Characterization of ESTs from black locust for gene discovery and marker development

The genetic diversity and population structure of Sophora alopecuroides (Faboideae) as determined by microsatellite markers developed from transcriptome

Sophora alopecuroides (Faboideae) is an endemic species, mainly distributed in northwest China. However, the limited molecular markers range for this species hinders breeding and genetic studies. A total of 20,324 simple sequence repeat (SSR) markers were identified from 118,197 assembled transcripts and 18 highly polymorphic SSR markers were used to explore the genetic diversity and population structure of S. alopecuroides from 23 different geographical populations. A relatively low genetic diversity was found in S. alopecuroides based on mean values of the number of effective alleles (Ne = 1.81), expected heterozygosity (He = 0.39) and observed heterozygosity (Ho = 0.55). The results of AMOVA indicated higher levels of variation within populations than between populations. Bayesian-based cluster analysis, principal coordinates analysis and Neighbor-Joining phylogeny analysis roughly divided all genotypes into four major groups with some admixtures. Meanwhile, geographic barriers would have restricted gene flow between the northern and southern regions (separated by Tianshan Mountains), wherein the two relatively ancestral and independent clusters of S. alopecuroides occur. History trade and migration along the Silk Road would together have promoted the spread of S. alopecuroides from the western to the eastern regions of the northwest plateau in China, resulting in the current genetic diversity and population structure. The transcriptomic SSR markers provide a valuable resource for understanding the genetic diversity and population structure of S. alopecuroides, and will assist effective conservation management.

Development and Application of EST-SSR Markers for DNA Fingerprinting and Genetic Diversity Analysis of the Main Cultivars of Black Locust (Robinia pseudoacacia L.) in China

Black locust (Robinia pseudoacacia L.) is an economically and ecologically important tree species which is used for pillar construction, honey production and soil improvement. More EST-SSR (Expressed sequence tag simple sequence repeat) markers of black locust can be used as a complement and improvement of Genomic-SSR markers for the identification of the function of gene and the construction of genetic map. Additionally, currently there is no simple method for identifying black locust cultivars. In this study, we obtained 2702 unigenes from 3095 expressed sequence tags (ESTs) from the National Center of Biotechnology Information (NCBI) database to identify simple sequence repeats (SSRs) in R. pseudoacacia samples. A total of 170 SSR loci were found to be distributed in 162 non-redundant sequences with a frequency of 6.29%. Dinucleotide repeats were the most predominant types among microsatellites (62.35%), followed by tri-nucleotide repeats (25.88%); the remaining SSRs accounted for less than 12%. The repeat motifs AG/TC (29.25%) and CT/GA (29.25%) were the most abundant among dinucleotides, and AAT/TTA (15.91%) was the most common among tri-nucleotides. A total of 62 primer pairs were designed to screen polymorphic and stable SSR loci. The resulting 25 EST-SSR markers capable of amplifying polymorphic, stable, and repeatable products. Eight newly developed EST-SSR markers and four published SSR markers were selected for DNA fingerprinting and genetic diversity analysis of the 123 main R. pseudoacacia cultivars in China. The 12 SSR loci amplified 102 alleles, with an average number of alleles per locus of 8.5 (range 4–15). The average polymorphism information content at the 12 SSR loci for the 123 cultivars was 0.670 (range 0.427–0.881). The 123 cultivars clustered into six main groups based on similarity coefficients, with most cultivars in one subgroup. Fingerprinting was performed using eight SSR markers; 110 black locust cultivars were distinguished. The results of this study increase the availability of EST-SSR markers in black locust and make it a simple method for checking the collection, the certification, and the correct attribution of clones and cultivars.

