High throughput mining and characterization of microsatellites from common carp genome

Microsatellite Characteristics of Silver Carp (Hypophthalmichthysmolitrix) Genome and Genetic Diversity Analysis in Four Cultured Populations

Hypophthalmichthys molitrix is one of the four most important fish in China and has high breeding potential. However, simple sequence repeat (SSR) markers developed on H. molitrix genome level for genetic diversity analysis are limited. In this study, the distribution characteristics of SSRs in the assembled H. molitrix genome were analyzed, and new markers were developed to preliminarily evaluate the genetic diversity of the four breeding populations. A total of 368,572 SSRs were identified from the H. molitrix genome. The total length of SSRs was 6,492,076 bp, accounting for 0.77% of the total length of the genome sequence. The total frequency and total density were 437.73 loci/Mb and 7713.16 bp/Mb, respectively. Among the 2–6 different nucleotide repeat types, SSRs were dominated by di-nucleotide repeats (204,873, 55.59%), and AC/GT was the most abundant motif. The number of SSRs on each chromosome was positively correlated with the length. The 13 pairs of markers developed were used to analyze the genetic diversity of four cultivated populations in Hubei Province. The results showed that the genetic diversity of the four populations was low, and the ranges of alleles (Na), effective alleles (Ne), observed heterozygosity (Ho), and Shannon’s index information (I) were 3.538–4.462, 2.045–2.461, 0.392–0.450, and 0.879–0.954, respectively. Genetic variation occurs mainly among individuals within populations (95.35%). UPGMA tree and Bayesian analysis showed that four populations could be divided into two different branches. Therefore, the genome-wide SSRs were effectively in genetic diversity analysis on H. molitrix.

Genome-wide identification of simple sequence repeats and development of polymorphic SSR markers in swamp eel (Monopterus albus)

OBJECTIVES

Swamp eel is one model species for sexual reversion and an aquaculture fish in China. One local strain with deep yellow and big spots of Monopterus albus has been selected for consecutive selective breeding. The objectives of this study were characterizing the Simple Sequence Repeats (SSRs) of M. albus in the assembled genome obtained recently, and developing polymorphic SSRs for future breeding programs.

METHODS

The genome wide SSRs were mined by using MISA software, and their types and genomic distribution patterns were investigated. Based on the available flanking sequences, primer pairs were batched developed, and Polymorphic SSRs were identified by using Polymorphic SSR Retrieval tool. The obtained polymorphic SSRs were validated by using e-PCR and capillary electrophoresis, then they were used to investigate genetic diversity of one breeding population.

RESULTS

A total of 364,802 SSRs were identified in assembled M. albus genome. The total length, density and frequency of SSRs were 8,204,641 bp, 10,259 bp/Mb, and 456.16 loci/Mb, respectively. Mononucleotide repeats were predominant among SSRs (33.33%), and AC and AAT repeats were the most abundant di- and tri-nucleotide repeats motifs. A total of 287,189 primer pairs were designed, and a high-density physical map was constructed (359.11 markers per Mb). A total of 871 polymorphic SSRs were identified, and 38 SSRs of 101 randomly selected ones were validated by using e-PCR and capillary electrophoresis. Using these 38 polymorphic SSRs, 201 alleles were detected and genetic diversity level (Na, PIC, HO, and He) was evaluated.

CONCLUSIONS

The genome-wide SSRs and newly developed SSR markers will provide a useful tool for genetic mapping, diversity analysis studies in swamp eel in the future. The high level of genetic diversity (Na = 5.29, PIC = 0.5068, HO = 0.4665, He = 0.5525) but excess of homozygotes (FIS = 0.155) in one breeding population provide baseline information for future breeding program.

