De novo assembly and characterization of the floral transcriptome of an economically important tree species, Lindera glauca (Lauraceae), including the development of EST-SSR markers for population genetics

Characterisation of the Cinnamomumparthenoxylon (Jack) Meisn (Lauraceae) transcriptome using Illumina paired-end sequencing and EST-SSR markers development for population genetics

Cinnamomumparthenoxylon is an endemic and endangered species with significant economic and ecological value in Vietnam. A better understanding of the genetic architecture of the species will be useful when planning management and conservation. We aimed to characterize the transcriptome of C.parthenoxylon, develop novel molecular markers, and assess the genetic variability of the species. First, transcriptome sequencing of five trees (C.parthenoxylon) based on root, leaf, and stem tissues was performed for functional annotation analysis and development of novel molecular markers. The transcriptomes of C.parthenoxylon were analyzed via an Illumina HiSeqTM 4000 sequencing system. A total of 27,363,199 bases were generated for C.parthenoxylon. De novo assembly indicated that a total of 160,435 unigenes were generated (average length = 548.954 bp). The 51,691 unigenes were compared against different databases, i.e. COG, GO, KEGG, KOG, Pfam, Swiss-Prot, and NR for functional annotation. Furthermore, a total of 12,849 EST-SSRs were identified. Of the 134 primer pairs, 54 were randomly selected for testing, with 15 successfully amplified across nine populations of C.parthenoxylon. We uncovered medium levels of genetic diversity (PIC = 0.52, Na = 3.29, Ne = 2.18, P = 94.07%, Ho = 0.56 and He = 0.47) within the studied populations. The molecular variance was 10% among populations and low genetic differentiation (Fst = 0.06) indicated low gene flow (Nm = 2.16). A reduction in the population size of C.parthenoxylon was detected using BOTTLENECK (VP population). The structure analysis suggested two optimal genetic clusters related to gene flow among the populations. Analysis of molecular variance (AMOVA) revealed higher genetic variation within populations (90%) than among populations (10%). The UPGMA approach and DAPC divided the nine populations into three main clusters. Our findings revealed a significant fraction of the transcriptome sequences and these newlydeveloped novel EST-SSR markers are a very efficient tool for germplasm evaluation, genetic diversity and molecular marker-assisted selection in C.parthenoxylon. This study provides comprehensive genetic resources for the breeding and conservation of different varieties of C.parthenoxylon.

De novo transcriptomes for two species of snappers from the Western Atlantic (Lutjanus griseus and Lutjanus synagris)

Transcriptomes are appropriate resources for studying species that lack sequenced genomes, as they can serve as references for a broad suite of genetic applications, including: phylogenetic assessments, population genomics, and evaluate responses to environmental fluctuations. Here, we present the transcriptomes of two species of marine fishes of commercial and ecological relevance in the Western Atlantic: Lutjanus griseus and L. synagris. This project represents a step forward on developing genomic resources for important species of the Atlantic Ocean.

Genetic diversity and population structure of Cinnamomum balansae Lecomte inferred by microsatellites

Cinnamomum balansae Lecomte (Lauraceae), an economically important forest tree, is distributed in the tropical forests of central and northern Vietnam, which has been threatened in recent decades due to the destruction of its habitat and over-exploitation. The genetic diversity and population structure of the species have not been fully evaluated. We used a set of 15 microsatellites to analyze 161 adult trees from 9 different populations, representing the geographical distribution of C. balansae. Ninety-two different alleles were identified. Here our results showed a low genetic diversity level with an average H_o = 0.246 and H_e = 0.262, and a high level of genetic differentiation (F_ST = 0.601). The bottleneck tests indicated evidence of a reduction in the population size of the two populations (TC and CP). Additionally, all three clustering methods (Bayesian analysis, principal coordinate analysis, and Neighbor-joining tree) were identified in the two genetic groups. The Mantel test showed a significant positive correlation between genetic distance and geographic distance (R² = 0.7331). This study will provide a platform for the conservation of C. balansae both in ex-situ and in-situ plans.

