Minimal marker: an algorithm and computer program for the identification of minimal sets of discriminating DNA markers for efficient variety identification

CoreSNP: an efficient pipeline for core marker profile selection from genome-wide SNP datasets in crops

BACKGROUND

DNA marker profiles play a crucial role in the identification and registration of germplasm, as well as in the distinctness, uniformity, and stability (DUS) testing of new plant variety protection. However, selecting minimal marker sets from large-scale SNP dataset can be challenging to distinguish a maximum number of samples.

RESULTS

Here, we developed the CoreSNP pipeline using a "divide and conquer" strategy and a "greedy" algorithm. The pipeline offers adjustable parameters to guarantee the distinction of each sample pair with at least two markers. Additionally, it allows datasets with missing loci as input. The pipeline was tested in barley, soybean, wheat, rice and maize. A few dozen of core SNPs were efficiently selected in different crops with SNP array, GBS, and WGS dataset, which can differentiate thousands of individual samples. The core SNPs were distributed across all chromosomes, exhibiting lower pairwise linkage disequilibrium (LD) and higher polymorphism information content (PIC) and minor allele frequencies (MAF). It was shown that both the genetic diversity of the population and the characteristics of the original dataset can significantly influence the number of core markers. In addition, the core SNPs capture a certain level of the original population structure.

CONCLUSIONS

CoreSNP is an efficiency way of core marker sets selection based on Genome-wide SNP datasets of crops. Combined with low-density SNP chip or genotyping technologies, it can be a cost-effective way to simplify and expedite the evaluation of genetic resources and differentiate different crop varieties. This tool is expected to have great application prospects in the rapid comparison of germplasm and intellectual property protection of new varieties.

Bioinformatics approach for developing a minimum set of SNP markers for identification of temperate japonica rice varieties cultivated in Spain

The use of molecular markers for plant variety identification and protection is increasing. For this purpose, SNP markers have provided a reliable and stable tool for plant genotyping. The availability of small and low-cost SNP panels to accelerate the identification of the cultivated rice varieties should be beneficial for breeders, seed certification entities and rice industry. With the intention of providing of such a facility, we first developed a simple and easy-handle bioinformatics tool based on the widely used and freely available software R to generate small sets of SNPs that can discriminate varieties, by selecting markers from a larger genotyping dataset. By applying this algorithm to data from a previously genotyped collection of temperate japonica varieties from different countries, we identified a minimal set of 31 SNPs markers to distinguish 210 varieties. In addition, we used this algorithm to discriminate the 43 most cultivated in Spain rice varieties with minimal sets of 8 SNPs. We then developed and tested 22 Kompetitive Allele-Specific PCR (KASP) assays for the markers included in these panels, and obtained reliable genotype patterns for rice varieties identification. The complete 22 markers panel and the rice genotypes data could offer a useful and low-cost tool for rice breeders and industry to identify varieties and therefore to guarantee the quality of rice. The provided R-based algorithm can be applied to other genomic resources to develop core sets of discriminating markers.

A target cultivar-specific identification system based on the chromatographic printed array strip method for eight prominent Japanese citrus cultivars

Citrus is a major cultivated crop in Japan, and new cultivars are of great interest in the Japanese and global market. Recently, the infringement of breeders' rights to citrus cultivars bred in Japan has become a problem related to the agricultural product export strategy promoted by the Japanese government. Cultivar identification systems using DNA markers are an effective tool for protecting breeders' rights. Here, a novel target cultivar-specific identification system using the chromatographic printed array strip method was developed for eight prominent Japanese citrus cultivars. A polymorphic InDel fragment specific to each cultivar was explored through the screening of published citrus InDel markers and next-generation sequencing of retrotransposon libraries. The cultivar-specific DNA marker set for each cultivar comprised 1-3 polymorphic InDel fragments in combination with a PCR-positive DNA marker for the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit gene. The DNA markers were detected within 3 hours from DNA extraction to the detection by the C-PAS4 membrane stick following multiplex PCR. The developed system is superior as a convenient, rapid, and cost-effective DNA diagnostic method during inspection. The proposed target cultivar-specific identification system is expected to serve as an efficient tool for the injunction of suspicious registered cultivars, contributing to the protection of breeders' rights.

