Development of simple sequence repeat markers and diversity analysis in alfalfa (Medicago sativa L.)

Efficient and robust molecular markers are essential for molecular breeding in plant. Compared to dominant and bi-allelic markers, multiple alleles of simple sequence repeat (SSR) markers are particularly informative and superior in genetic linkage map and QTL mapping in autotetraploid species like alfalfa. The objective of this study was to enrich SSR markers directly from alfalfa expressed sequence tags (ESTs). A total of 12,371 alfalfa ESTs were retrieved from the National Center for Biotechnology Information. Total 774 SSR-containing ESTs were identified from 716 ESTs. On average, one SSR was found per 7.7 kb of EST sequences. Tri-nucleotide repeats (48.8 %) was the most abundant motif type, followed by di—(26.1 %), tetra—(11.5 %), penta—(9.7 %), and hexanucleotide (3.9 %). One hundred EST–SSR primer pairs were successfully designed and 29 exhibited polymorphism among 28 alfalfa accessions. The allele number per marker ranged from two to 21 with an average of 6.8. The PIC values ranged from 0.195 to 0.896 with an average of 0.608, indicating a high level of polymorphism of the EST–SSR markers. Based on the 29 EST–SSR markers, assessment of genetic diversity was conducted and found that Medicago sativa ssp. sativa was clearly different from the other subspecies. The high transferability of those EST–SSR markers was also found for relative species.     

Development of polymorphic microsatellite markers in the medicinal plant Gardenia jasminoides (Rubiaceae)

A total of 12,960 simple sequence repeats (SSR) motifs were identified in the genome of the medicinal plant Gardenia jasminoides using Illumina-based EST sequences. Among the SSRs, mono-nucleotides were the most abundant (56.7%), followed by di- (19%) and tri-nucleotides (16.4%). AG/TC (60.2%), TTC/AAG (25.8%), and TTTC/GAAA (35.9%) repeat motifs were the most abundant of the di-, tri- and tetra-nucleotide motifs, respectively. Subsequently, twenty-five allelic, polymorphic primer pairs were identified and tested in 153 individuals from five natural populations of G. jasminoides. The number of alleles per polymorphic locus (A) ranged from two to eight. Observed and expected heterozygosity varied from 0.095 to 0.857 and 0.182 to 0.832, respectively. The PIC values for each locus ranged from 0.171 to 0.792. These new polymorphic EST-SSR markers will be useful for further genetic studies on this economically important plant.     

Development of EST‐SSRs in Cucumis sativus from sequence database

Simple sequence repeat markers derived from expressed sequence tags (EST‐SSR) are potentially valuable tools for plant breeding and germplasm collection conservation, and increasingly, efforts have been made for developing this type of marker. We have identified 20 polymorphic SSR markers from cucumber ESTs deposited in public sequence database. The average allele number was 3.3 per locus, ranging from two to six alleles during screening 20 cucumber genotypes with the mean expected heterozygosity of 0.477. Amplification products were also detected by 13 pairs of primer in Cucumis melo. These informative EST‐SSR markers can be used in cucumber genetic improvement projects.     

Development of EST SSRs in Finger Millet (Eleusine coracana ssp coracana) and their Transferability to Pearl Millet (Pennisetum glaucum)

EST-SSR markers were developed using sequence information from 1740 expressed sequence tags (ESTs) of finger millet available in the public domain. A set of 31 SSR markers were synthesized based on di, tri, tetra and penta-nucleotide repeat sequences. These were used for PCR analysis of 11 elite germplasm lines of finger millet of Indian and African origin. Out of 31 SSR markers, amplification products were obtained for 17 primer pairs. Of these nine were found polymorphic with two alleles per locus. These 17 SSR primer pairs were also tested for amplification in three varieties of pearl millet (Pennisetum glaucum) and 11 could be transferred to pearl millet. The informative EST SSR markers developed, can be used in finger millet as well as pearl millet genetic improvement projects.     

