Decoding the massive genome of loblolly pine using haploid DNA and novel assembly strategies

On the Evolution and Functional Diversity of Terpene Synthases in the Pinus Species: A Review

In the biosynthesis of terpenoids, the ample catalytic versatility of terpene synthases (TPS) allows the formation of thousands of different molecules. A steadily increasing number of sequenced plant genomes invariably show that the TPS gene family is medium to large in size, comprising from 30 to 100 functional members. In conifers, TPSs belonging to the gymnosperm-specific TPS-d subfamily produce a complex mixture of mono-, sesqui-, and diterpenoid specialized metabolites, which are found in volatile emissions and oleoresin secretions. Such substances are involved in the defence against pathogens and herbivores and can help to protect against abiotic stress. Oleoresin terpenoids can be also profitably used in a number of different fields, from traditional and modern medicine to fine chemicals, fragrances, and flavours, and, in the last years, in biorefinery too. In the present work, after summarizing the current views on the biosynthesis and biological functions of terpenoids, recent advances on the evolution and functional diversification of plant TPSs are reviewed, with a focus on gymnosperms. In such context, an extensive characterization and phylogeny of all the known TPSs from different Pinus species is reported, which, for such genus, can be seen as the first effort to explore the evolutionary history of the large family of TPS genes involved in specialized metabolism. Finally, an approach is described in which the phylogeny of TPSs in Pinus spp. has been exploited to isolate for the first time mono-TPS sequences from Pinus nigra subsp. laricio, an ecologically important endemic pine in the Mediterranean area.

Functional and morphological evolution in gymnosperms: A portrait of implicated gene families

Gymnosperms diverged from their sister plant clade of flowering plants 300 Mya. Morphological and functional divergence between the two major seed plant clades involved significant changes in their reproductive biology, water-conducting systems, secondary metabolism, stress defense mechanisms, and small RNA-mediated epigenetic silencing. The relatively recent sequencing of several gymnosperm genomes and the development of new genomic resources have enabled whole-genome comparisons within gymnosperms, and between angiosperms and gymnosperms. In this paper, we aim to understand how genes and gene families have contributed to the major functional and morphological differences in gymnosperms, and how this information can be used for applied breeding and biotechnology. In addition, we have analyzed the angiosperm versus gymnosperm evolution of the pleiotropic drug resistance (PDR) gene family with a wide range of functionalities in plants' interaction with their environment including defense mechanisms. Some of the genes reviewed here are newly studied members of gene families that hold potential for biotechnological applications related to commercial and pharmacological value. Some members of conifer gene families can also be exploited for their potential in phytoremediation applications.

Transcriptome-wide identification and profiling of miRNAs in a stress-tolerant conifer Sabina chinensis

miRNAs are important regulatory components involving in many biological processes, including plant development, vegetative and reproductive growth, and stress response. However, identification and characterization of miRNAs still remain limited for conifer species. In this study, with deep sequencing, we obtained 1,314,450 unique reads with 18-30 nt length from a stress-tolerant conifer, Sabina chinensis. We identified 37 conserved and 103 novel miRNAs, their unique characteristics were further analyzed, and 10 randomly selected were validated by qRT-PCR. Through miRNA target predictions and annotations, we found miRNA may have several targets as well a target could be regulated by several miRNAs, and a total of 2,397 mRNAs were predicted to be targets of the 140 miRNAs. These targets included not only important transcription factors such as auxin response factors, but also indispensable non-transcriptional factor proteins. Pathway-based analysis showed that S. chinensis miRNAs are involved in 172 metabolic pathways, of which 3 were discovered in adaptation-related pathways, indicating their possible relevance to the species' stress-tolerance characteristics. This study is expected to lay the foundation for exploring the regulative roles of miRNAs in development, growth, and response to environmental stresses of S. chinensis.

275 years of forestry meets genomics in Pinus sylvestris

Pinus sylvestris has a long history of basic and applied research that is relevant for both forestry and evolutionary studies. Its patterns of adaptive variation and role in forest economic and ecological systems have been studied extensively for nearly 275 years, detailed demography for a 100 years and mating system more than 50 years. However, its reference genome sequence is not yet available and genomic studies have been lagging compared to, for example, Pinus taeda and Picea abies, two other economically important conifers. Despite the lack of reference genome, many modern genomic methods are applicable for a more detailed look at its biological characteristics. For example, RNA-seq has revealed a complex transcriptional landscape and targeted DNA sequencing displays an excess of rare variants and geographically homogenously distributed molecular genetic diversity. Current DNA and RNA resources can be used as a reference for gene expression studies, SNP discovery, and further targeted sequencing. In the future, specific consequences of the large genome size, such as functional effects of regulatory open chromatin regions and transposable elements, should be investigated more carefully. For forest breeding and long-term management purposes, genomic data can help in assessing the genetic basis of inbreeding depression and the application of genomic tools for genomic prediction and relatedness estimates. Given the challenges of breeding (long generation time, no easy vegetative propagation) and the economic importance, application of genomic tools has a potential to have a considerable impact. Here, we explore how genomic characteristics of P. sylvestris, such as rare alleles and the low extent of linkage disequilibrium, impact the applicability and power of the tools.

Evaluating genomic data for management of local adaptation in a changing climate: A lodgepole pine case study

We evaluate genomic data, relative to phenotypic and climatic data, as a basis for assisted gene flow and genetic conservation. Using a seedling common garden trial of 281 lodgepole pine (Pinus contorta) populations from across western Canada, we compare genomic data to phenotypic and climatic data to assess their effectiveness in characterizing the climatic drivers and spatial scale of local adaptation in this species. We find that phenotype-associated loci are equivalent or slightly superior to climate data for describing local adaptation in seedling traits, but that climate data are superior to genomic data that have not been selected for phenotypic associations. We also find agreement between the climate variables associated with genomic variation and with 20-year heights from a long-term provenance trial, suggesting that genomic data may be a viable option for identifying climatic drivers of local adaptation where phenotypic data are unavailable. Genetic clines associated with the experimental traits occur at broad spatial scales, suggesting that standing variation of adaptive alleles for this and similar species does not require management at scales finer than those indicated by phenotypic data. This study demonstrates that genomic data are most useful when paired with phenotypic data, but can also fill some of the traditional roles of phenotypic data in management of species for which phenotypic trials are not feasible.

Structural and Functional Characteristics of Two Molecular Variants of the Nitrogen Sensor PII in Maritime Pine

High levels of nitrogen are stored as arginine during the last stages of seed formation in maritime pine (Pinus pinaster Aiton). The protein sensor PII regulates the feedback inhibition of arginine biosynthesis through interaction with the key enzyme N-acetylglutamate kinase (NAGK). In this study, the structural and functional characteristics of PII have been investigated in maritime pine to get insights into the regulation of arginine metabolism. Two different forms of PII have been identified, PpPIIa and PpPIIb, which differ in their amino acid sequence and most likely correspond to splicing variants of a single gene in the pine genome. Two PII variants are also present in other pine species but not in other conifers such as spruces. PpPIIa and PpPIIb are trimeric proteins for which structural modeling predicts similar tridimensional protein core structures. Both are located in the chloroplast, where the PII-target enzyme PpNAGK is also found. PpPIIa, PpPIIb, and PpNAGK have been recombinantly produced to investigate the formation of NAGK-PII complexes. The interaction of PpPIIa/PpPIIb and PpNAGK may be enhanced by glutamine and contribute to relieve the feedback inhibition of PpNAGK by arginine. Expression analysis of PpPII genes revealed that PpIIa transcripts were predominant during embryogenesis and germination. The potential roles of PpPIIa and PpPIIb in the regulation of arginine metabolism of maritime pine are discussed.

Playing the genome card

In the 1990s, prominent biologists and journalists predicted that by 2020 each of us would carry a genome card, which would allow physicians to access our entire genome sequence and routinely use this information to diagnose and treat common and debilitating conditions. This is not yet the case. Why not? Common and debilitating diseases are rarely caused by single-gene mutations, and this was recognized before these genome card predictions had been made. Debilitating conditions, including common psychiatric disorders, are typically caused either by rare mutations or by complex interactions of many genes, each having a small effect, and epigenetic, environmental, and microbial factors. In such cases, having a complete genome sequence may have limited utility in diagnosis and treatment. Genome sequencing technologies have transformed biological research in many ways, but had a much smaller effect than expected on treatments of common diseases. Thus, early proponents of genome sequencing effectively "mis-promised" its benefits. One reason may be that there are incentives for both biologists and journalists to tell simple stories, including the idea of relatively simple genetic causation of common, debilitating diseases. These incentives may have led to misleading predictions, which to some extent continue today. Although the Human Genome Project has facilitated biological research generally, the mis-promising of medical benefits, at least for treating common and debilitating disorders, could undermine support for scientific research over the long term.