GRAS-Di SNP-based molecular characterization and fingerprinting of a Turkish Corylus avellana core set provide insights into the cultivation and breeding of hazelnut in Turkey

Hazelnut (Corylus avellana L.) is an economically and socially important product for Turkey, the country that leads global production of this crop. The preservation of Turkish hazelnut genetic diversity and informed breeding of new cultivars are crucial for maintaining quality and crop yield stability. In this study, genotyping by random amplicon sequencing (GRAS-Di) was used to identify single-nucleotide polymorphisms (SNPs) in a panel of 96 individuals representing the Turkish national hazelnut collection. The resulting 7609 high-quality SNPs were physically mapped to the Tombul cultivar reference genome and used for population structure and diversity analyses. These analyses revealed that cultivars are not less diverse than wild accessions and that 44% of the panel had admixed ancestry. The results also indicated that recently released Turkish cultivars are highly similar to each other, suggesting that diversity analysis is an important tool that should be employed to prevent future genetic bottlenecks in this crop. A minimal marker algorithm was used to select a set of seven SNP markers that were capable of differentiating the panel accessions. These fingerprinting markers should be useful for the propagation of true-to-type elite cultivars that can be used to renew Turkey's aging hazelnut orchards.

Two fingerprinting sets for Humulus lupulus based on KASP and microsatellite markers

Verification of clonal identity of hop (Humulus lupulus L.) cultivars within breeding programs and germplasm collections is vital to conserving genetic resources. Accurate and economic DNA-based tools are needed in dioecious hop to confirm identity and parentage, neither of which can be reliably determined from morphological observations. In this study, we developed two fingerprinting sets for hop: a 9-SSR fingerprinting set containing high-core repeats that can be run in a single PCR reaction and a kompetitive allele specific PCR (KASP) assay of 25 single nucleotide polymorphisms (SNPs). The SSR set contains a sex-linked primer pair, HI-AGA7, that was used to genotype 629 hop accessions from the US Department of Agriculture (USDA) National Clonal Germplasm Repository (NCGR), the USDA Forage Seed and Cereal Research (FSCR), and the University of Nebraska-Lincoln (UNL) collections. The SSR set identified unique genotypes except for 89 sets of synonymous samples. These synonyms included: cultivars with different designations, the same cultivars from different sources, heat-treated clones, and clonal variants. Population structure analysis clustered accessions into wild North American (WNA) and cultivated groups. Diversity was slightly higher in the cultivated samples due to larger sample size. Parentage and sib-ship analyses were used to identify true-to-type cultivars. The HI-AGA7 marker generated two male- and nine female-specific alleles among the cultivated and WNA samples. The SSR and KASP fingerprinting sets were compared in 190 samples consisting of cultivated and WNA accession for their ability to confirm identity and assess diversity and population structure. The SSR fingerprinting set distinguished cultivars, selections and WNA accessions while the KASP assays were unable to distinguish the WNA samples and had lower diversity estimates than the SSR set. Both fingerprinting sets are valuable tools for identity confirmation and parentage analysis in hop for different purposes. The 9-SSR assay is cost efficient when genotyping a small number of wild and cultivated hop samples (<96) while the KASP assay is easy to interpret and cost efficient for genotyping a large number of cultivated samples (multiples of 96).

Potential application of simple easy-to-use insertion-deletion (InDel) markers in citrus cultivar identification

We previously developed insertion-deletion (InDel) markers that distinguish three genotypes (two homozygous and one heterozygous) of diverse citrus cultivars. These InDel markers were codominant and could be clearly detected by using simple agarose gel electrophoresis. We sought to establish a method for cultivar identification using these 28 InDel markers to genotype 31 citrus cultivars. The results revealed that a minimum of 6 markers were required to identify individuals using the three-genotype classification method. Furthermore, we found that a simple method for distinguishing between two genotypes (homozygous and heterozygous) could be used to identify individuals using a minimum of 7 markers. Our findings provide a basis for the development of simple and rapid citrus cultivar identification methods.