Development of a set of genomic microsatellite markers in tea (Camellia L.) (Camelliaceae)

Tea (Camellia L.) is the world’s most consumed health drink and is also important economically. Due to its self-incompatible and outcrossing nature, tea is composed of highly heterogeneous germplasm. It is a perennial, slow-growing crop and hence the successful release of new improved cultivars following conventional breeding methods takes years. In this context, a DNA marker-based molecular breeding approach holds great promise in accelerating genetic improvement programs in tea. Here we describe the isolation of a set of highly polymorphic genomic microsatellite markers using the enrichment approach, which may be useful for phylogenetic and marker-assisted breeding programs in tea. The enriched library comprising 3,205 clones was screened for the presence of microsatellites using a three-primer-based colony PCR method. Four hundred positive clones were selected and sequenced, to reveal 153 sequences containing simple sequence repeats. Seventy-eight primer pairs were designed from repeat-positive sequences, out of which 40 primer pairs produced successful amplifications. Twenty-two of these primer pairs, when tested on a panel of 21 diverse tea clones and accessions, were found to be highly polymorphic, resulting in 137 alleles with an average of 6.76 alleles per primer pair. The polymorphic information content (PIC), expected heterozygosity (H _e) and observed heterozygosity (H _o) of the polymorphic markers ranged from 0.1 to 0.9, 0.1–0.9 and 0.0–0.8, with average values of 0.6 ± 0.18, 0.7 ± 0.17 and 0.5 ± 0.22, respectively. These markers can be applied for various diversity analyses, mapping programs and genotyping of tea crop.     

Development and characterization of new microsatellite markers in <Emphasis Type="Italic">Panax</Emphasis><Emphasis Type="Italic">ginseng</Emphasis> (C.A. Meyer) from BAC end sequences

Panax ginseng, commonly known as Korean ginseng, is a valued source of herbal medicine in Korea and China. We have developed and characterized 35 microsatellite markers in P. ginseng from available BAC end sequences. Characterization of these 35 SSR primer pairs in 14 cultivars of P. ginseng showed 12 primer pairs to be polymorphic and 19 primer pairs to be monomorphic, while the remaining four primer pairs did not produce any product. The number of alleles amplified ranged from 4 to 8 per primer pair, with an average of six alleles per locus. The expected and observed heterozygosities ranged from 0.7500 to 0.9678 and 0.5645 to 0.7109, respectively. None of these loci deviated from Hardy–Weinberg equilibrium. All of the functional primer pairs of P. ginseng showed cross-species transferability with Panax quinquefolium. The cross-species transferable markers could be used for ginseng cultivar identification, for genomic studies, including mapping of specific trait QTL/genes, and for measuring conservation of ginseng.     

Development of SSR markers derived from SSR-enriched genomic library of eggplant (<Emphasis Type="Italic">Solanum melongena</Emphasis> L.)

Eggplant (Solanum melongena L.), also known as aubergine or brinjal, is an important vegetable in many countries. Few useful molecular markers have been reported for eggplant. We constructed simple sequence repeat (SSR)-enriched genomic libraries in order to develop SSR markers, and sequenced more than 14,000 clones. From these sequences, we designed 2,265 primer pairs to flank SSR motifs. We identified 1,054 SSR markers from amplification of 1,399 randomly selected primer pairs. The markers have an average polymorphic information content of 0.27 among eight lines of S. melongena. Of the 1,054 SSR markers, 214 segregated in an intraspecific mapping population. We constructed cDNA libraries from several eggplant tissues and obtained 6,144 expressed sequence tag (EST) sequences. From these sequences, we designed 209 primer pairs, 7 of which segregated in the mapping population. On the basis of the segregation data, we constructed a linkage map, and mapped the 236 segregating markers to 14 linkage groups. The linkage map spans a total length of 959.1 cM, with an average marker distance of 4.3 cM. The markers should be a useful resource for qualitative and quantitative trait mapping and for marker-assisted selection in eggplant breeding.     

