Development of a Genetic Map for Onion (Allium cepa L.) Using Reference-Free Genotyping-by-Sequencing and SNP Assays.

Single nucleotide polymorphisms (SNPs) play important roles as molecular markers in plant genomics and breeding studies. Although onion (Allium cepa L.) is an important crop globally, relatively few molecular marker resources have been reported due to its large genome and high heterozygosity. Genotyping-by-sequencing (GBS) offers a greater degree of complexity reduction followed by concurrent SNP discovery and genotyping for species with complex genomes. In this study, GBS was employed for SNP mining in onion, which currently lacks a reference genome. A segregating F2 population, derived from a cross between 'NW-001' and 'NW-002,' as well as multiple parental lines were used for GBS analysis. A total of 56.15 Gbp of raw sequence data were generated and 1,851,428 SNPs were identified from the de novo assembled contigs. Stringent filtering resulted in 10,091 high-fidelity SNP markers. Robust SNPs that satisfied the segregation ratio criteria and with even distribution in the mapping population were used to construct an onion genetic map. The final map contained eight linkage groups and spanned a genetic length of 1,383 centiMorgans (cM), with an average marker interval of 8.08 cM. These robust SNPs were further analyzed using the high-throughput Fluidigm platform for marker validation. This is the first study in onion to develop genome-wide SNPs using GBS. The resulting SNP markers and developed linkage map will be valuable tools for genetic mapping of important agronomic traits and marker-assisted selection in onion breeding programs.

De Novo Transcriptome Analysis of Cucumis melo L. var. makuwa.

Oriental melon (Cucumis melo L. var. makuwa) is one of six subspecies of melon and is cultivated widely in East Asia, including China, Japan, and Korea. Although oriental melon is economically valuable in Asia and is genetically distinct from other subspecies, few reports of genome-scale research on oriental melon have been published. We generated 30.5 and 36.8 Gb of raw RNA sequence data from the female and male flowers, leaves, roots, and fruit of two oriental melon varieties, Korean landrace (KM) and Breeding line of NongWoo Bio Co. (NW), respectively. From the raw reads, 64,998 transcripts from KM and 100,234 transcripts from NW were de novo assembled. The assembled transcripts were used to identify molecular markers (e.g., single-nucleotide polymorphisms and simple sequence repeats), detect tissue-specific expressed genes, and construct a genetic linkage map. In total, 234 single-nucleotide polymorphisms and 25 simple sequence repeats were screened from 7,871 and 8,052 candidates, respectively, between the KM and NW varieties and used for construction of a genetic map with 94 F2 population specimens. The genetic linkage map consisted of 12 linkage groups, and 248 markers were assigned. These transcriptome and molecular marker data provide information useful for molecular breeding of oriental melon and further comparative studies of the Cucurbitaceae family.

Qualitative and quantitative late blight resistance in the potato cultivar Sarpo Mira is determined by the perception of five distinct RXLR effectors.

Potato defends against Phytophthora infestans infection by resistance (R)-gene-based qualitative resistance as well as a quantitative field resistance. R genes are renowned to be rapidly overcome by this oomycete, and potato cultivars with a decent and durable resistance to current P. infestans populations are hardly available. However, potato cultivar Sarpo Mira has retained resistance in the field over several years. We dissected the resistance of 'Sarpo Mira' in a segregating population by matching the responses to P. infestans RXLR effectors with race-specific resistance to differential strains. The resistance is based on the combination of four pyramided qualitative R genes and a quantitative R gene that was associated with field resistance. The qualitative R genes include R3a, R3b, R4, and the newly identified Rpi-Smira1. The qualitative resistances matched responses to avirulence (AVR)3a, AVR3b, AVR4, and AVRSmira1 RXLR effectors and were overcome by particular P. infestans strains. The quantitative resistance was determined to be conferred by a novel gene, Rpi-Smira2. It was only detected under field conditions and was associated with responses to the RXLR effector AvrSmira2. We foresee that effector-based resistance breeding will facilitate selecting and combining qualitative and quantitative resistances that may lead to a more durable resistance to late blight.

Broad spectrum late blight resistance in potato differential set plants MaR8 and MaR9 is conferred by multiple stacked R genes.

