Development of EST-derived SSR markers using next-generation sequencing to reveal the genetic diversity of 50 chrysanthemum cultivars

Development and validation of SSR markers related to flower color based on full-length transcriptome sequencing in Chrysanthemum

Chrysanthemum (Chrysanthemum moriforlium Ramat.) is one of the most popular flowers worldwide, with very high ornamental and economic values. However, the limitations of available DNA molecular markers and the lack of full genomic sequences hinder the study of genetic diversity and the molecular breeding of chrysanthemum. Here, we developed simple sequence repeat (SSR) from the full-length transcriptome sequences of chrysanthemum cultivar 'Hechengxinghuo'. A total of 11,699 SSRs with mono-, di-, tri-, tetra-, penta- and hexanucleotide repeats were identified, of which eight out of eighteen SSR loci identified based on sixteen transcripts participated in carotenoid metabolism or anthocyanin synthesis were validated as polymorphic SSR markers. These SSRs were used to classify 117 chrysanthemum accessions with different flower colors at the DNA and cDNA levels. The results showed that four SSR markers of carotenoid metabolic pathway divided 117 chrysanthemum accessions into five groups at cDNA level and all purple chrysanthemum accessions were in the group III. Furthermore, the SSR marker CHS-3, LCYE-1 and 3MaT may be related to green color and the PSY-1b marker may be related to yellow color. Overall, our work may be provide a novel method for mining SSR markers associated with specific traits.

First genomic insights into the Mandevilla genus

Mandevilla (Apocynaceae) is a greatly appreciated genus in the world ornamental market. In this study, we attempted to address the poor genetic knowledge and the huge taxonomic gaps existing in this genus by analyzing a collection of 55 accessions. After cytometrically determining the triploid genome size (1,512.64 Mb) of a reference sample (variety "Mandevilla 2001"), the plastidial genome (cpDNA, 0.18 Mb) and a draft of the nuclear genome (nuDNA, 207 Mb) were assembled. While cpDNA was effective in reconstructing the phylogenesis of the Apocynaceae family based on a DNA superbarcoding approach, the nuDNA assembly length was found to be only 41% of the haploid genome size (506 Mb, predicted based on the K-mer frequency distribution). Its annotation enabled the prediction of 37,811 amino acid sequences, of which 10,562 resulted full length proteins. Among them, we identified nine proteins whose orthologs (in Catharanthus roseus) are involved in the biosynthesis of monoterpene indole alkaloids (MIAs), including catharanthine, tabersonine, and vincadifformine. The nuclear genome draft was also useful to develop a highly informative (average polymorphism information content, PIC = 0.62) set of 23 simple sequence repeat (SSR) markers that was validated on the Mandevilla collection. These results were integrated with cytometric measurements, nuclear ITS1 haplotyping and chloroplast DNA barcoding analyses to assess the origin, divergence and relationships existing among the 55 accessions object of the study. As expected, based on the scarce information available in the literature, the scenario was extremely intricate. A reasonable hypothesis is that most of the accessions represent interspecific hybrids sharing the same species as maternal parent (i.e., Mandevilla sanderi).

Genetic Divergence and Relationship Among Opisthopappus Species Identified by Development of EST-SSR Markers

Opisthopappus Shih is an endemic and endangered genus restricted to the Taihang Mountains that has important ornamental and economic value. According to the Flora Reipublicae Popularis Sinicae (FRPS, Chinese version), this genus contains two species (Opisthopappus longilobus and Opisthopappus taihangensis), whereas in the Flora of China (English version) only one species O. taihangensis is present. The interspecific phylogenetic relationship remains unclear and undefined, which might primarily be due to the lack of specific molecular markers for phylogenetic analysis. For this study, 2644 expressed sequence tag-simple sequence repeats (EST-SSRs) from 33,974 unigenes using a de novo transcript assembly of Opisthopappus were identified with a distribution frequency of 7.78% total unigenes. Thereinto, mononucleotides (1200, 45.39%) were the dominant repeat motif, followed by trinucleotides (992, 37.52%), and dinucleotides (410, 15.51%). The most dominant trinucleotide repeat motif was ACC/GGT (207, 20.87%). Based on the identified EST-SSRs, 245 among 1444 designed EST-SSR primers were selected for the development of potential molecular markers. Among these markers, 63 pairs of primers (25.71%) generated clear and reproducible bands with expected sizes. Eventually, 11 primer pairs successfully amplified all individuals from the studied populations. Through the EST-SSR markers, a high level of genetic diversity was detected between Opisthopappus populations. A significant genetic differentiation between the O. longilobus and O. taihangensis populations was found. All studied populations were divided into two clusters by UPGMA, NJ, STRUCTURE, and PCoA. These results fully supported the view of the FRPS, namely, that O. longilobus and O. taihangensis should be regarded as two distinct species. Our study demonstrated that transcriptome sequences, as a valuable tool for the quick and cost-effective development of molecular markers, was helpful toward obtaining comprehensive EST-SSR markers that could contribute to an in-depth assessment of the genetic and phylogenetic relationships between Opisthopappus species.

