SNP discovery by illumina-based transcriptome sequencing of the olive and the genetic characterization of Turkish olive genotypes revealed by AFLP, SSR and SNP markers

One hundred single-copy nuclear sequence markers for olive variety identification: a case of fingerprinting database construction in China

Olive is an ancient oil-producing tree, widely cultivated in Mediterranean countries, and now spread to other areas of the world, including China. Recently, several molecular databases were constructed in different countries and platforms for olive identification using simple sequence repeats (SSRs) or single-nucleotide polymorphisms (SNPs). However, comparing their results across laboratories was difficult. Herein, hundreds of polymorphic single-copy nuclear sequence markers were developed from the olive genome. Using the advantage of multiplex PCR amplification and high-throughput sequencing, a fingerprint database was constructed for the majority of olives cultivated in China. We used 100 high-quality sequence loci and estimated the genetic diversity and structure among all these varieties. We found that compared with that based on SSRs, the constructed fingerprint database based on these 100 sequences or a few of them, could provide a reliable olive variety identification platform in China, with high discrimination among different varieties using the principle of BLAST algorithm. An example of such identification platform based on this study was displayed on the web for the olive database in China (http://olivedb.cn/jianding). After resolving redundant genotypes, we identified 126 olive varieties with distinct genotypes in China. These varieties could be divided into two clusters, and it was revealed that the grouping of the varieties has a certain relationship with their origin. Herein, it is concluded that these single-copy orthologous nuclear sequences could be used to construct a universal fingerprint database of olives across different laboratories and platforms inexpensively. Based on such a database, variety identification can be performed easily by any laboratory, which would further facilitate olive breeding and variety exchange globally.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01434-9.

Genealogical tracing of Olea europaea species and pedigree relationships of var. europaea using chloroplast and nuclear markers

BACKGROUND

Olive is one of the most cultivated species in the Mediterranean Basin and beyond. Despite being extensively studied for its commercial relevance, the origin of cultivated olive and the history of its domestication remain open questions. Here, we present a genealogical and kinship relationships analysis by mean of chloroplast and nuclear markers of different genera, subgenus, species, subspecies, ecotypes, cultivated, ancient and wild types, which constitutes one of the most inclusive research to date on the diversity within Olea europaea species. A complete survey of the variability across the nuclear and plastid genomes of different genotypes was studied through single nucleotide polymorphisms, indels (insertions and deletions), and length variation.

RESULTS

Fifty-six different chlorotypes were identified among the Oleaceae family including Olea europaea, other species and genera. The chloroplast genome evolution, within Olea europaea subspecies, probably started from subsp. cuspidata, which likely represents the ancestor of all the other subspecies and therefore of wild types and cultivars. Our study allows us to hypothesize that, inside the subspecies europaea containing cultivars and the wild types, the ancestral selection from var. sylvestris occurred both in the eastern side of the Mediterranean and in the central-western part of Basin. Moreover, it was elucidated the origin of several cultivars, which depends on the introduction of eastern cultivars, belonging to the lineage E1, followed by crossing and replacement of the autochthonous olive germplasm of central-western Mediterranean Basin. In fact, our study highlighted that two main 'founders' gave the origin to more than 60% of analyzed olive cultivars. Other secondary founders, which strongly contributed to give origin to the actual olive cultivar diversity, were already detected.

CONCLUSIONS

The application of comparative genomics not only paves the way for a better understanding of the phylogenetic relationships within the Olea europaea species but also provides original insights into other elusive evolutionary processes, such as chloroplast inheritance and parentage inside olive cultivars, opening new scenarios for further research such as the association studies and breeding programs.

Genomic and Transcriptional Profiling Analysis and Insights into Rhodomyrtone Yield in Rhodomyrtus tomentosa (Aiton) Hassk

Rhodomyrtus tomentosa is a source of a novel antibiotic, rhodomyrtone. Because of the increasing industrial demand for this compound, germplasm with a high rhodomyrtone content is the key to sustainable future cultivation. In this study, rhodomyrtone genotypes were verified using the plastid genomic region marker matK and nuclear ribosomal internal transcribed spacer ITS. These two DNA barcodes proved to be useful tools for identifying different rhodomyrtone contents via the SNP haplotypes C569T and A561G, respectively. The results were correlated with rhodomyrtone content determined via HPLC. Subsequently, R. tomentosa samples with high- and low-rhodomyrtone genotypes were collected for de novo transcriptome and gene expression analyses. A total of 83,402 unigenes were classified into 25 KOG classifications, and 74,102 annotated unigenes were obtained. Analysis of differential gene expression between samples or groups using DESeq2 revealed highly expressed levels related to rhodomyrtone content in two genotypes. semiquantitative RT-PCR further revealed that the high rhodomyrtone content in these two genotypes correlated with expression of zinc transporter protein (RtZnT). In addition, we found that expression of RtZnT resulted in increased sensitivity of R. tomentosa under ZnSO4 stress. The findings provide useful information for selection of cultivation sites to achieve high rhodomyrtone yields in R. tomentosa.

Utility of EST-SNP Markers for Improving Management and Use of Olive Genetic Resources: A Case Study at the Worldwide Olive Germplasm Bank of Córdoba

Olive, the emblematic Mediterranean fruit crop, owns a great varietal diversity, which is maintained in ex situ field collections, such as the World Olive Germplasm Bank of Córdoba (WOGBC), Spain. Accurate identification of WOGBC, one of the world’s largest collections, is essential for efficient management and use of olive germplasm. The present study is the first report of the use of a core set of 96 EST-SNP markers for the fingerprinting of 1273 accessions from 29 countries, including both field and new acquired accessions. The EST-SNP fingerprinting made possible the accurate identification of 668 different genotypes, including 148 detected among the new acquired accessions. Despite the overall high genetic diversity found at WOGBC, the EST-SNPs also revealed the presence of remarkable redundant germplasm mostly represented by synonymy cases within and between countries. This finding, together with the presence of homonymy cases, may reflect a continuous interchange of olive cultivars, as well as a common and general approach for their naming. The structure analysis revealed a certain geographic clustering of the analysed germplasm. The EST-SNP panel under study provides a powerful and accurate genotyping tool, allowing for the foundation of a common strategy for efficient safeguarding and management of olive genetic resources.