Identification and characterization of 43 novel polymorphic EST-SSR markers for arum lily,Zantedeschia aethiopica (Araceae)

• Premise of the study: A new set of microsatellite or simple sequence repeat (SSR) markers from expressed sequence tags (ESTs) was developed for arum lily (Zantedeschia aethiopica), which is one of the most iconic and widely recognized ornamental plants in the world. • Methods and Results: Using 2175 unigenes derived from 4283 random ESTs in arum lily, 166 primer pairs were designed and tested for amplification in 24 accessions from Asia, Europe, and Africa. A total of 43 loci were polymorphic, with the number of alleles per locus ranging from two to 10. The observed heterozygosity, expected heterozygosity, and polymorphism information content ranged from 0.2313 to 0.8480, 0.3034 to 0.8648, and 0.1015 to 0.7364, respectively. • Conclusions: These novel polymorphic EST-SSR markers will facilitate future studies of genetic variation and molecular-assisted breeding systems in arum lily.     

Exploring genetic variability within lentil (Lens culinaris Medik.) and across related legumes using a newly developed set of microsatellite markers

Lentil (Lens culinaris Medik.) is an economically important grain legume, yet the genetic and genomic resources remain largely uncharacterized and unexploited in this crop. Microsatellites have become markers of choice for crop improvement applications. Hence, simple sequence repeat (SSR) markers were developed for lentil through the construction of genomic library enriched for GA/CT motifs. As a result 122 functional SSR primer pairs were developed from 151 microsatellite loci and validated in L. culinaris cv. Precoz. Thirty three SSR markers were utilized for the analysis of genetic relationships between cultivated and wild species of Lens and related legumes. A total of 123 alleles were amplified at 33 loci ranging from 2–5 alleles with an average of 3.73 alleles per locus. Polymorphic information content (PIC) for all the loci ranged from 0.13 to 0.99 with an average of 0.66 per locus. Varied levels of cross genera transferability were obtained ranging from 69.70 % across Pisum sativum to 12.12 % across Vigna radiata. The UPGMA based dendrogram was able to establish the uniqueness of each genotype and grouped them into two major clusters clearly resolving the genetic relationships within lentil and related species. The new set of SSR markers reported here were efficient and highly polymorphic and would add to the existing repertoire of lentil SSR markers to be utilized in molecular breeding. Moreover, the improved knowledge about intra- and inter-specific genetic relationships would facilitate germplasm utilization for lentil improvement.     

Highly Informative Single-Copy Nuclear Microsatellite DNA Markers Developed Using an AFLP-SSR Approach in Black Spruce (Picea mariana) and Red Spruce (P. rubens)

Background

Microsatellites or simple sequence repeats (SSRs) are highly informative molecular markers for various biological studies in plants. In spruce (Picea) and other conifers, the development of single-copy polymorphic genomic microsatellite markers is quite difficult, owing primarily to the large genome size and predominance of repetitive DNA sequences throughout the genome. We have developed highly informative single-locus genomic microsatellite markers in black spruce (Picea mariana) and red spruce (Picea rubens) using a simple but efficient method based on a combination of AFLP and microsatellite technologies.

Principal Findings

A microsatellite-enriched library was constructed from genomic AFLP DNA fragments of black spruce. Sequencing of the 108 putative SSR-containing clones provided 94 unique sequences with microsatellites. Twenty-two of the designed 34 primer pairs yielded scorable amplicons, with single-locus patterns. Fourteen of these microsatellite markers were characterized in 30 black spruce and 30 red spruce individuals drawn from many populations. The number of alleles at a polymorphic locus ranged from 2 to 18, with a mean of 9.3 in black spruce, and from 3 to 15, with a mean of 6.2 alleles in red spruce. The polymorphic information content or expected heterozygosity ranged from 0.340 to 0.909 (mean = 0.67) in black spruce and from 0.161 to 0.851 (mean = 0.62) in red spruce. Ten SSR markers showing inter-parental polymorphism inherited in a single-locus Mendelian mode, with two cases of distorted segregation. Primer pairs for almost all polymorphic SSR loci resolved microsatellites of comparable size in Picea glauca, P. engelmannii, P. sitchensis, and P. abies.

