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Sexual fate of the sawfly, Athalia rosae(Hymenoptera: Tenthredinidae) is determined by the complementary sex determination (CSD) mechanism as is the case in honeybees. However, to date, genes involved in sex determination have not been identified in this species. In this study, we attempted to identify orthologs of complementary sex-determiner(csd), feminizer(fem), and doublesex(dsx) from the A. rosaegenome, all of which are crucial components of the sex determination cascade in the honeybee. As a result, we identified a sawfly ortholog of dsx(designated as Ardsx). Rapid amplification of cDNA ends (RACE) using total RNA extracted from male and female larvae identified three male-specific variants and three female-specific variants. Comparison between the full-length ArdsxcDNAs and the genomic sequence revealed that exon 5 was differentially spliced between the male- and female-specific variants. RT-PCR analysis demonstrated that Ardsxpre-mRNA was spliced alternatively in a sex-dependent manner at almost all the developmental stages. RNAi-mediated knockdown of Ardsxin males caused severe defects in the reproductive organs and, notably, induced development of the ovipository apparatus containing the dorsal pair of blades and the sheath. These males also showed abnormalities in testes and seminal vesicles and lacked mature sperm. The present study provides the first direct evidence that dsxis essential for sexual development in hymenopteran species.