GYNOGENESIS IN ZEBRAFISH RESULTS IN ALL MALE PROGENIES

Zebrafish Danio rerio has rapidly become the model vertebrate of choice for developmental and biomedical research. Despite rigorous investigation, the sex determination system of zebrafish remains poorly understood. Current understanding suggests the system to be polygenic, and mapping studies only identify markers weakly associated with sex. Gynogenesis, which is inheritance of only maternal chromosomes, is a widely used technique for investigating the sex determining systems of fish. However, the literature describing gynogenesis in zebrafish is contradictory and marred by poor growth and yields of gynogens as well as the use of irradiated zebrafish sperm, not heterologous sperm, to activate oocytes.

We showed that common carp Cyprinus carpio sperm is capable of fertilizing zebrafish oocytes, and the resulting hybrids are inviable past the embryonic stage. Three meiotic and three mitotic gynogenetic progenies (defined based on the time of shock after insemination, 1.5 and 13 min, respectively; Streisinger et al. 1981) were obtained by applying physical shocks to oocytes after insemination with UV-irradiated common carp sperm. Using a novel larval rearing method, growth of the gynogens was comparable to controls (Fig. 1). All gynogens which reached reproductive maturity (n=46) were male. The fertility of gynogen males was first assessed by performing natural spawns; 60% of meiotic and 40% of mitotic gynogen males successfully fertilized oocytes compared to 100% of biparental males. In vitro fertilization (sperm collected by dissection and maceration of testes) was attempted with two meiotic and five mitotic gynogens which failed to fertilize oocytes by natural spawns. Sperm from both meiotic gynogens and three mitotic gynogens successfully fertilized oocytes in vitro. Fertilization rate was significantly lower among these fertile (in vitro) mitotic gynogens (50%±33) than in biparental controls (n=3, 75%±17) (paired t-test, p>0.05).

Sex ratio in progenies produced by meiotic gynogens and their biparental siblings was investigated. The distributions of progeny sex ratio were found to be markedly different with meiotic gynogens giving a multimodal distribution of both highly biased and balanced progenies, while biparental males gave a unimodal distribution centered around a balanced ratio. The results of these studies confirm that the sex determination system is polygenic but suggest that an additive model of inheritance is not accurate.