejiangensis and A. hemsleyana were generated. The chromosomes of A. zhejiangensis are confidently assigned to the two haplomes, and one of them originates from A. eriantha and the other originates from A. hemsleyana. Whole genome resequencing data reveal that A. zhejiangensis are mainly F1 hybrids of A. hemsleyana and A. eriantha and gene flow initiated about 0.98 million years ago, implying both strong genetic barriers and ongoing hybridization between these two deeply divergent kiwifruit species. Five inversions containing genes involved in pollen germination and pollen tube growth might account for the fertility breakdown of hybrids between A. hemsleyana and A. eriantha. Despite its distinct morphological traits and long recurrent hybrid origination, A. zhejiangensis does not initiate speciation. Collectively, our study provides new insights into homoploid hybridization in plants and provides genomic resources for evolutionary and functional genomic studies of kiwifruit.

Genomic insights into biased allele loss and increased gene numbers after genome duplication in autotetraploid Cyclocarya paliurus

BACKGROUND

Autopolyploidy is a valuable model for studying whole-genome duplication (WGD) without hybridization, yet little is known about the genomic structural and functional changes that occur in autopolyploids after WGD. Cyclocarya paliurus (Juglandaceae) is a natural diploid-autotetraploid species. We generated an allele-aware autotetraploid genome, a chimeric chromosome-level diploid genome, and whole-genome resequencing data for 106 autotetraploid individuals at an average depth of 60 × per individual, along with 12 diploid individuals at an average depth of 90 × per individual.

RESULTS

Autotetraploid C. paliurus had 64 chromosomes clustered into 16 homologous groups, and the majority of homologous chromosomes demonstrated similar chromosome length, gene numbers, and expression. The regions of synteny, structural variation and nonalignment to the diploid genome accounted for 81.3%, 8.8% and 9.9% of the autotetraploid genome, respectively. Our analyses identified 20,626 genes (69.18%) with four alleles and 9191 genes (30.82%) with one, two, or three alleles, suggesting post-polyploid allelic loss. Genes with allelic loss were found to occur more often in proximity to or within structural variations and exhibited a marked overlap with transposable elements. Additionally, such genes showed a reduced tendency to interact with other genes. We also found 102 genes with more than four copies in the autotetraploid genome, and their expression levels were significantly higher than their diploid counterparts. These genes were enriched in enzymes involved in stress response and plant defense, potentially contributing to the evolutionary success of autotetraploids. Our population genomic analyses suggested a single origin of autotetraploids and recent divergence (~ 0.57 Mya) from diploids, with minimal interploidy admixture.

CONCLUSIONS

Our results indicate the potential for genomic and functional reorganization, which may contribute to evolutionary success in autotetraploid C. paliurus.

Genomic Analysis of Plastid-Nuclear Interactions and Differential Evolution Rates in Coevolved Genes across Juglandaceae Species

The interaction between the nuclear and chloroplast genomes in plants is crucial for preserving essential cellular functions in the face of varying rates of mutation, levels of selection, and modes of transmission. Despite this, identifying nuclear genes that coevolve with chloroplast genomes at a genome-wide level has remained a challenge. In this study, we conducted an evolutionary rate covariation analysis to identify candidate nuclear genes coevolving with chloroplast genomes in Juglandaceae. Our analysis was based on 4,894 orthologous nuclear genes and 76 genes across seven chloroplast partitions in nine Juglandaceae species. Our results indicated that 1,369 (27.97%) of the nuclear genes demonstrated signatures of coevolution, with the Ycf1/2 partition yielding the largest number of hits (765) and the ClpP1 partition yielding the fewest (13). These hits were found to be significantly enriched in biological processes related to leaf development, photoperiodism, and response to abiotic stress. Among the seven partitions, AccD, ClpP1, MatK, and RNA polymerase partitions and their respective hits exhibited a narrow range, characterized by dN/dS values below 1. In contrast, the Ribosomal, Photosynthesis, Ycf1/2 partitions and their corresponding hits, displayed a broader range of dN/dS values, with certain values exceeding 1. Our findings highlight the differences in the number of candidate nuclear genes coevolving with the seven chloroplast partitions in Juglandaceae species and the correlation between the evolution rates of these genes and their corresponding chloroplast partitions.

Genome structure-based Juglandaceae phylogenies contradict alignment-based phylogenies and substitution rates vary with DNA repair genes

In lineages of allopolyploid origin, sets of homoeologous chromosomes may coexist that differ in gene content and syntenic structure. Presence or absence of genes and microsynteny along chromosomal blocks can serve to differentiate subgenomes and to infer phylogenies. We here apply genome-structural data to infer relationships in an ancient allopolyploid lineage, the walnut family (Juglandaceae), by using seven chromosome-level genomes, two of them newly assembled. Microsynteny and gene-content analyses yield identical topologies that place Platycarya with Engelhardia as did a 1980s morphological-cladistic study. DNA-alignment-based topologies here and in numerous earlier studies instead group Platycarya with Carya and Juglans, perhaps misled by past hybridization. All available data support a hybrid origin of Juglandaceae from extinct or unsampled progenitors nested within, or sister to, Myricaceae. Rhoiptelea chiliantha, sister to all other Juglandaceae, contains proportionally more DNA repair genes and appears to evolve at a rate 2.6- to 3.5-times slower than the remaining species.

Characteristics and potential functional effects of long insertions in Asian butternuts

Background

Structural variants (SVs) play important roles in adaptation evolution and species diversification. Especially, in plants, many phenotypes of response to the environment were found to be associated with SVs. Despite the prevalence and significance of SVs, long insertions remain poorly detected and studied in all but model species.

