An Arabidopsis introgression zone studied at high spatio-temporal resolution: interglacial and multiple genetic contact exemplified using whole nuclear and plastid genomes

Chromosome-level genome assembly provides insights into the genetic diversity, evolution, and flower development of Prunus conradinae

Prunus conradinae, a valuable flowering cherry belonging to the Rosaceae family subgenus Cerasus and endemic to China, has high economic and ornamental value. However, a high-quality P. conradinae genome is unavailable, which hinders our understanding of its genetic relationships and phylogenesis, and ultimately, the possibility of mining of key genes for important traits. Herein, we have successfully assembled a chromosome-scale P. conradinae genome, identifying 31,134 protein-coding genes, with 98.22% of them functionally annotated. Furthermore, we determined that repetitive sequences constitute 46.23% of the genome. Structural variation detection revealed some syntenic regions, inversions, translocations, and duplications, highlighting the genetic diversity and complexity of Cerasus. Phylogenetic analysis demonstrated that P. conradinae is most closely related to P. campanulata, from which it diverged ~ 19.1 million years ago (Mya). P. avium diverged earlier than P. cerasus and P. conradinae. Similar to the other Prunus species, P. conradinae underwent a common whole-genome duplication event at ~ 138.60 Mya. Furthermore, 79 MADS-box members were identified in P. conradinae, accompanied by the expansion of the SHORT VEGETATIVE PHASE subfamily. Our findings shed light on the complex genetic relationships, and genome evolution of P. conradinae and will facilitate research on the molecular breeding and functions of key genes related to important horticultural and economic characteristics of subgenus Cerasus.

Taxonomy of Dianthus (Caryophyllaceae) - overall phylogenetic relationships and assessment of species diversity based on a first comprehensive checklist of the genus

In this study, we present an overall phylogenetic framework for Dianthus using four plastid regions (matK-trnK-psbA, rpl32-trnL, trnQ-rps16) and nuclear ITS and a species-level checklist for the genus developed by using all available databases and the literature. The trees from the plastid dataset depict a clade of Dianthus that also includes Velezia and a few taxa of Petrorhagia. New combinations in Dianthus are provided for these species. The checklist of Dianthus in this new delimitation covers 1781 names, with 384 accepted species, 150 subspecies, 12 heterotypic varieties and two forms (not counting autonyms), 1050 synonyms, 22 hybrid names and 172 unresolved names, 3 names were excluded. Implications for the evolution of flower characters, life forms, biogeography, as well as sectional classification are discussed based on the phylogenetic framework.

Pulvinatusia (Brassicaceae), a new cushion genus from China and its systematic position

The new genus and species Pulvinatusiaxuegulaensis (Brassicaceae) are described and illustrated. The species is a cushion plant collected from Xuegu La, Xizang, China. Its vegetative parts are most similar to those of Arenariabryophylla (Caryophyllaceae) co-occurring in the same region, while its leaves and fruits closely resemble those of Xerodrabapatagonica (Brassicaceae) from Patagonian Argentina and Chile. Family-level phylogenetic analyses based on both nuclear ITS and plastome revealed that it is a member of the tribe Crucihimalayeae, but the infra-/intergeneric relationships within the tribe are yet to be resolved.

Adaptive introgression facilitate adaptation to high latitudes in European aspen (Populus tremula L.)

Understanding local adaptation has become a key research area given the ongoing climate challenge and the concomitant requirement to conserve genetic resources. Perennial plants, such as forest trees, are good models to study local adaptation given their wide geographic distribution, largely outcrossing mating systems, and demographic histories. We evaluated signatures of local adaptation in European aspen (Populus tremula) across Europe by means of whole-genome resequencing of a collection of 411 individual trees. We dissected admixture patterns between aspen lineages and observed a strong genomic mosaicism in Scandinavian trees, evidencing different colonization trajectories into the peninsula from Russia, Central and Western Europe. As a consequence of the secondary contacts between populations after the last glacial maximum, we detected an adaptive introgression event in a genome region of ∼500 kb in chromosome 10, harboring a large-effect locus that has previously been shown to contribute to adaptation to the short growing seasons characteristic of Northern Scandinavia. Demographic simulations and ancestry inference suggest an Eastern origin—probably Russian—of the adaptive Nordic allele which nowadays is present in a homozygous state at the north of Scandinavia. The strength of introgression and positive selection signatures in this region is a unique feature in the genome. Furthermore, we detected signals of balancing selection, shared across regional populations, that highlight the importance of standing variation as a primary source of alleles that facilitate local adaptation. Our results, therefore, emphasize the importance of migration–selection balance underlying the genetic architecture of key adaptive quantitative traits.

