Genome-wide distribution and organization of microsatellites in plants: an insight into marker development in Brachypodium

Whole-genome sequencing and identification of antimicrobial peptide coding genes in parsley (Petroselinum crispum), an important culinary and medicinal Apiaceae species

Parsley is a commonly cultivated Apiaceae species of culinary and medicinal importance. Parsley has several recognized health benefits and the species has been utilized in traditional medicine since ancient times. Although parsley is among the most commonly cultivated members of Apiaceae, no systematic genomic research has been conducted on parsley. In the present work, parsley genome was sequenced using the long-read HiFi (high fidelity) sequencing technology and a draft contig assembly of 1.57 Gb that represents 80.9% of the estimated genome size was produced. The assembly was highly repeat-rich with a repetitive DNA content of 81%. The assembly was phased into a primary and alternate assembly in order to minimize redundant contigs. Scaffolds were constructed with the primary assembly contigs, which were used for the identification of AMP (antimicrobial peptide) genes. Characteristic AMP domains and 3D structures were used to detect and verify antimicrobial peptides. As a result, 23 genes (PcAMP1-23) representing defensin, snakin, thionin, lipid transfer protein and vicilin-like AMP classes were identified. Bioinformatic analyses for the characterization of peptide physicochemical properties indicated that parsley AMPs are extracellular peptides, therefore, plausibly exert their antimicrobial effects through the most commonly described AMP action mechanism of membrane attack. AMPs are attracting increasing attention since they display their fast antimicrobial effects in small doses on both plant and animal pathogens with a significantly reduced risk of resistance development. Therefore, identification and characterization of AMPs is important for their incorporation into plant disease management protocols as well as medicinal research for the treatment of multi-drug resistant infections.

Comparative chloroplast genomes of Dactylicapnos species: insights into phylogenetic relationships

BACKGROUND

Dactylicapnos is a climbing herbaceous vine, distributed from the Himalayas to southwestern China, and some of the species have important medicinal values. However, the chloroplast genomes of Dactylicapnos have never been investigated. In this study, chloroplast genomes of seven Dactylicapnos species covering all three sections and one informal group of Dactylicapnos were sequenced and assembled, and the detailed comparative analyses of the chloroplast genome structure were provided for the first time.

RESULTS

The results showed that the chloroplast genomes of Dactylicapnos have a typical quadripartite structure with lengths from 172,344 bp to 176,370 bp, encoding a total of 133-140 genes, containing 88-94 protein-coding genes, 8 rRNAs and 37-39 tRNAs. 31 codons were identified as relative synonymous codon usage values greater than one in the chloroplast genome of Dactylicapnos genus based on 80 protein-coding genes. The results of the phylogenetic analysis showed that seven Dactylicapnos species can be divided into three main categories. Phylogenetic analysis revealed that seven species form three major clades which should be treated as three sections.

CONCLUSIONS

This study provides the initial report of the chloroplast genomes of Dactylicapnos, their structural variation, comparative genomic and phylogenetic analysis for the first time. The results provide important genetic information for development of medical resources, species identification, infrageneric classification and diversification of Dactylicapnos.

Comparative Analysis of Chloroplast Genome of Meconopsis (Papaveraceae) Provides Insights into Their Genomic Evolution and Adaptation to High Elevation

The Meconopsis species are widely distributed in the Qinghai-Tibet Plateau, Himalayas, and Hengduan Mountains in China, and have high medicinal and ornamental value. The high diversity of plant morphology in this genus poses significant challenges for species identification, given their propensity for highland dwelling, which makes it a question worth exploring how they cope with the harsh surroundings. In this study, we recently generated chloroplast (cp) genomes of two Meconopsis species, Meconopsis paniculata (M. paniculata) and M. pinnatifolia, and compared them with those of ten Meconopsis cp genomes to comprehend cp genomic features, their phylogenetic relationships, and what part they might play in plateau adaptation. These cp genomes shared a great deal of similarities in terms of genome size, structure, gene content, GC content, and codon usage patterns. The cp genomes were between 151,864 bp and 154,997 bp in length, and contain 133 predictive genes. Through sequence divergence analysis, we identified three highly variable regions (trnD-psbD, ccsA-ndhD, and ycf1 genes), which could be used as potential markers or DNA barcodes for phylogenetic analysis. Between 22 and 38 SSRs and some long repeat sequences were identified from 12 Meconopsis species. Our phylogenetic analysis confirmed that 12 species of Meconopsis clustered into a monophyletic clade in Papaveraceae, which corroborated their intrageneric relationships. The results indicated that M. pinnatifolia and M. paniculata are sister species in the phylogenetic tree. In addition, the atpA and ycf2 genes were positively selected in high-altitude species. The functions of these two genes might be involved in adaptation to the extreme environment in the cold and low CO2 concentration conditions at the plateau.

Full-length transcriptome and RNA-Seq analyses reveal the resistance mechanism of sesame in response to Corynespora cassiicola

BACKGROUND

Corynespora leaf spot is a common leaf disease occurring in sesame, and the disease causes leaf yellowing and even shedding, which affects the growth quality of sesame. At present, the mechanism of sesame resistance to this disease is still unclear. Understanding the resistance mechanism of sesame to Corynespora leaf spot is highly important for the control of infection. In this study, the leaves of the sesame resistant variety (R) and the sesame susceptible variety (S) were collected at 0-48 hpi for transcriptome sequencing, and used a combined third-generation long-read and next-generation short-read technology approach to identify some key genes and main pathways related to resistance.

RESULTS

The gene expression levels of the two sesame varieties were significantly different at 0, 6, 12, 24, 36 and 48 hpi, indicating that the up-regulation of differentially expressed genes in the R might enhanced the resistance. Moreover, combined with the phenotypic observations of sesame leaves inoculated at different time points, we found that 12 hpi was the key time point leading to the resistance difference between the two sesame varieties at the molecular level. The WGCNA identified two modules significantly associated with disease resistance, and screened out 10 key genes that were highly expressed in R but low expressed in S, which belonged to transcription factors (WRKY, AP2/ERF-ERF, and NAC types) and protein kinases (RLK-Pelle_DLSV, RLK-Pelle_SD-2b, and RLK-Pelle_WAK types). These genes could be the key response factors in the response of sesame to infection by Corynespora cassiicola. GO and KEGG enrichment analysis showed that specific modules could be enriched, which manifested as enrichment in biologically important pathways, such as plant signalling hormone transduction, plant-pathogen interaction, carbon metabolism, phenylpropanoid biosynthesis, glutathione metabolism, MAPK and other stress-related pathways.

CONCLUSIONS

This study provides an important resource of genes contributing to disease resistance and will deepen our understanding of the regulation of disease resistance, paving the way for further molecular breeding of sesame.

Genome-wide survey and characterization of microsatellites in cashew and design of a web-based microsatellite database: CMDB

The cashew is an edible tree nut crop having a wide range of food and industrial applications. Despite great economic importance, the genome-wide characterization of microsatellites [simple sequence repeats (SSRs)] in cashew is lacking. In this study, we carried out the first comprehensive genome-wide microsatellites/SSRs characterization in cashew and developed polymorphic markers and a web-based microsatellite database. A total of 54526 SSRs were discovered in the cashew genome, with a mean frequency of 153 SSRs/Mb. Among the mined genome-wide SSRs (2-6 bp size motifs), the dinucleotide repeat motifs were dominant (68.98%) followed by the trinucleotides (24.56%). The Class I type of SSRs (≥20 bp) were 45.10%, while Class II repeat motifs (≥12-<20 bp) were 54.89% of the total genomic SSRs discovered here. Further, the AT-rich SSRs occurred more frequently in the cashew genome (84%) compared to the GC-rich SSRs. The validation of the in silico-mined genome-wide SSRs by PCR screening in cashew genotypes resulted in the development of 59 polymorphic SSR markers, and the polymorphism information content (PIC) of the polymorphic SSR markers ranged from 0.19 to 0.84. Further, a web-based database, "Cashew Microsatellite Database (CMDB)," was constructed to provide access to the genome-wide SSRs mined in this study as well as transcriptome-based SSRs from our previous study to the research community through a user-friendly searchable interface. Besides, CMDB provides information on experimentally validated SSRs. CMDB permits the retrieval of SSR markers information with the customized search options. Altogether, the genome-wide SSRs characterization, the polymorphic markers and CMDB database developed in this study would serve as valuable marker resources for DNA fingerprinting, germplasm characterization, genetic studies, and molecular breeding in cashew and related Anacardium species.