Tissue-specific transposon-associated small RNAs in the gymnosperm tree, Norway spruce

BACKGROUND

Small RNAs (sRNAs) are regulatory molecules impacting on gene expression and transposon activity. MicroRNAs (miRNAs) are responsible for tissue-specific and environmentally-induced gene repression. Short interfering RNAs (siRNA) are constitutively involved in transposon silencing across different type of tissues. The male gametophyte in angiosperms has a unique set of sRNAs compared to vegetative tissues, including phased siRNAs from intergenic or genic regions, or epigenetically activated siRNAs. This is contrasted by a lack of knowledge about the sRNA profile of the male gametophyte of gymnosperms.

RESULTS

Here, we isolated mature pollen from male cones of Norway spruce and investigated its sRNA profiles. While 21-nt sRNAs is the major size class of sRNAs in needles, in pollen 21-nt and 24-nt sRNAs are the most abundant size classes. Although the 24-nt sRNAs were exclusively derived from TEs in pollen, both 21-nt and 24-nt sRNAs were associated with TEs. We also investigated sRNAs from somatic embryonic callus, which has been reported to contain 24-nt sRNAs. Our data show that the 24-nt sRNA profiles are tissue-specific and differ between pollen and cell culture.

CONCLUSION

Our data reveal that gymnosperm pollen, like angiosperm pollen, has a unique sRNA profile, differing from vegetative leaf tissue. Thus, our results reveal that angiosperm and gymnosperm pollen produce new size classes not present in vegetative tissues; while in angiosperm pollen 21-nt sRNAs are generated, in the gymnosperm Norway spruce 24-nt sRNAs are generated. The tissue-specific production of distinct TE-derived sRNAs in angiosperms and gymnosperms provides insights into the diversification process of sRNAs in TE silencing pathways between the two groups of seed plants.

Oleoresin defenses in conifers: chemical diversity, terpene synthases and limitations of oleoresin defense under climate change

Conifers have evolved complex oleoresin terpene defenses against herbivores and pathogens. In co-evolved bark beetles, conifer terpenes also serve chemo-ecological functions as pheromone precursors, chemical barcodes for host identification, or nutrients for insect-associated microbiomes. We highlight the genomic, molecular and biochemical underpinnings of the large chemical space of conifer oleoresin terpenes and volatiles. Conifer terpenes are predominantly the products of the conifer terpene synthase (TPS) gene family. Terpene diversity is increased by cytochromes P450 of the CYP720B class. Many conifer TPS are multiproduct enzymes. Multisubstrate CYP720B enzymes catalyse multistep oxidations. We summarise known terpenoid gene functions in various different conifer species with reference to the annotated terpenoid gene space in a spruce genome. Overall, biosynthesis of terpene diversity in conifers is achieved through a system of biochemical radiation and metabolic grids. Expression of TPS and CYP720B genes can be specific to individual cell types of constitutive or traumatic resin duct systems. Induced terpenoid transcriptomes in resin duct cells lead to dynamic changes of terpene composition and quantity to fend off herbivores and pathogens. While terpenoid defenses have contributed much to the evolutionary success of conifers, under new conditions of climate change, these defences may become inconsequential against range-expanding forest pests.

Genome-Wide Variant Identification and High-Density Genetic Map Construction Using RADseq for Platycladus orientalis (Cupressaceae)

Platycladus orientalis is an ecologically important native conifer in Northern China and exotic species in many parts of the world; however, knowledge about the species’ genetics and genome are very limited. The availability of well-developed battery of genetic markers, with large genome coverage, is a prerequisite for the species genetic dissection of adaptive attributes and efficient selective breeding. Here, we present a genome-wide genotyping method with double-digestion restriction site associated DNA sequencing (ddRAD-seq) that is effective in generating large number of Mendelian markers for genome mapping and other genetic applications. Using 139 megagametophytes collected from a single mother tree, we assembled 397,226 loci, of which 108,683 (27.4%) were polymorphic. After stringent filtering for 1:1 segregation ratio and missing rate of <20%, the remaining 23,926 loci (22% of the polymorphic loci) were ordered into 11 linkage groups (LGs) and distributed across 7,559 unique positions, with a total map length of 1,443 cM and an average spacing of 0.2 cM between adjacent unique positions. The 11 LGs correspond to the species’ 11 haploid genome chromosome number. This genetic map is among few high-density maps available for conifers to date, and represents the first genetic map for P. orientalis. The information generated serves as a solid foundation not only for marker-assisted breeding efforts, but also for comparative conifer genomic studies.

A Complete Transcriptional Landscape Analysis of Pinus elliottii Engelm. Using Third-Generation Sequencing and Comparative Analysis in the Pinus Phylogeny

The planting of Pinus elliottii Engelm. has now reached close to three million ha in China. Molecular breeding as part of the improvement program for P. elliottii in southern China has been carried out in recent years. Third-generation sequencing (Pacbio sequencing technology, TGS) was used to obtain the exome of P. elliottii for molecular breeding. A total of 35.8 Gb clean reads were generated using TGS. After removing the redundant reads, we obtained 80,339 high-accuracy transcripts. Significantly, a total of 76,411 transcripts (95.1%) were blasted to public annotation databases. We predicted 65,062 intact coding sequences (CDSs), 8916 alternative splicing events, 1937 long non-coding RNAs, and 22,109 simple sequence repeats (SSRs) based on these obtained transcripts. Using the public databases and the data obtained above, 23 orthologous single-copy genes were identified to analyze the phylogenetic relationships for Pinus firstly including P. elliottii. Many positive selection genes involved in important biological processes and metabolism pathways were identified between P. elliottii and other pines. These positive selection genes could be candidate genes to be researched on the genetic basis of superior performance. Our study is the first to reveal the full-length and well-annotated transcripts of P. elliottii, which could provide reference for short transcriptome sequences in the research of genetics, phylogenetics, and genetic improvement for the non-reference genome species.

Single-pollen-cell sequencing for gamete-based phased diploid genome assembly in plants

Genome assemblies from diploid organisms create mosaic sequences alternating between parental alleles, which can create erroneous gene models and other problems. In animals, a popular strategy to generate haploid genome-resolved assemblies has been the sampling of (haploid) gametes, and the advent of single-cell sequencing has further advanced such methods. However, several challenges for the isolation and amplification of DNA from plant gametes have limited such approaches in plants. Here, we combined a new approach for pollen protoplast isolation with a single-cell DNA amplification technique and then used a “barcoding” bioinformatics strategy to incorporate haploid-specific sequence data from 12 pollen cells, ultimately enabling the efficient and accurate phasing of the pear genome into its A and B haploid genomes. Beyond revealing that 8.12% of the genes in the pear reference genome feature mosaic assemblies and enabling a previously impossible analysis of allelic affects in pear gene expression, our new haploid genome assemblies provide high-resolution information about recombination during meiosis in pollen. Considering that outcrossing pear is an angiosperm species featuring very high heterozygosity, our method for rapidly phasing genome assemblies is potentially applicable to several yet-unsequenced outcrossing angiosperm species in nature.

Construction of genetic linkage map and identification of a novel major locus for resistance to pine wood nematode in Japanese black pine (Pinus thunbergii)

BACKGROUND

Pine wilt disease (PWD), which is caused by the pine wood nematode (PWN) Bursaphelenchus xylophilus, is currently the greatest threat to pine forests in Europe and East Asian countries including Japan. Constructing a detailed linkage map of DNA markers and identifying PWD resistance genes/loci lead to improved resistance in Pinus thunbergii, as well as other Pinus species that are also susceptible to PWD.