How to Choose a Good Marker to Analyze the Olive Germplasm (Olea europaea L.) and Derived Products

The olive tree (Olea europaea L.) is one of the most cultivated crops in the Mediterranean basin. Its economic importance is mainly due to the intense production of table olives and oil. Cultivated varieties are characterized by high morphological and genetic variability and present a large number of synonyms and homonyms. This necessitates the introduction of a rapid and accurate system for varietal identification. In the past, the recognition of olive cultivars was based solely on analysis of the morphological traits, however, these are highly influenced by environmental conditions. Therefore, over the years, several methods based on DNA analysis were developed, allowing a more accurate and reliable varietal identification. This review aims to investigate the evolving history of olive tree characterization approaches, starting from the earlier morphological methods to the latest technologies based on molecular markers, focusing on the main applications of each approach. Furthermore, we discuss the impact of the advent of next generation sequencing and the recent sequencing of the olive genome on the strategies used for the development of new molecular markers.

Microsatellite Markers in Olives (Olea europaea L.): Utility in the Cataloging of Germplasm, Food Authenticity and Traceability Studies

The olive fruit, a symbol of Mediterranean diets, is a rich source of antioxidants and oleic acid (55–83%). Olive genetic resources, including cultivated olives (cultivars), wild olives as well as related subspecies, are distributed widely across the Mediterranean region and other countries. Certain cultivars have a high commercial demand and economical value due to the differentiating organoleptic characteristics. This might result in economically motivated fraudulent practices and adulteration. Hence, tools to ensure the authenticity of constituent olive cultivars are crucial, and this can be achieved accurately through DNA-based methods. The present review outlines the applications of microsatellite markers, one of the most extensively used types of molecular markers in olive species, particularly referring to the use of these DNA-based markers in cataloging the vast olive germplasm, leading to identification and authentication of the cultivars. Emphasis has been given on the need to adopt a uniform platform where global molecular information pertaining to the details of available markers, cultivar-specific genotyping profiles (their synonyms or homonyms) and the comparative profiles of oil and reference leaf samples is accessible to researchers. The challenges of working with microsatellite markers and efforts underway, mainly advancements in genotyping methods which can be effectively incorporated in olive oil varietal testing, are also provided. Such efforts will pave the way for the development of more robust microsatellite marker-based olive agri-food authentication platforms.

Olive Fruit Development and Ripening: Break on through to the "-Omics" Side

The olive tree (Olea europaea L. subsp. europaea) is the most important perennial crop in the Mediterranean region, producing table olives and oil, both appreciated for their nutraceutical value. Although olive oil quality traits have been extensively studied, much less attention has been paid to olive drupe. Olive drupe ripening is an extremely complex process involving numerous physiological and molecular changes that are unique in this fruit crop species. This review underlines the contribution of "-omics" techniques and of the recent advances in bioinformatics and analytical tools, notably next-generation sequencing and mass spectrometry, for the characterization of the olive ripening syndrome. The usage of high-dimensional datasets, such as transcriptomics, proteomics, and metabolomics, will provide a systematical description of the molecular-specific processes regulating olive fruit development and ripening. However, the incomplete sequence of the O. europaea L. reference genome has largely hampered the utilization of omics tools towards olive drupe research. Due to this disadvantage, the most reported -omics studies on fruit trees concern metabolomics and only a few transcriptomics and proteomics. In this review, up-to-date applications of -omics technologies towards olive drupe biology are addressed, and future perspectives in olive fruit research are highlighted.

Applications of Microsatellite Markers for the Characterization of Olive Genetic Resources of Tunisia

Among the countries of the Mediterranean Basin, Tunisia is located at the crossroad for the immigration of several civilizations over the last two millennia, becoming a strategic place for gene flow, and a secondary center of diversity for olive species. Olive is one of the principal crop species in Tunisia and now it strongly characterizes the rural landscape of the country. In recent years, collecting missions on farm and in situ were carried out by various institutes, with special emphasis given to ex situ collections serving as a reference for the identification of olive germplasm. Simple Sequence Repeats (SSRs) represent the easiest and cheapest markers for olive genetic fingerprinting and have been the tool of choice for studying the genetic diversity of this crop in Tunisia, to resolve cases of homonymy and synonymy among the commercialized varieties, to identify rare cultivars, to improve knowledge about the genetic variability of this crop, to identify a hot spot of olive biodiversity in the Tunisian oasis of Degache, and to enrich the national reference collection of olive varieties. The present review describes the state of the art of the genetic characterization of the Tunisian olive germplasm and illustrate the progress obtained through the SSR markers, in individuating interesting genotypes that could be used for facing incoming problems determined by climate changes.

Genetic Diversity Assessment of a Chinese Medicinal Endemic Species, Aspidopterys obcordata var. obcordata, by Combined Molecular Marker Methods (ISSR & SRAP)

Aspidopterys obcordata var. obcordata, a medicinal plant endemic to China, is a narrowly distributed species and wild resources are extremely limited. To evaluate the genetic variability and degree of genetic divergence of A. obcordata var. obcordata, and to make rational scientific decisions on its harvest and germplasm conservation, we collected 122 samples from across nearly all of its distribution area and studied genetic diversity using inter-simple sequence repeats (ISSRs), sequence-related amplified polymorphisms (SRAPs), and a method combining the two techniques. The results revealed the high genetic diversity of A. obcordata var. obcordata, mainly due to its intra-population diversity, and the top two populations with the highest levels of intra-population diversity were ML and DH, individuals of which can serve as excellent germplasm candidates during the processing of germplasm screening and conservation. In general, the combining method was prior to the ISSR analyses and SRAP analyses results, except for a slight difference in the genetic structure of individual populations. Therefore, we suggest that a combination analysis of the two marker methods is ideal for evaluating the genetic diversity and genetic relationships of A. obcordata var. obcordata.

