Characterization of masson pine (Pinus massoniana Lamb.) microsatellite DNA by 454 genome shotgun sequencing

Microsatellites (simple sequence repeats, SSRs) are important genetic markers in tree breeding and conservation. Here we utilized high-throughput 454 sequencing technology to mine microsatellites from masson pine (MP) genomic DNA. First, we analyzed the characteristics of SSRs in all nonredundant MP reads (genome survey sequences, GSSs) and compared them with loblolly pine (LP) GSSs and BACs (bacterial artificial chromosome clone sequences), and three other nonconiferous species GSSs. Second, a set of MP GSS–SSR primer pairs were designed. There were extremely low overall GSS–SSR densities (28 SSR/Mb) in MP when compared with LP (48 SSR/Mb) and the other species. AT, AAT, AAAT, and AAAAAT were the richest motifs in di-, tri-, tetra-, and hexanucleotides, respectively. Two hundred forty GSS–SSR primer pairs were designed in total, and 20 novel polymorphic markers were identified using three populations (two natural and one clonal seed orchard) as evaluating samples. These markers should be useful for future MP population genetics studies.     

Comparative mapping in loblolly and radiata pine using RFLP and microsatellite markers

Genetic linkage maps were constructed for loblolly pine (Pinus taeda L.) and radiata pine (P. radiata D. Don) using a common set of RFLP and microsatellite markers. The map for loblolly pine combined data from two full-sib families and consisted of 20 linkage groups covering 1281 cM. The map for radiata pine had 14 linkage groups and covered 1223 cM. All of the RFLP probes readily hybridise between loblolly and radiata pine often producing similar hybridisation patterns. There were in total 60 homologous RFLP loci mapped in both species which could be used for comparative purposes. A set of 20 microsatellite markers derived from radiata pine were also assayed; however, only 9 amplified and revealed polymorphic loci in both species. Single-locus RFLP and microsatellite markers were used to match up linkage groups and compare order between species. Twelve syntenic groups were obtained each consisting of from 3 to 9 homologous loci. The order of homologous loci was colinear in most cases, suggesting no major chromosomal rearrangements in the evolution of these species. Comparative mapping between loblolly and radiata pine should facilitate genetic research in both species and provide a framework for mapping in other pine species. Received: 25 November 1998 / /Accepted: 19 December 1998     

Discovery and experimental analysis of microsatellites in an oil woody plant Camellia chekiangoleosa

Oil camellia trees are important woody plants for the production of high-quality cooking oil. On the contrary to their economic importance, their genetic and genomic resources are very limited, which greatly hamper the genetic studies on oil camellia trees. Microsatellites or simple sequence repeats (SSRs) have great value in many aspects of genetic analyses due to their high polymorphism and codominant inheritance. In this study, we report the large-scale development and characterization of SSR markers derived from genomic sequences of Camellia chekiangoleosa by high-throughput pyrosequencing technology. A total of 1,091,393 genomic shotgun reads were generated using Roche 454 FLX sequencer, the average read length was 319 bp, and the total sequence throughput was 347.9 Mb. These sequences were assembled into 35,315 contigs with total length of 14.8 Mb and the N50 contig size of 770 bp. By analyzing with microsatellite (MISA), a total of 5,844 perfect microsatellites were detected from the assembled sequences. Among them, tetranucleotide repeats were found to be the most frequent microsatellites in the genome of C. chekiangoleosa, and all the dominant repeat motifs for different types of SSRs were detected to be rich in A/T. Experimental analysis with 900 SSR primer pairs revealed that 66 % of them succeeded in PCR amplification. Further investigation with 345 SSR primer pairs showed that a relatively high percentage of primers amplified polymorphic loci (31.9 %). Experimental data also revealed that, overall, long microsatellite repeats (>20 bp) were more variable than the short ones (<20 bp) in the genome of oil camellia tree.     

