Upon the completion of whole genome sequencing, thorough genome annotation that associates genome sequences with biological meanings is essential. Genome annotation depends on the availability of transcript information as well as orthology information. In teleost fish, genome annotation is seriously hindered by genome duplication. Because of gene duplications, one cannot establish orthologies simply by homology comparisons. Rather intense phylogenetic analysis or structural analysis of orthologies is required for the identification of genes. To conduct phylogenetic analysis and orthology analysis, full-length transcripts are essential. Generation of large numbers of full-length transcripts using traditional transcript sequencing is very difficult and extremely costly.

Echinoderms exhibit several fascinating evolutionary innovations that are rarely seen in the animal kingdom, but how these animals attained such features is not well understood. Here we report the sequencing and analysis of the genome and extensive transcriptomes of the sea cucumberApostichopus japonicus, a species from a special echinoderm group with extraordinary potential for saponin synthesis, aestivation and organ regeneration. The sea cucumber does not possess a reorganizedHoxcluster as previously assumed for all echinoderms, and the spatial expression ofHox7andHox11/13bpotentially guides the embryo-to-larva axial transformation. Contrary to the typical production of lanosterol in animal cholesterol synthesis, the oxidosqualene cyclase of sea cucumber produces parkeol for saponin synthesis and has “plant-like” motifs suggestive of convergent evolution. The transcriptional factorsKlf2andEgr1are identified as key regulators of aestivation, probably exerting their effects through a clock gene-controlled process. Intestinal hypometabolism during aestivation is driven by the DNA hypermethylation of various metabolic gene pathways, whereas the transcriptional network of intestine regeneration involves diverse signaling pathways, including Wnt, Hippo and FGF. Decoding the sea cucumber genome provides a new avenue for an in-depth understanding of the extraordinary features of sea cucumbers and other echinoderms.

The sea cucumber (Apostichopus japonicus) occupies a basal position during the evolution of deuterostomes and is also an important aquaculture species. In order to identify more immune effectors, transcriptome sequencing of A. japonicus coelomocytes in response to lipopolysaccharide (LPS) challenge was performed using the Illumina HiSeq™ 2000 platform. One hundred and seven differentially expressed genes were selected and divided into four functional categories including pathogen recognition (25 genes), reorganization of cytoskeleton (27 genes), inflammation (41 genes) and apoptosis (14 genes). They were analyzed to elucidate the mechanisms of host-pathogen interactions and downstream signaling transduction. Quantitative real-time polymerase chain reactions (qRT-PCRs) of 10 representative genes validated the accuracy and reliability of RNA sequencing results with the correlation coefficients from 0.88 to 0.98 and p-value <0.05. Expression analysis of immune-related genes after LPS challenge will be useful in understanding the immune response mechanisms of A. japonicus against pathogen invasion and developing strategies for resistant markers selection.

Sea cucumber Apostichopus japonicus is an important economic species in China, which is affected by various diseases; skin ulceration syndrome (SUS) is the most serious. In this study, we characterized the transcriptomes in A. japonicus challenged with Vibrio splendidus to elucidate the changes in gene expression throughout the three stages of SUS progression. RNA sequencing of 21 cDNA libraries from various tissues and developmental stages of SUS-affected A. japonicus yielded 553 million raw reads, of which 542 million high-quality reads were generated by deep-sequencing using the Illumina HiSeq™ 2000 platform. The reference transcriptome comprised a combination of the Illumina reads, 454 sequencing data and Sanger sequences obtained from the public database to generate 93,163 unigenes (average length, 1,052 bp; N50 = 1,575 bp); 33,860 were annotated. Transcriptome comparisons between healthy and SUS-affected A. japonicus revealed greater differences in gene expression profiles in the body walls (BW) than in the intestines (Int), respiratory trees (RT) and coelomocytes (C). Clustering of expression models revealed stable up-regulation as the main pattern occurring in the BW throughout the three stages of SUS progression. Significantly affected pathways were associated with signal transduction, immune system, cellular processes, development and metabolism. Ninety-two differentially expressed genes (DEGs) were divided into four functional categories: attachment/pathogen recognition (17), inflammatory reactions (38), oxidative stress response (7) and apoptosis (30). Using quantitative real-time PCR, twenty representative DEGs were selected to validate the sequencing results. The Pearson’s correlation coefficient (R) of the 20 DEGs ranged from 0.811 to 0.999, which confirmed the consistency and accuracy between these two approaches. Dynamic changes in global gene expression occur during SUS progression in A. japonicus. Elucidation of these changes is important in clarifying the molecular mechanisms associated with the development of SUS in sea cucumber.