Cultivated Olive Diversification at Local and Regional Scales: Evidence From the Genetic Characterization of French Genetic Resources

Molecular characterization of crop genetic resources is a powerful approach to elucidate the origin of varieties and facilitate local cultivar management. Here we aimed to decipher the origin and diversification of French local olive germplasm. The 113 olive accessions of the ex situ collection of Porquerolles were characterized with 20 nuclear microsatellites plus their plastid haplotype. We then compared this collection to Mediterranean olive varieties from the Worldwide Olive Germplasm Bank of Marrakech, Morocco. High genetic diversity was observed within local French varieties, indicating a high admixture level, with an almost equal contribution from the three main Mediterranean gene pools. Nearly identical and closely related genotypes were observed among French and Italian/Spanish varieties. A high number of parent-offspring relationships were also detected among French varieties and between French and two Italian varieties ('Frantoio' and 'Moraiolo') and the Spanish variety ('Gordal Sevillana'). Our investigations indicated that French olive germplasm resulted from the diffusion of material from multiple origins followed by diversification based on parentage relationships between varieties. We strongly suggest that farmers have been actively selecting olives based on local French varieties. French olive agroecosystems more affected by unexpected frosts than southernmost regions could also be seen as incubators and as a bridge between Italy and Spain that has enhanced varietal olive diversification.

Characterization of Worldwide Olive Germplasm Banks of Marrakech (Morocco) and Córdoba (Spain): Towards management and use of olive germplasm in breeding programs

Olive (Olea europaea L.) is a major fruit crop in the Mediterranean Basin. Ex-situ olive management is essential to ensure optimal use of genetic resources in breeding programs. The Worldwide Olive Germplasm Bank of Córdoba (WOGBC), Spain, and Marrakech (WOGBM), Morocco, are currently the largest existing olive germplasm collections. Characterization, identification, comparison and authentication of all accessions in both collections could thus provide useful information for managing olive germplasm for its preservation, exchange within the scientific community and use in breeding programs. Here we applied 20 microsatellite markers (SSR) and 11 endocarp morphological traits to discriminate and authenticate 1091 olive accessions belonging to WOGBM and WOGBC (554 and 537, respectively). Of all the analyzed accessions, 672 distinct SSR profiles considered as unique genotypes were identified, but only 130 were present in both collections. Combining SSR markers and endocarp traits led to the identification of 535 cultivars (126 in common) and 120 authenticated cultivars. No significant differences were observed between collections regarding the allelic richness and diversity index. We concluded that the genetic diversity level was stable despite marked contrasts in varietal composition between collections, which could be explained by their different collection establishment conditions. This highlights the extent of cultivar variability within WOGBs. Moreover, we detected 192 mislabeling errors, 72 of which were found in WOGBM. A total of 228 genotypes as molecular variants of 74 cultivars, 79 synonyms and 39 homonyms as new cases were identified. Both collections were combined to define the nested core collections of 55, 121 and 150 sample sizes proposed for further studies. This study was a preliminary step towards managing and mining the genetic diversity in both collections while developing collaborations between olive research teams to conduct association mapping studies by exchanging and phenotyping accessions in contrasted environmental sites.

Genetic Diversity Analysis of Olive Germplasm (Olea europaea L.) With Genotyping-by-Sequencing Technology

Olive (Olea europaea L.) is a very important edible oil crop and has been cultivated for about 4,000 years in the Mediterranean area. Due to its nutritional and economic importance, researches on germplasm characterization received extensive attention. In this study, using the genotyping-by-sequencing (GBS) technology, we carried out genetic diversity analysis on 57 olive cultivars with different geographical origins. In total, 73,482 high-quality single-nucleotide polymorphisms (SNPs) with minor allele frequency (MAF) > 5%, call rate > 50%, and heterozygosity rate < 10% were obtained at the whole genome level. Genetic structure and phylogenetic analysis showed that the 57 olive cultivars could be classified into two groups (Group I and Group II). No clear geographical distributions of cultivars were observed generally between the two groups. The average nucleotide diversities (π) specific for Group I and Group II were 0.317 and 0.305. The fixation index (F ST) between Group I and Group II was 0.033. In Group II, cultivars could be further divided into two subgroups (Group IIa and Group IIb), which seem to be associated with their fruit sizes. The five Chinese-bred cultivars were all clustered in Group II, showing a closer genetic relationship with those from the central Mediterranean region and limited genetic background. It is therefore necessary for Chinese olive breeding programs to incorporate other genetic basis by utilizing germplasm from the other regions particularly from the east Mediterranean region as breeding parents. The results showed that GBS is an effective marker choice for cultivar characterization and genetic diversity analysis in olive and will help us better understand the genetic backgrounds of the crop.

Facing Climate Change: Biotechnology of Iconic Mediterranean Woody Crops

The Mediterranean basin is especially sensitive to the adverse outcomes of climate change and especially to variations in rainfall patterns and the incidence of extremely high temperatures. These two concurring adverse environmental conditions will surely have a detrimental effect on crop performance and productivity that will be particularly severe on woody crops such as citrus, olive and grapevine that define the backbone of traditional Mediterranean agriculture. These woody species have been traditionally selected for traits such as improved fruit yield and quality or alteration in harvesting periods, leaving out traits related to plant field performance. This is currently a crucial aspect due to the progressive and imminent effects of global climate change. Although complete genome sequence exists for sweet orange (Citrus sinensis) and clementine (Citrus clementina), olive tree (Olea europaea) and grapevine (Vitis vinifera), the development of biotechnological tools to improve stress tolerance still relies on the study of the available genetic resources including interspecific hybrids, naturally occurring (or induced) polyploids and wild relatives under field conditions. To this respect, post-genomic era studies including transcriptomics, metabolomics and proteomics provide a wide and unbiased view of plant physiology and biochemistry under adverse environmental conditions that, along with high-throughput phenotyping, could contribute to the characterization of plant genotypes exhibiting physiological and/or genetic traits that are correlated to abiotic stress tolerance. The ultimate goal of precision agriculture is to improve crop productivity, in terms of yield and quality, making a sustainable use of land and water resources under adverse environmental conditions using all available biotechnological tools and high-throughput phenotyping. This review focuses on the current state-of-the-art of biotechnological tools such as high throughput -omics and phenotyping on grapevine, citrus and olive and their contribution to plant breeding programs.