Construction of an AFLP genetic map with nearly complete genome coverage in Pinus taeda

  De novo construction of complete genetic linkage maps requires large mapping populations, large numbers of genetic markers, and efficient algorithms for ordering markers and evaluating order confidence. We constructed a complete genetic map of an individual loblolly pine (Pinus taeda L.) using amplified fragment length polymorphism (AFLP) markers segregating in haploid megagametophytes and PGRI mapping software. We generated 521 polymorphic fragments from 21 AFLP primer pairs. A total of 508 fragments mapped to 12 linkage groups, which is equal to the Pinus haploid chromosome number. Bootstrap locus order matrices and recombination matrices generated by PGRI were used to select 184 framework markers that could be ordered confidently. Order support was also evaluated using log likelihood criteria in MAPMAKER. Optimal marker orders from PGRI and MAPMAKER were identical, but the implied reliability of orders differed greatly. The framework map provides nearly complete coverage of the genome, estimated at approximately 1700 cM in length using a modified estimator. This map should provide a useful framework for merging existing loblolly pine maps and adding multiallelic markers as they become available. Map coverage with dominant markers in both linkage phases will make the map useful for subsequent quantitative trait locus mapping in families derived by self-pollination. Received: 7 August 1998 / Accepted: 27 October 1998     

Developing and characterising Ricinus communis SSR markers by data mining of whole-genome sequences

Ricinus communis is a versatile industrial oil crop that is cultivated worldwide. Genetic improvement and marker-assisted breeding of castor bean have been slowed owing to the lack of abundant and efficient molecular markers. As co-dominant markers, simple sequence repeats (SSRs) are useful for genetic evaluation and molecular breeding. The recently released whole-genome sequence of castor bean provides useful genomic resources for developing markers on a genome-wide scale. In the present study, the distribution and frequency of microsatellites in the castor bean genome were characterised and numerous SSR markers were developed using genomic data mining. In total, 18,647 SSR loci at a density of one SSR per 18.89 Kb in the castor bean genome sequence (representing approximately 352.27 Mb) were identified. Dinucleotide repeats were the most frequently observed microsatellites, although the AAT repeat motif was also prevalent. Using six cultivars as screening samples, 670 polymorphic SSR markers from 1,435 primer pairs (46.7 %) were developed. Trinucleotide motif loci contained a higher proportion of polymorphisms (48.5 %) than dinucleotide motif loci (39.2 %). The polymorphism level in the SSR loci was positively correlated with the increasing number of repeat units in the microsatellites. The phylogenetic relationship among 32 varieties was evaluated using the developed SSR markers. Cultivars developed at the same institute clustered together, suggesting that these cultivars have a narrow genetic background. The large number of SSR markers developed in this study will be useful for genetic mapping and for breeding improved castor-oil plants. These markers will also facilitate genetic and genomic studies of Euphorbiaceae.     

A genetic linkage map for Pinus radiata based on RFLP, RAPD, and microsatellite markers

A genetic linkage map for radiata pine (Pinus radiata D. Don) has been constructed using segregation data from a three-generation outbred pedigree. A total of 208 loci were analyzed including 165 restriction fragment length polymorphism (RFLP), 41 random amplified polymorphic DNA (RAPD) and 2 microsatellite markers. The markers were assembled into 22 linkage groups of 2 or more loci and covered a total distance of 1382 cM. Thirteen loci were unlinked to any other marker. Of the RFLP loci that were mapped, 93 were detected by loblolly pine (P. taeda L.) cDNA probes that had been previously mapped or evaluated in that species. The remaining 72 RFLP loci were detected by radiata pine probes from a PstI genomic DNA library. Two hundred and eighty RAPD primers were evaluated, and 41 loci which were segregating in a 11 ratio were mapped. Two microsatellite markers were also placed on the map. This map and the markers derived from it will have wide applicability to genetic studies in P. radiata and other pine species.     

