RNA sequencing reveals a diverse and dynamic repertoire of the Xenopus tropicalis transcriptome over development

A genome-wide survey of maternal and embryonic transcripts during Xenopus tropicalis development

Background

Dynamics of polyadenylation vs. deadenylation determine the fate of several developmentally regulated genes. Decay of a subset of maternal mRNAs and new transcription define the maternal-to-zygotic transition, but the full complement of polyadenylated and deadenylated coding and non-coding transcripts has not yet been assessed in Xenopus embryos.

Results

To analyze the dynamics and diversity of coding and non-coding transcripts during development, both polyadenylated mRNA and ribosomal RNA-depleted total RNA were harvested across six developmental stages and subjected to high throughput sequencing. The maternally loaded transcriptome is highly diverse and consists of both polyadenylated and deadenylated transcripts. Many maternal genes show peak expression in the oocyte and include genes which are known to be the key regulators of events like oocyte maturation and fertilization. Of all the transcripts that increase in abundance between early blastula and larval stages, about 30% of the embryonic genes are induced by fourfold or more by the late blastula stage and another 35% by late gastrulation. Using a gene model validation and discovery pipeline, we identified novel transcripts and putative long non-coding RNAs (lncRNA). These lncRNA transcripts were stringently selected as spliced transcripts generated from independent promoters, with limited coding potential and a codon bias characteristic of noncoding sequences. Many lncRNAs are conserved and expressed in a developmental stage-specific fashion.

Conclusions

These data reveal dynamics of transcriptome polyadenylation and abundance and provides a high-confidence catalogue of novel and long non-coding RNAs.

RNA-sequencing reveals previously unannotated protein- and microRNA-coding genes expressed in aleurone cells of rice seeds

The rice genome annotation has been greatly improved in recent years, largely due to the availability of full length cDNA sequences derived from many tissues. Among those yet to be studied is the aleurone layer, which produces hydrolases for mobilization of seed storage reserves during seed germination and post germination growth. Herein, we report transcriptomes of aleurone cells treated with the hormones abscisic acid, gibberellic acid, or both. Using a comprehensive approach, we identified hundreds of novel genes. To minimize the number of false positives, only transcripts that did not overlap with existing annotations, had a high level of expression, and showed a high level of uniqueness within the rice genome were considered to be novel genes. This approach led to the identification of 553 novel genes that encode proteins and/or microRNAs. The transcriptome data reported here will help to further improve the annotation of the rice genome.

Combining different mRNA capture methods to analyze the transcriptome: analysis of the Xenopus laevis transcriptome

mRNA sequencing (mRNA-seq) is a commonly used technique to survey gene expression from organisms with fully sequenced genomes. Successful mRNA-seq requires purification of mRNA away from the much more abundant ribosomal RNA, which is typically accomplished by oligo-dT selection. However, mRNAs with short poly-A tails are captured poorly by oligo-dT based methods. We demonstrate that combining mRNA capture via oligo-dT with mRNA capture by the 5’ 7-methyl guanosine cap provides a more complete view of the transcriptome and can be used to assay changes in mRNA poly-A tail length on a genome-wide scale. We also show that using mRNA-seq reads from both capture methods as input for de novo assemblers provides a more complete reconstruction of the transcriptome than either method used alone. We apply these methods of mRNA capture and de novo assembly to the transcriptome of Xenopus laevis, a well-studied frog that currently lacks a finished sequenced genome, to discover transcript sequences for thousands of mRNAs that are currently absent from public databases. The methods we describe here will be broadly applicable to many organisms and will provide insight into the transcriptomes of organisms with sequenced and unsequenced genomes.

Transcriptome profile of the green odorous frog (Odorrana margaretae)

Transcriptome profiles provide a practical and inexpensive alternative to explore genomic data in non-model organisms, particularly in amphibians where the genomes are very large and complex. The odorous frog Odorranamargaretae (Anura: Ranidae) is a dominant species in the mountain stream ecosystem of western China. Limited knowledge of its genetic background has hindered research on this species, despite its importance in the ecosystem and as biological resources. Here we report the transcriptome of O. margaretae in order to establish the foundation for genetic research. Using an Illumina sequencing platform, 62,321,166 raw reads were acquired. After a de novo assembly, 37,906 transcripts were obtained, and 18,933 transcripts were annotated to 14,628 genes. We functionally classified these transcripts by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). A total of 11,457 unique transcripts were assigned to 52 GO terms, and 1,438 transcripts were assigned to 128 KEGG pathways. Furthermore, we identified 27 potential antimicrobial peptides (AMPs), 50,351 single nucleotide polymorphism (SNP) sites, and 2,574 microsatellite DNA loci. The transcriptome profile of this species will shed more light on its genetic background and provide useful tools for future studies of this species, as well as other species in the genus Odorrana. It will also contribute to the accumulation of amphibian genomic data.

lincRNAs: genomics, evolution, and mechanisms

Long intervening noncoding RNAs (lincRNAs) are transcribed from thousands of loci in mammalian genomes and might play widespread roles in gene regulation and other cellular processes. This Review outlines the emerging understanding of lincRNAs in vertebrate animals, with emphases on how they are being identified and current conclusions and questions regarding their genomics, evolution and mechanisms of action.