Phylogeny of Euclidieae (Brassicaceae) based on plastome and nuclear ribosomal DNA data

Euclidieae, a morphologically diverse tribe in the family Brassicaceae (Cruciferae), consists of 29 genera and more than 150 species distributed mainly in Asia. Prior phylogenetic analyses on Euclidieae are inadequate. In this study, sequence data from the plastid genome and nuclear ribosomal DNA of 72 species in 27 genera of Euclidieae were used to infer the inter- and intra-generic relationships within. The well-resolved and strongly supported plastome phylogenies revealed that Euclidieae could be divided into five clades. Both Cymatocarpus and Neotorularia are polyphyletic in nuclear and plastome phylogenies. Besides, the conflicts of systematic positions of three species of Braya and two species of Solms-laubachia s.l. indicated that hybridization and or introgression might have happened during the evolutionary history of the tribe. Results from divergence-time analyses suggested an early Miocene origin of Euclidieae, and it probably originated from the Central Asia, Pamir Plateau and West Himalaya. In addition, multiple ndh genes loss and pseudogenization were detected in eight species based on comparative genomic study.

Genomics of Evolutionary Novelty in Hybrids and Polyploids

It has long been recognized that hybridization and polyploidy are prominent processes in plant evolution. Although classically recognized as significant in speciation and adaptation, recognition of the importance of interspecific gene flow has dramatically increased during the genomics era, concomitant with an unending flood of empirical examples, with or without genome doubling. Interspecific gene flow is thus increasingly thought to lead to evolutionary innovation and diversification, via adaptive introgression, homoploid hybrid speciation and allopolyploid speciation. Less well understood, however, are the suite of genetic and genomic mechanisms set in motion by the merger of differentiated genomes, and the temporal scale over which recombinational complexity mediated by gene flow might be expressed and exposed to natural selection. We focus on these issues here, considering the types of molecular genetic and genomic processes that might be set in motion by the saltational event of genome merger between two diverged species, either with or without genome doubling, and how these various processes can contribute to novel phenotypes. Genetic mechanisms include the infusion of new alleles and the genesis of novel structural variation including translocations and inversions, homoeologous exchanges, transposable element mobilization and novel insertional effects, presence-absence variation and copy number variation. Polyploidy generates massive transcriptomic and regulatory alteration, presumably set in motion by disrupted stoichiometries of regulatory factors, small RNAs and other genome interactions that cascade from single-gene expression change up through entire networks of transformed regulatory modules. We highlight both these novel combinatorial possibilities and the range of temporal scales over which such complexity might be generated, and thus exposed to natural selection and drift.

A novel allele of ASY3 is associated with greater meiotic stability in autotetraploid Arabidopsis lyrata

In this study we performed a genotype-phenotype association analysis of meiotic stability in 10 autotetraploid Arabidopsis lyrata and A. lyrata/A. arenosa hybrid populations collected from the Wachau region and East Austrian Forealps. The aim was to determine the effect of eight meiosis genes under extreme selection upon adaptation to whole genome duplication. Individual plants were genotyped by high-throughput sequencing of the eight meiosis genes (ASY1, ASY3, PDS5b, PRD3, REC8, SMC3, ZYP1a/b) implicated in synaptonemal complex formation and phenotyped by assessing meiotic metaphase I chromosome configurations. Our results reveal that meiotic stability varied greatly (20-100%) between individual tetraploid plants and associated with segregation of a novel ASYNAPSIS3 (ASY3) allele derived from A. lyrata. The ASY3 allele that associates with meiotic stability possesses a putative in-frame tandem duplication (TD) of a serine-rich region upstream of the coiled-coil domain that appears to have arisen at sites of DNA microhomology. The frequency of multivalents observed in plants homozygous for the ASY3 TD haplotype was significantly lower than in plants heterozygous for ASY3 TD/ND (non-duplicated) haplotypes. The chiasma distribution was significantly altered in the stable plants compared to the unstable plants with a shift from proximal and interstitial to predominantly distal locations. The number of HEI10 foci at pachytene that mark class I crossovers was significantly reduced in a plant homozygous for ASY3 TD compared to a plant heterozygous for ASY3 ND/TD. Fifty-eight alleles of the 8 meiosis genes were identified from the 10 populations analysed, demonstrating dynamic population variability at these loci. Widespread chimerism between alleles originating from A. lyrata/A. arenosa and diploid/tetraploids indicates that this group of rapidly evolving genes may provide precise adaptive control over meiotic recombination in the tetraploids, the very process that gave rise to them.