Phytophthora infestans is the causal agent of late blight in potato. The Mexican species Solanum demissum is well known as a good resistance source. Among the 11 R gene differentials, which were introgressed from S. demissum, especially R8 and R9 differentials showed broad spectrum resistance both under laboratory and under field conditions. In order to gather more information about the resistance of the R8 and R9 differentials, F1 and BC1 populations were made by crossing Mastenbroek (Ma) R8 and R9 clones to susceptible plants. Parents and offspring plants were examined for their pathogen recognition specificities using agroinfiltration with known Avr genes, detached leaf assays (DLA) with selected isolates, and gene-specific markers. An important observation was the discrepancy between DLA and field trial results for Pi isolate IPO-C in all F1 and BC1 populations, so therefore also field trial results were included in our characterization. It was shown that in MaR8 and MaR9, respectively, at least four (R3a, R3b, R4, and R8) and seven (R1, Rpi-abpt1, R3a, R3b, R4, R8, R9) R genes were present. Analysis of MaR8 and MaR9 offspring plants, that contained different combinations of multiple resistance genes, showed that R gene stacking contributed to the Pi recognition spectrum. Also, using a Pi virulence monitoring system in the field, it was shown that stacking of multiple R genes strongly delayed the onset of late blight symptoms. The contribution of R8 to this delay was remarkable since a plant that contained only the R8 resistance gene still conferred a delay similar to plants with multiple resistance genes, like, e.g., cv Sarpo Mira. Using this "de-stacking" approach, many R gene combinations can be made and tested in order to select broad spectrum R gene stacks that potentially provide enhanced durability for future application in new late blight resistant varieties.

Trichome density of main stem is tightly linked to PepMoV resistance in chili pepper (Capsicum annuum L.).

A relationship between pepper trichome and pepper mottle virus (PepMoV) resistance was examined. In an intraspecific F(2) mapping population from the cross between Capsicum annuum CM334 (trichome-bearing and PepMoV resistant) and Chilsungcho (glabrous and PepMoV susceptible), major QTLs for both traits were identified by composite interval mapping in linkage group (LG) 24 corresponding a telomere region on pepper chromosome 10. Ptel1 of putative trichome enhancing locus was a common major QTL for trichome density on the main stem and calyx. Ptel1 apart from HpmsE031 at a 1.03 cM interval was specifically associated to the trichome density on the main stem, whereas Ptel2 near m104 marker on LG2 was specific for the calyx trichome. Epistatic analysis indicated that Ptel1 engaged in controlling the trichome density by mutual interactions with the organ-specific QTLs. For PepMoV resistance, two QTLs (Pep1 and Pep2) were identified on the LG 24. Pep1 was located with Ptel1 in the R-gene cluster (RGC) for potyvirus resistance including Pvr4 with broad spectrum resistance to potyviruses. Pep1 flanking TG420 marker seemed to be the major factors determining correlation with PepMoV resistance. These results indicate that the level of trichome density on pepper main stem can be used as a morphological marker for Pvr4 in pepper breeding.

Construction of an integrated pepper map using RFLP, SSR, CAPS, AFLP, WRKY, rRAMP, and BAC end sequences.

Map-based cloning to find genes of interest, markerassisted selection (MAS), and marker-assisted breeding (MAB) all require good genetic maps with high reproducible markers. For map construction as well as chromosome assignment, development of single copy PCR-based markers and map integration process are necessary. In this study, the 132 markers (57 STS from BAC-end sequences, 13 STS from RFLP, and 62 SSR) were newly developed as single copy type PCR-based markers. They were used together with 1830 markers previously developed in our lab to construct an integrated map with the Joinmap 3.0 program. This integrated map contained 169 SSR, 354 RFLP, 23 STS from BAC-end sequences, 6 STS from RFLP, 152 AFLP, 51 WRKY, and 99 rRAMP markers on 12 chromosomes. The integrated map contained four genetic maps of two interspecific (Capsicum annuum 'TF68' and C. chinense 'Habanero') and two intraspecific (C. annuum 'CM334' and C. annuum 'Chilsungcho') populations of peppers. This constructed integrated map consisted of 805 markers (map distance of 1858 cM) in interspecific populations and 745 markers (map distance of 1892 cM) in intraspecific populations. The used pepper STS were first developed from end sequences of BAC clones from Capsicum annuum 'CM334'. This integrated map will provide useful information for construction of future pepper genetic maps and for assignment of linkage groups to pepper chromosomes.

Marker development for erect versus pendant-orientated fruit in Capsicum annuum L.

The erect habit of fruit setting is a unique characteristic of ornamental peppers and wild pepper species. The erect habit is known to be controlled by the up locus on pepper (Capsicum annuum L.) chromosome 12. The result of a genetic analysis using Saengryeog 211 (pendant), Saengryeog 213 (erect), and their F1 and BC1 progeny demonstrated that up is a recessive gene. To develop an up-linked marker, bulked segregant analysis (BSA) and amplified fragment length polymorphism (AFLP) were employed using 108 F2:3 individuals. The closest AFLP marker, A2C79, was located at a genetic distance of 1.7 cM from the up locus and was converted into a cleaved amplified polymorphic sequence (CAPS) marker. This marker was mapped at a genetic distance of 4.3 cM from the up locus. When the CAPS was applied to seven ornamental lines and 27 breeding lines with erect fruit, these genotypes of 28 lines were correctly predicted. Thus, the CAPS marker will be useful for marker-assisted selection (MAS) of pepper breeding lines with the up allele at the early seedling stage.