High Levels of Variation Within Gene Sequences of Olea europaea L

Gene sequence variation in cultivated olive (Olea europaea L. subsp. europaea var. europaea), the most important oil tree crop of the Mediterranean basin, has been poorly evaluated up to now. A deep sequence analysis of fragments of four genes, OeACP1, OeACP2, OeLUS and OeSUT1, in 90 cultivars, revealed a wide range of polymorphisms along all recognized allele forms and unexpected allele frequencies and genotype combinations. High linkage values among most polymorphisms were recorded within each gene fragment. The great sequence variability corresponded to a low number of alleles and, surprisingly, to a small fraction of genotype combinations. The distribution, frequency, and combination of the different alleles at each locus is possibly due to natural and human pressures, such as selection, ancestrality, or fitness. Phylogenetic analyses of allele sequences showed distant and complex patterns of relationships among cultivated olives, intermixed with other related forms, highlighting an evolutionary connection between olive cultivars and the O. europaea subspecies cuspidata and cerasiformis. This study demonstrates how a detailed and complete sequence analysis of a few gene portions and a thorough genotyping on a representative set of cultivars can clarify important issues related to sequence polymorphisms, reconstructing the phylogeny of alleles, as well as the genotype combinations. The identification of regions representing blocks of recombination could reveal polymorphisms that represent putatively functional markers. Indeed, specific mutations found on the analyzed OeACP1 and OeACP2 fragments seem to be correlated to the fruit weight.

GBS-derived SNP catalogue unveiled wide genetic variability and geographical relationships of Italian olive cultivars

Information on the distribution of genetic variation is essential to preserve olive germplasm from erosion and to recover alleles lost through selective breeding. In addition, knowledge on population structure and genotype-phenotype associations is crucial to support modern olive breeding programs that must respond to new environmental conditions imposed by climate change and novel biotic/abiotic stressors. To further our understanding of genetic variation in the olive, we performed genotype-by-sequencing on a panel of 94 Italian olive cultivars. A reference-based and a reference-independent SNP calling pipeline generated 22,088 and 8,088 high-quality SNPs, respectively. Both datasets were used to model population structure via parametric and non parametric clustering. Although the two pipelines yielded a 3-fold difference in the number of SNPs, both described wide genetic variability among our study panel and allowed individuals to be grouped based on fruit weight and the geographical area of cultivation. Multidimensional scaling analysis on identity-by-state allele-sharing values as well as inference of population mixtures from genome-wide allele frequency data corroborated the clustering pattern we observed. These findings allowed us to formulate hypotheses about geographical relationships of Italian olive cultivars and to confirm known and uncover novel cases of synonymy.

On the origins and domestication of the olive: a review and perspectives

Background

Unravelling domestication processes is crucial for understanding how species respond to anthropogenic pressures, forecasting crop responses to future global changes and improving breeding programmes. Domestication processes for clonally propagated perennials differ markedly from those for seed-propagated annual crops, mostly due to long generation times, clonal propagation and recurrent admixture with local forms, leading to a limited number of generations of selection from wild ancestors. However, additional case studies are required to document this process more fully.

Scope

The olive is an iconic species in Mediterranean cultural history. Its multiple uses and omnipresence in traditional agrosystems have made this species an economic pillar and cornerstone of Mediterranean agriculture. However, major questions about the domestication history of the olive remain unanswered. New paleobotanical, archeological, historical and molecular data have recently accumulated for olive, making it timely to carry out a critical re-evaluation of the biogeography of wild olives and the history of their cultivation. We review here the chronological history of wild olives and discuss the questions that remain unanswered, or even unasked, about their domestication history in the Mediterranean Basin. We argue that more detailed ecological genomics studies of wild and cultivated olives are crucial to improve our understanding of olive domestication. Multidisciplinary research integrating genomics, metagenomics and community ecology will make it possible to decipher the evolutionary ecology of one of the most iconic domesticated fruit trees worldwide.

Conclusion

The olive is a relevant model for improving our knowledge of domestication processes in clonally propagated perennial crops, particularly those of the Mediterranean Basin. Future studies on the ecological and genomic shifts linked to domestication in olive and its associated community will provide insight into the phenotypic and molecular bases of crop adaptation to human uses.

Usefulness of a New Large Set of High Throughput EST-SNP Markers as a Tool for Olive Germplasm Collection Management

Germplasm collections are basic tools for conservation, characterization, and efficient use of olive genetic resources. The identification of the olive cultivars maintained in the collections is an important ongoing task which has been performed by both, morphological and molecular markers. In the present study, based on the sequencing results of previous genomic projects, a new set of 1,043 EST-SNP markers has been identified. In order to evaluate its discrimination capacity and utility in diversity studies, this set of markers was used in a representative number of accessions from 20 different olive growing countries and maintained at the World Olive Germplasm Collection of IFAPA Centre 'Alameda del Obispo' (Córdoba, Spain), one of the world's largest olive germplasm bank. Thus, the cultivated material included: cultivars belonging to previously defined core collections by means of SSR markers and agronomical traits, well known homonymy cases, possible redundancies previously identified in the collection, and recently introduced accessions. Marker stability was tested in repeated analyses of a selected number of accessions, as well as in different trees and accessions belonging to the same cultivar. In addition, 15 genotypes from a cross 'Picual' × 'Arbequina' cultivars from the IFAPA olive breeding program and a set of 89 wild genotypes were also included in the study. Our results indicate that, despite their relatively wide variability, the new set of EST-SNPs displayed lower levels of genetic diversity than SSRs in the set of olive core collections tested. However, the EST-SNP markers displayed consistent and reliable results from different plant material sources and plant propagation events. The EST-SNPs revealed a clear cut off between inter- and intra-cultivar variation in olive. Besides, they were able to reliably discriminate among different accessions, to detect possible homonymy cases as well as efficiently ascertain the presence of redundant germplasm in the collection. Additionally, these markers were highly transferable to the wild genotypes. These results, together with the low genotyping error rates and the easy and fully automated procedure used to get the genotyping data, validate the new set of EST-SNPs as possible markers of choice for olive cultivar identification.

