Testicular Development Induced by a Recessive Mutation during Gonadal Differentiation of Female Common Carp (Cyprinus carpio, L.): (sex determination/germ cells/testes/common carp/teleost).

The phenotypic effects of a new recessive mutation mas , which in homozygous condition induces testicular development in XX animals of common carp (Cyprinus carpio L.), are described. Sexual differentiation of XX; mas /mas and XX; mas /mas animals was compared with the gonad development of XX wild type females and XY males. In XX females gonadal differentiation starts with the formation of an ovarian cavity and entry into meiosis of germ cells at around 80 days post hatching (ph). Male gonads remain quiescent until 120 days ph during which period they develop a network of loose connective tissue. Spermatogenesis starts with tubule formation and the differentiation of germ cells into spermatogonia type B. Heterozygous XX; mas /mas animals developed as normal females, but in homozygous XX; mas /mas animals two types of gonad development were observed. In the first type, germ cells did not enter meiosis until 100 days ph when they differentiated as spermatogonia. An ovarian cavity was not formed but male specific connective tissue developed instead. These gonad developed as normal testes. In the second type, germ cells differentiated at 80 days ph as either oocytes or spermatocytes, which resulted in the gonads developing as ovotestes. The formation of an ovarian cavity was in most cases incomplete. The phenotypic effects of mas are interpreted as a timing mismatch between mas activation and female sex differentiation.