Small cardamom genome: development and utilization of microsatellite markers from a draft genome sequence of Elettaria cardamomum Maton

Small cardamom (Elettaria cardamomum Maton), the queen of spices, is the third most expensive spice in the world after saffron and vanilla, valued highly for its aroma and taste. This perennial herbaceous plant is a native of coastal parts of Southern India and displays a significant amount of morphological diversity. Its genetic potential has not been exploited due to lack of genomic resources limiting our understanding of the genome and important metabolic pathways which give it the economic advantage in the spice industry. Here, we report upon the de novo assembled, draft whole genome sequence of cardamom variety, Njallani Green Gold. We used a hybrid assembly strategy using the reads from the Oxford Nanopore, Illumina and 10x Genomics GemCode sequencing chemistries. The assembled genome length was 1.06 Gb (gigabases) which is close to the estimated genome size of cardamom. More than 75% of the genome was captured in 8000 scaffolds with a N50 of 0.15 Mb. The genome appears to have a high repeat content and 68055 gene models were predicted. The genome is close to Musa species and displays an expansion and contraction in different gene families. The draft assembly was used for in silico mining of simple sequence repeats (SSRs). A total of 2,50,571 SSRs were identified of which 2,18,270 were perfect SSRs and 32,301 were compound SSRs. Among the perfect SSRs, trinucleotides were most abundant (1,25,329) and hexanucleotide repeats appear least (2,380). From the 2,50,571 SSRs mined, 2,27,808 primer pairs were designed based on flanking sequence information. Wet lab validation was performed for 246 SSR loci and based on their amplification profiles, 60 SSR markers were used for diversity analysis of a set of 60 diverse cardamom accessions. The average number of alleles detected per locus were 14.57 with a minimum of 4 and maximum of 30 alleles. Population structure analysis revealed the presence of high degree of admixtures which could primarily be due to cross-pollination prevalent in this species. The SSR markers identified would help in the development of gene or trait-linked markers which can be subsequently used for marker-assisted breeding for crop improvement in cardamom. The information on utilization of the SSR loci for generation of markers has been developed into a public database, 'cardamomSSRdb' that is freely available for use by the cardamom community.

Genome-wide characterization and development of SSR markers for genetic diversity analysis in northwestern Himalayas Walnut (Juglans regia L.)

In the present study, we designed and validated genome-wide polymorphic SSR markers (110 SSRs) by mining the walnut genome. A total of 198,924 SSR loci were identified. Among these, successful primers were designed for 162,594 (81.73%) SSR loci. Dinucleotides were the most predominant accounting for 88.40% (175,075) of total SSRs. The SSR frequency was 377.312 SSR/Mb and it showed a decreasing trend from dinucleotide to octanucleotide motifs. We identified 20 highly polymorphic SSR markers and used them to genotype 72 walnut accessions. Over all, we obtained 118 alleles that ranged from 2 to 12 with an average value of 5.9. The higher SSR PIC values indicate their robustness in discriminating walnut genotypes. Heat map, PCA, and population structure categorized 72 walnut genotypes into 2 distinct clusters. The genetic variation within population was higher than among population as inferred by analysis of molecular variance (AMOVA). For walnut improvement, it is necessary to have a large repository of SSRs with high discriminative power. The present study reports 150,000 SSRs, which is the largest SSR repository for this important nut crop. Scientific communities may use this repository for walnut improvement such as QTL mapping, genetic studies, linkage map construction, and marker-assisted selection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03563-6.

Optimization and application of non-native Phragmites australis transcriptome assemblies

Phragmites australis (common reed) has a cosmopolitan distribution and has been suggested as a model organism for the study of invasive plant species. In North America, the non-native subspecies (ssp. australis) is widely distributed across the contiguous 48 states in the United States and large parts of Canada. Even though millions of dollars are spent annually on Phragmites management, insufficient knowledge of P. australis impeded the efficiency of management. To solve this problem, transcriptomic information generated from multiple types of tissue could be a valuable resource for future studies. Here, we constructed forty-nine P. australis transcriptomes assemblies via different assembly tools and multiple parameter settings. The optimal transcriptome assembly for functional annotation and downstream analyses was selected among these transcriptome assemblies by comprehensive assessments. For a total of 422,589 transcripts assembled in this transcriptome assembly, 319,046 transcripts (75.5%) have at least one functional annotation. Within the transcriptome assembly, we further identified 1,495 transcripts showing tissue-specific expression pattern, 10,828 putative transcription factors, and 72,165 candidates for simple sequence repeats markers. The identification and analyses of predicted transcripts related to herbicide- and salinity-resistant genes were shown as two applications of the transcriptomic information to facilitate further research on P. australis. Transcriptome assembly and selection would be important for the transcriptome annotation. With this optimal transcriptome assembly and all relative information from downstream analyses, we have helped to establish foundations for future studies on the mechanisms underlying the invasiveness of non-native P. australis subspecies.

THE WALNUT - CONSTRAINTS AND ADVANTAGES FOR A SUSTAINABLE DEVELOPMENT

The Persian walnut or English walnut (Juglans regia L.) is a commonly grown species for nut production and noble wood. The nut is one of the oldest food and traditional medicine sources. The native and commercial walnut genotypes present a large diversity that differ widely in nut productivity and quality. However, genetic erosion poses a serious threat to this tree. Several researches of walnut genetic diversity are being carried out utilizing morphological, biochemical, and molecular approaches in order to select superior walnut cultivars of different agroclimatic areas to increase nut production and quality. Genetic resource evaluation and agrodiversity conservation have a major role in ensuring food security for future generations through a continuous supply of new rootstocks and improved cultivars.

Hybrid Assembly and Annotation of the Genome of the Indian Punica granatum, a Superfood

The wonder fruit pomegranate (Punica granatum, family Lythraceae) is one of India’s economically important fruit crops that can grow in different agro-climatic conditions ranging from tropical to temperate regions. This study reports high-quality de novo draft hybrid genome assembly of diploid Punica cultivar “Bhagwa” and identifies its genomic features. This cultivar is most common among the farmers due to its high sustainability, glossy red color, soft seed, and nutraceutical properties with high market value. The draft genome assembly is about 361.76 Mb (N50 = 40 Mb), ∼9.0 Mb more than the genome size estimated by flow cytometry. The genome is 90.9% complete, and only 26.68% of the genome is occupied by transposable elements and has a relative abundance of 369.93 SSRs/Mb of the genome. A total of 30,803 proteins and their putative functions were predicted. Comparative whole-genome analysis revealed Eucalyptus grandis as the nearest neighbor. KEGG-KASS annotations indicated an abundance of genes involved in the biosynthesis of flavonoids, phenylpropanoids, and secondary metabolites, which are responsible for various medicinal properties of pomegranate, including anticancer, antihyperglycemic, antioxidant, and anti-inflammatory activities. The genome and gene annotations provide new insights into the pharmacological properties of the secondary metabolites synthesized in pomegranate. They will also serve as a valuable resource in mining biosynthetic pathways for key metabolites, novel genes, and variations associated with disease resistance, which can facilitate the breeding of new varieties with high yield and superior quality.

Construction and characterization of a de novo draft genome of garden cress (Lepidium sativum L.)

Garden cress (Lepidium sativum L.) is a Brassicaceae crop recognized as a healthy vegetable and a medicinal plant. Lepidium is one of the largest genera in Brassicaceae, yet, the genus has not been a focus of extensive genomic research. In the present work, garden cress genome was sequenced using the long read high-fidelity sequencing technology. A de novo, draft genome assembly that spans 336.5 Mb was produced, corresponding to 88.6% of the estimated genome size and representing 90% of the evolutionarily expected orthologous gene content. Protein coding gene content was structurally predicted and functionally annotated, resulting in the identification of 25,668 putative genes. A total of 599 candidate disease resistance genes were identified by predicting resistance gene domains in gene structures, and 37 genes were detected as orthologs of heavy metal associated protein coding genes. In addition, 4289 genes were assigned as "transcription factor coding." Six different machine learning algorithms were trained and tested for their performance in classifying miRNA coding genomic sequences. Logistic regression proved the best performing trained algorithm, thus utilized for pre-miRNA coding loci identification in the assembly. Repetitive DNA analysis involved the characterization of transposable element and microsatellite contents. L. sativum chloroplast genome was also assembled and functionally annotated. Data produced in the present work is expected to constitute a foundation for genomic research in garden cress and contribute to genomics-assisted crop improvement and genome evolution studies in the Brassicaceae family.