Genetic Diversity and Genetic Relationships of Purple Willow (Salix purpurea L.) from Natural Locations

In this study, the genetic diversity and structure of 13 natural locations of Salix purpurea were determined with the use of AFLP (amplified length polymorphism), RAPD (randomly amplified polymorphic DNA) and ISSR (inter-simple sequence repeats). The genetic relationships between 91 examined S. purpurea genotypes were evaluated by analyses of molecular variance (AMOVA), principal coordinates analyses (PCoA) and UPGMA (unweighted pair group method with arithmetic mean) dendrograms for both single marker types and a combination of all marker systems. The locations were assigned to distinct regions and the analysis of AMOVA (analysis of molecular variance) revealed a high genetic diversity within locations. The genetic diversity between both regions and locations was relatively low, but typical for many woody plant species. The results noted for the analyzed marker types were generally comparable with few differences in the genetic relationships among S. purpurea locations. A combination of several marker systems could thus be ideally suited to understand genetic diversity patterns of the species. This study makes the first attempt to broaden our knowledge of the genetic parameters of the purple willow (S. purpurea) from natural location for research and several applications, inter alia breeding purposes.

Genetic Diversity and Genetic Relationships of Purple Willow (Salix purpurea L.) from Natural Locations

In this study, the genetic diversity and structure of 13 natural locations of Salix purpurea were determined with the use of AFLP (amplified length polymorphism), RAPD (randomly amplified polymorphic DNA) and ISSR (inter-simple sequence repeats). The genetic relationships between 91 examined S. purpurea genotypes were evaluated by analyses of molecular variance (AMOVA), principal coordinates analyses (PCoA) and UPGMA (unweighted pair group method with arithmetic mean) dendrograms for both single marker types and a combination of all marker systems. The locations were assigned to distinct regions and the analysis of AMOVA (analysis of molecular variance) revealed a high genetic diversity within locations. The genetic diversity between both regions and locations was relatively low, but typical for many woody plant species. The results noted for the analyzed marker types were generally comparable with few differences in the genetic relationships among S. purpurea locations. A combination of several marker systems could thus be ideally suited to understand genetic diversity patterns of the species. This study makes the first attempt to broaden our knowledge of the genetic parameters of the purple willow (S. purpurea) from natural location for research and several applications, inter alia breeding purposes.

Recent developments in olive (Olea europaea L.) genetics and genomics: applications in taxonomy, varietal identification, traceability and breeding

The latest results in DNA markers application and genomic studies in olive. Olive (Olea europaea L.) is among the most ancient tree crops worldwide and the source of oil beneficial for human health. Despite this, few data on olive genetics are available in comparison with other cultivated plant species. Molecular information is mainly linked to molecular markers and their application to the study of DNA variation in the Olea europaea complex. In terms of genomic research, efforts have been made in sequencing, heralding the era of olive genomic. The present paper represents an update of a previous review work published in this journal in 2011. The review is again mainly focused on DNA markers, whose application still constitutes a relevant percentage of the most recently published researches. Since the olive genomic era has recently started, the latest results in this field are also being discussed.

Transcriptomic Analysis of L. Roots during the Early Infection Process

Olive cultivation is affected by a wide range of biotic constraints. Verticillium wilt of olive is one of the most devastating diseases affecting this woody crop, inflicting major economic losses in many areas, particularly within the Mediterranean Basin. Little is known about gene-expression changes during plant infection by of woody plants such as olive. A complete RNA-seq transcriptomic analysis of olive tree roots was made. Trinity assembler proved to be the best option to assemble the olive and transcriptomes. The olive transcriptome (Oleup) consisted of 68,259 unigenes (254,252 isoforms/transcripts), and the transcriptome (Vedah) consisted of 37,425 unigenes (52,119 isoforms/transcripts). Most unigenes of the Oleup transcriptome corresponded to cellular processes (12,339), metabolic processes (10,974), single-organism processes (7263), and responses to stimuli (5114). As for the Vedah transcriptome, most unigenes correspond to metabolic processes (25,372), cellular processes (23,718), localization (6385), and biological regulation (4801). Differential gene-expression analysis of both transcriptomes was made at 2 and 7 d post-infection. The induced genes of both organisms during the plant-pathogen interaction were clustered in six subclusters, depending on the expression patterns during the infection. Subclusters A to C correspond to plant genes, and subcluster D to F correspond to genes. A relevant finding was that the differentially expressed gene (DEGs) included in subclusters B and C were highly enriched in proteolysis as well as protein-folding and biosynthesis genes. In addition, a reactive oxygen species (ROS) defense was induced first in the pathogen and later in the plant roots.

Transcriptome-based SNP discovery by GBS and the construction of a genetic map for olive

Molecular markers located in the genic regions of plants are valuable tools for the identification of candidate genes of economically important traits and consequent use in marker-assisted selection (MAS). In the past, simple sequence repeat markers (SSRs) and single-nucleotide polymorphisms (SNPs) located in expressed sequence tags (ESTs) were developed by sequencing RNA derived from different plant tissues, which involves laborious RNA extraction, mRNA isolation, and cDNA synthesis. In order to develop SNP markers located in olive transcriptomes, we used the recently developed genotyping-by-sequencing (GBS) technique. An analysis was done for 125 olive DNA samples (123 DNA samples from a cross-pollinated F₁ mapping population, and two samples from parents). From 45 to 66% of Illumina reads from GBS analysis were aligned to the olive transcriptome. A total of 22,033 transcriptome-based SNP markers were identified, and 3384 of these were mapped in the olive genome. The genetic linkage map constructed in this study consists of 1 cleaved amplified polymorphic sequence (CAPS), 19 SSR, and 3384 transcriptome-based SNP markers. The map covers 3340.8 cM of the olive genome in 23 linkage groups, with the length of the linkage groups ranging from 55.6 to 248.7 cM. Average map distance between flanking markers was 0.98 cM. This genetic linkage map is a saturated genetic map and will be a useful tool for the localization of quantitative trait loci (QTLs) and gene(s) of interest and for the identification of candidate genes for economically important traits.