RESULTS

A total F₁ mapping population of 188 individuals derived from a cross between the PWD-resistant P. thunbergii varieties 'Tanabe 54' (resistant rank 2 to PWD) and 'Tosashimizu 63' (resistant rank 4 to PWD) was inoculated with PWN, and was evaluated for disease symptoms. To perform linkage analysis for PWN resistance, a set of three maps was constructed; two parental maps generated using the integrated two-way pseudo-testcross method, and a consensus map with population-type cross-pollination. The linkage map of 'Tanabe 54' consisted of 167 loci, and covered 14 linkage groups (LGs), with a total genetic distance of 1214.6 cM. The linkage map of 'Tosashimizu 63' consisted of 252 loci, and covered 14 LGs, with a total genetic distance of 1422.1 cM. The integrated consensus map comprised 12 LGs with the basic chromosome number of P. thunbergii, and a total genetic distance of 1403.6 cM. Results from quantitative trait loci (QTL) analysis using phenotype data and linkage maps indicated that PWN resistance is controlled by a single dominant allele, which was derived from the 'Tanabe 54' female parent. This major QTL was located on linkage group 3 and was designated PWD1 for PINE WILT DISEASE 1.

CONCLUSIONS

The PWD1 locus is a major resistance QTL located on the Pinus consensus LG03 that acts in a dominant manner to confer pine wood nematode resistance. Information from the present study will be useful for P. thunbergii breeding programs to improve resistance to PWD, and also to help identify susceptibility genes in Pinus species.

Utilization of Tissue Ploidy Level Variation in de Novo Transcriptome Assembly of Pinus sylvestris

Compared to angiosperms, gymnosperms lag behind in the availability of assembled and annotated genomes. Most genomic analyses in gymnosperms, especially conifer tree species, rely on the use of de novo assembled transcriptomes. However, the level of allelic redundancy and transcript fragmentation in these assembled transcriptomes, and their effect on downstream applications have not been fully investigated. Here, we assessed three assembly strategies for short-reads data, including the utility of haploid megagametophyte tissue during de novo assembly as single-allele guides, for six individuals and five different tissues in Pinus sylvestris. We then contrasted haploid and diploid tissue genotype calls obtained from the assembled transcriptomes to evaluate the extent of paralog mapping. The use of the haploid tissue during assembly increased its completeness without reducing the number of assembled transcripts. Our results suggest that current strategies that rely on available genomic resources as guidance to minimize allelic redundancy are less effective than the application of strategies that cluster redundant assembled transcripts. The strategy yielding the lowest levels of allelic redundancy among the assembled transcriptomes assessed here was the generation of SuperTranscripts with Lace followed by CD-HIT clustering. However, we still observed some levels of heterozygosity (multiple gene fragments per transcript reflecting allelic redundancy) in this assembled transcriptome on the haploid tissue, indicating that further filtering is required before using these assemblies for downstream applications. We discuss the influence of allelic redundancy when these reference transcriptomes are used to select regions for probe design of exome capture baits and for estimation of population genetic diversity.

Core set construction and association analysis of Pinus massoniana from Guangdong province in southern China using SLAF-seq

Germplasm resource collection and utilization are important in forestry species breeding. High-through sequencing technologies have been playing increasing roles in forestry breeding. In this study, specific-locus amplified fragment sequencing (SLAF-seq) was employed to analyze 149 masson pine (Pinus massoniana) accessions collected from Guangdong in China. A large number of 471,660 SNPs in the total collection were identified from 599,164 polymorphic SLAF tags. Population structure analysis showed that 149 masson pines could not be obviously divided into subpopulations. Two core sets, containing 29 masson pine accessions for increasing resin and wood yield respectively, were obtained from the total collection. Phenotypic analyses of five traits showed abundant variations, 25 suggestive and 9 significant SNPs were associated with the resin-yielding capacity (RYC') and volume of wood (VW) using EMMAX and FaST-LMM; 22 suggestive and 11 significant SNPs were associated with RYC' and VW using mrMLM and FASTmrMLM. Moreover, a large number of associated SNPs were detected in trait HT, DBH, RW and RYC using mrMLM, FASTmrMLM, FASTmrEMMA and ISIS EM-BLASSO. The core germplasm sets would be a valuable resource for masson pine improvement and breeding. In addition, the associated SNP markers would be meaningful for masson pine resource selection.

Chinese Fir Breeding in the High-Throughput Sequencing Era: Insights from SNPs

Knowledge on population diversity and structure is of fundamental importance for conifer breeding programs. In this study, we concentrated on the development and application of high-density single nucleotide polymorphism (SNP) markers through a high-throughput sequencing technique termed as specific-locus amplified fragment sequencing (SLAF-seq) for the economically important conifer tree species, Chinese fir (Cunninghamia lanceolata). Based on the SLAF-seq, we successfully established a high-density SNP panel consisting of 108,753 genomic SNPs from Chinese fir. This SNP panel facilitated us in gaining insight into the genetic base of the Chinese fir advance breeding population with 221 genotypes for its genetic variation, relationship and diversity, and population structure status. Overall, the present population appears to have considerable genetic variability. Most (94.15%) of the variability was attributed to the genetic differentiation of genotypes, very limited (5.85%) variation occurred on the population (sub-origin set) level. Correspondingly, low FST (0.0285–0.0990) values were seen for the sub-origin sets. When viewing the genetic structure of the population regardless of its sub-origin set feature, the present SNP data opened a new population picture where the advanced Chinese fir breeding population could be divided into four genetic sets, as evidenced by phylogenetic tree and population structure analysis results, albeit some difference in membership of the corresponding set (cluster vs. group). It also suggested that all the genetic sets were admixed clades revealing a complex relationship of the genotypes of this population. With a step wise pruning procedure, we captured a core collection (core 0.650) harboring 143 genotypes that maintains all the allele, diversity, and specific genetic structure of the whole population. This generalist core is valuable for the Chinese fir advanced breeding program and further genetic/genomic studies.

Ultra-fast genome comparison for large-scale genomic experiments

In the last decade, a technological shift in the bioinformatics field has occurred: larger genomes can now be sequenced quickly and cost effectively, resulting in the computational need to efficiently compare large and abundant sequences. Furthermore, detecting conserved similarities across large collections of genomes remains a problem. The size of chromosomes, along with the substantial amount of noise and number of repeats found in DNA sequences (particularly in mammals and plants), leads to a scenario where executing and waiting for complete outputs is both time and resource consuming. Filtering steps, manual examination and annotation, very long execution times and a high demand for computational resources represent a few of the many difficulties faced in large genome comparisons. In this work, we provide a method designed for comparisons of considerable amounts of very long sequences that employs a heuristic algorithm capable of separating noise and repeats from conserved fragments in pairwise genomic comparisons. We provide software implementation that computes in linear time using one core as a minimum and a small, constant memory footprint. The method produces both a previsualization of the comparison and a collection of indices to drastically reduce computational complexity when performing exhaustive comparisons. Last, the method scores the comparison to automate classification of sequences and produces a list of detected synteny blocks to enable new evolutionary studies.

A Reference Genome Sequence for the European Silver Fir (Abies alba Mill.): A Community-Generated Genomic Resource

Silver fir (Abies alba Mill.) is a keystone conifer of European montane forest ecosystems that has experienced large fluctuations in population size during during the Quaternary and, more recently, due to land-use change. To forecast the species' future distribution and survival, it is important to investigate the genetic basis of adaptation to environmental change, notably to extreme events. For this purpose, we here provide a first draft genome assembly and annotation of the silver fir genome, established through a community-based initiative. DNA obtained from haploid megagametophyte and diploid needle tissue was used to construct and sequence Illumina paired-end and mate-pair libraries, respectively, to high depth. The assembled A. alba genome sequence accounted for over 37 million scaffolds corresponding to 18.16 Gb, with a scaffold N50 of 14,051 bp. Despite the fragmented nature of the assembly, a total of 50,757 full-length genes were functionally annotated in the nuclear genome. The chloroplast genome was also assembled into a single scaffold (120,908 bp) that shows a high collinearity with both the A. koreana and A. sibirica complete chloroplast genomes. This first genome assembly of silver fir is an important genomic resource that is now publicly available in support of a new generation of research. By genome-enabling this important conifer, this resource will open the gate for new research and more precise genetic monitoring of European silver fir forests.