The stage-specific long non-coding RNAs and mRNAs identification and analysis during early development of common carp, Cyprinus carpio

Cyprinus carpio is considered an alternative vertebrate fish model to zebrafish. However, systemic times-series research on the lncRNAs and mRNAs during early development of C. carpio has not been reported yet. This study provides the first long non-coding RNA (lncRNA)-mRNA expression profiles during six main early development stages (2 h post-fertilization hpf, 6 hpf, 12 hpf, 20 hpf, 64 hpf and 1 day post-hatching). A total of 51,979 lncRNAs were identified. We screened the top 10 abundance lncRNAs and mRNAs and stage-specific lncRNAs and mRNAs (specificity measure SPM > 0.9). We identified significant differentially expressed lncRNAs and mRNAs (|log2 (fold change)| ≥ 1 and false discovery rate FDR of <0.05). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified numerous signaling pathways. Additionally, the lncRNA-mRNA co-regulated network analysis of two lncRNAs (lncrps25 and malat1) and two mRNAs (mitf and troponin T) were investigated. Our results provide new insight into the role of lncRNAs and mRNAs, and would advance the understanding of lncRNA-mediated mechanisms in early development of fish.

Common carp pentraxin gene: Evidence for its role in ovarian differentiation and growth

Pentraxins (PTX), belong to an evolutionarily conserved family, containing a PTX protein domain, having role in acute immunological responses and fertility in higher vertebrates. However, information regarding the action of ptx on reproduction is extremely limited in fish. To study this, ptx cDNA was cloned for downstream analysis. Tissue distribution and ontogeny expression analysis indicated the prevalence of ptx in ovary. Varied phase-wise expression during carp ovarian cycle and elevated ptx expression after human chorionic gonadotropin induction, in vitro and in vivo, indicated probable regulation of gonadotropin. In situ hybridization and immunohistochemistry revealed the presence of ptx transcript and protein in the follicular layer of stage-III/IV oocytes indicating a role in ovarian growth. To assess the functional significance of ptx, transient silencing was performed using follicular primary cell culture, in vitro and in common carp, in vivo, through ovary-targeted injection of PEI-siRNA. Transient silencing of ptx-siRNA reduced the expression of various genes/factors related to oogenesis such as transcription factors, several steroidogenic enzymes, and esrs genes. These alterations in expression suggested a plausible role for ptx in ovarian steroidogenesis either, directly or indirectly, which is evident from the changes in the serum estradiol-17β (E₂) and 17α,20β-dihydroxyprogesterone levels. Furthermore, downregulation of aromatase activity was also noticed after transient silencing. Increased ptx expression after E₂ induced sex reversal to juvenile carp showed the correlative role of ptx during ovarian differentiation and development. Taken together, these findings suggest that ptx exerts an important role during ovarian growth, maturation and/or recrudescence of common carp.

Identification, expression profiling and localization of thoc in common carp ovary: Influence of thoc3-siRNA transient silencing

THO complex is a multisubunit family with a function in transcription and mRNA export. In the present study, transcripts of THO complex (thoc) were identified in developing ovary of common carp and their role during ovarian development and growth has been characterized for the first time in a teleost using expression profiling and transient siRNA silencing. Thoc expression revealed a spatiotemporal pattern in the gonads with high levels at 120 days post-hatch, with moderately high levels thereafter. In situ hybridization and immunohistochemical localization revealed the presence of thoc3 in follicular layer of stage-III/IV oocytes. High levels of thoc3, thoc5, and thoc7 genes in the follicular layer suggest a possible role in ovarian growth. Reduced levels of serum estradiol-17β and 17α, 20β-dihydroxypregn-4-en-3-one after thoc3 transient silencing indicated differential action on steroidogenic enzyme, transcription factor, and growth factor genes. Furthermore, transient silencing of thoc3, in vivo and in vitro, downregulated ad4bp/sf1, amh, cyp19a1a, foxl2, hsd3b, hsd11b1, hsd20b, hsd17b1, rspo1, and vtg. Incidentally, gdf9 and igf1 were upregulated, while no change was seen in esr1/2, nanos, and vasa. These observations imply that thoc3 seems to regulate ovarian function including steroidogenesis, either directly or indirectly.