The First Molecular Identification of an Olive Collection Applying Standard Simple Sequence Repeats and Novel Expressed Sequence Tag Markers

Germplasm collections of tree crop species represent fundamental tools for conservation of diversity and key steps for its characterization and evaluation. For the olive tree, several collections were created all over the world, but only few of them have been fully characterized and molecularly identified. The olive collection of Perugia University (UNIPG), established in the years' 60, represents one of the first attempts to gather and safeguard olive diversity, keeping together cultivars from different countries. In the present study, a set of 370 olive trees previously uncharacterized was screened with 10 standard simple sequence repeats (SSRs) and nine new EST-SSR markers, to correctly and thoroughly identify all genotypes, verify their representativeness of the entire cultivated olive variation, and validate the effectiveness of new markers in comparison to standard genotyping tools. The SSR analysis revealed the presence of 59 genotypes, corresponding to 72 well known cultivars, 13 of them resulting exclusively present in this collection. The new EST-SSRs have shown values of diversity parameters quite similar to those of best standard SSRs. When compared to hundreds of Mediterranean cultivars, the UNIPG olive accessions were splitted into the three main populations (East, Center and West Mediterranean), confirming that the collection has a good representativeness of the entire olive variability. Furthermore, Bayesian analysis, performed on the 59 genotypes of the collection by the use of both sets of markers, have demonstrated their splitting into four clusters, with a well balanced membership obtained by EST respect to standard SSRs. The new OLEST (Olea expressed sequence tags) SSR markers resulted as effective as the best standard markers. The information obtained from this study represents a high valuable tool for ex situ conservation and management of olive genetic resources, useful to build a common database from worldwide olive cultivar collections, also based on recently developed markers.

Evolution and perspectives of cultivar identification and traceability from tree to oil and table olives by means of DNA markers

In recent years, an increasing number of typicality marks has been awarded to high-quality olive oils produced from local cultivars. In this case, quality control requires effective varietal checks of the starting materials. Moreover, accurate cultivar identification is essential in vegetative-propagated plants distributed by nurseries and is a pre-requisite to register new cultivars. Food genomics provides many tools for cultivar identification and traceability from tree to oil and table olives. The results of the application of different classes of DNA markers to olive with the purpose of checking cultivar identity and variability of plant material are extensively discussed in this review, with special regard to repeatability issues and polymorphism degree. The characterization of olive germplasm from all countries of the Mediterranean basin and from less studied geographical areas is described and innovative high-throughput molecular tools to manage reference collections are reviewed. Then the transferability of DNA markers to processed products - virgin olive oils and table olives - is overviewed to point out strengths and weaknesses, with special regard to (i) the influence of processing steps and storage time on the quantity and quality of residual DNA, (ii) recent advances to overcome the bottleneck of DNA extraction from processed products, (iii) factors affecting whole comparability of DNA profiles between fresh plant materials and end-products, (iv) drawbacks in the analysis of multi-cultivar versus single-cultivar end-products and (v) the potential of quantitative polymerase chain reaction (PCR)-based techniques. © 2016 Society of Chemical Industry.

Genetic Diversity Among Historical Olive (Olea europaea L.) Genotypes from Southern Anatolia Based on SSR Markers

Olive (Olea europaea) is an ancient and important crop in both olive oil production and table use. It is important to identify the genetic diversity of olive genetic resources for cultivar development and evaluation of olive germplasm. In the study, 14 microsatellite markers (UDO4, UDO8, UDO9, UDO11, UDO12, UDO22, UDO24, UDO26, UDO28, DCA9, DCA11, DCA13, DCA15, and DCA18) were used to assess the genetic variation on 76 olive (Olea europaea L.) genotypes from Mardin province together with 6 well-known Turkish and 4 well-known foreign reference cultivars. All microsatellite markers showed polymorphism and the number of alleles varied between 9 and 22, with an average of 14.57. The most informative loci were DCA 11 (22 alleles) and DCA 9 (21 alleles). Dendrogram based on genetic distances was constructed for the 86 olive genotypes/cultivars, which revealed the existence of different clusters. The high genetic similarity was evident between Bakırkire2 and Zinnar5 (0.74) genotypes, while the most genetically divergent genotypes were Gürmeşe5 and Yedikardeşler2 (0.19). It was concluded that there was abundant SSR polymorphism in olive germplasm in southern Anatolia in Turkey and could be important for future breeding activities.

Association Mapping in Turkish Olive Cultivars Revealed Significant Markers Related to Some Important Agronomic Traits

Olive (Olea europaea L.) is one of the most important fruit trees especially in the Mediterranean countries due to high consumption of table olive and olive oil. In olive breeding, the phenotypic traits associated to fruit are the key factors that determine productivity. Association mapping has been used in some tree species and a lot of crop plant species, and here, we perform an initial effort to detect marker-trait associations in olive tree. In the current study, a total of 96 olive genotypes, including both oil and table olive genotypes from Turkish Olive GenBank Resources, were used to examine marker-trait associations. For olive genotyping, SNP, AFLP, and SSR marker data were selected from previously published study and association analysis was performed between these markers and 5 yield-related traits. Three different approaches were used to check for false-positive results in association tests, and association results obtained from these models were compared. Using the model utilizing both population structure and relative kinship, eleven associations were significant with FDR ≤ 0.05. The largest number of significant associations was detected for fruit weight and stone weight. Our results suggested that association mapping could be an effective approach for identifying marker-trait associations in olive genotypes, without the development of mapping populations. This study shows for the first time the use of association mapping for identifying molecular markers linked to important traits in olive tree.