Physcomitrella STEMIN transcription factor induces stem cell formation with epigenetic reprogramming

Epigenetic modifications, including histone modifications, stabilize cell-specific gene expression programmes to maintain cell identities in both metazoans and land plants^1-3. Notwithstanding the existence of these stable cell states, in land plants, stem cells are formed from differentiated cells during post-embryonic development and regeneration^4-6, indicating that land plants have an intrinsic ability to regulate epigenetic memory to initiate a new gene regulatory network. However, it is less well understood how epigenetic modifications are locally regulated to influence the specific genes necessary for cellular changes without affecting other genes in a genome. In this study, we found that ectopic induction of the AP2/ERF transcription factor STEMIN1 in leaf cells of the moss Physcomitrella patens decreases a repressive chromatin mark, histone H3 lysine 27 trimethylation (H3K27me3), on its direct target genes before cell division, resulting in the conversion of leaf cells to chloronema apical stem cells. STEMIN1 and its homologues positively regulate the formation of secondary chloronema apical stem cells from chloronema cells during development. Our results suggest that STEMIN1 functions within an intrinsic mechanism underlying local H3K27me3 reprogramming to initiate stem cell formation.

The unique photosynthetic apparatus of Pinaceae: analysis of photosynthetic complexes in Picea abies

Pinaceae are the predominant photosynthetic species in boreal forests, but so far no detailed description of the protein components of the photosynthetic apparatus of these gymnosperms has been available. In this study we report a detailed characterization of the thylakoid photosynthetic machinery of Norway spruce (Picea abies (L.) Karst). We first customized a spruce thylakoid protein database from translated transcript sequences combined with existing protein sequences derived from gene models, which enabled reliable tandem mass spectrometry identification of P. abies thylakoid proteins from two-dimensional large pore blue-native/SDS-PAGE. This allowed a direct comparison of the two-dimensional protein map of thylakoid protein complexes from P. abies with the model angiosperm Arabidopsis thaliana. Although the subunit composition of P. abies core PSI and PSII complexes is largely similar to that of Arabidopsis, there was a high abundance of a smaller PSI subcomplex, closely resembling the assembly intermediate PSI* complex. In addition, the evolutionary distribution of light-harvesting complex (LHC) family members of Pinaceae was compared in silico with other land plants, revealing that P. abies and other Pinaceae (also Gnetaceae and Welwitschiaceae) have lost LHCB4, but retained LHCB8 (formerly called LHCB4.3). The findings reported here show the composition of the photosynthetic apparatus of P. abies and other Pinaceae members to be unique among land plants.

Cyberinfrastructure to Improve Forest Health and Productivity: The Role of Tree Databases in Connecting Genomes, Phenomes, and the Environment

Despite tremendous advancements in high throughput sequencing, the vast majority of tree genomes, and in particular, forest trees, remain elusive. Although primary databases store genetic resources for just over 2,000 forest tree species, these are largely focused on sequence storage, basic genome assemblies, and functional assignment through existing pipelines. The tree databases reviewed here serve as secondary repositories for community data. They vary in their focal species, the data they curate, and the analytics provided, but they are united in moving toward a goal of centralizing both data access and analysis. They provide frameworks to view and update annotations for complex genomes, interrogate systems level expression profiles, curate data for comparative genomics, and perform real-time analysis with genotype and phenotype data. The organism databases of today are no longer simply catalogs or containers of genetic information. These repositories represent integrated cyberinfrastructure that support cross-site queries and analysis in web-based environments. These resources are striving to integrate across diverse experimental designs, sequence types, and related measures through ontologies, community standards, and web services. Efficient, simple, and robust platforms that enhance the data generated by the research community, contribute to improving forest health and productivity.

Multiomics analysis reveals a distinct mechanism of oleaginousness in the emerging model alga Chromochloris zofingiensis

Chromochloris zofingiensis, featured due to its capability to simultaneously synthesize triacylglycerol (TAG) and astaxanthin, is emerging as a leading candidate alga for production uses. To better understand the oleaginous mechanism of this alga, we conducted a multiomics analysis by systematically integrating time-resolved transcriptomes, lipidomes and metabolomes in response to nitrogen deprivation. The data analysis unraveled the distinct mechanism of TAG accumulation, which involved coordinated stimulation of multiple biological processes including supply of energy and reductants, carbon reallocation from protein and starch, and 'pushing' and 'pulling' carbon to TAG synthesis. Unlike the model alga Chlamydomonas, de novo fatty acid synthesis in C. zofingiensis was promoted, together with enhanced turnover of both glycolipids and phospholipids, supporting the drastic need of acyls for TAG assembly. Moreover, genomewide analysis identified many key functional enzymes and transcription factors that had engineering potential for TAG modulation. Two genes encoding glycerol-3-phosphate acyltransferase (GPAT), the first committed enzyme for TAG assembly, were found in the C. zofingiensis genome; in vivo functional characterization revealed that extrachloroplastic GPAT instead of chloroplastic GPAT played a central role in TAG synthesis. These findings illuminate distinct oleaginousness mechanisms in C. zofingiensis and pave the way towards rational manipulation of this alga to becone an emerging model for trait improvements.

Mosaic modularity: an updated perspective and research agenda for the evolution of vascular cambial growth

Secondary growth from a vascular cambium, present today only in seed plants and isoetalean lycophytes, has a 400-million-yr evolutionary history that involves considerably broader taxonomic diversity, most of it hidden in the fossil record. Approaching vascular cambial growth as a complex developmental process, we review data from living plants and fossils that reveal diverse modes of secondary growth. These are consistent with a modular nature of secondary growth, when considered as a tracheophyte-wide structural feature. This modular perspective identifies putative constituent developmental modules of cambial growth, for which we review developmental anatomy and regulation. Based on these data, we propose a hypothesis that explains the sources of diversity of secondary growth, considered across the entire tracheophyte clade, and opens up new avenues for exploring the origin of secondary growth. In this hypothesis, various modes of secondary growth reflect a mosaic pattern of expression of different developmental-regulatory modules among different lineages. We outline an approach that queries three information systems (living seed plants, living seed-free plants, and fossils) and integrates data on developmental regulation, anatomy, gene evolution and phylogeny to test the mosaic modularity hypothesis and its implications, and to inform efforts aimed at understanding the evolution of secondary growth.

A Family of NAI2-Interacting Proteins in the Biogenesis of the ER Body and Related Structures

Plants produce different types of endoplasmic reticulum (ER)-derived vesicles that accumulate and transport proteins, lipids, and metabolites. In the Brassicales, a distinct ER-derived structure called the ER body is found throughout the epidermis of cotyledons, hypocotyls, and roots. NAI2 is a key factor for ER body formation in Arabidopsis (Arabidopsis thaliana). Homologs of NAI2 are found only in the Brassicales and therefore may have evolved specifically to enable ER body formation. Here, we report that three related Arabidopsis NAI2-interacting proteins (NAIP1, NAIP2, and NAIP3) play a critical role in the biogenesis of ER bodies and related structures. Analysis using GFP fusions revealed that all three NAIPs are components of the ER bodies found in the cotyledons, hypocotyls, and roots. Genetic analysis with naip mutants indicates that they have a critical and redundant role in ER body formation. NAIP2 and NAIP3 are also components of other vesicular structures likely derived from the ER that are formed independent of NAI2 and are present not only in the cotyledons, hypocotyls, and roots, but also in the rosettes. Thus, while NAIP1 is a specialized ER body component, NAIP2 and NAIP3 are components of different types of ER-derived structures. Analysis of chimeric NAIP proteins revealed that their N-terminal domains play a major role in the functional specialization between NAIP1 and NAIP3. Unlike NAI2, NAIPs have homologs in all plants; therefore, NAIP-containing ER structures, from which the ER bodies in the Brassicales may have evolved, are likely to be present widely in plants.

Trans-lineage polymorphism and nonbifurcating diversification of the genus Picea

Nonbifurcating divergence caused by introgressive hybridization is continuously reported for groups of closely related species. In this study, we aimed to reconstruct the genome-scale classification of deep lineages of the conifer genus Picea, establish their phylogenetic relationships and test the bifurcating hypothesis between deeply branching lineages based on genomic data. We sequenced the transcriptomes of 35 individuals of 27 taxa covering all main lineages of the genus. Four major lineages, comprising three to 12 taxa each, largely consistent with morphological evidence, were recovered across the coalescent and integrated nuclear phylogeny. However, many of the individual gene trees recovered contradict one another. Moreover, the well-supported coalescent tree inferred here differs from previous studies based on various DNA markers, with respect to topology and inter-lineage relationships. We identified the shared polymorphisms between four major lineages. ABBA-BABA tests confirmed the inter-lineage gene flow and thus violated the bifurcating divergence model. Gene flow occurred more frequently between lineages distributed in the same continent than those disjunct between continents. Our results indicate that introgression and nonbifurcating diversification apply, even between deeply branching lineages of the conifer genus Picea.