Results

We used whole-genome resequencing of paired reads from 80 Asian butternuts to detect long insertions and further analyse their characteristics and potential functional effects. By combining of mapping-based and de novo assembly-based methods, we obtained a multiple related species pangenome representing higher taxonomic groups. We obtained 89,312 distinct contigs totaling 147,773,999 base pair (bp) of new sequences, of which 347 were putative long insertions placed in the reference genome. Most of the putative long insertions appeared in multiple species; in contrast, only 62 putative long insertions appeared in one species, which may be involved in the response to the environment. 65 putative long insertions fell into 61 distinct protein-coding genes involved in plant development, and 105 putative long insertions fell into upstream of 106 distinct protein-coding genes involved in cellular respiration. 3,367 genes were annotated in 2,606 contigs. We propose PLAINS (https://github.com/CMB-BNU/PLAINS.git), a streamlined, comprehensive pipeline for the prediction and analysis of long insertions using whole-genome resequencing.

Conclusions

Our study lays down an important foundation for further whole-genome long insertion studies, allowing the investigation of their effects by experiments.

Species discrimination in Schima (Theaceae): Next-generation super-barcodes meet evolutionary complexity

Plastid genome and nrDNA arrays, proposed recently as "super barcodes", might provide additional discriminatory power and overcome the limitations of traditional barcoding loci, yet super barcodes need to be tested for their effectiveness in more plant groups. Morphological homoplasy among Schima species makes the genus a model for testing the efficacy of super barcodes. In this study, we generated multiple datasets comprising standard DNA barcodes (matK, rbcL, trnH-psbA, nrITS) and super-barcodes (plastid genome, nrDNA arrays) across 58 individuals from 12 out of 13 species of Schima from China. No samples were correctly assigned to species using standard DNA barcodes and nrDNA arrays, while only 27.27% of species with multiple accessions were distinguished using the plastid genome and its partitioned datasets-the lowest estimated rate of super barcode success in the literature so far. For Schima and other taxa with similarly recently divergence and low levels of genetic variation, incomplete lineage sorting, hybridization, or taxonomic oversplitting are all possible causes of the failure. Taken together, our study suggests that by no means are super barcodes immune to the challenges imposed by evolutionary complexity. We therefore call for developing multi-locus nuclear markers for species discrimination in plant groups.

Population-genomic analyses reveal bottlenecks and asymmetric introgression from Persian into iron walnut during domestication

Background

Persian walnut, Juglans regia, occurs naturally from Greece to western China, while its closest relative, the iron walnut, Juglans sigillata, is endemic in southwest China; both species are cultivated for their nuts and wood. Here, we infer their demographic histories and the time and direction of possible hybridization and introgression between them.

Results

We use whole-genome resequencing data, different population-genetic approaches (PSMC and GONE), and isolation-with-migration models (IMa3) on individuals from Europe, Iran, Kazakhstan, Pakistan, and China. IMa3 analyses indicate that the two species diverged from each other by 0.85 million years ago, with unidirectional gene flow from eastern J. regia and its ancestor into J. sigillata, including the shell-thickness gene. Within J. regia, a western group, located from Europe to Iran, and an eastern group with individuals from northern China, experienced dramatically declining population sizes about 80 generations ago (roughly 2400 to 4000 years), followed by an expansion at about 40 generations, while J. sigillata had a constant population size from about 100 to 20 generations ago, followed by a rapid decline.

Conclusions

Both J. regia and J. sigillata appear to have suffered sudden population declines during their domestication, suggesting that the bottleneck scenario of plant domestication may well apply in at least some perennial crop species. Introgression from introduced J. regia appears to have played a role in the domestication of J. sigillata.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02720-z.

plotsr: visualizing structural similarities and rearrangements between multiple genomes

SUMMARY

Third-generation genome sequencing technologies have led to a sharp increase in the number of high-quality genome assemblies. This allows the comparison of multiple assembled genomes of individual species and demands new tools for visualizing their structural properties. Here, we present plotsr, an efficient tool to visualize structural similarities and rearrangements between genomes. It can be used to compare genomes on chromosome level or to zoom in on any selected region. In addition, plotsr can augment the visualization with regional identifiers (e.g. genes or genomic markers) or histogram tracks for continuous features (e.g. GC content or polymorphism density).

AVAILABILITY AND IMPLEMENTATION

plotsr is implemented as a python package and uses the standard matplotlib library for plotting. It is freely available under the MIT license at GitHub (https://github.com/schneebergerlab/plotsr) and bioconda (https://anaconda.org/bioconda/plotsr).

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Demographic history and natural selection shape patterns of deleterious mutation load and barriers to introgression across Populus genome

Hybridization and resulting introgression are important processes shaping the tree of life and appear to be far more common than previously thought. However, how the genome evolution was shaped by various genetic and evolutionary forces after hybridization remains unresolved. Here we used whole-genome resequencing data of 227 individuals from multiple widespread Populus species to characterize their contemporary patterns of hybridization and to quantify genomic signatures of past introgression. We observe a high frequency of contemporary hybridization and confirm that multiple previously ambiguous species are in fact F₁ hybrids. Seven species were identified, which experienced different demographic histories that resulted in strikingly varied efficacy of selection and burdens of deleterious mutations. Frequent past introgression has been found to be a pervasive feature throughout the speciation of these Populus species. The retained introgressed regions, more generally, tend to contain reduced genetic load and to be located in regions of high recombination. We also find that in pairs of species with substantial differences in effective population size, introgressed regions are inferred to have undergone selective sweeps at greater than expected frequencies in the species with lower effective population size, suggesting that introgression likely have higher potential to provide beneficial variation for species with small populations. Our results, therefore, illustrate that demography and recombination have interplayed with both positive and negative selection in determining the genomic evolution after hybridization.