Long non-coding RNA identification over mouse brain development by integrative modeling of chromatin and genomic features

In silico prediction of genomic long non-coding RNAs (lncRNAs) is prerequisite to the construction and elucidation of non-coding regulatory network. Chromatin modifications marked by chromatin regulators are important epigenetic features, which can be captured by prevailing high-throughput approaches such as ChIP sequencing. We demonstrate that the accuracy of lncRNA predictions can be greatly improved when incorporating high-throughput chromatin modifications over mouse embryonic stem differentiation toward adult Cerebellum by logistic regression with LASSO regularization. The discriminating features include H3K9me3, H3K27ac, H3K4me1, open reading frames and several repeat elements. Importantly, chromatin information is suggested to be complementary to genomic sequence information, highlighting the importance of an integrated model. Applying integrated model, we obtain a list of putative lncRNAs based on uncharacterized fragments from transcriptome assembly. We demonstrate that the putative lncRNAs have regulatory roles in vicinity of known gene loci by expression and Gene Ontology enrichment analysis. We also show that the lncRNA expression specificity can be efficiently modeled by the chromatin data with same developmental stage. The study not only supports the biological hypothesis that chromatin can regulate expression of tissue-specific or developmental stage-specific lncRNAs but also reveals the discriminating features between lncRNA and coding genes, which would guide further lncRNA identifications and characterizations.

Comprehensive transcriptome profiling and functional analysis of the frog (Bombina maxima) immune system

Amphibians occupy a key phylogenetic position in vertebrates and evolution of the immune system. But, the resources of its transcriptome or genome are still little now. Bombina maxima possess strong ability to survival in very harsh environment with a more mature immune system. We obtained a comprehensive transcriptome by RNA-sequencing technology. 14.3% of transcripts were identified to be skin-specific genes, most of which were not isolated from skin secretion in previous works or novel non-coding RNAs. 27.9% of transcripts were mapped into 242 predicted KEGG pathways and 6.16% of transcripts related to human disease and cancer. Of 39 448 transcripts with the coding sequence, at least 1501 transcripts (570 genes) related to the immune system process. The molecules of immune signalling pathway were almost presented, several transcripts with high expression in skin and stomach. Experiments showed that lipopolysaccharide or bacteria challenge stimulated pro-inflammatory cytokine production and activation of pro-inflammatory caspase-1. These frog's data can remarkably expand the existing genome or transcriptome resources of amphibians, especially immunity data. The entity of the data provides a valuable platform for further investigation on more detailed immune response in B. maxima and a comparative study with other amphibians.

Transcriptome sequencing and de novo analysis of a cytoplasmic male sterile line and its near-isogenic restorer line in chili pepper (Capsicum annuum L.)

BACKGROUND

The use of cytoplasmic male sterility (CMS) in F1 hybrid seed production of chili pepper is increasingly popular. However, the molecular mechanisms of cytoplasmic male sterility and fertility restoration remain poorly understood due to limited transcriptomic and genomic data. Therefore, we analyzed the difference between a CMS line 121A and its near-isogenic restorer line 121C in transcriptome level using next generation sequencing technology (NGS), aiming to find out critical genes and pathways associated with the male sterility.

RESULTS

We generated approximately 53 million sequencing reads and assembled de novo, yielding 85,144 high quality unigenes with an average length of 643 bp. Among these unigenes, 27,191 were identified as putative homologs of annotated sequences in the public protein databases, 4,326 and 7,061 unigenes were found to be highly abundant in lines 121A and 121C, respectively. Many of the differentially expressed unigenes represent a set of potential candidate genes associated with the formation or abortion of pollen.

CONCLUSIONS

Our study profiled anther transcriptomes of a chili pepper CMS line and its restorer line. The results shed the lights on the occurrence and recovery of the disturbances in nuclear-mitochondrial interaction and provide clues for further investigations.

Validation of novel reference genes for RT-qPCR studies of gene expression in Xenopus tropicalis during embryonic and post-embryonic development

BACKGROUND

Accurate interpretation of transcriptome profiling by quantitative PCR requires the establishment of species-specific standards. However, the selection of reference genes for assessing RNA expression profiles in Xenopus laevis and Xenopus tropicalis was mostly based on historical reasons and they often only reflect the traditions of a laboratory.

RESULTS

We investigated the expression stability of 10 genes (dicer1, drosha, eef1a1, elavl3, gsc, h4, odc1, rpl8, smn2, tbp), 8 of which are commonly used as internal controls in published RT-qPCR experiments. We defined specific primer pairs and evaluated their suitability as reference genes by performing RT-qPCR expression profiling in Xenopus tropicalis. Gene expression stability was assayed in a set of 15 developmental stages from the egg to the froglet, and in dissected embryos.

CONCLUSIONS

Overall, we determined a set of qualified reference genes for distinct developmental periods. We recommend the use of dicer1, drosha, eef1a1, and smn2 from early embryonic development up to the end of metamorphosis. During early embryogenesis drosha, eef1a1, smn2 are suitable. For the whole post-embryonic development and for metamorphic stages including pro-metamorphosis and metamorphic climax, we recommend the use of drosha and smn2. These reference genes should prove their usefulness for data comparison across studies.