Characterization of Microsatellites in the Akebia trifoliata Genome and Their Transferability and Development of a Whole Set of Effective, Polymorphic, and Physically Mapped Simple Sequence Repeat Markers

Akebia trifoliata is a perennial climbing woody liana plant with a high potential for commercial exploitation and theoretical research. Similarly, microsatellites (simple sequence repeats, SSRs) also have dual roles: as critical markers and as essential elements of the eukaryotic genome. To characterize the profile of SSRs and develop molecular markers, the high-quality assembled genome of A. trifoliata was used. Additionally, to determine the potential transferability of SSR loci, the genomes of Amborella trichopoda, Oryza sativa, Vitis vinifera, Arabidopsis thaliana, Papaver somniferum, and Aquilegia coerulea were also used. We identified 434,293 SSRs with abundant short repeats, such as 290,868 (66.98%) single-nucleotide repeats (SNRs) and 113,299 (26.09%) dinucleotide repeats (DNRs) in the A. trifoliata genome. 398,728 (91.81%) SSRs on 344,283 loci were physically mapped on the chromosomes, and a positive correlation (r = 0.98) was found between the number of SSRs and chromosomal length. Additionally, 342,916 (99.60%) potential SSR markers could be designed from the 344,283 physically mapped loci, while only 36,160 could be viewed as high-polymorphism-potential (HPP) markers, findings that were validated by PCR. Finally, SSR loci exhibited broad potential transferability, particularly DNRs such as the "AT/AT" and "AG/CT" loci, among all angiosperms, a finding that was not related to the genetic divergence distance. Practically, we developed a whole set of effective, polymorphic, and physically anchored molecular markers and found that, evolutionarily, DNRs could be responsible for microsatellite origin and protecting gene function.

Genome survey sequencing and characterization of simple sequence repeat (SSR) markers in Platostoma palustre (Blume) A.J.Paton (Chinese mesona)

Platostoma palustre (Blume) A.J.Paton is an annual herbaceous persistent plant of the Labiatae family. However, there is a lack of genomic data for this plant, which severely restricts its genetic improvement. In this study, we performed genome survey sequencing of P. palustre and developed simple sequence repeat (SSR) markers based on the resulting sequence. K-mer analysis revealed that the assembled genome size was approximately 1.21 Gb. A total of 15,498 SSR motifs were identified and characterized in this study; among them, dinucleotide, and hexanucleotide repeats had the highest and lowest, respectively. Among the dinucleotide repeat motifs, AT/TA repeat motifs were the most abundant, and GC/CG repeat motifs were rather rare, accounting for 44.28% and 0.63%, respectively. Genetic similarity coefficient analysis by the UPMGA methods clustered 12 clones, of P. palustre and related species into two subgroups. These results provide helpful information for further research on P. palustre resources and variety improvements.

Comparative and Phylogenetic Analysis Based on the Chloroplast Genome of Coleanthus subtilis (Tratt.) Seidel, a Protected Rare Species of Monotypic Genus

Coleanthus subtilis (Tratt.) Seidel (Poaceae) is an ephemeral grass from the monotypic genus Coleanthus Seidl, which grows on wet muddy areas such as fishponds or reservoirs. As a rare species with strict habitat requirements, it is protected at international and national levels. In this study, we sequenced its whole chloroplast genome for the first time using the next-generation sequencing (NGS) technology on the Illumina platform, and performed a comparative and phylogenetic analysis with the related species in Poaceae. The complete chloroplast genome of C. subtilis is 135,915 bp in length, with a quadripartite structure having two 21,529 bp inverted repeat regions (IRs) dividing the entire circular genome into a large single copy region (LSC) of 80,100 bp and a small single copy region (SSC) of 12,757 bp. The overall GC content is 38.3%, while the GC contents in LSC, SSC, and IR regions are 36.3%, 32.4%, and 43.9%, respectively. A total of 129 genes were annotated in the chloroplast genome, including 83 protein-coding genes, 38 tRNA genes, and 8 rRNA genes. The accD gene and the introns of both clpP and rpoC1 genes were missing. In addition, the ycf1, ycf2, ycf15, and ycf68 were pseudogenes. Although the chloroplast genome structure of C. subtilis was found to be conserved and stable in general, 26 SSRs and 13 highly variable loci were detected, these regions have the potential to be developed as important molecular markers for the subfamily Pooideae. Phylogenetic analysis with species in Poaceae indicated that Coleanthus and Phippsia were sister groups, and provided new insights into the relationship between Coleanthus, Zingeria, and Colpodium. This study presents the initial chloroplast genome report of C. subtilis, which provides an essential data reference for further research on its origin.

An empirical analysis of mtSSRs: could microsatellite distribution patterns explain the evolution of mitogenomes in plants?

Microsatellites (SSRs) are tandem repeat sequences in eukaryote genomes, including plant cytoplasmic genomes. The mitochondrial genome (mtDNA) has been shown to vary in size, number, and distribution of SSRs among different plant groups. Thus, SSRs contribute with genomic diversity in mtDNAs. However, the abundance, distribution, and evolutionary significance of SSRs in mtDNA from a wide range of algae and plants have not been explored. In this study, the mtDNAs of 204 plant and algal species were investigated related to the presence of SSRs. The number of SSRs was positively correlated with genome size. Its distribution is dependent on plant and algal groups analyzed, although the cluster analysis indicates the conservation of some common motifs in algal and terrestrial plants that reflect common ancestry of groups. Many SSRs in coding and non-coding regions can be useful for molecular markers. Moreover, mitochondrial SSRs are highly abundant, representing an important source for natural or induced genetic variation, i.e., for biotechnological approaches that can modulate mtDNA gene regulation. Thus, this comparative study increases the understanding of the plant and algal SSR evolution and brings perspectives for further studies.

Genic microsatellite marker characterization and development in little millet (Panicum sumatrense) using transcriptome sequencing

Little millet is a climate-resilient and high-nutrient value plant. The lack of molecular markers severely limits the adoption of modern genomic approaches in millet breeding studies. Here the transcriptome of three samples were sequenced. A total of 4443 genic-SSR motifs were identified in 30,220 unigene sequences. SSRs were found at a rate of 12.25 percent, with an average of one SSR locus per 10 kb. Among different repeat motifs, tri-nucleotide repeat (66.67) was the most abundant one, followed by di- (27.39P), and tetra- (3.83P) repeats. CDS contained fewer motifs with the majority of tri-nucleotides, while 3' and 5' UTR carry more motifs but have shorter repeats. Functional annotation of unigenes containing microsatellites, revealed that most of them were linked to metabolism, gene expression regulation, and response to environmental stresses. Fifty primers were randomly chosen and validated in five little millet and 20 minor millet genotypes; 48% showed polymorphism, with a high transferability (70%) rate. Identified microsatellites can be a noteworthy resource for future research into QTL-based breeding, genetic resource conservation, MAS selection, and evolutionary genetics.

Development of a new set of genic SSR markers in the genus Gentiana: in silico mining, characterization and validation

Gentiana is an important genus of around 360 medicinally important species, majority of which are not well characterized. Despite its importance, very few genomic resources are available for Gentiana L. Till date, the number of informative and robust simple sequence repeat (SSR)-based markers is limited and very few efforts have been made for their development. A set of robust, freely accessible and informative SSR markers for Gentiana is a pre-requisite for any molecular systematic as well as improvement studies in this group of pharmacologically valuable plants. In view of the importance of these plants, Expressed Sequence Tag (EST) sequences of 18 Gentiana species were surveyed for the development of a large set of non-redundant SSR markers. A total of 5808 perfect SSR with an average length of 17 bp and relative abundance of 214 loci/Mb were identified in the analysed 47,487 EST sequences using Krait software. Mapping of the ESTs resulted in gene ontology annotations of 49.14% of the sequences. Based on these perfect SSRs, 2902 primer pairs were designed, and 60 markers were randomly selected and validated on a set of Gentiana kurroo Royle accessions. Among the screened markers, 39 (65%) were found to be cross-species transferable. This is the first report of the largest set of functional, novel genic SSR markers in Gentiana, which will be a valuable resource for future characterization, genotype identification, conservation and genomic studies in the various species of this group of important medicinal plants.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02969-4.

Genome-wide microsatellites in amaranth: development, characterization, and cross-species transferability

Amaranth (Amaranthus spp.) belonging to Amaranthaceae, is known as "the crop of the future" because of its incredible nutritional quality. Amaranthus spp. (> 70) have a huge diversity in terms of their plant morphology, production and nutritional quality; however, these species are not well characterized at molecular level due to unavailability of robust and reproducible molecular markers, which is essential for crop improvement programs. In the present study, 13,051 genome-wide microsatellite motifs were identified and subsequently utilized for marker development using A. hypochondriacus (L.) genome (JPXE01.1). Out of those, 1538 motifs were found with flanking sequences suitable for primer designing. Among designed primers, 225 were utilized for validation of which 119 (52.89%) primers were amplified. Cross-species transferability and evolutionary relatedness among ten species of Amaranthus (A. hypochondriacus, A. caudatus, A. retroflexus, A. cruentus, A. tricolor, A. lividus, A. hybridus, A. viridis, A. edulis, and A. dubius) were also studied using 45 microsatellite motifs. The maximum (86.67%) and minimum (28.89%) cross-species transferability were observed in A. caudatus and A. dubius, respectively, that indicated high variability present across the Amaranthus spp. Total 97 alleles were detected among 10 species of Amaranthus. The averages of major allele frequency, gene diversity, heterozygosity and PIC were 0.733, 0.347, 0.06, and 0.291, respectively. Nei's genetic dissimilarity coefficients ranged from 0.0625 (between A. tricolor and A. hybridus) to 0.7918 (between A. viridis and A. lividus). The phylogenetic tree grouped ten species into three major clusters. Genome-wide development of microsatellite markers and their transferability revealed relationships among amaranth species which ultimately can be useful for species identification, DNA fingerprinting, and QTLs/gene(s) identification.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02930-5.