Gonadal transcriptome analysis of the common carp, Cyprinus carpio: Identification of differentially expressed genes and SSRs

Common carp (Cyprinus carpio) is a world-wide freshwater fish of eutrophic waters. C. carpio, have various reproductive traits, including early sexual maturity, that may make them excellent, large, realistic, aquaculture model species. In the present work, de novo assembly of gonadal (testicular and ovarian) transcriptomes from juvenile common carp was performed to identify genes involved in gonadal development. A total of 81,757 and 43,257 transcripts with average lengths of 769 and 856 bp, were obtained from the immature testicular and ovarian transcriptomes, respectively. About 84,367 unigenes were constructed after removing redundancy involving representation of transcripts in both gonadal transcriptomes. Gene ontology (39,171 unigenes), clusters of orthologous group's analysis (6651 unigenes) and Kyoto encyclopedia of genes, and genomes automatic annotation server analysis (4783 unigenes) were performed to identify potential genes along with their functions. Furthermore, 18,342 (testis) and 8693 (ovary) simple sequence repeats were identified. About 298 differentially expressed genes were identified, of which 171 and 127 genes were up-regulated in testis and ovary, respectively. Quantitative real-time reverse transcription PCR was performed to validate differential expression of selected genes in testis and ovary. Nearly 809 genes related to reproduction were identified, sex-wise expression pattern of genes related to steroid synthesis, endocrine regulation, germ cell maintenance and others factors related to gonadal differentiation was observed, and expression analysis of nanos, ad4bp/sf-1, and gdf9 was performed. The present study identified certain important genes/factors involved in the gonadal development of C. carpio which may provide insights into the understanding of sex-differentiation and gonadal development processes.

A high-resolution genetic linkage map and QTL fine mapping for growth-related traits and sex in the Yangtze River common carp (Cyprinus carpio haematopterus)

Background

A high-density genetic linkage map is essential for QTL fine mapping, comparative genome analysis, identification of candidate genes and marker-assisted selection for economic traits in aquaculture species. The Yangtze River common carp (Cyprinus carpio haematopterus) is one of the most important aquacultured strains in China. However, quite limited genetics and genomics resources have been developed for genetic improvement of economic traits in such strain.

Results

A high-resolution genetic linkage map was constructed by using 7820 2b-RAD (2b-restriction site-associated DNA) and 295 microsatellite markers in a F2 family of the Yangtze River common carp (C. c. haematopterus). The length of the map was 4586.56 cM with an average marker interval of 0.57 cM. Comparative genome mapping revealed that a high proportion (70%) of markers with disagreed chromosome location was observed between C. c. haematopterus and another common carp strain (subspecies) C. c. carpio. A clear 2:1 relationship was observed between C. c. haematopterus linkage groups (LGs) and zebrafish (Danio rerio) chromosomes. Based on the genetic map, 21 QTLs for growth-related traits were detected on 12 LGs, and contributed values of phenotypic variance explained (PVE) ranging from 16.3 to 38.6%, with LOD scores ranging from 4.02 to 11.13. A genome-wide significant QTL (LOD = 10.83) and three chromosome-wide significant QTLs (mean LOD = 4.84) for sex were mapped on LG50 and LG24, respectively. A 1.4 cM confidence interval of QTL for all growth-related traits showed conserved synteny with a 2.06 M segment on chromosome 14 of D. rerio. Five potential candidate genes were identified by blast search in this genomic region, including a well-studied multi-functional growth related gene, Apelin.

Conclusions

We mapped a set of suggestive and significant QTLs for growth-related traits and sex based on a high-density genetic linkage map using SNP and microsatellite markers for Yangtze River common carp. Several candidate growth genes were also identified from the QTL regions by comparative mapping. This genetic map would provide a basis for genome assembly and comparative genomics studies, and those QTL-derived candidate genes and genetic markers are useful genomic resources for marker-assisted selection (MAS) of growth-related traits in the Yangtze River common carp.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4613-1) contains supplementary material, which is available to authorized users.