An RFLP linkage map for loblolly pine based on a three-generation outbred pedigree

A genetic linkage map for loblolly pine (Pinus taeda L.) was constructed using segregation data from a three-generation outbred pedigree consisting of four grandparents, two parents, and 95 F₂ progeny. The map was based predominantly on restriction fragment length polymorphism (RFLP) loci detected by cDNA probes. Sixty-five cDNA and three genomic DNA probes revealed 90 RFLP loci. Six polymorphic isozyme loci were also scored. One-fourth (24%) of the cDNA probes detected more than 1 segregating locus, an indication that multigene families are common in pines. As many as six alleles were observed at a single segregating locus among grandparents and it was not unusual for the progeny to segregate for three or four alleles per locus. Multipoint linkage analysis placed 73 RFLP and 2 isozyme loci into 20 linkage groups; the remaining 17 RFLP and 4 isozyme loci were unlinked. The mapped RFLP probes provide a new set of codominant markers for genetic analyses in loblolly pine.     

Simple sequence repeat marker development from bacterial artificial chromosome end sequences and expressed sequence tags of flax (Linum usitatissimum L.)

Flax is an important oilseed crop in North America and is mostly grown as a fibre crop in Europe. As a self-pollinated diploid with a small estimated genome size of ~370 Mb, flax is well suited for fast progress in genomics. In the last few years, important genetic resources have been developed for this crop. Here, we describe the assessment and comparative analyses of 1,506 putative simple sequence repeats (SSRs) of which, 1,164 were derived from BAC-end sequences (BESs) and 342 from expressed sequence tags (ESTs). The SSRs were assessed on a panel of 16 flax accessions with 673 (58 %) and 145 (42 %) primer pairs being polymorphic in the BESs and ESTs, respectively. With 818 novel polymorphic SSR primer pairs reported in this study, the repertoire of available SSRs in flax has more than doubled from the combined total of 508 of all previous reports. Among nucleotide motifs, trinucleotides were the most abundant irrespective of the class, but dinucleotides were the most polymorphic. SSR length was also positively correlated with polymorphism. Two dinucleotide (AT/TA and AG/GA) and two trinucleotide (AAT/ATA/TAA and GAA/AGA/AAG) motifs and their iterations, different from those reported in many other crops, accounted for more than half of all the SSRs and were also more polymorphic (63.4 %) than the rest of the markers (42.7 %). This improved resource promises to be useful in genetic, quantitative trait loci (QTL) and association mapping as well as for anchoring the physical/genetic map with the whole genome shotgun reference sequence of flax.     

Development of Juglans Regia SSR Markers by Data Mining of the EST Database

Walnut (Juglans regia), an economically important woody plant, is widely cultivated in temperate regions for its timber and nutritional fruits. Despite abundant studies in germplasm, systemic molecular evaluations of walnut are sparsely reported mainly due to the limited molecular markers available. Expressed sequence tags (EST) provide a valuable resource for developing simple sequence repeat (SSR) markers. In this study, a total of 5,025 walnut ESTs (covering 16.41 Mb) were retrieved from the National Center for Biotechnology Information database. The SSR motifs were then analyzed by the SSRHunter software. In total, 398 SSRs were obtained with an average frequency of 1/4.08 kb. Dinucleotide (di-) repeat motifs accounted for 69.85% of all SSRs, followed by trinucleotide (tri-) with a frequency of 27.64%, while low frequency (2.51%) of tetranucleotide (tetra-) to hexanucleotide (hexa-) was observed. Meanwhile, GCA and TC motifs were prevalent among di- and tri- loci, respectively. Subsequently, a total of 123 primer pairs were designed from the non-redundant SSR-containing unigenes with the selection threshold of SSR length set to 10 bp or more. To examine the efficiency of candidate markers, seven DNA pools were collected from geographically different accessions. Results demonstrated that 41 SSR primer sets could generate high polymorphic amplification products (33.3%), and these polymorphic loci were mainly located in the 3′-untranslated region. Annotation analysis revealed that only two of these 41 loci were located inside open reading frames of characterized proteins (E ≤ 1E−30).     