SNP Discovery by GBS in Olive and the Construction of a High-Density Genetic Linkage Map

Genetic linkage maps are valuable tools for genetic, genomic, and crop breeding studies. Several genetic linkage maps were constructed for the olive (Olea europaea L.) genome, mainly using amplified fragment length polymorphisms (AFLPs) and simple sequence repeat (SSR) markers. However, AFLPs and SSR markers were not enough to develop a high-density olive linkage map. Genotyping-by-sequencing (GBS), a recently developed single-nucleotide polymorphism (SNP) identification methodology based on next-generation sequencing (NGS) technologies, has been demonstrated to be useful for the identification of a high number of SNP markers and the construction of high-density genetic linkage maps. In the present study, we identified a total of 10,941 SNPs from a cross between the olive cultivars 'Gemlik' and 'Edincik Su' using GBS and de novo SNP discovery implemented in the computer program "Stacks." A high-density genetic linkage map for the olive genome was constructed using 121 cross-pollinated full-sib F1 progeny and 5643 markers (21 SSRs, 203 AFLPs, and 5736 SNPs). This linkage map was composed of 25 linkage groups, covering 3049 cM of the olive genome, and the mean distance between the flanking markers was 0.53 cM. To the best of our knowledge, this map is the most saturated genetic linkage map in olive to date. We demonstrated that GBS is a valuable tool for the identification of thousands of SNPs for the construction of a saturated genetic linkage map in olive. The high-density genetic map developed in this study is a useful tool for locating quantitative trait loci and other economically important traits in the olive genome.

Identifying Litchi (Litchi chinensis Sonn.) Cultivars and Their Genetic Relationships Using Single Nucleotide Polymorphism (SNP) Markers

Litchi is an important fruit tree in tropical and subtropical areas of the world. However, there is widespread confusion regarding litchi cultivar nomenclature and detailed information of genetic relationships among litchi germplasm is unclear. In the present study, the potential of single nucleotide polymorphism (SNP) for the identification of 96 representative litchi accessions and their genetic relationships in China was evaluated using 155 SNPs that were evenly spaced across litchi genome. Ninety SNPs with minor allele frequencies above 0.05 and a good genotyping success rate were used for further analysis. A relatively high level of genetic variation was observed among litchi accessions, as quantified by the expected heterozygosity (He = 0.305). The SNP based multilocus matching identified two synonymous groups, 'Heiye' and 'Wuye', and 'Chengtuo' and 'Baitangli 1'. A subset of 14 SNPs was sufficient to distinguish all the non-redundant litchi genotypes, and these SNPs were proven to be highly stable by repeated analyses of a selected group of cultivars. Unweighted pair-group method of arithmetic averages (UPGMA) cluster analysis divided the litchi accessions analyzed into four main groups, which corresponded to the traits of extremely early-maturing, early-maturing, middle-maturing, and late-maturing, indicating that the fruit maturation period should be considered as the primary criterion for litchi taxonomy. Two subpopulations were detected among litchi accessions by STRUCTURE analysis, and accessions with extremely early- and late-maturing traits showed membership coefficients above 0.99 for Cluster 1 and Cluster 2, respectively. Accessions with early- and middle-maturing traits were identified as admixture forms with varying levels of membership shared between the two clusters, indicating their hybrid origin during litchi domestication. The results of this study will benefit litchi germplasm conservation programs and facilitate maximum genetic gains in litchi breeding programs.

Cultivar origin and admixture detection in Turkish olive oils by SNP-based CAPS assays

The aim of this study was to establish a DNA-based identification key to ascertain the cultivar origin of Turkish monovarietal olive oils. To reach this aim, we sequenced short fragments from five olive genes for SNP (single nucleotide polymorphism) identification and developed CAPS (cleaved amplified polymorphic DNA) assays for SNPs that alter restriction enzyme recognition motifs. When applied on the oils of 17 olive cultivars, a maximum of five CAPS assays were necessary to discriminate the varietal origin of the samples. We also tested the efficiency and limit of our approach for detecting olive oil admixtures. As a result of the analysis, we were able to detect admixing down to a limit of 20%. The SNP-based CAPS assays developed in this work can be used for testing and verification of the authenticity of Turkish monovarietal olive oils, for olive tree certification, and in germplasm characterization and preservation studies.

Development of genetic markers in Eucalyptus species by target enrichment and exome sequencing

The advent of next-generation sequencing has facilitated large-scale discovery, validation and assessment of genetic markers for high density genotyping. The present study was undertaken to identify markers in genes supposedly related to wood property traits in three Eucalyptus species. Ninety four genes involved in xylogenesis were selected for hybridization probe based nuclear genomic DNA target enrichment and exome sequencing. Genomic DNA was isolated from the leaf tissues and used for on-array probe hybridization followed by Illumina sequencing. The raw sequence reads were trimmed and high-quality reads were mapped to the E. grandis reference sequence and the presence of single nucleotide variants (SNVs) and insertions/ deletions (InDels) were identified across the three species. The average read coverage was 216X and a total of 2294 SNVs and 479 InDels were discovered in E. camaldulensis, 2383 SNVs and 518 InDels in E. tereticornis, and 1228 SNVs and 409 InDels in E. grandis. Additionally, SNV calling and InDel detection were conducted in pair-wise comparisons of E. tereticornis vs. E. grandis, E. camaldulensis vs. E. tereticornis and E. camaldulensis vs. E. grandis. This study presents an efficient and high throughput method on development of genetic markers for family– based QTL and association analysis in Eucalyptus.