Genome Wide Identification, Phylogeny, and Expression of Aquaporin Genes in Common Carp (Cyprinus carpio)

Background

Aquaporins (Aqps) are integral membrane proteins that facilitate the transport of water and small solutes across cell membranes. Among vertebrate species, Aqps are highly conserved in both gene structure and amino acid sequence. These proteins are vital for maintaining water homeostasis in living organisms, especially for aquatic animals such as teleost fish. Studies on teleost Aqps are mainly limited to several model species with diploid genomes. Common carp, which has a tetraploidized genome, is one of the most common aquaculture species being adapted to a wide range of aquatic environments. The complete common carp genome has recently been released, providing us the possibility for gene evolution of aqp gene family after whole genome duplication.

Results

In this study, we identified a total of 37 aqp genes from common carp genome. Phylogenetic analysis revealed that most of aqps are highly conserved. Comparative analysis was performed across five typical vertebrate genomes. We found that almost all of the aqp genes in common carp were duplicated in the evolution of the gene family. We postulated that the expansion of the aqp gene family in common carp was the result of an additional whole genome duplication event and that the aqp gene family in other teleosts has been lost in their evolution history with the reason that the functions of genes are redundant and conservation. Expression patterns were assessed in various tissues, including brain, heart, spleen, liver, intestine, gill, muscle, and skin, which demonstrated the comprehensive expression profiles of aqp genes in the tetraploidized genome. Significant gene expression divergences have been observed, revealing substantial expression divergences or functional divergences in those duplicated aqp genes post the latest WGD event.

Conclusions

To some extent, the gene families are also considered as a unique source for evolutionary studies. Moreover, the whole set of common carp aqp gene family provides an essential genomic resource for future biochemical, toxicological, physiological, and evolutionary studies in common carp.

Quantitative trait loci for morphometric traits in multiple families of common carp (Cyprinus carpio)

Quantitative trait locus (QTL) mapping is frequently used to understand the genetic architecture of quantitative traits. Herein, we performed a genome scan for QTL affecting the morphometric characters in eight full-sib families containing 522 individuals using different statistical methods (Sib-pair and half-sib model). A total of 194 QTLs were detected in 25 different regions on 10 linkage groups (LGs). Among them, 37 QTLs on five LGs (eight, 13, 24, 40 and 45) were significant (5% genome-wide level), while the remaining 40 (1% chromosome-wide level) and 117 (5% chromosome-wide level) indicated suggestive effect on those traits. Heritabilities for most morphometric traits were moderate to high, ranging from 0.21 to 0.66, with generally strong phenotypic and genetic correlations between the traits. A large number of QTLs for morphometric traits were co-located, consistent with their high correlations, and may reflect pleiotropic effect on the same genes. Biological pathways were mapped for possible candidate genes on QTL regions. One significantly enriched pathway was identified on LG45, which had a P-value of 0.04 and corresponded to the "regulation of actin cytoskeleton pathway". The results are expected to be useful in marker-assisted selection (MAS) and provide valuable information for the study of gene pathway for morphometric and growth traits of the common carp.

An ultra-high density linkage map and QTL mapping for sex and growth-related traits of common carp (Cyprinus carpio)

High density genetic linkage maps are essential for QTL fine mapping, comparative genomics and high quality genome sequence assembly. In this study, we constructed a high-density and high-resolution genetic linkage map with 28,194 SNP markers on 14,146 distinct loci for common carp based on high-throughput genotyping with the carp 250 K single nucleotide polymorphism (SNP) array in a mapping family. The genetic length of the consensus map was 10,595.94 cM with an average locus interval of 0.75 cM and an average marker interval of 0.38 cM. Comparative genomic analysis revealed high level of conserved syntenies between common carp and the closely related model species zebrafish and medaka. The genome scaffolds were anchored to the high-density linkage map, spanning 1,357 Mb of common carp reference genome. QTL mapping and association analysis identified 22 QTLs for growth-related traits and 7 QTLs for sex dimorphism. Candidate genes underlying growth-related traits were identified, including important regulators such as KISS2, IGF1, SMTLB, NPFFR1 and CPE. Candidate genes associated with sex dimorphism were also identified including 3KSR and DMRT2b. The high-density and high-resolution genetic linkage map provides an important tool for QTL fine mapping and positional cloning of economically important traits, and improving common carp genome assembly.

