Comparative transcriptome analysis of Phascolosoma esculenta under different salinities

Salinity is an important parameter that influences the developmental progress of aquatic organisms. The larvae of Phascolosoma esculenta developed into the early stage trochosphere, the late stage trochosphere and the early stage pelagosphere in the low-salinity group (15), the middle group (22.5) and the high-salinity group (30), respectively, after being incubated for 50 h, according to morphological observation. The results showed that high salinity could accelerate the cellular development of P. esculenta. To elucidate the molecular mechanism of larval development of P. esculenta under different salinities, we performed transcriptome analysis of the larvae in P. esculenta. Nine transcriptome libraries of P. esculenta larvae were constructed, i.e., the low-salinity group (A1, A2 and A3), the middle group (B1, B2 and B3) and the high-salinity group (C1, C2 and C3), and each group with three repeats. These nine paired-end libraries were sequenced using the Illumina HiSeq 2500 platform. In total, 57.63 Gb clean data were obtained and de novo assembled into 320,527 unigenes with mean lengths of 482.62 bp and N50 of 529 bp. A total of 249,162 unigenes were significantly matched to known unique proteins. The gene ontology (GO) annotation and the Kyoto Encyclopedia of Genes and Genomes (KEGG) were used to determine the regulation pathways and candidate genes that are associated with the process of larval development. A total number of 12,811, 16,991 and 1735 transcripts were identified as differentially expressed genes (DEGs) from the comparison of A vs. B, A vs. C, and B vs. C, respectively. Of these differentially expressed genes, many DEGs that were upregulated in the high-salinity group were involved in the function that controls the larval development of P. esculenta, such as the neuroactive ligand-receptor interaction and ECM-receptor interaction. In addition, the results of the transcriptome analysis were verified by quantitative real-time PCR (qRT-PCR). After morphological observation and transcriptome analysis, we think high salinity (30) is more suitable than medium (22.5) or low (15) salinity for the development of P. esculenta larvae. This information provides guidance for artificial breeding and paves the way to the further research on the individual development of P. esculenta under different salinities.