Transcriptome sequencing, de novo assembly and annotation of the freeze tolerant earthworm, Dendrobaena octaedra

The lumbricid earthworm Dendrobaena octaedra, widely distributed in the northern hemisphere, is a freeze tolerant worm species known for its ability to acclimate to the extreme cold condition. However, at the genome and transcriptome level, the species has not been explored to a great extent. The present study focuses on transcriptome analysis of Dendrobaena octaedra to identify and functionally annotate the potential transcripts and biological pathways associated with the freeze tolerance of the species. The Roche 454 pyrosequencing generated 181,856 reads for warm acclimated worms and 190,209 reads for cold acclimated worms which were de novo assembled to 6336 isotigs using the Newbler 2.3 assembler. In total, 2665 isotigs were annotated against the NCBI nr database and of them, 1753 isotigs were assigned to 9546 GO terms. The conserved domain analysis using InterProScan annotated 6309 isotigs into 1307 domains with EF-hand domain as the most highly represented domain. Further, the COG annotation classified 1426 isotigs into 25 functional categories and 895 isotigs were mapped to 90 KEGG pathways. Among them, 22 pathways were functionally enriched in the cold acclimated worms. The RNA seq analysis showed 96 differentially expressed genes of which 50 genes were upregulated in the cold acclimated worms. In addition, we identified 72 molecular chaperones which may have effect in regulating the cold tolerance of the species. The complete transcriptome resource of Dendrobaena octaedra can be effectively utilized further to interpret the key genetic, molecular and physiological mechanisms regulating the freeze tolerance and cold acclimation process of the species.