QTL variations for growth-related traits in eight distinct families of common carp (Cyprinus carpio)

BACKGROUND

Comparing QTL analyses of multiple pair-mating families can provide a better understanding of important allelic variations and distributions. However, most QTL mapping studies in common carp have been based on analyses of individual families. In order to improve our understanding of heredity and variation of QTLs in different families and identify important QTLs, we performed QTL analysis of growth-related traits in multiple segregating families.

RESULTS

We completed a genome scan for QTLs that affect body weight (BW), total length (TL), and body thickness (BT) of 522 individuals from eight full-sib families using 250 microsatellites evenly distributed across 50 chromosomes. Sib-pair and half-sib model mapping identified 165 QTLs on 30 linkage groups. Among them, 10 (genome-wide P <0.01 or P < 0.05) and 28 (chromosome-wide P < 0.01) QTLs exhibited significant evidence of linkage, while the remaining 127 exhibited a suggestive effect on the above three traits at a chromosome-wide (P < 0.05) level. Multiple QTLs obtained from different families affect BW, TL, and BT and locate at close or identical positions. It suggests that same genetic factors may control variability in these traits. Furthermore, the results of the comparative QTL analysis of multiple families showed that one QTL was common in four of the eight families, nine QTLs were detected in three of the eight families, and 26 QTLs were found common to two of the eight families. These common QTLs are valuable candidates in marker-assisted selection.

CONCLUSION

A large number of QTLs were detected in the common carp genome and associated with growth-related traits. Some of the QTLs of different growth-related traits were identified at similar chromosomal regions, suggesting a role for pleiotropy and/or tight linkage and demonstrating a common genetic basis of growth trait variations. The results have set up an example for comparing QTLs in common carp and provided insights into variations in the identified QTLs affecting body growth. Discovery of these common QTLs between families and growth-related traits represents an important step towards understanding of quantitative genetic variation in common carp.

Discovery of Genome-Wide Microsatellite Markers in Scombridae: A Pilot Study on Albacore Tuna

Recent developments in sequencing technologies and bioinformatics analysis provide a greater amount of DNA sequencing reads at a low cost. Microsatellites are the markers of choice for a variety of population genetic studies, and high quality markers can be discovered in non-model organisms, such as tuna, with these recent developments. Here, we use a high-throughput method to isolate microsatellite markers in albacore tuna, Thunnus alalunga, based on coupling multiplex enrichment and next-generation sequencing on 454 GS-FLX Titanium pyrosequencing. The crucial minimum number of polymorphic markers to infer evolutionary and ecological processes for this species has been described for the first time. We provide 1670 microsatellite design primer pairs, and technical and molecular genetics selection resulting in 43 polymorphic microsatellite markers. On this panel, we characterized 34 random and selectively neutral markers («neutral») and 9 «non-neutral» markers. The variability of «neutral» markers was screened with 136 individuals of albacore tuna from southwest Indian Ocean (42), northwest Indian Ocean (31), South Africa (31), and southeast Atlantic Ocean (32). Power analysis demonstrated that the panel of genetic markers can be applied in diversity and population genetics studies. Global genetic diversity for albacore was high with a mean number of alleles at 16.94; observed heterozygosity 66% and expected heterozygosity 77%. The number of individuals was insufficient to provide accurate results on differentiation. Of the 9 «non-neutral» markers, 3 were linked to a sequence of known function. The one is located to a sequence having an immunity function (ThuAla-Tcell-01) and the other to a sequence having energy allocation function (ThuAla-Hki-01). These two markers were genotyped on the 136 individuals and presented different diversity levels. ThuAla-Tcell-01 has a high number of alleles (20), heterozygosity (87–90%), and assignment index. ThuAla-Hki-01 has a lower number of alleles (9), low heterozygosity (24–27%), low assignment index and significant inbreeding. Finally, the 34 «neutral» and 3 «non-neutral» microsatellites markers were tested on four economically important Scombridae species—Thunnus albacares, Thunnus thynnus, Thunnus obesus, and Acanthocybium solandri.

