Characterization of the transcriptome and gene expression of four different tissues in the ecologically relevant sea urchin Arbacia lixula using RNA-seq.

The sea urchin Arbacia lixula is a keystone species in Mediterranean ecosystems that drive landscape changes in littoral communities. However, genomic information available for the whole order Arbacioida is very limited. Using RNA-seq techniques, we have characterized the transcriptome of four different tissue types in A. lixula: the 'somatic' tissues (coelomocytes and digestive tissue) and the 'reproductive' tissues (ovary and testis), from two replicated cDNA libraries for each sample. Additionally, we performed a de novo assembly to build the 'reference' transcriptome, pooling reads of the four tissues, to analyse the differential expression (DE) in pairwise comparisons between tissues. The complete de novo assembly yielded 186,084 transcripts, with a sequence size limit of 100 nt, being 31% of them spliced isoforms. Approximately 21% of the transcripts had blast hits against proteins of metazoans (E < 10(-5) ), being less than 2.2% functionally annotated. Between coelomocytes and digestive, 30,794 transcripts showed DE (~11.8% of them with blast hit), and 19,567 transcripts did so between testis and ovary (~28.5% of them with blast hit). Major GO-term categories upregulated in somatic tissues were those related to muscle contraction and energy generation in digestive, and lipid metabolism associated with immune response in coelomocytes. Between reproductive tissues, the major upregulated GO categories were related to energy generation in testis, and negative regulation of nucleotide metabolism in ovary. We particularly screened for a collection of target genes in each tissue because of their relevance for further studies on evolution and adaptation of echinoids.