Genome-Wide Comparative Analyses of Microsatellites in Papaya

Microsatellites, or simple sequence repeats (SSRs), are highly polymorphic and universally distributed in eukaryotes. SSRs have been used extensively as sequence tagged markers in genetic studies. Recently, the functional and evolutionary importance of SSRs has received considerable attention. Here we report the mining and characterization of the SSRs in papaya genome. We analyzed SSRs from 277.4 Mb of whole genome shotgun (WGS) sequences, 51.2 Mb bacterial artificial chromosome (BAC) end sequences (BES), and 13.4 Mb expressed sequence tag (EST) sequences. The papaya SSR density was one SSR per 0.7 kb of DNA sequence in the WGS, which was higher than that in BES and EST sequences. SSR abundance was dramatically reduced as the repeat length increased. According to SSR motif length, dinucleotide repeats were the most common motif in class I, whereas hexanucleotides were the most copious in class II SSRs. The tri- and hexanucleotide repeats of both classes were greater in EST sequences compared to genomic sequences. In class I SSR, AT and AAT were the most frequent motifs in BES and WGS sequences. By contrast, AG and AAG were the most abundant in EST sequences. For SSR marker development, 9,860 primer pairs were surveyed for amplification and polymorphism. Successful amplification and polymorphic rates were 66.6% and 17.6%, respectively. The highest polymorphic rates were achieved by AT, AG, and ATG motifs. The genome wide analysis of microsatellites revealed their frequency and distribution in papaya genome, which varies among plant genomes. This complete set of SSRs markers throughout the genome will assist diverse genetic studies in papaya and related species.     

Combination of multipoint maximum likelihood (MML) and regression mapping algorithms to construct a high-density genetic linkage map for loblolly pine (Pinus taeda L.)

Genetic maps have been successfully applied to assist in the dissection of complex traits, provide insight on genome structure, and estimate recombination in conjunction with physical maps. Despite an extensive list of genetic maps developed for loblolly pine (Pinus taeda L.) over the past two decades, a high-density consensus map has not yet been constructed. In this study, we used two reference three-generation outbred pedigrees, base and qtl, obtained from the North Carolina State University Cooperative Tree Improvement Program, to obtain a high-density genetic consensus map. Both populations were genotyped with ≈ 7,000 different markers (restriction fragment length polymorphisms, expressed sequence tag polymorphisms, simple sequence repeats, SNPs). The grouping, ordering, and spacing of the markers on each linkage group were performed with JoinMap® 4.1, which implements the multipoint maximum likelihood algorithm for outbred populations. The final consensus map contains 2,466 markers, with a total length of 1,476 centimorgans (cM). The average marker density across the 12 linkage groups was 0.62 cM/marker. This high-density map provides an important resource for breeders and geneticists and will enable comparative studies across species, as well as improve the loblolly pine genome sequence assembly.     

Development,characterization and utilization of GenBank microsatellite markers in <Emphasis Type="Italic">Phyllostachys pubescens</Emphasis> and related species

Public sequence databases provide a rapid, simple and cost-effective source of microsatellite markers. We analyzed 1,532 bamboo (Phyllostachys pubescens) sequences available in public domain DNA databases, and found 3,241 simple sequence repeat (SSR) loci comprising repeats of two or more nucleotides in 920 genomic survey sequences (GSSs) and 68 cDNA sequences. This corresponded to one SSR per 336 bp of GSS DNA and one SSR per 363 bp of cDNA. The SSRs consisted of 76.6 and 74.5% dinucleotide repeats, 20.0 and 22.3% trinucleotide repeats, and 3.4 and 3.2% higher-number repeats in the GSS DNA and cDNA sequences, respectively. The repeat motif AG/CT (or GA/TC) was the most abundant. Nineteen microsatellite markers were developed from Class I and Class II SSRs, showing that the limited polymorphism in Ph. pubescens cultivars and provenances could be attributed to clonal propagation of the bamboo plant. The transferability of the microsatellites reached 75.3%, and the polymorphism of loci successfully transferred was 66.7% for six additional Phyllostachys species. Microsatellite PBM014 transferred successfully to all six species, showed rich polymorphism, and could serve as species-specific alleles for the identification of Phyllostachys interspecies hybrids.     

