Transcriptomic Analysis Revealed Candidate Genes Involved in Pseudomale Sperm Abnormalities in Chinese Tongue Sole (Cynoglossus semilaevis)

SIMPLE SUMMARY: Chinese tongue sole (Cynoglossus semilaevis) is an important aquatic fish in northeast Asia that displays sexual growth morphism, with females being 2–4 times larger than males. Tongue sole has a ZZ/ZW sex determination system, and the genotypic female (ZW) can be sex-reversed into phenotypic males, namely, pseudomales. Pseudomales show similar growth to male fish and are disadvantageous in aquaculture. Moreover, pseudomale fish can produce sperm but only Z-type sperm, and its epigenetic information makes its ZW offspring prone to become pseudomale. Thus, screening the key genes for W absence would provide clues for producing high female ratio or all female fry, benefiting the tongue sole industry. In this study, we compared the transcriptomic profiles of pseudomale and male sperm and found that the FoxO signalling pathway, especially the foxo3a and foxo6-like genes, may play important roles in spermatogenesis. In addition, we identified an ~1 M bp area on the Z chromosome enriched with eight DEGs. The study has provided valuable data for screening candidate genes involved in pseudomale sperm abnormity, and their functional study in the future would shed light on sex control in the tongue sole industry. ABSTRACT: Chinese tongue sole (Cynoglossus semilaevis) has a ZZ/ZW sex determination system, but the genotypic female (ZW) can be sex-reversed into phenotypic males, namely, pseudomales. Pseudomale fish can produce only Z-type sperm but not W sperm. However, the molecular mechanism is unclear. To screen the key genes involved in pseudomale sperm abnormalities, we analysed the transcriptomic profiles of pseudomale and male sperm. In comparison to male sperm, 592 differentially expressed genes (DEGs) were identified in pseudomale sperm, including 499 upregulated and 93 downregulated genes. KEGG analysis indicated that the FoxO signalling pathway, especially the foxo3a and foxo6-like genes, may play an important role in spermatogenesis. The DEGs were mainly distributed on sex chromosomes, with 158 downregulated genes on the Z chromosome and 41 upregulated genes on the W chromosome. A specific area (14–15 M) on the Z chromosome was identified, which enriched eight DEGs inside the ~1 M region. In addition, there were five gene alleles on the sex chromosomes, which showed the opposite transcription pattern (upregulated for the W allele, downregulated for the Z allele). This study has provided valuable data for screening candidate genes involved in the pseudomale sperm abnormality.