Selection and validation of reference genes for quantitative gene expression analyses in black locust (Robinia pseudoacacia L.) using real-time quantitative PCR

Black locust (Robinia pseudoacacia L.) is an easy to raise, fast growing, medium-sized deciduous tree species highly tolerant to harsh eco-conditions, i.e., drought and harsh winters, and it is widely adaptable to sandy, loamy, and marshy soils. The basis for this adaptability remains to be investigated at the transcriptomic level using real-time quantitative PCR (qPCR). Selection of a reliable gene for the normalization of qPCR data is important for obtaining accurate results in gene expression. The goal of this study was to identify an appropriate reference gene from 12 candidate genes for gene expression analysis in black locust exposed to various stressors such as abscisic acid (ABA), NaCl, polyethylene glycol (PEG) and varying temperatures. In GeNorm and NormFinder analyses, ACT (actin) and GAPDH (glyceraldehyde-3-phosphate dehydrogenase) gene expression were the most stable in all conditions except heat stress, but in BestKeeper analysis, GAPDH and helicase gene expression were the most stable under NaCl and heat stress. In contrast, ACT and GAPDH were highest under abscisic acid (ABA), GAPDH and βTUB (beta tubulin) under cold stress, and helicase and EF1α (elongation factor 1 alpha) under PEG stress. We found that the most stable reference gene combination for all conditions was ACT and GAPDH. Additionally, the expression pattern of NAC2 (a transcription factor) and BGL2 in different tissues and under different stress conditions was analyzed relative to ACT and GAPDH and UBQ (ubiquitin) the least stably expressed gene. NAC2 and BGL2 both had highest expression in flowers and pods under ABA stress at 48h. This study provides useful reference genes for future gene expression studies in black locust.

Development and Evaluation of a Novel Set of EST-SSR Markers Based on Transcriptome Sequences of Black Locust (Robinia pseudoacacia L.)

Black locust (Robinia pseudoacacia L. of the family Fabaceae) is an ecologically and economically important deciduous tree. However, few genomic resources are available for this forest species, and few effective expressed sequence tag-derived simple sequence repeat (EST-SSR) markers have been developed to date. In this study, paired-end sequencing was used to sequence transcriptomes of R. pseudoacacia by the Illumina HiSeq TM2000 platform, and EST-SSR loci were identified by de novo assembly. Furthermore, a total of 1697 primer pairs were successfully designed, from which 286 primers met the selection screening criteria; 94 pairs were randomly selected and tested for validation using polymerase chain reaction amplification. Forty-five primers were verified as polymorphic, with clear bands. The polymorphism information content values were 0.033–0.765, the number of alleles per locus ranged from 2 to 10, and the observed and expected heterozygosities were 0.000–0.931 and 0.035–0.810, respectively, indicating a high level of informativeness. Subsequently, 45 polymorphic EST-SSR loci were tested for amplification efficiency, using the verified primers, in an additional nine species of Leguminosae, 23 loci were amplified in more than three species, of which two loci were amplified successfully in all species. These EST-SSR markers provide a valuable tool for investigating the genetic diversity and population structure of R. pseudoacacia, constructing a DNA fingerprint database, performing quantitative trait locus mapping, and preserving genetic information.

De novo Transcriptome Generation and Annotation for Two Korean Endemic Land Snails, Aegista chejuensis and Aegista quelpartensis, Using Illumina Paired-End Sequencing Technology

Aegista chejuensis and Aegista quelpartensis (Family-Bradybaenidae) are endemic to Korea, and are considered vulnerable due to declines in their population. The limited genetic resources for these species restricts the ability to prioritize conservation efforts. We sequenced the transcriptomes of these species using Illumina paired-end technology. Approximately 257 and 240 million reads were obtained and assembled into 198,531 and 230,497 unigenes for A. chejuensis and A. quelpartensis, respectively. The average and N50 unigene lengths were 735.4 and 1073 bp, respectively, for A. chejuensis, and 705.6 and 1001 bp, respectively, for A. quelpartensis. In total, 68,484 (34.5%) and 77,745 (33.73%) unigenes for A. chejuensis and A. quelpartensis, respectively, were annotated to databases. Gene Ontology terms were assigned to 23,778 (11.98%) and 26,396 (11.45) unigenes, for A. chejuensis and A. quelpartensis, respectively, while 5050 and 5838 unigenes were mapped to 117 and 124 pathways in the Kyoto Encyclopedia of Genes and Genomes database. In addition, we identified and annotated 9542 and 10,395 putative simple sequence repeats (SSRs) in unigenes from A. chejuensis and A. quelpartensis, respectively. We designed a list of PCR primers flanking the putative SSR regions. These microsatellites may be utilized for future phylogenetics and conservation initiatives.