De novo transcriptome analysis of white teak (Gmelina arborea Roxb) wood reveals critical genes involved in xylem development and secondary metabolism

BACKGROUND

Gmelina arborea Roxb is a fast-growing tree species of commercial importance for tropical countries due to multiple industrial uses of its wood. Wood is primarily composed of thick secondary cell walls of xylem cells which imparts the strength to the wood. Identification of the genes involved in the secondary cell wall biosynthesis as well as their cognate regulators is crucial to understand how the production of wood occurs and serves as a starting point for developing breeding strategies to produce varieties with improved wood quality, better paper pulping or new potential uses such as biofuel production. In order to gain knowledge on the molecular mechanisms and gene regulation related with wood development in white teak, a de novo sequencing and transcriptome assembly approach was used employing secondary cell wall synthesizing cells from young white teak trees.

RESULTS

For generation of transcriptome, RNA-seq reads were assembled into 110,992 transcripts and 49,364 genes were functionally annotated using plant databases; 5071 GO terms and 25,460 SSR markers were identified within xylem transcripts and 10,256 unigenes were assigned to KEGG database in 130 pathways. Among transcription factor families, C2H2, C3H, bLHLH and MYB were the most represented in xylem. Differential gene expression analysis using leaves as a reference was carried out and a total of 20,954 differentially expressed genes were identified including monolignol biosynthetic pathway genes. The differential expression of selected genes (4CL, COMT, CCoAOMT, CCR and NST1) was validated using qPCR.

CONCLUSIONS

We report the very first de novo transcriptome of xylem-related genes in this tropical timber species of commercial importance and constitutes a valuable extension of the publicly available transcriptomic resource aimed at fostering both basic and breeding studies.

Development, characterization, functional annotation and validation of genomic and genic-SSR markers using de novo next generation sequencing in Melia dubia Cav

Melia dubia Cav. (Meliaceae), a fast-growing tropical tree finds use in plywood, pulp and high-value solid wood products. To increase its productivity, we must essentially capture genetic diversity and identify genotypes with superior wood properties. This study aimed to develop novel microsatellite markers from genomic data and validate the markers in M. dubia. Direct Seq-to-SSR approach was adopted and using an in-house Perl script, 426,390 SSR markers identified. For validation, selected 151 markers, of which 50 were genomic markers chosen randomly, and 101 were genic markers identified through BLAST2GO. Amplification was observed in all loci, and 81.4% generated high-quality, reproducible amplicons of the expected size. Out of 50 genomic markers, we used ten highly polymorphic markers to assess genetic diversity among 75 genotypes from three populations. One hundred fourteen alleles were recorded, with a moderate level of diversity and a positive fixation index. Twenty-nine genic markers representing 13 enzymes showing polymorphism for wood stiffness were selected for diversity assessment of 24 genotypes (12 genotypes each with high and low-stress wave velocity). The product size ranged from 87 to 279, covering the majority of the genome. Cluster and structure analysis segregated ~ 80% of the genotypes based on the trait. This is the first report of the development of genic markers from a genomic survey and has proved efficient in differentiating genotypes based on the trait. The markers developed in this study will be useful for genetic mapping, diversity estimation, marker-assisted selection for desired traits and breeding for wood traits in M. dubia.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02858-w.

Uncovering genomic regions controlling plant architectural traits in hexaploid wheat using different GWAS models

Wheat is a major food crop worldwide. The plant architecture is a complex trait mostly influenced by plant height, tiller number, and leaf morphology. Plant height plays a crucial role in lodging and thus affects yield and grain quality. In this study, a wheat population was genotyped by using Illumina iSelect 90K single nucleotide polymorphism (SNP) assay and finally 22,905 high-quality SNPs were used to perform a genome-wide association study (GWAS) for plant architectural traits employing four multi-locus GWAS (ML-GWAS) and three single-locus GWAS (SL-GWAS) models. As a result, 174 and 97 significant SNPs controlling plant architectural traits were detected by ML-GWAS and SL-GWAS methods, respectively. Among these SNP makers, 43 SNPs were consistently detected, including seven across multiple environments and 36 across multiple methods. Interestingly, five SNPs (Kukri_c34553_89, RAC875_c8121_1490, wsnp_Ex_rep_c66315_64480362, Ku_c5191_340, and tplb0049a09_1302) consistently detected across multiple environments and methods, played a role in modulating both plant height and flag leaf length. Furthermore, candidate SNPs (BS00068592_51, Kukri_c4750_452 and BS00022127_51) constantly repeated in different years and methods associated with flag leaf width and number of tillers. We also detected several SNPs (Jagger_c6772_80, RAC875_c8121_1490, BS00089954_51, Excalibur_01167_1207, and Ku_c5191_340) having common associations with more than one trait across multiple environments. By further appraising these GWAS methods, the pLARmEB and FarmCPU models outperformed in SNP detection compared to the other ML-GWAS and SL-GWAS methods, respectively. Totally, 152 candidate genes were found to be likely involved in plant growth and development. These finding will be helpful for better understanding of the genetic mechanism of architectural traits in wheat.

De novo assembly and characterization of the first draft genome of quince (Cydonia oblonga Mill.)

Quince (Cydonia oblonga Mill.) is the sole member of the genus Cydonia in the Rosacea family and closely related to the major pome fruits, apple (Malus domestica Borkh.) and pear (Pyrus communis L.). In the present work, whole genome shotgun paired-end sequencing was employed in order to assemble the first draft genome of quince. A genome assembly that spans 488.4 Mb of sequence corresponding to 71.2% of the estimated genome size (686 Mb) was produced in the study. Gene predictions via ab initio and homology-based sequence annotation strategies resulted in the identification of 25,428 and 30,684 unique putative protein coding genes, respectively. 97.4 and 95.6% of putative homologs of Arabidopsis and rice transcription factors were identified in the ab initio predicted genic sequences. Different machine learning algorithms were tested for classifying pre-miRNA (precursor microRNA) coding sequences, identifying Support Vector Machine (SVM) as the best performing classifier. SVM classification predicted 600 putative pre-miRNA coding loci. Repetitive DNA content of the assembly was also characterized. The first draft assembly of the quince genome produced in this work would constitute a foundation for functional genomic research in quince toward dissecting the genetic basis of important traits and performing genomics-assisted breeding.

New evidence confirming the CD genomic constitutions of the tetraploid Avena species in the section Pachycarpa Baum

The tetraploid Avena species in the section Pachycarpa Baum, including A. insularis, A. maroccana, and A. murphyi, are thought to be involved in the evolution of hexaploid oats; however, their genome designations are still being debated. Repetitive DNA sequences play an important role in genome structuring and evolution, so understanding the chromosomal organization and distribution of these sequences in Avena species could provide valuable information concerning genome evolution in this genus. In this study, the chromosomal organizations and distributions of six repetitive DNA sequences (including three SSR motifs (TTC, AAC, CAG), one 5S rRNA gene fragment, and two oat A and C genome specific repeats) were investigated using non-denaturing fluorescence in situ hybridization (ND-FISH) in the three tetraploid species mentioned above and in two hexaploid oat species. Preferential distribution of the SSRs in centromeric regions was seen in the A and D genomes, whereas few signals were detected in the C genomes. Some intergenomic translocations were observed in the tetraploids; such translocations were also detected between the C and D genomes in the hexaploids. These results provide robust evidence for the presence of the D genome in all three tetraploids, strongly suggesting that the genomic constitution of these species is DC and not AC, as had been thought previously.