Variety Discrimination Power: An Appraisal Index for Loci Combination Screening Applied to Plant Variety Discrimination

Molecular marker technology is used widely in plant variety discrimination, molecular breeding, and other fields. To lower the cost of testing and improve the efficiency of data analysis, molecular marker screening is very important. Screening usually involves two phases: the first to control loci quality and the second to reduce loci quantity. To reduce loci quantity, an appraisal index that is very sensitive to a specific scenario is necessary to select loci combinations. In this study, we focused on loci combination screening for plant variety discrimination. A loci combination appraisal index, variety discrimination power (VDP), is proposed, and three statistical methods, probability-based VDP (P-VDP), comparison-based VDP (C-VDP), and ratio-based VDP (R-VDP), are described and compared. The results using the simulated data showed that VDP was sensitive to statistical populations with convergence toward the same variety, and the total probability of discrimination power (TDP) method was effective only for partial populations. R-VDP was more sensitive to statistical populations with convergence toward various varieties than P-VDP and C-VDP, which both had the same sensitivity; TDP was not sensitive at all. With the real data, R-VDP values for sorghum, wheat, maize and rice data begin to show downward tendency when the number of loci is 20, 7, 100, 100 respectively, while in the case of P-VDP and C-VDP (which have the same results), the number is 6, 4, 9, 19 respectively and in the case of TDP, the number is 6, 4, 4, 11 respectively. For the variety threshold setting, R-VDP values of loci combinations with different numbers of loci responded evenly to different thresholds. C-VDP values responded unevenly to different thresholds, and the extent of the response increased as the number of loci decreased. All the methods gave underestimations when data were missing, with systematic errors for TDP, C-VDP, and R-VDP going from smallest to biggest. We concluded that VDP was a better loci combination appraisal index than TDP for plant variety discrimination and the three VDP methods have different applications. We developed the software called VDPtools, which can calculate the values of TDP, P-VDP, C-VDP, and R-VDP. VDPtools is publicly available at https://github.com/caurwx1/VDPtools.git.

Allelic composition of carotenoid metabolic genes in 13 founders influences carotenoid composition in juice sac tissues of fruits among Japanese citrus breeding population

To enrich carotenoids, especially β-cryptoxanthin, in juice sac tissues of fruits via molecular breeding in citrus, allele mining was utilized to dissect allelic variation of carotenoid metabolic genes and identify an optimum allele on the target loci characterized by expression quantitative trait (eQTL) analysis. SNPs of target carotenoid metabolic genes in 13 founders of the Japanese citrus breeding population were explored using the SureSelect target enrichment method. An independent allele was determined based on the presence or absence of reliable SNPs, using trio analysis to confirm inheritability between parent and offspring. Among the 13 founders, there were 7 PSY alleles, 7 HYb alleles, 11 ZEP alleles, 5 NCED alleles, and 4 alleles for the eQTL that control the transcription levels of PDS and ZDS among the ancestral species, indicating that some founders acquired those alleles from them. The carotenoid composition data of 263 breeding pedigrees in juice sac tissues revealed that the phenotypic variance of carotenoid composition was similar to that in the 13 founders, whereas the mean of total carotenoid content increased. This increase in total carotenoid content correlated with the increase in either or both β-cryptoxanthin and violaxanthin in juice sac tissues. Bayesian statistical analysis between allelic composition of target genes and carotenoid composition in 263 breeding pedigrees indicated that PSY-a and ZEP-e alleles at PSY and ZEP loci had strong positive effects on increasing the total carotenoid content, including β-cryptoxanthin and violaxanthin, in juice sac tissues. Moreover, the pyramiding of these alleles also increased the β-cryptoxanthin content. Interestingly, the offset interaction between the alleles with increasing and decreasing effects on carotenoid content and the epistatic interaction among carotenoid metabolic genes were observed and these interactions complexed carotenoid profiles in breeding population. These results revealed that allele composition would highly influence the carotenoid composition in citrus fruits. The allelic genotype information for the examined carotenoid metabolic genes in major citrus varieties and the trio-tagged SNPs to discriminate the optimum alleles (PSY-a and ZEP-e) from the rest would promise citrus breeders carotenoid enrichment in fruit via molecular breeding.

