Germ cells are not essential for sexual dimorphism of gonads in common carp, C. carpio L

Common carp is the fourth most cultivated fish species in the world. Sexually dimorphic growth of common carp makes its sex differentiation an important research topic. The main objective of this study was to investigate the role of germ cells in sex differentiation of common carp. We generated the germ cell-deficient common carp through knocking down dead end gene with a morpholino antisense oligonucleotide during the embryogenesis. In the common carp without germ cells, we found about half of the gonads (type I gonads) contain granulosa cells and theca cells, which were the same with somatic cells we observed in the wild-type ovaries. The remaining half of the gonads (type II gonads) contain Sertoli cells and Leydig cells, which were the same with somatic cells in the wild-type testes. Furthermore, the expression of sex differentiation related genes was detected in the gonads without germ cells. It was found cyp19a1a was mainly expressed in the type I gonads, and the cyp19a1a mRNA level is much higher in each type I gonad than in the type II gonad. While dmrt1 was mainly expressed in the type II gonads, and the dmrt1 mRNA level is much higher in each type II gonad than in the type I gonad. Our comprehensive analysis showed that the gonadal structure and expression of genes related with sex differentiation were sexually dimorphic in the germ cell-deficient carp. These findings are valuable for discovering the underlying mechanism of common carp sex differentiation which is crucial for cultivating monosexual carp for aquaculture.