Significance

The AFLP-based microsatellite-enriched library appears to be a rapid, cost-effective approach for isolating and developing single-locus informative genomic microsatellite markers in black spruce. The markers developed should be useful in black spruce, red spruce and other Picea species for various genetics, genomics, breeding, forensics, conservation studies and applications.     

Development and characterization of simple sequence repeat (SSR) markers in the water lotus (Nelumbo nucifera)

Simple sequence repeat (SSR) markers were developed in the water lotus (Nelumbo nucifera Gaertn.) from an SSR-enriched genomic library. Of the SSR markers tested, 11 primer pairs produced clearly distinguishable DNA banding patterns. Forty-three alleles were detected with the 11 markers. The allele number per locus ranged from 2 to 5 with an average of 3.9. Polymorphism values ranged from 0.11 to 0.66 with an average of 0.51. These primers were also applicable to another Nelumbo species, Nelumbo lutea (Willd.) Pers. (American lotus) and hybrids between N. nucifera and N. lutea. These results indicate that the SSR markers developed in this study are informative and will be useful for genetic analysis in Nelumbo species.     

Analysis of genetic relationship on Amygdalus mira (koehne) Ricker with other peach species using simple sequence repeat (SSR)

Amygdalus mira (Koehne) Ricker is native to China and has many good economical traits. However, its genetic diversity information has not been extensively studied. In this study, to assess the genetic diversity and relationships of A. mira and other peach species (nineteen accessions from Zhengzhou, Henan Province and seven accessions from Harbin) we used simple sequence repeat (SSR) markers. Here, 10 SSR primers were used, and 100% of the SSR primers were polymorphic, with an average of 5.5 alleles per primer pairs, suggesting that these primers were informative for this study. Additionally, polymorphism information content (PIC) value ranged from 0.82 to 0.96 with an average of 0.91. All the accessions were clustered into two groups (cluster 1 and cluster 2) based on SSR data. Principal coordinate analysis recovered similar results that all accessions were divided into two major clusters. The genetic variations within and among populations were 63.9% and 36.1%, respectively. In conclusion, A. mira maintains high genetic variation levels. This research will be potentially useful to aid breeding and enhance the economic and ornamental value of this wild peach.     

Genetic Diversity and Population Structure Among Oat Cultivars and Landraces

In this study, genetic diversity among 177 oat (Avena sativa L.) accessions including both white and red oat landraces and 36 commercial cultivars was studied for simple sequence repeat (SSR) loci. Thirty-one genomic and expressed sequence tags (EST)-derived primer pairs were selected according to high polymorphism from an initial 66 SSR batch. Markers revealed a high level of polymorphism, detecting a total of 454 alleles. The average gene diversity for the whole sample was 0.29. Genetic similarity, calculated using the Dice coefficient, was used for cluster analysis, and principal component analysis was also applied. In addition, population structure using a Bayesian clustering approach identified discrete subpopulation based on allele frequency and showed similar clustering of oat genotypes in four groups. Accessions could be classified into four main clusters that clearly separated the commercial cultivars, the red oat landraces and two clusters of white oat landraces. Cultivars showed less diversity than the landraces indicating a reduction of genetic diversity during breeding, whereas white oat landraces showed higher diversity than red ones. The average polymorphic information content of 0.80 for the SSR loci indicated the usefulness of many of the SSR for genotype identification. In particular, two markers, MAMA5 and AM04, with a total of 50 alleles and a high discrimination power (>0.90), were sufficient to discriminate among all commercial cultivars studied highlighting their potential use for variety identification.