Exome Resequencing Reveals Evolutionary History, Genomic Diversity, and Targets of Selection in the Conifers Pinus taeda and Pinus elliottii

Loblolly pine (Pinus taeda) and slash pine (Pinus elliottii) are ecologically and economically important pine species that dominate many forest ecosystems in the southern United States, but like all conifers, the study of their genetic diversity and demographic history has been hampered by their large genome size. A small number of studies mainly based on candidate-gene sequencing have been reported for P. taeda to date, whereas none are available for P. elliottii. Targeted exome resequencing has recently enabled population genomics studies for conifers, approach used here to assess genomic diversity, signatures of selection, population structure, and demographic history of P. elliottii and P. taeda. Extensive similarities were revealed between these species: both species feature rapid linkage disequilibrium decay and high levels of genetic diversity. Moreover, genome-wide positive correlations for measures of genetic diversity between the species were also observed, likely due to shared structural genomic constraints. Also, positive selection appears to be targeting a common set of genes in both pines. Demographic history differs between both species, with only P. taeda being affected by a dramatic bottleneck during the last glacial period. The ability of P. taeda to recover from a dramatic reduction in population size while still retaining high levels of genetic diversity shows promise for other pines facing environmental stressors associated with climate change, indicating that these too may be able to adapt successfully to new future conditions even after a drastic population size contraction.

Novel Insights into Plant Genome Evolution and Adaptation as Revealed through Transposable Elements and Non-Coding RNAs in Conifers

Plant genomes are punctuated by repeated bouts of proliferation of transposable elements (TEs), and these mobile bursts are followed by silencing and decay of most of the newly inserted elements. As such, plant genomes reflect TE-related genome expansion and shrinkage. In general, these genome activities involve two mechanisms: small RNA-mediated epigenetic repression and long-term mutational decay and deletion, that is, genome-purging. Furthermore, the spatial relationships between TE insertions and genes are an important force in shaping gene regulatory networks, their downstream metabolic and physiological outputs, and thus their phenotypes. Such cascading regulations finally set up a fitness differential among individuals. This brief review demonstrates factual evidence that unifies most updated conceptual frameworks covering genome size, architecture, epigenetic reprogramming, and gene expression. It aims to give an overview of the impact that TEs may have on genome and adaptive evolution and to provide novel insights into addressing possible causes and consequences of intimidating genome sizes (20⁻30 Gb) in a taxonomic group, conifers.

Pinus massoniana Introgression Hybrids Display Differential Expression of Reproductive Genes

Pinus massoniana and P. hwangshanensis are two conifer species located in southern China, which are of both economic and ornamental value. Around the middle and lower reaches of the Yangtze River, P. massoniana occurs mainly at altitudes below 700 m, while P. hwangshanensis can be found above 900 m. At altitudes where the distribution of both pines overlaps, a natural introgression hybrid exists, which we will further refer to as the Z pine. This pine has a morphological character that shares attributes of both P. massoniana and P. hwangshanensis. However, compared to the other two pines, its reproductive structure, the pinecone, has an ultra-low ripening rate with seeds that germinate poorly. In this study, we aimed to find the reason for the impaired cone maturation by comparing transcriptome libraries of P. massoniana and Z pine cones at seven successive growth stages. After sequencing and assembly, we obtained unigenes and then annotated them against NCBI’s non-redundant nucleotide and protein sequences, Swiss-Prot, Clusters of Orthologous Groups, Gene Ontology and KEGG Orthology databases. Gene expression levels were estimated and differentially expressed genes (DEGs) of the two pines were mined and analyzed. We found that several of them indeed relate to reproductive process. At every growth stage, these genes are expressed at a higher level in P. massoniana than in the Z pine. These data provide insight into understanding which molecular mechanisms are altered between P. massoniana and the Z pine that might cause changes in the reproductive process.

Genomic architecture of complex traits in loblolly pine

Dissecting the genetic and genomic architecture of complex traits is essential to understand the forces maintaining the variation in phenotypic traits of ecological and economical importance. Whole-genome resequencing data were used to generate high-resolution polymorphic single nucleotide polymorphism (SNP) markers and genotype individuals from common gardens across the loblolly pine (Pinus taeda) natural range. Genome-wide associations were tested with a large phenotypic dataset comprising 409 variables including morphological traits (height, diameter, carbon isotope discrimination, pitch canker resistance), and molecular traits such as metabolites and expression of xylem development genes. Our study identified 2335 new SNP × trait associations for the species, with many SNPs located in physical clusters in the genome of the species; and the genomic location of hotspots for metabolic × genotype associations. We found a highly polygenic basis of quantitative inheritance, with significant differences in number, effects size, genomic location and frequency of alleles contributing to variation in phenotypes in the different traits. While mutation-selection balance might be shaping the genetic variation in metabolic traits, balancing selection is more likely to shape the variation in expression of xylem development genes. Our work contributes to the study of complex traits in nonmodel plant species by identifying associations at a whole-genome level.

Gene duplication and evolution in recurring polyploidization-diploidization cycles in plants

BACKGROUND

The sharp increase of plant genome and transcriptome data provide valuable resources to investigate evolutionary consequences of gene duplication in a range of taxa, and unravel common principles underlying duplicate gene retention.

RESULTS

We survey 141 sequenced plant genomes to elucidate consequences of gene and genome duplication, processes central to the evolution of biodiversity. We develop a pipeline named DupGen_finder to identify different modes of gene duplication in plants. Genes derived from whole-genome, tandem, proximal, transposed, or dispersed duplication differ in abundance, selection pressure, expression divergence, and gene conversion rate among genomes. The number of WGD-derived duplicate genes decreases exponentially with increasing age of duplication events-transposed duplication- and dispersed duplication-derived genes declined in parallel. In contrast, the frequency of tandem and proximal duplications showed no significant decrease over time, providing a continuous supply of variants available for adaptation to continuously changing environments. Moreover, tandem and proximal duplicates experienced stronger selective pressure than genes formed by other modes and evolved toward biased functional roles involved in plant self-defense. The rate of gene conversion among WGD-derived gene pairs declined over time, peaking shortly after polyploidization. To provide a platform for accessing duplicated gene pairs in different plants, we constructed the Plant Duplicate Gene Database.

CONCLUSIONS

We identify a comprehensive landscape of different modes of gene duplication across the plant kingdom by comparing 141 genomes, which provides a solid foundation for further investigation of the dynamic evolution of duplicate genes.

Apollo: Democratizing genome annotation

Genome annotation is the process of identifying the location and function of a genome's encoded features. Improving the biological accuracy of annotation is a complex and iterative process requiring researchers to review and incorporate multiple sources of information such as transcriptome alignments, predictive models based on sequence profiles, and comparisons to features found in related organisms. Because rapidly decreasing costs are enabling an ever-growing number of scientists to incorporate sequencing as a routine laboratory technique, there is widespread demand for tools that can assist in the deliberative analytical review of genomic information. To this end, we present Apollo, an open source software package that enables researchers to efficiently inspect and refine the precise structure and role of genomic features in a graphical browser-based platform. Some of Apollo’s newer user interface features include support for real-time collaboration, allowing distributed users to simultaneously edit the same encoded features while also instantly seeing the updates made by other researchers on the same region in a manner similar to Google Docs. Its technical architecture enables Apollo to be integrated into multiple existing genomic analysis pipelines and heterogeneous laboratory workflow platforms. Finally, we consider the implications that Apollo and related applications may have on how the results of genome research are published and made accessible.

Stepwise large genome assembly approach: a case of Siberian larch (Larix sibirica Ledeb)

Background

De novo assembling of large genomes, such as in conifers (~ 12–30 Gbp), which also consist of ~ 80% of repetitive DNA, is a very complex and computationally intense endeavor. One of the main problems in assembling such genomes lays in computing limitations of nucleotide sequence assembly programs (DNA assemblers). As a rule, modern assemblers are usually designed to assemble genomes with a length not exceeding the length of the human genome (3.24 Gbp). Most assemblers cannot handle the amount of input sequence data required to provide sufficient coverage needed for a high-quality assembly.