Comprehensive Characterization and Validation of Chromosome-Specific Highly Polymorphic SSR Markers From Pomegranate (Punica granatum L.) cv. Tunisia Genome

The simple sequence repeat (SSR) survey of 'Tunisia' genome (296.85 Mb) identified a total of 365,279 perfect SSRs spanning eight chromosomes, with a mean marker density of 1,230.6 SSRs/Mb. We found a positive trend in chromosome length and the SSR abundance as marker density enhanced with a shorter chromosome length. The highest number of SSRs (60,708) was mined from chromosome 1 (55.56 Mb), whereas the highest marker density (1,294.62 SSRs/Mb) was recorded for the shortest chromosome 8 (27.99 Mb). Furthermore, we categorized all SSR motifs into three major classes based on their tract lengths. Across the eight chromosomes, the class III had maximum number of SSR motifs (301,684, 82.59%), followed by the class II (31,056, 8.50%) and the class I (5,003, 1.37%). Examination of the distribution of SSR motif types within a chromosome suggested the abundance of hexanucleotide repeats in each chromosome followed by dinucleotides, and these results are consistent with 'Tunisia' genome features as a whole. Concerning major repeat types, AT/AG was the most frequent (14.16%), followed by AAAAAT/AAAAAG (7.89%), A/C (7.54%), AAT/AAG (5.23%), AAAT/AAAG (4.37%), and AAAAT/AAAAG (1.2%) types. We designed and validated a total of 3,839 class I SSRs in the 'Tunisia' genome through electronic polymerase chain reaction (ePCR) and found 1,165 (30.34%) SSRs producing a single amplicon. Then, we selected 906 highly variable SSRs (> 40 nt) from the ePCR-verified class I SSRs and in silico validated across multiple draft genomes of pomegranate, which provided us a subset of 265 highly polymorphic SSRs. Of these, 235 primers were validated on six pomegranate genotypes through wet-lab experiment. We found 221 (94%) polymorphic SSRs on six genotypes, and 187 of these SSRs had ≥ 0.5 PIC values. The utility of the developed SSRs was demonstrated by analyzing genetic diversity of 30 pomegranate genotypes using 16 HvSSRs spanning eight pomegranate chromosomes. In summary, we developed a comprehensive set of highly polymorphic genome-wide SSRs. These chromosome-specific SSRs will serve as a powerful genomic tool to leverage future genetic studies, germplasm management, and genomics-assisted breeding in pomegranate.

The landscape of microsatellites in the enset (Ensete ventricosum) genome and web-based marker resource development

Ensete ventricosum (Musaceae, enset) is an Ethiopian food security crop. To realize the potential of enset for rural livelihoods, further knowledge of enset diversity, genetics and genomics is required to support breeding programs and conservation. This study was conducted to explore the enset genome to develop molecular markers, genomics resources, and characterize enset landraces while giving insight into the organization of the genome. We identified 233 microsatellites (simple sequence repeats, SSRs) per Mbp in the enset genome, representing 0.28% of the genome. Mono- and di-nucleotide repeats motifs were found in a higher proportion than other classes of SSR-motifs. In total, 154,586 non-redundant enset microsatellite markers (EMM) were identified and 40 selected for primer development. Marker validation by PCR and low-cost agarose gel electrophoresis revealed that 92.5% were polymorphic, showing a high PIC (Polymorphism Information Content; 0.87) and expected heterozygosity (He = 0.79-0.82). In silico analysis of genomes of closely related species showed 46.86% of the markers were transferable among enset species and 1.90% were transferable to Musa. The SSRs are robust (with basic PCR methods and agarose gel electrophoresis), informative, and applicable in measuring enset diversity, genotyping, selection and potentially breeding. Enset SSRs are available in a web-based database at https://enset-project.org/EnMom@base.html (or https://enset.aau.edu.et/index.html , downloadable from Figshare).

Evolutionary dynamics of the chloroplast genome sequences of six Colobanthus species

The complete plastome sequences of six species were sequenced to better understand the evolutionary relationships and mutation patterns in the chloroplast genome of the genus Colobanthus. The length of the chloroplast genome sequences of C. acicularis, C. affinis, C. lycopodioides, C. nivicola, C. pulvinatus and C. subulatus ranged from 151,050 to 151,462 bp. The quadripartite circular structure of these genome sequences has the same overall organization and gene content with 73 protein-coding genes, 30 tRNA genes, four rRNA genes and five conserved chloroplast open reading frames. A total of 153 repeat sequences were revealed. Forward repeats were dominant, whereas complementary repeats were found only in C. pulvinatus. The mononucleotide SSRs composed of A/T units were most common, and hexanucleotide SSRs were detected least often. Eleven highly variable regions which could be utilized as potential markers for phylogeny reconstruction, species identification or phylogeography were identified within Colobanthus chloroplast genomes. Seventy-three protein-coding genes were used in phylogenetic analyses. Reconstructed phylogeny was consistent with the systematic position of the studied species, and the representatives of the same genus were grouped in one clade. All studied Colobanthus species formed a single group and C. lycopodioides was least similar to the remaining species.

Comparative Analyses of Five Complete Chloroplast Genomes from the Genus Pterocarpus (Fabacaeae)

Pterocarpus is a genus of trees mainly distributed in tropical Asia, Africa, and South America. Some species of Pterocarpus are rosewood tree species, having important economic value for timber, and for some species, medicinal value as well. Up to now, information about this genus with regard to the genomic characteristics of the chloroplasts has been limited. Based on a combination of next-generation sequencing (Illumina Hiseq) and long-read sequencing (PacBio), the whole chloroplast genomes (cp genomes) of five species (rosewoods) in Pterocarpus (Pterocarpus macrocarpus, P. santalinus, P. indicus, P. pedatus, P. marsupium) have been assembled. The cp genomes of five species in Pterocarpus have similar structural characteristics, gene content, and sequence to other flowering plants. The cp genomes have a typical four-part structure, containing 110 unique genes (77 protein coding genes, 4 rRNAs, 29 tRNAs). Through comparative genomic analysis, abundant simple sequence repeat (SSR)loci (333-349) were detected in Pterocarpus, among which A /T single nucleotide repeats accounted for the highest proportion (72.8-76.4%). In the five cp genomes of Pterocarpus, eight hypervariable regions, including trnH-GUG_psbA, trnS-UGA_psbC, accD-psaI, ndhI-exon2_ndhI-exon1, ndhG_ndhi-exon2, rpoC2-exon2, ccsA, and trnfM-CAU, are proposed for use as DNA barcode regions. In the comparison of gene selection pressures (P. santalinus as the reference genome), purifying selection was inferred as the primary mode of selection in maintaining important biological functions. Phylogenetic analysis shows that Pterocarpus is a monophyletic group. The species P. tinctorius is resolved as early diverging in the genus. Pterocarpus was resolved as sister to the genus Tipuana.

Draft genome of multiple resistance donor plant Sinapis alba: An insight into SSRs, annotations and phylogenetics

BACKGROUND

Sinapis alba is a wild member of the Brassicaceae family reported to possess genetic resistance against major biotic and abiotic stresses of oilseed brassicas. However, the resistance nature of S. alba was not exploited generously due to the unavailability of usable genome sequences in public databases. Therefore, the present study was conducted to assemble the first draft genome from raw whole genome shotgun sequences with annotation and develop simple sequence repeat markers for molecular genetics and marker-assisted breeding.

RESULTS

The raw genome sequences had 96x coverage on the Illumina platform with 170 Gbp data. The developed assembly by SOAPdenovo2 has ~459 Mbp genome size covered in 403,423 contigs with an average size of 1138.04 bp. The assembly was BLASTX with Arabidopsis thaliana which showed 32.9% positive hits between both plants. The top hit species distribution analysis showed the highest similarity with A. thaliana. A total of 809,597 GO level annotations were recorded after BLASTX results, and 34,012 sequences were annotated with different enzyme codes grouped under seven classes. The gene prediction tool AUGUSTUS identified 113,107 probable genes with an average size of 684 bp. The biochemical pathway annotation assigned 16,119 potential genes to 152 KEGG maps and 1751 enzyme codes. The development of potential SSRs from the de-novo assembly yielded 70731 unique primer pairs. Out of 159 randomly selected SSR markers for validation, 149 successfully amplified in S. alba. However, 10 SSR markers did not amplify during the validation experiment.

CONCLUSION

The annotated genome assembly with a large number of SSRs was developed in the present study. To the best of our knowledge, this is the first report of S. alba genome assembly development, annotation, and SSRs mining to date. The data presented here will be a very important resource for future crop improvement programs, especially for resistant breeding.

Genome-wide characterization of simple sequence repeats in Palmae genomes

Background

Microsatellites or simple sequence repeats (SSRs) have become the most significant DNA marker technology used in genetic research. The availability of complete draft genomes for a number of Palmae species has made it possible to perform genome-wide analysis of SSRs in these species. Palm trees are tropical and subtropical plants with agricultural and economic importance due to the nutritional value of their fruit cultivars.

Objective

This is the first comprehensive study examining and comparing microsatellites in completely-sequenced draft genomes of Palmae species.

Methods

We identified and compared perfect SSRs with 1–6 bp nucleotide motifs to characterize microsatellites in Palmae species using PERF v0.2.5. We analyzed their relative abundance, relative density, and GC content in five palm species: Phoenix dactylifera, Cocos nucifera, Calamus simplicifolius, Elaeis oleifera, and Elaeis guineensis.

Results

A total of 118241, 328189, 450753, 176608, and 70694 SSRs were identified, respectively. The six repeat types were not evenly distributed across the five genomes. Mono- and dinucleotide SSRs were the most abundant, and GC content was highest in tri- and hexanucleotide SSRs.