Phylogeny and Evolution of Multiple Common Carp (Cyprinus carpio L.) Populations Clarified by Phylogenetic Analysis Based on Complete Mitochondrial Genomes

Common carp (Cyprinus carpio L.) is one of the oldest, most widely farmed commercially important freshwater fish in the world. However, many undetermined phylogenetic relationships and origins of common carp lineages remain, which are obstacles to conservation and genetic breeding of this species. Phylogenetic analyses based on molecular tools are helpful to distinguish the origin of species, understand and clarify their evolutionary history, and provide a genetic basis for selective breeding. In this study, we demonstrated a method to extract complete mitochondrial genome sequences from whole-genome resequencing data using the Illumina platform. The complete mitochondrial genome sequences of 26 individuals representing nine strains were obtained and subjected to a phylogenetic analysis. We reconstructed the phylogenetic topologies of the nine strains and analyzed the haplotypes. Results from both analyses suggested that the genome sequences belonged to two distinct subspecies from Europe and East Asia. We also estimated the time of divergence of the nine strains, which was up to 100 KYA. The phylogenetic results clarified the breeding history of Songpu mirror carp and suggest that this species may be hybrid of paternal European mirror carp and maternal Xingguo red carp. The results also support a previous hypothesis that koi may have originated from or have close ancestry with Oujiang color carp in China.

Development and evaluation of the first high-throughput SNP array for common carp (Cyprinus carpio)

Background

A large number of single nucleotide polymorphisms (SNPs) have been identified in common carp (Cyprinus carpio) but, as yet, no high-throughput genotyping platform is available for this species. C. carpio is an important aquaculture species that accounts for nearly 14% of freshwater aquaculture production worldwide. We have developed an array for C. carpio with 250,000 SNPs and evaluated its performance using samples from various strains of C. carpio.

Results

The SNPs used on the array were selected from two resources: the transcribed sequences from RNA-seq data of four strains of C. carpio, and the genome re-sequencing data of five strains of C. carpio. The 250,000 SNPs on the resulting array are distributed evenly across the reference C.carpio genome with an average spacing of 6.6 kb. To evaluate the SNP array, 1,072 C. carpio samples were collected and tested. Of the 250,000 SNPs on the array, 185,150 (74.06%) were found to be polymorphic sites. Genotyping accuracy was checked using genotyping data from a group of full-siblings and their parents, and over 99.8% of the qualified SNPs were found to be reliable. Analysis of the linkage disequilibrium on all samples and on three domestic C.carpio strains revealed that the latter had the longer haplotype blocks. We also evaluated our SNP array on 80 samples from eight species related to C. carpio, with from 53,526 to 71,984 polymorphic SNPs. An identity by state analysis divided all the samples into three clusters; most of the C. carpio strains formed the largest cluster.

Conclusions

The Carp SNP array described here is the first high-throughput genotyping platform for C. carpio. Our evaluation of this array indicates that it will be valuable for farmed carp and for genetic and population biology studies in C. carpio and related species.

Analysis of the skin transcriptome in two oujiang color varieties of common carp

Background

Body color and coloration patterns are important phenotypic traits to maintain survival and reproduction activities. The Oujiang color varieties of common carp (Cyprinus carpio var. color), with a narrow distribution in Zhejiang Province of China and a history of aquaculture for over 1,200 years, consistently exhibit a variety of body color patterns. The molecular mechanism underlying diverse color patterns in these variants is unknown. To the practical end, it is essential to develop molecular markers that can distinguish different phenotypes and assist selective breeding.

Methodology/Principal Findings

In this exploratory study, we conducted Roche 454 transcriptome sequencing of two pooled skin tissue samples of Oujiang common carp, which correspond to distinct color patterns, red with big black spots (RB) and whole white (WW), and a total of 737,525 sequence reads were generated. The reads obtained in this study were co-assembled jointly with common carp Roche 454 sequencing reads downloaded from NCBI SRA database, resulting in 43,923 isotigs and 546,676 singletons. Over 31 thousand (31,445; 71.6%) isotigs were found with significant BLAST matches (E<1e-10) to the nr protein database, which corresponds to 12,597 annotated zebrafish genes. A total of 70,947 isotigs and singletons (transcripts) were annotated with Gene Ontology, and 60,221 transcripts were found with corresponding EC numbers. Out of 145 zebrafish pigmentation genes, orthologs for 117 were recovered in Oujiang color carp transcriptome, including 18 found only among singletons. Our transcriptome analysis revealed over 52,902 SNPs in Oujiang common carp, and identified 63 SNP markers that are putatively unique either for RB or WW.

