Identification of genes expressed differentially in female and male gametes ofSipunculus nudus

The sipunculid wormSipunculus nudusis a widely distributed, commercially important marine mollusc. Oogenesis and sex determination/differentiation are important steps inS. nudusreproduction. However, the molecular mechanisms of oogenesis and sex determination/differentiation inS. nudusare unclear. We successfully generated a high-qualityde novoassembled and annotated transcriptome of oocytes from coelomic fluid (OC), oocytes from nephridium (ON) and spermatozeugmata from coelomic fluid (SC) ofS. nudususing the Illumina sequencing platform. There were 17,720 and 17,030 differentially expressed genes (DEGs) in OC vs. ON and OC vs. SC respectively. KEGG pathway analysis showed that the DEGs in these two groups were enriched in ribosome, thyroid hormone synthesis, apelin signalling pathway and cellular senescence. Critical genes that play essential roles in sex determination/differentiation were found, includingfoxl2,fem-1,dmrt1,ftz-f1andsox9. This may largely contribute to the understanding of the molecular mechanisms of oogenesis and sex determination/differentiation inS. nudusand laid a foundation for further in-depth studies.