Conclusion

We envisage that this analysis would further substantiate more in-depth computational, biochemical, and molecular studies on the roles SSRs may play in the genome organization of the palm species. The current study contributes a detailed characterization of simple sequence repeats in palm genomes.

Electronic supplementary material

The online version of this article (10.1007/s13258-020-00924-w) contains supplementary material, which is available to authorized users.

Identification of Genic SSRs Provide a Perspective for Studying Environmental Adaptation in the Endemic Shrub Tetraena mongolica

Tetraena mongolica is a xerophytic shrub endemic to desert regions in Inner Mongolia. This species has evolved distinct survival strategies that allow it to adapt to hyper-drought and heterogeneous habitats. Simple sequence repeats (SSRs) may provide a molecular basis in plants for fast adaptation to environmental change. Thus, identifying SSRs and their possible effects on gene behavior has the potential to provide valuable information for studies of adaptation. In this study, we sequenced six individual transcriptomes of T. mongolica from heterogeneous habitats, focused on SSRs located in genes, and identified 811 polymorphic SSRs. Of the identified SSRs, 172, 470, and 76 were located in 5' UTRs, CDSs, and 3' UTRs in 591 transcripts; and AG/CT, AAC/GTT, and AT/AT were the most abundant repeats in each gene region. Functional annotation showed that many of the identified polymorphic SSRs were in genes that were enriched in several GO terms and KEGG pathways, suggesting the functional significance of these genes in the environmental adaptation process. The identification of polymorphic genic SSRs in our study lays a foundation for future studies investigating the contribution of SSRs to regulation of genes in natural populations of T. mongolica and their importance for adaptive evolution of this species.

Development of genome-wide polymorphic microsatellite markers for Trichinella spiralis

Background

Trichinella nematodes are globally distributed food-borne pathogens, in which Trichinella spiralis is the most common species in China. Microsatellites are a powerful tool in population genetics and phylogeographic analysis. However, only a few microsatellite markers were reported in T. spiralis. Thus, there is a need to develop and validate genome-wide microsatellite markers for T. spiralis.

Methods

Microsatellites were selected from shotgun genomic sequences using MIcroSAtellite identification tool (MISA). The identified markers were validated in 12 isolates of T. spiralis in China.

Results

A total of 93,140 microsatellites were identified by MISA from 9267 contigs in T. spiralis genome sequences, in which 16 polymorphic loci were selected for validation by PCR with single larvae from 12 isolates of T. spiralis in China. There were 7–19 alleles per locus (average 11.25 alleles per locus). The observed heterozygosity (H_O) and expected heterozygosity (H_E) ranged from 0.325 to 0.750 and 0.737 to 0.918, respectively. The polymorphism information content (PIC) ranged from 0.719 to 0.978 (average 0.826). Among the 16 loci, markers for 10 loci could be amplified from all 12 international standard strains of Trichinella spp.

Conclusions

Sixteen highly polymorphic markers were selected and validated for T. spiralis. Primary phylogenetic analysis showed that these markers might serve as a useful tool for genetic studies of Trichinella parasites.

Comprehensive functional analysis and mapping of SSR markers in the chickpea genome (Cicer arietinum L.)

Plant molecular breeding largely depends on the relationship between molecular markers and major traits. Herein, a total of 32,962 genomic simple sequence repeats (SSRs) were detected in the whole genome of chickpea with an average density of 94.93 SSRs/Mb. Chickpea chromosomes uniformity test indicated that the genomic SSRs (gSSRs) were steadily distributed across the genome. Moreover, 48,667 transcriptome sequences were analyzed and 1949 SSR-containing transcript assembly contigs (TACs) were identified. The analysis showed that di- and trinucleotide SSRs were the most frequent SSR motifs within the transcriptome sequences. Among them, AT and TTA and AG and TTC motifs within the transcriptome showed the highest frequencies among di- and trinucleotide repeat motifs, respectively. The SSRs-containing TACs were compared to the GenBank non-redundant database using BLASTX, and subsequently, gene ontology (GO) analysis was performed using QuickGO browser to reduce complexity and highlight biological processes associated with the SSRs-containing TACs. The identified SSRs-containing TACs were categorized into 35 enriched functional-related gene group. The mapping of characterized SSRs-containing TACs onto chickpea chromosomes was performed using BLASTN. The mapping result showed that, a total of 1798 SSRs-containing TACs were mapped onto the chickpea genome. Based on the functional analysis result, 249 and 242 of the mapped SSRs-containing TACs were found in the genes encoding for putative stress-related proteins and transcription factors, respectively. The results presented here can be applied to improve and speed up the chickpea breeding programs.

Advances in Omics Approaches for Abiotic Stress Tolerance in Tomato

Tomato, one of the most important crops worldwide, has a high demand in the fresh fruit market and processed food industries. Despite having considerably high productivity, continuous supply as per the market demand is hard to achieve, mostly because of periodic losses occurring due to biotic as well as abiotic stresses. Although tomato is a temperate crop, it is grown in almost all the climatic zones because of widespread demand, which makes it challenge to adapt in diverse conditions. Development of tomato cultivars with enhanced abiotic stress tolerance is one of the most sustainable approaches for its successful production. In this regard, efforts are being made to understand the stress tolerance mechanism, gene discovery, and interaction of genetic and environmental factors. Several omics approaches, tools, and resources have already been developed for tomato growing. Modern sequencing technologies have greatly accelerated genomics and transcriptomics studies in tomato. These advancements facilitate Quantitative trait loci (QTL) mapping, genome-wide association studies (GWAS), and genomic selection (GS). However, limited efforts have been made in other omics branches like proteomics, metabolomics, and ionomics. Extensive cataloging of omics resources made here has highlighted the need for integration of omics approaches for efficient utilization of resources and a better understanding of the molecular mechanism. The information provided here will be helpful to understand the plant responses and the genetic regulatory networks involved in abiotic stress tolerance and efficient utilization of omics resources for tomato crop improvement.

Comparative analyses of simple sequence repeats (SSRs) in 23 mosquito species genomes: Identification, characterization and distribution (Diptera: Culicidae)

Simple sequence repeats (SSRs) exist in both eukaryotic and prokaryotic genomes and are the most popular genetic markers, but the SSRs of mosquito genomes are still not well understood. In this study, we identified and analyzed the SSRs in 23 mosquito species using Drosophila melanogaster as reference at the whole-genome level. The results show that SSR numbers (33 076-560 175/genome) and genome sizes (574.57-1342.21 Mb) are significantly positively correlated (R² = 0.8992, P < 0.01), but the correlation in individual species varies in these mosquito species. In six types of SSR, mono- to trinucleotide SSRs are dominant with cumulative percentages of 95.14%-99.00% and densities of 195.65/Mb-787.51/Mb, whereas tetra- to hexanucleotide SSRs are rare with 1.12%-4.22% and 3.76/Mb-40.23/Mb. The (A/T)n, (AC/GT)n and (AGC/GCT)n are the most frequent motifs in mononucleotide, dinucleotide and trinucleotide SSRs, respectively, and the motif frequencies of tetra- to hexanucleotide SSRs appear to be species-specific. The 10-20 bp length of SSRs are dominant with the number of 110 561 ± 93 482 and the frequency of 87.25% ± 5.73% on average, and the number and frequency decline with the increase of length. Most SSRs (83.34% ± 7.72%) are located in intergenic regions, followed by intron regions (11.59% ± 5.59%), exon regions (3.74% ± 1.95%), and untranslated regions (1.32% ± 1.39%). The mono-, di- and trinucleotide SSRs are the main SSRs in both gene regions (98.55% ± 0.85%) and exon regions (99.27% ± 0.52%). An average of 42.52% of total genes contains SSRs, and the preference for SSR occurrence in different gene subcategories are species-specific. The study provides useful insights into the SSR diversity, characteristics and distribution in 23 mosquito species of genomes.

Microsatellite Borders and Micro-sequence Conservation in Juglans

Walnuts (Juglans spp.) are economically important nut and timber species with a worldwide distribution. Using the published Persian walnut genome as a reference for the assembly of short reads from six Juglans species and several interspecific hybrids, we identified simple sequence repeats in 12 Juglans nuclear and organellar genomes. The genome-wide distribution and polymorphisms of nuclear and organellar microsatellites (SSRs) for most Juglans genomes have not been previously studied. We compared the frequency of nuclear SSR motifs and their lengths across Juglans, and identified section-specific chloroplast SSR motifs. Primer pairs were designed for more than 60,000 SSR-containing sequences based on alignment against assembled scaffold sequences. Of the >60,000 loci, 39,000 were validated by e-PCR using unique primer pairs. We identified primers containing 100% sequence identity in multiple species. Across species, sequence identity in the SSR-flanking regions was generally low. Although SSRs are common and highly dispersed in the genome, their flanking sequences are conserved at about 90 to 95% identity within Juglans and within species. In a few rare cases, flanking sequences are identical across species of Juglans. This comprehensive report of nuclear and organellar SSRs in Juglans and the generation of validated SSR primers will be a useful resource for future genetic analyses, walnut breeding programs, high-level taxonomic evaluations, and genomic studies in Juglandaceae.