SSR markers developed using next-generation sequencing technology in pineapple, Ananas comosus (L.) Merr

Simple sequence repeat (SSR) markers provide a reliable tool for the identification of accessions and the construction of genetic linkage maps because of their co-dominant inheritance. In the present study, we developed new SSR markers with next-generation sequencing using the Roche 454 GS FLX+ platform. Five hundred SSR primer sets were tested for the genetic identification of pineapple, including 100 each for the di-, tri-, tetra-, penta-, and hexa-nucleotide motif SSRs. In total, 160 SSR markers successfully amplified fragments and exhibited polymorphism among accessions. The SSR markers revealed the number of alleles per locus (ranging from 2 to 13), the expected heterozygosity (ranging from 0.041 to 0.823), and the observed heterozygosity (ranging from 0 to 0.875). A total of 117 SSR markers with tri- or greater nucleotide motifs were shown to be effective at facilitating accurate genotyping. Using the SSR markers, 25 accessions were distinguished genetically, with the exception of accessions 'MD-2' and 'Yonekura'. The developed SSR markers could facilitate the establishment of efficient and accurate genetic identification systems and the construction of genetic linkage maps in the future.

TaqMan-MGB SNP genotyping assay to identify 48 citrus cultivars distributed in the Japanese market

A citrus cultivar identification system using CAPS marker has been developed on nursery trees, but this needs to be extended to include various product types, such as imported fruits and processed products. Here, we developed a new cultivar identification system using TaqMan-MGB SNP genotyping assay. Eight probe and primer sets were designed to amplify PCR fragments <100 bp to enable the genotyping of fresh and processed fruits in which predicted that insufficient quantities of DNA and residual impurities in the DNA extracts. The TaqMan-MGB SNP genotyping assay was stable and reproducible, and were confirmed to apply various sample sources, including leaves, fresh fruit, juice, canned fruit, and dry fruit. They could provide at least a single differentiating SNP to discriminate any paired combination among 48 citrus cultivars. Minimal marker subsets to identify the target cultivar were listed for each of 18 registered cultivars with valid patent. The allelic SNP genotypes of 48 citrus cultivars, which cover more than 98% of all citrus fruit shipment produced in Japan, is valuable for the referencing information in the DNA-based identification for fresh and processed fruits. This identification system will help protect registered cultivars and facilitate food fraud inspections.

SR4R: An Integrative SNP Resource for Genomic Breeding and Population Research in Rice

The information commons for rice (IC4R) database is a collection of 18 million single nucleotide polymorphisms (SNPs) identified by resequencing of 5152 rice accessions. Although IC4R offers ultra-high density rice variation map, these raw SNPs are not readily usable for the public. To satisfy different research utilizations of SNPs for population genetics, evolutionary analysis, association studies, and genomic breeding in rice, raw genotypic data of these 18 million SNPs were processed by unified bioinformatics pipelines. The outcomes were used to develop a daughter database of IC4R - SnpReady for Rice (SR4R). SR4R presents four reference SNP panels, including 2,097,405 hapmapSNPs after data filtration and genotype imputation, 156,502 tagSNPs selected from linkage disequilibrium-based redundancy removal, 1180 fixedSNPs selected from genes exhibiting selective sweep signatures, and 38 barcodeSNPs selected from DNA fingerprinting simulation. SR4R thus offers a highly efficient rice variation map that combines reduced SNP redundancy with extensive data describing the genetic diversity of rice populations. In addition, SR4R provides rice researchers with a web interface that enables them to browse all four SNP panels, use online toolkits, as well as retrieve the original data and scripts for a variety of population genetics analyses on local computers. SR4R is freely available to academic users at http://sr4r.ic4r.org/.