Conclusions

The transcriptome of Oujiang color varieties of common carp obtained through this study, along with the pigmentation genes recovered and the color pattern-specific molecular markers developed, will facilitate future research on the molecular mechanism of color patterns and promote aquaculture of Oujiang color varieties of common carp through molecular marker assisted-selective breeding.

Genome-wide microsatellite identification in the fungus Anisogramma anomala using Illumina sequencing and genome assembly

High-throughput sequencing has been dramatically accelerating the discovery of microsatellite markers (also known as Simple Sequence Repeats). Both 454 and Illumina reads have been used directly in microsatellite discovery and primer design (the “Seq-to-SSR” approach). However, constraints of this approach include: 1) many microsatellite-containing reads do not have sufficient flanking sequences to allow primer design, and 2) difficulties in removing microsatellite loci residing in longer, repetitive regions. In the current study, we applied the novel “Seq-Assembly-SSR” approach to overcome these constraints in Anisogramma anomala. In our approach, Illumina reads were first assembled into a draft genome, and the latter was then used in microsatellite discovery. A. anomala is an obligate biotrophic ascomycete that causes eastern filbert blight disease of commercial European hazelnut. Little is known about its population structure or diversity. Approximately 26 M 146 bp Illumina reads were generated from a paired-end library of a fungal strain from Oregon. The reads were assembled into a draft genome of 333 Mb (excluding gaps), with contig N₅₀ of 10,384 bp and scaffold N₅₀ of 32,987 bp. A bioinformatics pipeline identified 46,677 microsatellite motifs at 44,247 loci, including 2,430 compound loci. Primers were successfully designed for 42,923 loci (97%). After removing 2,886 loci close to assembly gaps and 676 loci in repetitive regions, a genome-wide microsatellite database of 39,361 loci was generated for the fungus. In experimental screening of 236 loci using four geographically representative strains, 228 (96.6%) were successfully amplified and 214 (90.7%) produced single PCR products. Twenty-three (9.7%) were found to be perfect polymorphic loci. A small-scale population study using 11 polymorphic loci revealed considerable gene diversity. Clustering analysis grouped isolates of this fungus into two clades in accordance with their geographic origins. Thus, the “Seq-Assembly-SSR” approach has proven to be a successful one for microsatellite discovery.

A genome-wide analysis of simple sequence repeats in maize and the development of polymorphism markers from next-generation sequence data

Background

Maize (Zea mays ssp. mays L.), as the most important plant for staple food of several million people, animal feed and bioenergy productions, is widely cultivated around the world. Simple sequence repeats (SSRs) are widely used as molecular markers in maize genetics and breeding, but only two thousands pairs of SSRs have been published currently, which hardly satisfies for the increasing needs of geneticists and breeders. Furthermore, the increasing studies have revealed that SSRs also play a vital role in functional regulation and evolution. It is fortunate that the development of sequencing technology and bio-software provides the basis for characterization and development of SSRs in maize.

Results

In this study, MISA was applied to identify overall 179,681 SSRs in maize reference genome B73, with an average distance of 11.46 Kbp. Their distributions within the genome in different regions were non-random, and the density followed in a descending order of UTR, promotor, intron, intergenic and CDS. Meanwhile, 82,694 (46.02%) SSRs with unique flanking sequences were selected, and then applied to analyze the polymorphism of next-generation sequencing data from 345 maize inbred lines and data from maize reference genome B73. There were 58,946 SSRs with length information results in ten or more than ten genomes, accounting for 71.28% of SSRs with unique flanking sequences, while 55,621 SSRs had polymorphism, with an average PIC value of 0.498. 250 pairs of SSR primers in different genomic regions covering all maize chromosomes were randomly chosen for the experimental validation, with an average PIC value of 0.63 in 11 elite maize inbred lines.