The Quaternary evolutionary history of Bristol rock cress (Arabis scabra, Brassicaceae), a Mediterranean element with an outpost in the north-western Atlantic region

BACKGROUND AND AIMS

Bristol rock cress is among the few plant species in the British Isles considered to have a Mediterranean-montane element. Spatiotemporal patterns of colonization of the British Isles since the last interglacial and after the Last Glacial Maximum (LGM) from mainland Europe are underexplored and have not yet included such floristic elements. Here we shed light on the evolutionary history of a relic and outpost metapopulation of Bristol rock cress in the south-western UK.

METHODS

Amplified fragment length polymorphisms (AFLPs) were used to identify distinct gene pools. Plastome assembly and respective phylogenetic analysis revealed the temporal context. Herbarium material was largely used to exemplify the value of collections to obtain a representative sampling covering the entire distribution range.

KEY RESULTS

The AFLPs recognized two distinct gene pools, with the Iberian Peninsula as the primary centre of genetic diversity and the origin of lineages expanding before and after the LGM towards mountain areas in France and Switzerland. No present-day lineages are older than 51 ky, which is in sharp contrast to the species stem group age of nearly 2 My, indicating severe extinction and bottlenecks throughout the Pleistocene. The British Isles were colonized after the LGM and feature high genetic diversity.

CONCLUSIONS

The short-lived perennial herb Arabis scabra, which is restricted to limestone, has expanded its distribution range after the LGM, following corridors within an open landscape, and may have reached the British Isles via the desiccated Celtic Sea at about 16 kya. This study may shed light on the origin of other rare and peculiar species co-occurring in limestone regions in the south-western British Isles.

Analysis of complete chloroplast genomes of Curcuma and the contribution to phylogeny and adaptive evolution

Curcuma is an important member of Zingiberaceae. Many species of this genus are widely used in traditional medicine and have important cultural value in East Asia. Among them, C. longa is considered to be the main source of curcumin and has a very wide range of uses. The rapid development of molecular phylogeny has deepened our understanding of taxonomy and evolution of Curcuma. However, little is known about the chloroplast genome phylogeny and the genetic bases of adaptative evolution. In this work, we sequenced the complete chloroplast genome of 4 Curcuma species. Curcuma chloroplast genomes showed highly conserved structures and the length ranged from 159,423 bp to 152,723 bp. A total of 133 genes were observed. Multiple repeats and simple sequence repeats (SSRs) were detected. By comparing with related species, 7 highly variable regions were identified as potential specific DNA barcodes for species identification. Phylogenetic analysis of complete plastome sequences and specific data sets revealed discordance with expected genus boundary. Chloroplast phylogenetic relationships were better predicted by geography than by morphological and nuclear DNA, indicating a substantial existence of introgression. 9 genes were proved to have high posteriori probability in positive selection analysis, and 4 of them (psbA, psbD, PetA and rbcL) closely related to photosynthesis, implying that chloroplast genes may had undergone positive selection pressure in evolution. These results are of great significance for us to understand the genetic basis, phylogeny and adaptive evolution of Curcuma chloroplast.

Interspecific introgression mediates adaptation to whole genome duplication

Adaptive gene flow is a consequential phenomenon across all kingdoms. Although recognition is increasing, there is no study showing that bidirectional gene flow mediates adaptation at loci that manage core processes. We previously discovered concerted molecular changes among interacting members of the meiotic machinery controlling crossover number upon adaptation to whole-genome duplication (WGD) in Arabidopsis arenosa. Here we conduct a population genomic study to test the hypothesis that adaptation to WGD has been mediated by adaptive gene flow between A. arenosa and A. lyrata. We find that A. lyrata underwent WGD more recently than A. arenosa, suggesting that pre-adapted alleles have rescued nascent A. lyrata, but we also detect gene flow in the opposite direction at functionally interacting loci under the most extreme levels of selection. These data indicate that bidirectional gene flow allowed for survival after WGD, and that the merger of these species is greater than the sum of their parts.