Results

An original stepwise method of de novo assembly by parts (sets), which allows to bypass the limitations of modern assemblers associated with a huge amount of data being processed, is presented in this paper. The results of numerical assembling experiments conducted using the model plant Arabidopsis thaliana, Prunus persica (peach) and four most popular assemblers, ABySS, SOAPdenovo, SPAdes, and CLC Assembly Cell, showed the validity and effectiveness of the proposed stepwise assembling method.

Conclusion

Using the new stepwise de novo assembling method presented in the paper, the genome of Siberian larch, Larix sibirica Ledeb. (12.34 Gbp) was completely assembled de novo by the CLC Assembly Cell assembler. It is the first genome assembly for larch species in addition to only five other conifer genomes sequenced and assembled for Picea abies, Picea glauca, Pinus taeda, Pinus lambertiana, and Pseudotsuga menziesii var. menziesii.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2570-y) contains supplementary material, which is available to authorized users.

De novo genome assembly of the stress tolerant forest species Casuarina equisetifolia provides insight into secondary growth

Casuarina equisetifolia (C. equisetifolia), a conifer-like angiosperm with resistance to typhoon and stress tolerance, is mainly cultivated in the coastal areas of Australasia. C. equisetifolia, making it a valuable model to study secondary growth associated genes and stress-tolerance traits. However, the genome sequence is unavailable and therefore wood-associated growth rate and stress resistance at the molecular level is largely unexplored. We therefore constructed a high-quality draft genome sequence of C. equisetifolia by a combination of Illumina second-generation sequencing reads and Pacific Biosciences single-molecule real-time (SMRT) long reads to advance the investigation of this species. Here, we report the genome assembly, which contains approximately 300 megabases (Mb) and scaffold size of N50 is 1.06 Mb. Additionally, gene annotation, assisted by a combination of prediction and RNA-seq data, generated 29 827 annotated protein-coding genes and 1983 non-coding genes, respectively. Furthermore, we found that the total number of repetitive sequences account for one-third of the genome assembly. Here we also construct the genome-wide map of DNA modification, such as two novel forms N⁶ -adenine (6mA) and N4-methylcytosine (4mC) at the level of single-nucleotide resolution using single-molecule real-time (SMRT) sequencing. Interestingly, we found that 17% of 6mA modification genes and 15% of 4mC modification genes also included alternative splicing events. Finally, we investigated cellulose, hemicellulose, and lignin-related genes, which were associated with secondary growth and contained different DNA modifications. The high-quality genome sequence and annotation of C. equisetifolia in this study provide a valuable resource to strengthen our understanding of the diverse traits of trees.

Selective single molecule sequencing and assembly of a human Y chromosome of African origin

Mammalian Y chromosomes are often neglected from genomic analysis. Due to their inherent assembly difficulties, high repeat content, and large ampliconic regions, only a handful of species have their Y chromosome properly characterized. To date, just a single human reference quality Y chromosome, of European ancestry, is available due to a lack of accessible methodology. To facilitate the assembly of such complicated genomic territory, we developed a novel strategy to sequence native, unamplified flow sorted DNA on a MinION nanopore sequencing device. Our approach yields a highly continuous assembly of the first human Y chromosome of African origin. It constitutes a significant improvement over comparable previous methods, increasing continuity by more than 800%. Sequencing native DNA also allows to take advantage of the nanopore signal data to detect epigenetic modifications in situ. This approach is in theory generalizable to any species simplifying the assembly of extremely large and repetitive genomes.

Due to various structural and sequence complexities, the human Y chromosome is challenging to sequence and characterize. Here, the authors develop a strategy to sequence native, unamplified flow sorted Y chromosomes with a nanopore sequencing platform, and report the first assembly of a human Y chromosome of African origin.

Assessment of flux through oleoresin biosynthesis in epithelial cells of loblolly pine resin ducts

The shoot system of pines contains abundant resin ducts, which harbor oleoresins that play important roles in constitutive and inducible defenses. In a pilot study, we assessed the chemical diversity of oleoresins obtained from mature tissues of loblolly pine trees (Pinus taeda L.). Building on these data sets, we designed experiments to assess oleoresin biosynthesis in needles of 2-year-old saplings. Comparative transcriptome analyses of single cell types indicated that genes involved in the biosynthesis of oleoresins are significantly enriched in isolated epithelial cells of resin ducts, compared with those expressed in mesophyll cells. Simulations using newly developed genome-scale models of epithelial and mesophyll cells, which incorporate our data on oleoresin yield and composition as well as gene expression patterns, predicted that heterotrophic metabolism in epithelial cells involves enhanced levels of oxidative phosphorylation and fermentation (providing redox and energy equivalents). Furthermore, flux was predicted to be more evenly distributed across the metabolic network of mesophyll cells, which, in contrast to epithelial cells, do not synthesize high levels of specialized metabolites. Our findings provide novel insights into the remarkable specialization of metabolism in epithelial cells.

Diversity and Evolution of Sensor Histidine Kinases in Eukaryotes

Histidine kinases (HKs) are primary sensor proteins that act in cell signaling pathways generically referred to as "two-component systems" (TCSs). TCSs are among the most widely distributed transduction systems used by both prokaryotic and eukaryotic organisms to detect and respond to a broad range of environmental cues. The structure and distribution of HK proteins are now well documented in prokaryotes, but information is still fragmentary for eukaryotes. Here, we have taken advantage of recent genomic resources to explore the structural diversity and the phylogenetic distribution of HKs in the prominent eukaryotic supergroups. Searches of the genomes of 67 eukaryotic species spread evenly throughout the phylogenetic tree of life identified 748 predicted HK proteins. Independent phylogenetic analyses of predicted HK proteins were carried out for each of the major eukaryotic supergroups. This allowed most of the compiled sequences to be categorized into previously described HK groups. Beyond the phylogenetic analysis of eukaryotic HKs, this study revealed some interesting findings: 1) characterization of some previously undescribed eukaryotic HK groups with predicted functions putatively related to physiological traits; 2) discovery of HK groups that were previously believed to be restricted to a single kingdom in additional supergroups, and 3) indications that some evolutionary paths have led to the appearance, transfer, duplication, and loss of HK genes in some phylogenetic lineages. This study provides an unprecedented overview of the structure and distribution of HKs in the Eukaryota and represents a first step toward deciphering the evolution of TCS signaling in living organisms.

Combining mitochondrial and nuclear genome analyses to dissect the effects of colonization, environment, and geography on population structure in Pinus tabuliformis

The phylogeographic histories of plants in East Asia are complex and shaped by both past large-scale climatic oscillations and dramatic tectonic events. The impact of these historic events, as well as ecological adaptation, on the distribution of biodiversity remains to be elucidated. Pinus tabuliformis is the dominant coniferous tree in northern China, with a large distribution across wide environmental gradients. We examined genetic variation in this species using genotyping-by-sequencing and mitochondrial (mt) DNA markers. We found population structure on both nuclear and mt genomes with a geographic pattern that corresponds well with the landscape of northern China. To understand the contributions of environment, geography, and colonization history to the observed population structure, we performed ecological niche modeling and partitioned the among-population genomic variance into isolation by environment (IBE), isolation by distance (IBD), and isolation by colonization (IBC). We used mtDNA, which is transmitted by seeds in pine, to reflect colonization. We found little impact of IBE, IBD, and IBC on variation in neutral SNPs, but significant impact of IBE on a group of outlier loci. The lack of IBC illustrates that the maternal history can be quickly eroded from the nuclear genome by high rates of gene flow. Our results suggest that genomic variation in P. tabuliformis is largely affected by neutral and stochastic processes, and the signature of local adaptation is visible only at robust outlier loci. This study enriches our understanding on the complex evolutionary forces that shape the distribution of genetic variation in plant taxa in northern China, and guides breeding, conservation, and reforestation programs for P. tabuliformis.