Transcriptome sequencing and microsatellite marker discovery in Ailanthus altissima (Mill.) Swingle (Simaroubaceae)

Ailanthus altissima Swingle, is a tree species native to East Asia and has a great potential in decorative, bioenergy and industrial applications in many countries. To date, despite its commercial importance, the genomic and genetic resources available for this species are still insufficient. In this study, we characterized the transcriptome of A. altissima and developed thirteen EST-SSRs (expressed sequence tag-simple sequence repeats) based on Illumina paired-end RNA sequencing (RNA-seq). Besides, we developed ten polymorphic chloroplast microsatellite (cpSSR) markers using the available chloroplast genome of A. altissima. The transcriptome data produced 87,797 unigenes, of which 64,891 (73.91%) unigenes were successfully annotated in at least one protein database. For cpSSR markers the number of detected alleles (N) per marker varied from three at cpSSR12 to twelve at cpSSR8, the unbiased haploid diversity indices (uh) varied from 0.111 to 0.485, and haploid diversity indices (h) ranged from 0.101 to 0.444 with an average unbiased haploid diversity index (uh) of 0.274. Overall, a total of 65 different cpSSR alleles were identified at the ten loci among 165 individuals of A. altissima. The allele number per locus for EST-SSRs varied from 2.143 to 9.357, and the values of observed and expected heterozygosity ranged from 0.312 to 1.000 and 0.505 to 0.826, respectively. The molecular markers developed in this study will facilitate future genetic diversity, population structure, long distance-gene transfer and pollen-based gene flow analyses of A. altissima populations from its known distribution ranges in China focusing on planted and natural forest stands.

Population genetic structure and variability in Lindera glauca (Lauraceae) indicates low levels of genetic diversity and skewed sex ratios in natural populations in mainland China

Lindera glauca (Lauraceae) is a tree of economic and ecological significance that reproduces sexually and asexually via apomictic seeds. It is widely distributed in the low-altitude montane forests of East Asia. Despite the potential implications of a mixed reproductive system in terms of genetic diversity, few studies have focused on this aspect. In this study, the genetic structure of wild populations of L. glauca was investigated via genetic analyses. Overall, 13 nuclear microsatellites (nSSRs) and five chloroplast microsatellites (cpSSRs) were used to genotype 300 individual plants, taken from 20 wild populations (a small sample size in some wild populations is due to the limitation of its specific reproduction, leading to certain limitations in the results of this study) and two cultivated populations ranging across nearly the entire natural distribution of mainland China. The populations exhibited low levels of genetic diversity (nSSR: A_R = 1.75, Ho = 0.32, He = 0.36; cpSSR: Nb = 2.01, Hrs = 0.40), and no significant effect of isolation by distance between populations existed, regardless of marker type (nSSR: R² = 0.0401, P = 0.068; cpSSR: R² = 0.033, P = 0.091). Haplotype networks showed complex relationships among populations, and the H12 haplotype was predominant in most populations. Analyses of molecular variance obtained with nuclear markers (Fsc = 0.293, F_ST = 0.362) and chloroplast markers (Fsc = 0.299, F_ST = 0.312) were similar. The migration ratio of pollen flow versus seed flow in this study was negative (r = −1.149). Results suggest that weak barriers of dispersal between populations and/or the similarity of founders shared between neighbors and distant populations are indicative of the gene flow between populations more likely involving seeds. Wild L. glauca in mainland China was inferred to have highly skewed sex ratios with predominant females. In addition, some populations experienced a recent bottleneck effect, especially in Gujianshan, Chongqing, and southwest China (population GJS). It is suggested that few wild male individuals should be conserved in order to maintain overall genetic diversity in the wild populations of this species. These findings provide important information for the sustainable utilization and preservation of the overall genetic diversity of L. glauca.

Development of 20 chloroplast microsatellite primers in wuyao (Lindera aggregata, Lauraceae)

PREMISE OF THE STUDY

To investigate the population genetics and evolutionary history of wuyao (Lindera aggregata, Lauraceae) and other Lindera species, polymorphic chloroplast microsatellite (cpSSR) primers were developed.

METHODS AND RESULTS

The complete chloroplast genome of L. glauca was used to design 74 primers for L. aggregata. Agarose gel electrophoresis and Sanger sequencing were used to select 20 polymorphic primers. These primers were then tested in 70 individuals from three populations of L. aggregata. The number of alleles ranged from two to four, and the unbiased haploid diversity index ranged from 0.457 to 0.685. Up to 17 primers successfully amplified in four other Lindera species: L. prattii, L. chunii, L. lungshengensis, and L. pulcherrima var. hemsleyana.

CONCLUSIONS

The 20 primers reported here can potentially be used for future genetic studies of L. aggregata and other Lindera species.