Whole genome duplication (WGD) presents new challenges to the establishment of optimal allelic combinations and to the meiotic machinery. Here, the authors show that adaptive gene flow from Arabidopsis arenosa could rescue the nascent A. lyrata from extinction following WGD.

The genomic basis of adaptation to calcareous and siliceous soils in Arabidopsis lyrata

Edaphic conditions are important determinants of plant fitness. While much has been learnt in recent years about plant adaptation to heavy metal contaminated soils, the genomic basis underlying adaptation to calcareous and siliceous substrates remains largely unknown. We performed a reciprocal germination experiment and whole-genome resequencing in natural calcareous and siliceous populations of diploid Arabidopsis lyrata to test for edaphic adaptation and detect signatures of selection at loci associated with soil-mediated divergence. In parallel, genome scans on respective diploid ecotypes from the Arabidopsis arenosa species complex were undertaken, to search for shared patterns of adaptive genetic divergence. Soil ecotypes of A. lyrata display significant genotype-by-treatment responses for seed germination. Sequence (SNPs) and copy-number variants (CNVs) point towards loci involved in ion transport as the main targets of adaptive genetic divergence. Two genes exhibiting high differentiation among soil types in A. lyrata further share trans-specific single nucleotide polymorphisms with A. arenosa. This work applies experimental and genomic approaches to study edaphic adaptation in A. lyrata and suggests that physiological response to elemental toxicity and deficiency underlies the evolution of calcareous and siliceous ecotypes. The discovery of shared adaptive variation between sister species indicates that ancient polymorphisms contribute to adaptive evolution.

Restriction associated DNA-genotyping at multiple spatial scales in Arabidopsis lyrata reveals signatures of pathogen-mediated selection

Background

Genome scans based on outlier analyses have revolutionized detection of genes involved in adaptive processes, but reports of some forms of selection, such as balancing selection, are still limited. It is unclear whether high throughput genotyping approaches for identification of single nucleotide polymorphisms have sufficient power to detect modes of selection expected to result in reduced genetic differentiation among populations. In this study, we used Arabidopsis lyrata to investigate whether signatures of balancing selection can be detected based on genomic smoothing of Restriction Associated DNA sequencing (RAD-seq) data. We compared how different sampling approaches (both within and between subspecies) and different background levels of polymorphism (inbreeding or outcrossing populations) affected the ability to detect genomic regions showing key signatures of balancing selection, specifically elevated polymorphism, reduced differentiation and shifts towards intermediate allele frequencies. We then tested whether candidate genes associated with disease resistance (R-gene analogs) were detected more frequently in these regions compared to other regions of the genome.

Results

We found that genomic regions showing elevated polymorphism contained a significantly higher density of R-gene analogs predicted to be under pathogen-mediated selection than regions of non-elevated polymorphism, and that many of these also showed evidence for an intermediate site-frequency spectrum based on Tajima’s D. However, we found few genomic regions that showed both elevated polymorphism and reduced F_ST among populations, despite strong background levels of genetic differentiation among populations. This suggests either insufficient power to detect the reduced population structure predicted for genes under balancing selection using sparsely distributed RAD markers, or that other forms of diversifying selection are more common for the R-gene analogs tested.

Conclusions

Genome scans based on a small number of individuals sampled from a wide range of populations were sufficient to confirm the relative scarcity of signatures of balancing selection across the genome, but also identified new potential disease resistance candidates within genomic regions showing signatures of balancing selection that would be strong candidates for further sequencing efforts.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4806-7) contains supplementary material, which is available to authorized users.

Polyploid Arabidopsis species originated around recent glaciation maxima

Polyploidy may provide adaptive advantages and is considered to be important for evolution and speciation. Polyploidy events are found throughout the evolutionary history of plants, however they do not seem to be uniformly distributed along the time axis. For example, many of the detected ancient whole-genome duplications (WGDs) seem to cluster around the K/Pg boundary (∼66Mya), which corresponds to a drastic climate change event and a mass extinction. Here, we discuss more recent polyploidy events using Arabidopsis as the most developed plant model at the level of the entire genus. We review the history of the origin of allotetraploid species A. suecica and A. kamchatica, and tetraploid lineages of A. lyrata, A. arenosa and A. thaliana, and discuss potential adaptive advantages. Also, we highlight an association between recent glacial maxima and estimated times of origins of polyploidy in Arabidopsis. Such association might further support a link between polyploidy and environmental challenge, which has been observed now for different time-scales and for both ancient and recent polyploids.