LTR Retrotransposons Show Low Levels of Unequal Recombination and High Rates of Intraelement Gene Conversion in Large Plant Genomes

The accumulation and removal of transposable elements (TEs) is a major driver of genome size evolution in eukaryotes. In plants, long terminal repeat (LTR) retrotransposons (LTR-RTs) represent the majority of TEs and form most of the nuclear DNA in large genomes. Unequal recombination (UR) between LTRs leads to removal of intervening sequence and formation of solo-LTRs. UR is a major mechanism of LTR-RT removal in many angiosperms, but our understanding of LTR-RT-associated recombination within the large, LTR-RT-rich genomes of conifers is quite limited. We employ a novel read-based methodology to estimate the relative rates of LTR-RT-associated UR within the genomes of four conifer and seven angiosperm species. We found the lowest rates of UR in the largest genomes studied, conifers and the angiosperm maize. Recombination may also resolve as gene conversion, which does not remove sequence, so we analyzed LTR-RT-associated gene conversion events (GCEs) in Norway spruce and six angiosperms. Opposite the trend for UR, we found the highest rates of GCEs in Norway spruce and maize. Unlike previous work in angiosperms, we found no evidence that rates of UR correlate with retroelement structural features in the conifers, suggesting that another process is suppressing UR in these species. Recent results from diverse eukaryotes indicate that heterochromatin affects the resolution of recombination, by favoring gene conversion over crossing-over, similar to our observation of opposed rates of UR and GCEs. Control of LTR-RT proliferation via formation of heterochromatin would be a likely step toward large genomes in eukaryotes carrying high LTR-RT content.

Breeding and Engineering Trees to Accumulate High Levels of Terpene Metabolites for Plant Defense and Renewable Chemicals

Plants evolved the capacity to synthesize highly diverse sets of secondary metabolites which are important for plant adaptation and health. In forest trees, many classes of compounds, particularly ones related to defense against insects, fungi, and bacteria accumulate to levels that enable their recovery and commercial use. One of the oldest examples is conifer terpenes, but terpenes are important secondary products from other tree species including eucalypts. Because terpenes, latex, and natural gums are synthesized and stored in specialized secretory glands, ducts, and laticifers in mostly pure forms they can be collected from live trees in addition to being extracted during industrial processing of wood. This minireview describes the potential of breeding and genetic engineering approaches to increase the quantities of terpene secondary metabolites to increase the amount of secondary products and thereby increasing the value of planted and managed forest trees. I advance the view that breeding and genetic engineering of metabolic pathways and specialized cell secretory structures can dramatically increase tissue terpene content.

Genome assembly of the Pink Ipê (Handroanthus impetiginosus, Bignoniaceae), a highly valued, ecologically keystone Neotropical timber forest tree

Background

Handroanthus impetiginosus (Mart. ex DC.) Mattos is a keystone Neotropical hardwood tree widely distributed in seasonally dry tropical forests of South and Mesoamerica. Regarded as the “new mahogany,” it is the second most expensive timber, the most logged species in Brazil, and currently under significant illegal trading pressure. The plant produces large amounts of quinoids, specialized metabolites with documented antitumorous and antibiotic effects. The development of genomic resources is needed to better understand and conserve the diversity of the species, to empower forensic identification of the origin of timber, and to identify genes for important metabolic compounds.

Findings

The genome assembly covers 503.7 Mb (N50 = 81 316 bp), 90.4% of the 557-Mbp genome, with 13 206 scaffolds. A repeat database with 1508 sequences was developed, allowing masking of ∼31% of the assembly. Depth of coverage indicated that consensus determination adequately removed haplotypes assembled separately due to the extensive heterozygosity of the species. Automatic gene prediction provided 31 688 structures and 35 479 messenger RNA transcripts, while external evidence supported a well-curated set of 28 603 high-confidence models (90% of total). Finally, we used the genomic sequence and the comprehensive gene content annotation to identify genes related to the production of specialized metabolites.

Conclusions

This genome assembly is the first well-curated resource for a Neotropical forest tree and the first one for a member of the Bignoniaceae family, opening exceptional opportunities to empower molecular, phytochemical, and breeding studies. This work should inspire the development of similar genomic resources for the largely neglected forest trees of the mega-diverse tropical biomes.

Independent assessment and improvement of wheat genome sequence assemblies using Fosill jumping libraries

Background

The accurate sequencing and assembly of very large, often polyploid, genomes remains a challenging task, limiting long-range sequence information and phased sequence variation for applications such as plant breeding. The 15-Gb hexaploid bread wheat (Triticum aestivum) genome has been particularly challenging to sequence, and several different approaches have recently generated long-range assemblies. Mapping and understanding the types of assembly errors are important for optimising future sequencing and assembly approaches and for comparative genomics.

Results

Here we use a Fosill 38-kb jumping library to assess medium and longer–range order of different publicly available wheat genome assemblies. Modifications to the Fosill protocol generated longer Illumina sequences and enabled comprehensive genome coverage. Analyses of two independent Bacterial Artificial Chromosome (BAC)-based chromosome-scale assemblies, two independent Illumina whole genome shotgun assemblies, and a hybrid Single Molecule Real Time (SMRT-PacBio) and short read (Illumina) assembly were carried out. We revealed a surprising scale and variety of discrepancies using Fosill mate-pair mapping and validated several of each class. In addition, Fosill mate-pairs were used to scaffold a whole genome Illumina assembly, leading to a 3-fold increase in N50 values.

Conclusions

Our analyses, using an independent means to validate different wheat genome assemblies, show that whole genome shotgun assemblies based solely on Illumina sequences are significantly more accurate by all measures compared to BAC-based chromosome-scale assemblies and hybrid SMRT-Illumina approaches. Although current whole genome assemblies are reasonably accurate and useful, additional improvements will be needed to generate complete assemblies of wheat genomes using open-source, computationally efficient, and cost-effective methods.

Exploring the genetic basis of gene transcript abundance and metabolite levels in loblolly pine (Pinus taeda L.) using association mapping and network construction

BACKGROUND

Identifying genetic variations that shape important complex traits is fundamental to the genetic improvement of important forest tree species, such as loblolly pine (Pinus taeda L.), which is one of the most commonly planted forest tree species in the southern U.S. Gene transcripts and metabolites are important regulatory intermediates that link genetic variations to higher-order complex traits such as wood development and drought response. A few prior studies have associated intermediate phenotypes including mRNA expression and metabolite levels with a limited number of molecular markers, but the identification of genetic variations that regulate intermediate phenotypes needs further investigation.

RESULTS

We identified 1841 single nucleotide polymorphisms (SNPs) associated with 191 gene expression mRNA phenotypes and 524 SNPs associated with 53 metabolite level phenotypes using 2.8 million exome-derived SNPs. The identified SNPs reside in genes with a wide variety of functions. We further integrated the identified SNPs and the associated expressed genes and metabolites into networks. We described the SNP-SNP interactions that significantly impacted the gene transcript abundance and metabolite level in the networks. Key loci and genes in the wood development and drought response networks were identified and analyzed.

CONCLUSIONS

This work provides new candidate genes for research on the genetic basis of gene expression and metabolism linked to wood development and drought response in loblolly pine and highlights the efficiency of using association-mapping-based networks to discover candidate genes with important roles in complex biological processes.

Molecular signatures of divergence and selection in closely related pine taxa

Efforts to detect loci under selection in plants have mostly focussed on single species. However, assuming that intraspecific divergence may lead to speciation, comparisons of genetic variation within and among recently diverged taxa can help to locate such genes. In this study, coalescent and outlier detection methods were used to assess nucleotide polymorphism and divergence at 79 nuclear gene fragments (1212 SNPs) in 16 populations (153 individuals) of the closely related, but phenotypically and ecologically distinct, pine taxa Pinus mugo, P. uliginosa and P. uncinata across their European distributions. Simultaneously, mitochondrial DNA markers, which are maternally inherited in pines and distributed by seeds at short geographic distance, were used to assess genetic relationships of the focal populations and taxa. The majority of nuclear loci showed homogenous patterns of variation between the taxa due to a high number of shared SNPs and haplotypes, similar levels of polymorphism, and low net divergence. However, against this common genetic background and an overall low population structure within taxa at mitochondrial markers, we identified several genes showing signatures of selection, accompanied by significant intra- and interspecific divergence. Our results indicate that loci involved in species divergence may be involved in intraspecific local adaptation.

Modularity of genes involved in local adaptation to climate despite physical linkage

BACKGROUND

Linkage among genes experiencing different selection pressures can make natural selection less efficient. Theory predicts that when local adaptation is driven by complex and non-covarying stresses, increased linkage is favored for alleles with similar pleiotropic effects, with increased recombination favored among alleles with contrasting pleiotropic effects. Here, we introduce a framework to test these predictions with a co-association network analysis, which clusters loci based on differing associations. We use this framework to study the genetic architecture of local adaptation to climate in lodgepole pine, Pinus contorta, based on associations with environments.