Conclusions

Our work provided insight into the non-random distribution spatterns and compositions of SSRs in different regions of maize genome, and also developed more polymorphic SSR markers using next-generation sequencing reads. The genome-wide SSRs polymorphism markers could be useful for genetic analysis and marker-assisted selection in breeding practice, and it was also proved to be high efficient for molecular marker development via next-generation sequencing reads.

A dense genetic linkage map for common carp and its integration with a BAC-based physical map

Background

Common carp (Cyprinus carpio) is one of the most important aquaculture species with an annual global production of 3.4 million metric tons. It is also an important ornamental species as well as an important model species for aquaculture research. To improve the economically important traits of this fish, a number of genomic resources and genetic tools have been developed, including several genetic maps and a bacterial artificial chromosome (BAC)-based physical map. However, integrated genetic and physical maps are not available to study quantitative trait loci (QTL) and assist with fine mapping, positional cloning and whole genome sequencing and assembly. The objective of this study was to integrate the currently available BAC-based physical and genetic maps.

Results

The genetic map was updated with 592 novel markers, including 312 BAC-anchored microsatellites and 130 SNP markers, and contained 1,209 genetic markers on 50 linkage groups, spanning 3,565.9 cM in the common carp genome. An integrated genetic and physical map of the common carp genome was then constructed, which was composed of 463 physical map contigs and 88 single BACs. Combined lengths of the contigs and single BACs covered a physical length of 498.75 Mb, or around 30% of the common carp genome. Comparative analysis between common carp and zebrafish genomes was performed based on the integrated map, providing more insights into the common carp specific whole genome duplication and segmental rearrangements in the genome.

Conclusion

We integrated a BAC-based physical map to a genetic linkage map of common carp by anchoring BAC-associated genetic markers. The density of the genetic linkage map was significantly increased. The integrated map provides a tool for both genetic and genomic studies of common carp, which will help us to understand the genomic architecture of common carp and facilitate fine mapping and positional cloning of economically important traits for genetic improvement and modification.

Genome-Wide SNP Discovery from Transcriptome of Four Common Carp Strains

Background

Single nucleotide polymorphisms (SNPs) have been used as genetic marker for genome-wide association studies in many species. Gene-associated SNPs could offer sufficient coverage in trait related research and further more could themselves be causative SNPs for traits. Common carp (Cyprinus carpio) is one of the most important aquaculture species in the world accounting for nearly 14% of freshwater aquaculture production. There are various strains of common carp with different economic traits, however, the genetic mechanism underlying the different traits have not been elucidated yet. In this project, we identified a large number of gene-associated SNPs from four strains of common carp using next-generation sequencing.

Results

Transcriptome sequencing of four strains of common carp (mirror carp, purse red carp, Xingguo red carp, Yellow River carp) was performed with Solexa HiSeq2000 platform. De novo assembled transcriptome was used as reference for alignments, and SNP calling was done through BWA and SAMtools. A total of 712,042 Intra-strain SNPs were discovered in four strains, of which 483,276 SNPs for mirror carp, 486,629 SNPs for purse red carp, 478,028 SNPs for Xingguo red carp and 488,281 SNPs for Yellow River carp were discovered, respectively. Besides, 53,893 inter-SNPs were identified. Strain-specific SNPs of four strains were 53,938, 53,866, 48,701, 40,131 in mirror carp, purse red carp, Xingguo red carp and Yellow River carp, respectively. GO and KEGG pathway analysis were done to reveal strain-specific genes affected by strain-specific non-synonymous SNPs. Validation of selected SNPs revealed that 48% percent of SNPs (12 of 25) were tested to be true SNPs.

Conclusions

Transcriptome analysis of common carp using RNA-Seq is a cost-effective way of generating numerous reads for SNP discovery. After validation of identified SNPs, these data will provide a solid base for SNP array designing and genome-wide association studies.