DNA markers based on retrotransposon insertion polymorphisms can detect short DNA fragments for strawberry cultivar identification

In this study, DNA markers were developed for discrimination of strawberry (Fragaria × ananassa L.) cultivars based on retrotransposon insertion polymorphisms. We performed a comprehensive genomic search to identify retrotransposon insertion sites and subsequently selected one retrotransposon family, designated CL3, which provided reliable discrimination among strawberry cultivars. Through analyses of 75 strawberry cultivars, we developed eight cultivar-specific markers based on CL3 retrotransposon insertion sites. Used in combination with 10 additional polymorphic markers, we differentiated 35 strawberry cultivars commonly cultivated in Japan. In addition, we demonstrated that the retrotransposon-based markers were effective for PCR detection of DNA extracted from processed food materials, whereas a SSR marker was ineffective. These results indicated that the retrotransposon-based markers are useful for cultivar discrimination for processed food products, such as jams, in which DNA may be fragmented or degraded.

A new SNP genotyping technology Target SNP-seq and its application in genetic analysis of cucumber varieties

To facilitate the utility of SNP-based genotyping, we developed a new method called target SNP-seq which combines the advantages of multiplex PCR amplification and high throughput sequencing. Compared with KASP, Microarrays, GBS and other SNP genotyping methods, target SNP-seq is flexible both in SNPs and samples, yields high accuracy, especially when genotyping genome wide perfect SNPs with high polymorphism and conserved flanking sequences, and is cost-effective, requiring 3 days and $7 for per DNA sample to genotype hundreds of SNP loci. The present study established a DNA fingerprint of 261 cucumber varieties by target SNP-seq with 163 perfect SNPs from 4,612,350 SNPs based on 182 cucumber resequencing datasets. Four distinct subpopulations were found in 261 Chinese cucumber varieties: the north China type, the south China type, the Europe type, and the Xishuangbanna type. The north China type and Xishuangbanna type harbored lower genetic diversity, indicating greater risk of genetic erosion in these two subpopulations. Furthermore, a core set of 24 SNPs was able to distinguish 99% of the 261 cucumber varieties. 29 core cucumber backbone varieties in China were identified. Therefore, target SNP-seq provides a new way to screen out core SNP loci from the whole genome for DNA fingerprinting of crop varieties. The high efficiency and low cost of target SNP-seq is more competitive than the current SNP genotyping methods, and it has excellent application prospects in genetic research, as well as in promoting plant breeding processes in the near future.

Genetic diversity and relatedness of mango cultivars assessed by SSR markers

Assessment of genetic diversity and relatedness is an essential component of germplasm characterization and use. We analyzed 120 mango (Mangifera indica L.) genetic resources in Japan for their parentage, cultivar identification, genetic relatedness, and genetic diversity, using 46 polymorphic simple sequence repeat (SSR) markers. Ten sets of three SSR markers could successfully distinguish 83 genotypes with the exception of synonymous and identical accessions. We successfully assessed parentage, newly identifying or reconfirming both parents of 11 accessions, and revealing over 30 cultivars as offspring of 'Haden'. Genetic relatedness and diversity analyses revealed three distinct clusters. Two clusters correspond to the groups of USA and India, which are closely related. The other includes accessions from Southeast and East Asia. The results agree with the previous identification of genetically distinct Indian and Southeast Asian types, and suggest that the Florida accessions, which originated from hybrids between those two types, are more closely related to the Indian type.

Development of model web-server for crop variety identification using throughput SNP genotyping data

Crop varieties or genotypes of a given species are pivotal for agricultural production and ownership, management and improvement of their germplasm is a great challenge. Its morphological identification requires time, cost and descriptors are often compromised statistically due to phenotypic plasticity. Development of DNA based signature of varieties can overcome these limitations. There is a global need to implement world trade organization (WTO) and intellectual property rights (IPR) guidelines of Plant Breeders Rights (PBR) where DUS (distinctness, uniformity and stability) testing can be supplemented by DNA profile. Universalization and minimization of SNP number without compromising identification accuracy is the major challenge in development of varietal profile by rapid genotype assay. Besides this, there is no server-based approach reducing computational skill with global accessibility of referral phenotypic and genotypic data. We report world’s first model web server for crop variety identification using >350 Indian wheat varieties and Axiom 35 K SNP chip data. Standard filtering and linkage disequilibrium approach were used to develop varietal signature in Linux using HTML, Java, PHP and MySQL with provision of QR code generator to facilitate bar-coding. Phylogenetic tree constructed by selected SNPs confirms six major trait based clusters of varieties and their pedigree. Our user friendly server based tool, VISTa (Variety Identification System of Triticum aestivum) (http://webtom.cabgrid.res.in/vista) can be used in DUS testing having dispute resolution of sovereignty and access benefit sharing (ABS) issues. This model approach can be used in other crops with pan-global level management of crop germplasm in endeavour of crop productivity.