BAC-derived markers converted from RFLP linked to Phytophthora capsici resistance in pepper (Capsicum annuum L.).

Phytophthora capsici Leonian, an oomycete pathogen, is a serious problem in pepper worldwide. Its resistance in pepper is controlled by quantitative trait loci (QTL). To detect QTL associated with P. capsici resistance, a molecular linkage map was constructed using 100 F(2) individuals from a cross between Capsicum annuum 'CM334' and C. annuum 'Chilsungcho'. This linkage map consisted of 202 restriction fragment length polymorphisms (RFLPs), 6 WRKYs and 1 simple sequence repeat (SSR) covering 1482.3 cM, with an average interval marker distance of 7.09 cM. QTL mapping of Phytophthora root rot and damping-off resistance was performed in F(2:3) originated from a cross between resistant Mexican landrace C. annuum 'CM334' and susceptible Korean landrace C. annuum 'Chilsungcho' using composite interval mapping (CIM) analysis. Four QTL explained 66.3% of the total phenotypic variations for root rot resistance and three 44.9% for damping-off resistance. Of these QTL loci, two were located close to RFLP markers CDI25 on chromosome 5 (P5) and CT211A on P9. A bacterial artificial chromosome (BAC) library from C. annuum 'CM334' was screened with these two RFLP probes to obtain sequence information around the RFLP marker loci for development of PCR-based markers. CDI25 and CT211 probes identified seven and eight BAC clones, respectively. Nine positive BAC clones containing probe regions were sequenced and used for cytogenetic analysis. One single-nucleotide amplified polymorphism (SNAP) for the CDI25 locus, and two SSRs and cleaved amplified polymorphic sequence (CAPS) for CT211 were developed using sequences of the positive BAC clones. These markers will be valuable for rapid selection of genotypes and map-based cloning for resistance genes against P. capsici.

Reverse random amplified microsatellite polymorphism reveals enhanced polymorphisms in the 3' end of simple sequence repeats in the pepper genome.

Microsatellites or simple sequence repeats (SSR) are widely distributed in eukaryotic genomes and are informative genetic markers. Despite many advantages of SSR markers such as a high degree of allelic polymorphisms, co-dominant inheritance, multi-allelism, and genome-wide coverage in various plant species, they also have shortcomings such as low polymorphic rates between genetically close lines, especially in Capsicum annuum. We developed an alternative technique to SSR by normalizing and alternating anchored primers in random amplified microsatellite polymorphisms (RAMP). This technique, designated reverse random amplified microsatellite polymorphism (rRAMP), allows the detection of nucleotide variation in the 3' region flanking an SSR using normalized anchored and random primer combinations. The reproducibility and frequency of polymorphic loci in rRAMP was vigorously enhanced by translocation of the 5' anchor of repeat sequences to the 3' end position and selective use of moderate arbitrary primers. In our study, the PCR banding pattern of rRAMP was highly dependent on the frequency of repeat motifs and primer combinations with random primers. Linkage analysis showed that rRAMP markers were well scattered on an intra-specific pepper map. Based on these results, we suggest that this technique is useful for studying genetic diversity, molecular fingerprinting, and rapidly constructing molecular maps for diverse plant species.

Marker production by PCR amplification with primer pairs from conserved sequences of WRKY genes in chili pepper.

Despite increasing awareness of the importance of WRKY genes in plant defense signaling, the locations of these genes in the Capsicum genome have not been established. To develop WRKY-based markers, primer sequences were deduced from the conserved sequences of the DNA binding motif within the WRKY domains of tomato and pepper genes. These primers were derived from upstream and downstream parts of the conserved sequences of the three WRKY groups. Six primer combinations of each WRKY group were tested for polymorphisms between the mapping parents, C. annuum 'CM334' and C. annuum 'Chilsungcho'. DNA fragments amplified by primer pairs deduced from WRKY Group II genes revealed high levels of polymorphism. Using 32 primer pairs to amplify upstream and downstream parts of the WRKY domain of WRKY group II genes, 60 polymorphic bands were detected. Polymorphisms were not detected with primer pairs from downstream parts of WRKY group II genes. Half of these primers were subjected to F2 genotyping to construct a linkage map. Thirty of 41 markers were located evenly spaced on 20 of the 28 linkage groups, without clustering. This linkage map also consisted of 199 AFLP and 26 SSR markers. This WRKY-based marker system is a rapid and simple method for generating sequence-specific markers for plant gene families.