Next Generation Sequencing Technologies: The Doorway to the Unexplored Genomics of Non-Model Plants

Non-model plants i.e., the species which have one or all of the characters such as long life cycle, difficulty to grow in the laboratory or poor fecundity, have been schemed out of sequencing projects earlier, due to high running cost of Sanger sequencing. Consequently, the information about their genomics and key biological processes are inadequate. However, the advent of fast and cost effective next generation sequencing (NGS) platforms in the recent past has enabled the unearthing of certain characteristic gene structures unique to these species. It has also aided in gaining insight about mechanisms underlying processes of gene expression and secondary metabolism as well as facilitated development of genomic resources for diversity characterization, evolutionary analysis and marker assisted breeding even without prior availability of genomic sequence information. In this review we explore how different Next Gen Sequencing platforms, as well as recent advances in NGS based high throughput genotyping technologies are rewarding efforts on de-novo whole genome/transcriptome sequencing, development of genome wide sequence based markers resources for improvement of non-model crops that are less costly than phenotyping.

Large-scale SNP discovery through RNA sequencing and SNP genotyping by targeted enrichment sequencing in cassava (Manihot esculenta Crantz)

Cassava (Manihot esculenta Crantz) is one of the most important crop species being the main source of dietary energy in several countries. Marker-assisted selection has become an essential tool in plant breeding. Single nucleotide polymorphism (SNP) discovery via transcriptome sequencing is an attractive strategy for genome complexity reduction in organisms with large genomes. We sequenced the transcriptome of 16 cassava accessions using the Illumina HiSeq platform and identified 675,559 EST-derived SNP markers. A subset of those markers was subsequently genotyped by capture-based targeted enrichment sequencing in 100 F1 progeny segregating for starch viscosity phenotypes. A total of 2,110 non-redundant SNP markers were used to construct a genetic map. This map encompasses 1,785 cM and consists of 19 linkage groups. A major quantitative trait locus (QTL) controlling starch pasting properties was identified and shown to coincide with the QTL previously reported for this trait. With a high-density SNP-based linkage map presented here, we also uncovered a novel QTL associated with starch pasting time on LG 10.

Novel microRNA-like viral small regulatory RNAs arising during human hepatitis A virus infection

MicroRNAs (miRNAs), including host miRNAs and viral miRNAs, play vital roles in regulating host-virus interactions. DNA viruses encode miRNAs that regulate the viral life cycle. However, it is generally believed that cytoplasmic RNA viruses do not encode miRNAs, owing to inaccessible cellular miRNA processing machinery. Here, we provide a comprehensive genome-wide analysis and identification of miRNAs that were derived from hepatitis A virus (HAV; Hu/China/H2/1982), which is a typical cytoplasmic RNA virus. Using deep-sequencing and in silico approaches, we identified 2 novel virally encoded miRNAs, named hav-miR-1-5p and hav-miR-2-5p. Both of the novel virally encoded miRNAs were clearly detected in infected cells. Analysis of Dicer enzyme silencing demonstrated that HAV-derived miRNA biogenesis is Dicer dependent. Furthermore, we confirmed that HAV mature miRNAs were generated from viral miRNA precursors (pre-miRNAs) in host cells. Notably, naturally derived HAV miRNAs were biologically and functionally active and induced post-transcriptional gene silencing (PTGS). Genomic location analysis revealed novel miRNAs located in the coding region of the viral genome. Overall, our results show that HAV naturally generates functional miRNA-like small regulatory RNAs during infection. This is the first report of miRNAs derived from the coding region of genomic RNA of a cytoplasmic RNA virus. These observations demonstrate that a cytoplasmic RNA virus can naturally generate functional miRNAs, as DNA viruses do. These findings also contribute to improved understanding of host-RNA virus interactions mediated by RNA virus-derived miRNAs.

Sequence analysis of single-copy genes in two wild olive subspecies: nucleotide diversity and potential use for testing admixture

The wild olive distribution extends from the Mediterranean region to south Asia and Austral Africa. The species is also invasive, particularly in Australia. Here, we investigated the sequence variation at five nuclear single-copy genes in 41 native and invasive accessions of the Mediterranean and African olive subspecies. The nucleotide diversity was assessed and the phylogenetic relationships between alleles were depicted with haplotype networks. A Bayesian clustering method (STRUCTURE) was applied to identify the main gene pools. We found an average of 18.4 alleles per locus. Native Mediterranean and African olives only share one allele, which testifies for ancient admixture on the Red Sea hills. The presence of divergent alleles in the Mediterranean olive, as well as the identification of two main genetic clusters, suggests a complex origin with two highly differentiated gene pools from the eastern and western Mediterranean that recently admixed. In the invasive range, relatively high nucleotide diversity is observed as a consequence of the introduction of alleles from two subspecies. Our data confirm that four invasive individuals are early-generation hybrids. Finally, the utility of single-copy gene sequences in olive population genomic and phylogenetic studies is briefly discussed.

Single-nucleotide polymorphism markers from de-novo assembly of the pomegranate transcriptome reveal germplasm genetic diversity

Pomegranate is a valuable crop that is grown commercially in many parts of the world. Wild species have been reported from India, Turkmenistan and Socotra. Pomegranate fruit has a variety of health-beneficial qualities. However, despite this crop's importance, only moderate effort has been invested in studying its biochemical or physiological properties or in establishing genomic and genetic infrastructures. In this study, we reconstructed a transcriptome from two phenotypically different accessions using 454-GS-FLX Titanium technology. These data were used to explore the functional annotation of 45,187 fully annotated contigs. We further compiled a genetic-variation resource of 7,155 simple-sequence repeats (SSRs) and 6,500 single-nucleotide polymorphisms (SNPs). A subset of 480 SNPs was sampled to investigate the genetic structure of the broad pomegranate germplasm collection at the Agricultural Research Organization (ARO), which includes accessions from different geographical areas worldwide. This subset of SNPs was found to be polymorphic, with 10.7% loci with minor allele frequencies of (MAF<0.05). These SNPs were successfully used to classify the ARO pomegranate collection into two major groups of accessions: one from India, China and Iran, composed of mainly unknown country origin and which was more of an admixture than the other major group, composed of accessions mainly from the Mediterranean basin, Central Asia and California. This study establishes a high-throughput transcriptome and genetic-marker infrastructure. Moreover, it sheds new light on the genetic interrelations between pomegranate species worldwide and more accurately defines their genetic nature.