Genome-Wide Characterization of Simple Sequence Repeat (SSR) Loci in Chinese Jujube and Jujube SSR Primer Transferability

Chinese jujube (Ziziphus jujuba), an economically important species in the Rhamnaceae family, is a popular fruit tree in Asia. Here, we surveyed and characterized simple sequence repeats (SSRs) in the jujube genome. A total of 436,676 SSR loci were identified, with an average distance of 0.93 Kb between the loci. A large proportion of the SSRs included mononucleotide, dinucleotide and trinucleotide repeat motifs, which accounted for 64.87%, 24.40%, and 8.74% of all repeats, respectively. Among the mononucleotide repeats, A/T was the most common, whereas AT/TA was the most common dinucleotide repeat. A total of 30,565 primer pairs were successfully designed and screened using a series of criteria. Moreover, 725 of 1,000 randomly selected primer pairs were effective among 6 cultivars, and 511 of these primer pairs were polymorphic. Sequencing the amplicons of two SSRs across three jujube cultivars revealed variations in the repeats. The transferability of jujube SSR primers proved that 35/64 SSRs could be transferred across family boundary. Using jujube SSR primers, clustering analysis results from 15 species were highly consistent with the Angiosperm Phylogeny Group (APGIII) System. The genome-wide characterization of SSRs in Chinese jujube is very valuable for whole-genome characterization and marker-assisted selection in jujube breeding. In addition, the transferability of jujube SSR primers could provide a solid foundation for their further utilization.     

Root regeneration as an indicator of aluminium toxicity in loblolly pine

Loblolly pine (Pinus taeda L.) seedlings were grown in sand culture and exposed to solution Al concentrations of 0, 5, 10, 20, 40, and 80 mg.l⁻¹ in two separate studies. Root regeneration potential (RRP) of loblolly pine was found to be very sensitive to Al. Both the total number and total length of new white roots produced during a 24-day period declined with as little as 5 mg Al.l⁻¹ in solution.     

Genetic mapping of 66 new microsatellite (SSR) loci in bread wheat

In hexaploid bread wheat ( Triticum aestivum L. em. Thell), ten members of the IWMMN ( International Wheat Microsatellites Mapping Network) collaborated in extending the microsatellite (SSR = simple sequence repeat) genetic map. Among a much larger number of microsatellite primer pairs developed as a part of the WMC ( Wheat Microsatellite Consortium), 58 out of 176 primer pairs tested were found to be polymorphic between the parents of the ITMI ( International Triticeae Mapping Initiative) mapping population W7984 x Opata 85 (ITMI pop). This population was used earlier for the construction of RFLP ( Restriction Fragment Length Polymorphism) maps in bread wheat (ITMI map). Using the ITMI pop and a framework map (having 266 anchor markers) prepared for this purpose, a total of 66 microsatellite loci were mapped, which were distributed on 20 of the 21 chromosomes (no marker on chromosome 6D). These 66 mapped microsatellite (SSR) loci add to the existing 384 microsatellite loci earlier mapped in bread wheat.     

Citrullus simple sequence repeats markers from sequence databases

Publically available cDNA sequence data of Citrullus lanatus were searched for simple sequence repeats (SSRs). Nineteen microsatellites were identified and primer pairs were designed to amplify those loci. Primers were evaluated for their ability to detect polymorphisms within a set of several watermelon varieties and local landraces, C. colocynthis, and interspecific hybrids. Eighteen polymorphic SSR loci were identified. These polymorphic loci can be used for varietal identification and other uses.