RESULTS

We identify many clusters of candidate genes and SNPs associated with distinct environments, including aspects of aridity and freezing, and discover low recombination rates among some candidate genes in different clusters. Only a few genes contain SNPs with effects on more than one distinct aspect of climate. There is limited correspondence between co-association networks and gene regulatory networks. We further show how associations with environmental principal components can lead to misinterpretation. Finally, simulations illustrate both benefits and caveats of co-association networks.

CONCLUSIONS

Our results support the prediction that different selection pressures favor the evolution of distinct groups of genes, each associating with a different aspect of climate. But our results went against the prediction that loci experiencing different sources of selection would have high recombination among them. These results give new insight into evolutionary debates about the extent of modularity, pleiotropy, and linkage in the evolution of genetic architectures.

Transcriptome sequencing of Pinus kesiya var. langbianensis and comparative analysis in the Pinus phylogeny

Background

Pines are widely distributed in the Northern Hemisphere and have a long evolutionary history. The availability of transcriptome data has facilitated comparative transcriptomics for studying the evolutionary patterns associated with the different geographical distributions of species in the Pinus phylogeny.

Results

The transcriptome of Pinus kesiya var. langbianensis was sequenced using the Illumina HiSeq 2000 platform, and a total of 68,881 unigenes were assembled by Trinity. Transcriptome sequences of another 12 conifer species were downloaded from public databases. All of the pairwise orthologues were identified by comparative transcriptome analysis in 13 conifer species, from which the rate of diversification was calculated and a phylogenetic tree inferred. All of the fast-evolving positive selection sequences were identified, and some salt-, drought-, and abscisic acid-resistance genes were discovered.

Conclusions

mRNA sequences of P. kesiya var. langbianensis were obtained by transcriptome sequencing, and a large number of simple sequence repeat and short nucleotide polymorphism loci were detected. These data can be used in molecular marker-assisted selected in pine breeding. Divergence times were estimated in the 13 conifer species using comparative transcriptomic analysis. A number of positive selection genes were found to be related to environmental factors. Salt- and abscisic acid-related genes exhibited different selection patterns between coastal and inland Pinus. Our findings help elucidate speciation patterns in the Pinus lineage.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5127-6) contains supplementary material, which is available to authorized users.

Multiple haploids, triploids, and tetraploids found in modern-day "living fossil" Ginkgo biloba

Ginkgo biloba, the last extant representative of a lineage of Mesozoic gymnosperms, is one of the few seed plants with an exceptionally long (~300 Myr) evolutionary history free of genome-wide duplications (polyploidy). Despite this genome conservatism, we have recently found a viable spontaneous tetraploid Ginkgo sapling during routine screening of several plants, demonstrating that natural polyploidy is possible in Ginkgo. Here we provide a much wider flow cytometry survey of ploidy in some European Ginkgo collections, and own seedlings (>2200 individuals and ~200 cultivars). We found a surprisingly high level of ploidy variation in modern-day Ginkgo and documented altogether 13 haploid, 3 triploid, and 10 tetraploid Ginkgo plants or cultivars, most of them being morphologically distinct from common diploids. Haploids frequently produced polyploid (dihaploid) buds or branches. Tetraploids showed some genome size variation. The surveyed plants provide a unique resource for future Ginkgo research and breeding, and they might be used to accelerate the modern diversification of this nearly extinct plant lineage.

Draft genome of a high value tropical timber tree, Teak (Tectona grandis L. f): insights into SSR diversity, phylogeny and conservation

Teak (Tectona grandis L. f.) is one of the precious bench mark tropical hardwood having qualities of durability, strength and visual pleasantries. Natural teak populations harbour a variety of characteristics that determine their economic, ecological and environmental importance. Sequencing of whole nuclear genome of teak provides a platform for functional analyses and development of genomic tools in applied tree improvement. A draft genome of 317 Mb was assembled at 151× coverage and annotated 36, 172 protein-coding genes. Approximately about 11.18% of the genome was repetitive. Microsatellites or simple sequence repeats (SSRs) are undoubtedly the most informative markers in genotyping, genetics and applied breeding applications. We generated 182,712 SSRs at the whole genome level, of which, 170,574 perfect SSRs were found; 16,252 perfect SSRs showed in silico polymorphisms across six genotypes suggesting their promising use in genetic conservation and tree improvement programmes. Genomic SSR markers developed in this study have high potential in advancing conservation and management of teak genetic resources. Phylogenetic studies confirmed the taxonomic position of the genus Tectona within the family Lamiaceae. Interestingly, estimation of divergence time inferred that the Miocene origin of the Tectona genus to be around 21.4508 million years ago.

Application of EST-SSR markers developed from the transcriptome of Torreya grandis (Taxaceae), a threatened nut-yielding conifer tree

Torreya grandis (Taxaceae) is an ancient conifer species endemic to southeast China. Because of its nutrient-rich and delicious seeds, this species has been utilized for centuries by the Chinese. However, transcriptome data and transcriptome-derived microsatellite markers for population genetics studies are still insufficient for understanding of this species’ genetic basis. In this study, a transcriptome from T. grandis leaves was generated using Illumina sequencing. A total of 69,920 unigenes were generated after de novo assembly, and annotated by searching against seven protein databases. In addition, 2,065 expressed sequence tag–simple sequence repeats (EST-SSRs) were detected, with the distribution frequency of 2.75% of total unigenes and average number of 0.03 SSRs per unigene. Among these EST-SSRs, 1,339 primer pairs were successfully designed, and 106 primer pairs were randomly selected for the development of potential molecular markers. Among them, 11 EST-SSR markers revealed a moderate level of genetic diversity, and were used to investigate the population structure of T. grandis. Two different genetic groups within this species were revealed using these EST-SSR markers, indicating that these markers developed in this study can be effectively applied to the population genetic analysis of T. grandis.

Potential of Genome-Wide Studies in Unrelated Plus Trees of a Coniferous Species, Cryptomeria japonica (Japanese Cedar)

A genome-wide association study (GWAS) was conducted on more than 30,000 single nucleotide polymorphisms (SNPs) in unrelated first-generation plus tree genotypes from three populations of Japanese cedar Cryptomeria japonica D. Don with genomic prediction for traits of growth, wood properties and male fecundity. Among the assessed populations, genetic characteristics including the extent of linkage disequilibrium (LD) and genetic structure differed and these differences are considered to be due to differences in genetic background. Through population-independent GWAS, several significant SNPs found close to the regions associated with each of these traits and shared in common across the populations were identified. The accuracies of genomic predictions were dependent on the traits and populations and reflected the genetic architecture of traits and genetic characteristics. Prediction accuracies using SNPs selected based on GWAS results were similar to those using all SNPs for several combinations of traits and populations. We discussed the application of genome-wide studies for C. japonica improvement.

Structural variation and rates of genome evolution in the grass family seen through comparison of sequences of genomes greatly differing in size

Homology was searched with genes annotated in the Aegilops tauschii pseudomolecules against genes annotated in the pseudomolecules of tetraploid wild emmer wheat, Brachypodium distachyon, sorghum and rice. Similar searches were performed with genes annotated in the rice pseudomolecules. Matrices of collinear genes and rearrangements in their order were constructed. Optical BioNano genome maps were constructed and used to validate rearrangements unique to the wild emmer and Ae. tauschii genomes. Most common rearrangements were short paracentric inversions and short intrachromosomal translocations. Intrachromosomal translocations outnumbered segmental intrachromosomal duplications. The densities of paracentric inversion lengths were approximated by exponential distributions in all six genomes. Densities of collinear genes along the Ae. tauschii chromosomes were highly correlated with meiotic recombination rates but those of rearrangements were not, suggesting different causes of the erosion of gene collinearity and evolution of major chromosome rearrangements. Frequent rearrangements sharing breakpoints suggested that chromosomes have been rearranged recurrently at some sites. The distal 4 Mb of the short arms of rice chromosomes Os11 and Os12 and corresponding regions in the sorghum, B. distachyon and Triticeae genomes contain clusters of interstitial translocations including from 1 to 7 collinear genes. The rates of acquisition of major rearrangements were greater in the large wild emmer wheat and Ae. tauschii genomes than in the lineage preceding their divergence or in the B. distachyon, rice and sorghum lineages. It is suggested that synergy between large quantities of dynamic transposable elements and annual growth habit have been the primary causes of the fast evolution of the Triticeae genomes.