SSR identification and marker development for sago palm based on NGS genome data

Sago palm (Metroxylon sagu Rottb.) is one of the most productive carbohydrate-producing crops. Unfortunately, only limited information regarding sago palm genetics is available. This study aimed to develop simple sequence repeat (SSR) markers using sago palm NGS genomic data and use these markers to evaluate the genetic diversity of sago palm from Indonesia. De novo assembly of partial sago palm genomic data and subsequent SSR mining identified 29,953 contigs containing 31,659 perfect SSR loci and 31,578 contigs with 33,576 imperfect SSR loci. The perfect SSR loci density was 132.57/Mb, and AG, AAG and AAAT were the most frequent SSR motifs. Five hundred perfect SSR loci were randomly selected and used for designing SSR primers; 93 SSR primer pairs were identified. After synteny analysis using rice genome sequences, 20 primer pairs were validated using 11 sago palm accessions, and seven primers generated polymorphic alleles. Genetic diversity analysis of 41 sago palm accessions from across Indonesia using polymorphic SSR loci indicated the presence of three clusters. These results demonstrated the success of SSR identification and marker development for sago palm based on NGS genome data, which can be further used for assisting sago palm breeding in the future.

Development and validation of whole genome-wide and genic microsatellite markers in oil palm (Elaeis guineensis Jacq.): First microsatellite database (OpSatdb)

The availability of large expressed sequence tag (EST) and whole genome databases of oil palm enabled the development of a data base of microsatellite markers. For this purpose, an EST database consisting of 40,979 EST sequences spanning 27 Mb and a chromosome-wise whole genome databases were downloaded. A total of 3,950 primer pairs were identified and developed from EST sequences. The tri and tetra nucleotide repeat motifs were most prevalent (each 24.75%) followed by di-nucleotide repeat motifs. Whole genome-wide analysis found a total of 245,654 SSR repeats across the 16 chromosomes of oil palm, of which 38,717 were compound microsatellite repeats. A web application, OpSatdb, the first microsatellite database of oil palm, was developed using the PHP and MySQL database ( https://ssr.icar.gov.in/index.php ). It is a simple and systematic web-based search engine for searching SSRs based on repeat motif type, repeat type, and primer details. High synteny was observed between oil palm and rice genomes. The mapping of ESTs having SSRs by Blast2GO resulted in the identification of 19.2% sequences with gene ontology (GO) annotations. Randomly, a set of ten genic SSRs and five genomic SSRs were used for validation and genetic diversity on 100 genotypes belonging to the world oil palm genetic resources. The grouping pattern was observed to be broadly in accordance with the geographical origin of the genotypes. The identified genic and genome-wide SSRs can be effectively useful for various genomic applications of oil palm, such as genetic diversity, linkage map construction, mapping of QTLs, marker-assisted selection, and comparative population studies.

Comparative chloroplast genomes of Paris Sect. Marmorata: insights into repeat regions and evolutionary implications

Background

Species of Paris Sect. Marmorata are valuable medicinal plants to synthesize steroidal saponins with effective pharmacological therapy. However, the wild resources of the species are threatened by plundering exploitation before the molecular genetics studies uncover the genomes and evolutionary significance. Thus, the availability of complete chloroplast genome sequences of Sect. Marmorata is necessary and crucial to the understanding the plastome evolution of this section and facilitating future population genetics studies. Here, we determined chloroplast genomes of Sect. Marmorata, and conducted the whole chloroplast genome comparison.

Results

This study presented detailed sequences and structural variations of chloroplast genomes of Sect. Marmorata. Over 40 large repeats and approximately 130 simple sequence repeats as well as a group of genomic hotspots were detected. Inverted repeat contraction of this section was inferred via comparing the chloroplast genomes with the one of P. verticillata. Additionally, almost all the plastid protein coding genes were found to prefer ending with A/U. Mutation bias and selection pressure predominately shaped the codon bias of most genes. And most of the genes underwent purifying selection, whereas photosynthetic genes experienced a relatively relaxed purifying selection.

Conclusions

Repeat sequences and hotspot regions can be scanned to detect the intraspecific and interspecific variability, and selected to infer the phylogenetic relationships of Sect. Marmorata and other species in subgenus Daiswa. Mutation and natural selection were the main forces to drive the codon bias pattern of most plastid protein coding genes. Therefore, this study enhances the understanding about evolution of Sect. Marmorata from the chloroplast genome, and provide genomic insights into genetic analyses of Sect. Marmorata.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5281-x) contains supplementary material, which is available to authorized users.

Molecular Cytogenetics of Pisum sativum L. Grown under Spaceflight-Related Stress

The ontogenesis and reproduction of plants cultivated aboard a spacecraft occur inside the unique closed ecological system wherein plants are subjected to serious abiotic stresses. For the first time, a comparative molecular cytogenetic analysis of Pisum sativum L. (Fabaceae) grown on board the RS ISS during the Expedition-14 and Expedition-16 and also plants of their succeeding (F1 and F2) generations cultivated on Earth was performed in order to reveal possible structural chromosome changes in the pea genome. The karyotypes of these plants were studied by multicolour fluorescence in situ hybridization (FISH) with five different repeated DNA sequences (45S rDNA, 5S rDNA, PisTR-B/1, microsatellite motifs (AG)12, and (GAA)9) as probes. A chromosome aberration was revealed in one F1 plant. Significant changes in distribution of the examined repeated DNAs in karyotypes of the "space grown" pea plants as well as in F1 and F2 plants cultivated on Earth were not observed if compared with control plants. Additional oligo-(GAA)9 sites were detected on chromosomes 6 and 7 in karyotypes of F1 and F2 plants. The detected changes might be related to intraspecific genomic polymorphism or plant cell adaptive responses to spaceflight-related stress factors. Our findings suggest that, despite gradual total trace contamination of the atmosphere on board the ISS associated with the extension of the space station operating life, exposure to the space environment did not induce serious chromosome reorganizations in genomes of the "space grown" pea plants and generations of these plants cultivated on Earth.

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.

Identification, analysis and development of salt responsive candidate gene based SSR markers in wheat

BACKGROUND

Salinity severely limits wheat production in many parts of the world. Development of salt tolerant varieties represents the most practical option for enhancing wheat production from these areas. Application of marker assisted selection may assist in fast tracking development of salt tolerant wheat varieties. However, SSR markers available in the public domain are not specifically targeted to functional regions of wheat genome, therefore large numbers of these need to be analysed for identification of markers associated with traits of interest. With the availability of a fully annotated wheat genome assembly, it is possible to develop SSR markers specifically targeted to genic regions. We performed extensive analysis to identify candidate gene based SSRs and assessed their utility in characterizing molecular diversity in a panel of wheat genotypes.

RESULTS

Our analysis revealed, 161 SSR motifs in 94 salt tolerance candidate genes of wheat. These SSR motifs were nearly equally distributed on the three wheat sub-genomes; 29.8% in A, 35.7% in B and 34.4% in D sub-genome. The maximum number of SSR motifs was present in exons (31.1%) followed by promoters (29.8%), 5'UTRs (21.1%), introns (14.3%) and 3'UTRs (3.7%). Out of the 65 candidate gene based SSR markers selected for validation, 30 were found polymorphic based on initial screening and employed for characterizing genetic diversity in a panel of wheat genotypes including salt tolerant and susceptible lines. These markers generated an average of 2.83 alleles/locus. Phylogenetic analysis revealed four clusters. Salt susceptible genotypes were mainly represented in clusters I and III, whereas high and moderate salt tolerant genotypes were distributed in the remaining two clusters. Population structure analysis revealed two sub-populations, sub-population 1 contained the majority of salt tolerant whereas sub-population 2 contained majority of susceptible genotypes. Moreover, we observed reasonably higher transferability of SSR markers to related wheat species.

CONCLUSION

We have developed salt responsive gene based SSRs in wheat for the first time. These were highly useful in unravelling functional diversity among wheat genotypes with varying responses to salt stress. The identified gene based SSR markers will be valuable genomic resources for genetic/association mapping of salinity tolerance in wheat.