SSR marker development and intraspecific genetic divergence exploration of Chrysanthemum indicum based on transcriptome analysis

BACKGROUND

Chrysanthemum indicum L., an important ancestral species of the flowering plant chrysanthemum, can be used as medicine and for functional food development. Due to the lack of hereditary information for this species and the difficulty of germplasm identification, we herein provide new genetic insight from the perspective of intraspecific transcriptome comparison and present single sequence repeat (SSR) molecular marker recognition technology.

RESULTS

Through the study of a diploid germplasm (DIWNT) and a tetraploid germplasm (DIWT), the following outcome were obtained. (1) A significant difference in Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations for specific homologous genes was observed using the OrthoMCL method for the identification of homologous gene families between the two cytotypes. Ka/Ks analysis of common, single-copy homologous family members also revealed a greater difference among genes that experienced positive selection than among those experiencing positive selection. (2) Of more practical value, 2575 SSR markers were predicted and partly verified. We used TaxonGap as a visual tool to inspect genotype uniqueness and screen for high-performance molecular loci; we recommend four primers of 65 randomly selected primers with a combined identification success rate of 88.6% as priorities for further development of DNA fingerprinting of C. indicum germplasm.

CONCLUSIONS

The SSR technology based on next-generation sequencing was proved to be successful in the identification of C. indicum germplasms. And the information on the intraspecfic genetic divergence generated by transcriptome comparison deepened the understanding of this complex species' nature.

Mining and Development of Novel SSR Markers Using Next Generation Sequencing (NGS) Data in Plants

Microsatellites, or simple sequence repeats (SSRs), are one of the most informative and multi-purpose genetic markers exploited in plant functional genomics. However, the discovery of SSRs and development using traditional methods are laborious, time-consuming, and costly. Recently, the availability of high-throughput sequencing technologies has enabled researchers to identify a substantial number of microsatellites at less cost and effort than traditional approaches. Illumina is a noteworthy transcriptome sequencing technology that is currently used in SSR marker development. Although 454 pyrosequencing datasets can be used for SSR development, this type of sequencing is no longer supported. This review aims to present an overview of the next generation sequencing, with a focus on the efficient use of de novo transcriptome sequencing (RNA-Seq) and related tools for mining and development of microsatellites in plants.

A Set of 20 New SSR Markers Developed and Evaluated in Mandevilla Lindl

Mandevilla is an ornamental crop with a bright future worldwide because of its high commercial acceptance and added value. However, as with most ornamental species, there are few molecular tools to support cultivar breeding and innovation. In this work, we report the development and analysis of 20 new Simple Sequence Repeat (SSR) markers in Mandevilla. Microsatellites were isolated from two enriched small-insert genomic libraries of Mandevilla × amabilis. The diversity parameters estimated after their amplification in a group of 11 commercial genotypes illustrate the effect of two opposite drifts: the high relatedness of cultivars belonging to the same commercial group and the high divergence of other cultivars, especially M. × amabilis. Based on their different band patterns, six genotypes were uniquely distinguished, and two groups of sport mutations remained undistinguishable. The amplification of the SSRs in three wild species suggested the existence of unexploited diversity available to be introgressed into the commercial pool. This is the first report of available microsatellites in Mandevilla. The development process has provided some clues concerning the genome structure of the species, and the SSRs obtained will help to create new products and to protect existing and upcoming plant innovations.