The newly developed single nucleotide polymorphism (SNP) markers for a potentially medicinal plant, Crepidiastrum denticulatum (Asteraceae), inferred from complete chloroplast genome data

Medicinal effects of Crepidiastrum denticulatum have been previously reported. However, the genomic resources of this species and its applications have not been studied. In this study, based on the next generation sequencing method (Miseq sequencing system), we characterize the chloroplast genome of C. denticulatum which contains a large single copy (84,112 bp) and a small single copy (18,519 bp), separated by two inverted repeat regions (25,074 bp). This genome consists of 80 protein-coding gene, 30 tRNAs, and four rRNAs. Notably, the trnT_GGU is pseudogenized because of a small insertion within the coding region. Comparative genomic analysis reveals a high similarity among Asteraceae taxa. However, the junctions between LSC, SSC, and IRs locate in different positions within rps19 and ycf1 among examined species. Also, we describe a newly developed single nucleotide polymorphism (SNP) marker for C. denticulatum based on amplification-refractory mutation system (ARMS) technique. The markers, inferred from SNP in rbcL and matK genes, show effectiveness to recognize C. denticulatum from other related taxa through simple PCR protocol. The chloroplast genome-based molecular markers are effective to distinguish a potentially medicinal species, C. denticulatum, from other related taxa. Additionally, the complete chloroplast genome of C. denticulatum provides initial genomic data for further studies on phylogenomics, population genetics, and evolutionary history of Crepidiastrum as well as other taxa in Asteraceae.

Generation and classification of transcriptomes in two Croomia species and molecular evolution of CYC/TB1 genes in Stemonaceae

The genus Croomia (Stemonaceae) is an excellent model for studying the evolution of the Eastern Asia (EA)-Eastern North America (ENA) floristic disjunction and the genetic mechanisms of floral zygomorphy formation. In addition to the presence of both actinomorphic and zygomorphic flowers within the genus, species are disjunctively distributed between EA and ENA. However, due to the limited availability of genomic resources, few studies of Croomia have examined these questions. In this study, we sequenced the floral and leaf transcriptomes of the zygomorphic flowered C roomia heterosepala and the actinomorphic flowered Croomia japonica, and used comparative genomic approaches to investigate the transcriptome evolution of the two closely related species. The sequencing and de novo assembly of transcriptomes from flowers of C. heterosepala (ChFlower), flowers of C. japonica (CjFlower), and leaves of C. japonica (CjLeaf) yielded 57,193, 62,131 and 64,448 unigenes, respectively. In addition, estimation of Ka/Ks ratios for 11,566 potential orthologous groups between ChFlower and CjFlower revealed that only six pairs had Ka/Ks ratios significantly greater than 1 and are likely under positive selection. A total of 429 single copy nuclear genes (SCNGs) and 21,460 expression sequence tags-simple sequence repeats (EST-SSRs) were identified in this study. Specifically, we identified seven CYC/TB1-like genes from Stemonaceae. Phylogenetic and molecular evolution analyses indicated that these CYC/TB1-like genes formed a monophyletic clade (SteTBL1) and were subject to strong purifying selection. The shifts of floral symmetry in Stemonaceae do not appear to be correlated with TBL copy number.

Characterization of the Lycium barbarum fruit transcriptome and development of EST-SSR markers

Lycium barbarum, commonly known as goji, is important in Chinese herbal medicine and its fruit is a very important agricultural and biological product. However, the molecular mechanism of formation of its fruit and associated medicinal and nutritional components is unexplored. Moreover, this species lacks SSR markers due to lack of genomic and transcriptomic information. In this study, a total of 139,333 unigenes with average length of 1049 bp and N50 of 1579 bp are obtained by trinity assembly from Illumina sequencing reads. A total of 92,498 (66.38%) unigenes showed similarities in at least one database including Nr (46.15%), Nt (56.56%), KO (15.56%), Swiss-prot (33.34%), Pfam (33.43%), GO (33.62%) and KOG/COG (17.55%). Genes in flavonoid and taurine biosynthesis pathways were found and validated by RT-qPCR. A total of 50,093 EST-SSRs were identified from 38,922 unigenes, and 22,537 EST-SSR primer pairs were designed. Four hundred pairs of SSR markers were randomly selected to validate assembly quality, of which 352 (88%) were successful in PCR amplification of genomic DNA from 11 Lycium accessions and 210 produced polymorphisms. The polymorphic loci showed that the genetic similarity of the 11 Lycium accessions ranged from 0.50 to 0.99 and the accessions could be divided into 4 groups. These results will facilitate investigations of the molecular mechanism of formation of L. barbarum fruit and associated medicinal and nutritional components, and will be of value to novel gene discovery and functional genomic studies. The EST-SSR markers will be useful for genetic diversity evaluation, genetic mapping and marker-assisted breeding.