A gene-rich fraction analysis of the Passiflora edulis genome reveals highly conserved microsyntenic regions with two related Malpighiales species

Passiflora edulis is the most widely cultivated species of passionflowers, cropped mainly for industrialized juice production and fresh fruit consumption. Despite its commercial importance, little is known about the genome structure of P. edulis. To fill in this gap in our knowledge, a genomic library was built, and now completely sequenced over 100 large-inserts. Sequencing data were assembled from long sequence reads, and structural sequence annotation resulted in the prediction of about 1,900 genes, providing data for subsequent functional analysis. The richness of repetitive elements was also evaluated. Microsyntenic regions of P. edulis common to Populus trichocarpa and Manihot esculenta, two related Malpighiales species with available fully sequenced genomes were examined. Overall, gene order was well conserved, with some disruptions of collinearity identified as rearrangements, such as inversion and translocation events. The microsynteny level observed between the P. edulis sequences and the compared genomes is surprising, given the long divergence time that separates them from the common ancestor. P. edulis gene-rich segments are more compact than those of the other two species, even though its genome is much larger. This study provides a first accurate gene set for P. edulis, opening the way for new studies on the evolutionary issues in Malpighiales genomes.

Transcriptome Analysis and Microsatellite Markers Development of a Traditional Chinese Medicinal Herb Halenia elliptica D. Don (Gentianaceae)

Halenia elliptica is a popular Chinese medicinal herb that is used to treat jaundice disease and virus hepatitis, and its wild populations have been reduced significantly due to overharvesting recently. However, effective conservation could not be implemented because of the lack of genomic information and genetic markers. In this study, a de novo transcriptome of H elliptica was sequenced using the NGS Illumina, and 132 695 unigenes with the length >200 bp (base pairs) were obtained. Among them, a total of 32 109 unigenes were scanned to develop simple sequence repeats (SSRs). Based on NCBI (National Center for Biotechnology Information) nonredundant database (Nr), these SSR sequences were annotated and assigned into gene ontology categories. In addition, we designed 126 pairs of SSR primers for polymerase chain reaction amplification, of which 12 pairs were identified to be polymorphic among 40 individuals from 8 populations. We then used the 12 polymorphic SSRs to construct a UPGMA dendrogram of the 40 individuals. In addition, a significant correlation between the genetic relationship and the geographic distance was found, suggesting a phylogeographic structure in H elliptica. Moreover, 2 of these SSRs were also successfully amplified in a related species Veratrilla baillonii, suggesting their cross-species transferability. Generally, the SSR markers with high polymorphisms identified in this study provide valuable genetic resources and represent an initial step for exploring the genetic diversity and population histories of H elliptica and its related species.

Comparative genome-wide characterization leading to simple sequence repeat marker development for Nicotiana

BACKGROUND

Simple sequence repeats (SSRs) are tandem repeats of DNA that have been used to develop robust genetic markers. These molecular markers are powerful tools for basic and applied studies such as molecular breeding. In the model plants in Nicotiana genus e.g. N. benthamiana, a comprehensive assessment of SSR content has become possible now because several Nicotiana genomes have been sequenced. We conducted a genome-wide SSR characterization and marker development across seven Nicotiana genomes.

RESULTS

Here, we initially characterized 2,483,032 SSRs (repeat units of 1-10 bp) from seven genomic sequences of Nicotiana and developed SSR markers using the GMATA® software package. Of investigated repeat units, mono-, di- and tri-nucleotide SSRs account for 98% of all SSRs in Nicotiana. More complex SSR motifs, although rare, are highly variable between Nicotiana genomes. A total of 1,224,048 non-redundant Nicotiana (NIX) markers were developed, of which 99.98% are novel. An efficient and uniform genotyping protocol for NIX markers was developed and validated. We created a web-based database of NIX marker information including amplicon sizes of alleles in each genome for downloading and online analysis.

CONCLUSIONS

The present work constitutes the first deep characterization of SSRs in seven genomes of Nicotiana, and the development of NIX markers for these SSRs. Our online marker database and an efficient genotyping protocol facilitate the application of these markers. The NIX markers greatly expand Nicotiana marker resources, thus providing a useful tool for future research and breeding. We demonstrate a novel protocol for SSR marker development and utilization at the whole genome scale that can be applied to any lineage of organisms. The Tobacco Markers & Primers Database (TMPD) is available at http://biodb.sdau.edu.cn/tmpd/index.html.

Genome survey sequencing of Dioscorea zingiberensis

Dioscorea zingiberensis (Dioscoreceae) is the main plant source of diosgenin (steroidal sapogenins), the precursor for the production of steroid hormones in the pharmaceutical industry. Despite its large economic value, genomic information of the genus Dioscorea is currently unavailable. Here, we present an initial survey of the D. zingiberensis genome performed by next-generation sequencing technology together with a genome size investigation inferred by flow cytometry. The whole genome survey of D. zingiberensis generated 31.48 Gb of sequence data with approximately 78.70× coverage. The estimated genome size is 800 Mb, with a high level of heterozygosity based on K-mer analysis. These reads were assembled into 334 288 contigs with a N50 length of 1079 bp, which were further assembled into 92 163 scaffolds with a total length of 173.46 Mb. A total of 4935 genes, 81 tRNAs, 69 rRNAs, and 661 miRNAs were predicted by the genome analysis, and 263 484 repeated sequences were obtained with 419 372 simple sequence repeats (SSRs). Among these SSRs, the mononucleotide repeat type was the most abundant (up to 54.60% of the total SSRs), followed by the dinucleotide (29.60%), trinucleotide (11.37%), tetranucleotide (3.53%), pentanucleotide (0.65%), and hexanucleotide (0.25%) repeat types. The 1C-value of D. zingiberensis was calibrated against Salvia miltiorrhiza and calculated as 0.87 pg (851 Mb) by flow cytometry, which was very close to the result of the genome survey. This is the first report of genome-wide characterization within this taxon.

The complete chloroplast genome of Colobanthus apetalus (Labill.) Druce: genome organization and comparison with related species

Colobanthus apetalus is a member of the genus Colobanthus, one of the 86 genera of the large family Caryophyllaceae which groups annual and perennial herbs (rarely shrubs) that are widely distributed around the globe, mainly in the Holarctic. The genus Colobanthus consists of 25 species, including Colobanthus quitensis, an extremophile plant native to the maritime Antarctic. Complete chloroplast (cp) genomes are useful for phylogenetic studies and species identification. In this study, next-generation sequencing (NGS) was used to identify the cp genome of C. apetalus. The complete cp genome of C. apetalus has the length of 151,228 bp, 36.65% GC content, and a quadripartite structure with a large single copy (LSC) of 83,380 bp and a small single copy (SSC) of 17,206 bp separated by inverted repeats (IRs) of 25,321 bp. The cp genome contains 131 genes, including 112 unique genes and 19 genes which are duplicated in the IRs. The group of 112 unique genes features 73 protein-coding genes, 30 tRNA genes, four rRNA genes and five conserved chloroplast open reading frames (ORFs). A total of 12 forward repeats, 10 palindromic repeats, five reverse repeats and three complementary repeats were detected. In addition, a simple sequence repeat (SSR) analysis revealed 41 (mono-, di-, tri-, tetra-, penta- and hexanucleotide) SSRs, most of which were AT-rich. A detailed comparison of C. apetalus and C. quitensis cp genomes revealed identical gene content and order. A phylogenetic tree was built based on the sequences of 76 protein-coding genes that are shared by the eleven sequenced representatives of Caryophyllaceae and C. apetalus, and it revealed that C. apetalus and C. quitensis form a clade that is closely related to Silene species and Agrostemma githago. Moreover, the genus Silene appeared as a polymorphic taxon. The results of this study expand our knowledge about the evolution and molecular biology of Caryophyllaceae.

Genetic Evaluation of Natural Populations of the Endangered Conifer Thuja koraiensis Using Microsatellite Markers by Restriction-Associated DNA Sequencing

Thuja koraiensis Nakai is an endangered conifer of high economic and ecological value in Jilin Province, China. However, studies on its population structure and conservation genetics have been limited by the lack of genomic data. Here, 37,761 microsatellites (simple sequence repeat, SSR) were detected based on 875,792 de novo-assembled contigs using a restriction-associated DNA (RAD) approach. Among these SSRs, 300 were randomly selected to test for polymorphisms and 96 obtained loci were able to amplify a fragment of expected size. Twelve polymorphic SSR markers were developed to analyze the genetic diversity and population structure of three natural populations. High genetic diversity (mean NA = 5.481, HE = 0.548) and moderate population differentiation (pairwise Fst = 0.048–0.078, Nm = 2.940–4.958) were found in this species. Molecular variance analysis suggested that most of the variation (83%) existed within populations. Combining the results of STRUCTURE, principal coordinate, and neighbor-joining analysis, the 232 individuals were divided into three genetic clusters that generally correlated with their geographical distributions. Finally, appropriate conservation strategies were proposed to protect this species. This study provides genetic information for the natural resource conservation and utilization of T. koraiensis and will facilitate further studies of the evolution and phylogeography of the species.