Citrus breeding, genetics and genomics in Japan

Citrus is one of the most cultivated fruits in the world, and satsuma mandarin (Citrus unshiu Marc.) is a major cultivated citrus in Japan. Many excellent cultivars derived from satsuma mandarin have been released through the improvement of mandarins using a conventional breeding method. The citrus breeding program is a lengthy process owing to the long juvenility, and it is predicted that marker-assisted selection (MAS) will overcome the obstacle and improve the efficiency of conventional breeding methods. To promote citrus molecular breeding in Japan, a genetic mapping was initiated in 1987, and the experimental tools and resources necessary for citrus functional genomics have been developed in relation to the physiological analysis of satsuma mandarin. In this paper, we review the progress of citrus breeding and genome researches in Japan and report the studies on genetic mapping, expression sequence tag cataloguing, and molecular characterization of breeding characteristics, mainly in terms of the metabolism of bio-functional substances as well as factors relating to, for example, fruit quality, disease resistance, polyembryony, and flowering.

Simple SNP-based minimal marker genotyping for Humulus lupulus L. identification and variety validation

Background

Hop is an economically important crop for the Pacific Northwest USA as well as other regions of the world. It is a perennial crop with rhizomatous or clonal propagation system for varietal distribution. A big concern for growers as well as brewers is variety purity and questions are regularly posed to public agencies concerning the availability of genotype testing. Current means for genotyping are based upon 25 microsatellites that provides relatively accurate genotyping but cannot always differentiate sister-lines. In addition, numerous PCR runs (25) are required to complete this process and only a few laboratories exist that perform this service. A genotyping protocol based upon SNPs would enable rapid accurate genotyping that can be assayed at any laboratory facility set up for SNP-based genotyping. The results of this study arose from a larger project designed for whole genome association studies upon the USDA-ARS hop germplasm collection consisting of approximately 116 distinct hop varieties and germplasm (female lines) from around the world.

Results

The original dataset that arose from partial sequencing of 121 genotypes resulted in the identification of 374,829 SNPs using TASSEL-UNEAK pipeline. After filtering out genotypes with more than 50 % missing data (5 genotypes) and SNP markers with more than 20 % missing data, 32,206 highly filtered SNP markers across 116 genotypes were identified and considered for this study. Minor allele frequency (MAF) was calculated for each SNP and ranked according to the most informative to least informative. Only those markers without missing data across genotypes as well as 60 % or less heterozygous gamete calls were considered for further analysis. Genetic distances among individuals in the study were calculated using the marker with the highest MAF value, then by using a combination of the two markers with highest MAF values and so on. This process was reiterated until a set of markers was identified that allowed for all genotypes in the study to be genetically differentiated from each other. Next, we compared genetic matrices calculated from the minimal marker sets [(Table 2; 6-, 7-, 8-, 10- and 12-marker set matrices] and that of a matrix calculated from a set of markers with no missing data across all 116 samples (1006 SNP markers). The minimum number of markers required to meet both specifications was a set of 7-markers (Table 3). These seven SNPs were then aligned with a genome assembly, and DNA sequence both upstream and downstream were used to identify primer sequences that can be used to develop seven amplicons for high resolution melting curve PCR detection or other SNP-based PCR detection methods.

Conclusions

This study identifies a set of 7 SNP markers that may prove useful for the identification and validation of hop varieties and accessions. Variety validation of unknown samples assumes that the variety under question has been included a priori in a discovery panel. These results are based upon in silica studies and markers need to be validated using different SNP marker technology upon a differential set of hop genotypes. The marker sequence data and suggested primer sets provide potential means to fingerprint hop varieties in most genetic laboratories utilizing SNP-marker technology.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-015-1492-2) contains supplementary material, which is available to authorized users.