Application of the scale cover gene (N) to identification of type of gynogenesis and determination of ploidy in common carp

Eggs from nude common carp females heterozygous for the scale cover gene N (ssNn) were used for induction of mitotic gynogenesis and triploidy by heat shock. The type of scale cover for reared fingerling progeny was identified. Among gynogenetic progeny, 99.8% of fish were scattered (ssnn) and 0.02% were nude (ssNn). As the N gene has a very high frequency of recombination relative to the centromere, the occurrence of homozygous nn scattered fishes (NN homozygotes are inviable) proved the mitotic character of induced gynogenesis. In the triploidy experiment, eggs from nude females were inseminated with sperm collected from scattered males. In progeny produced with heat shock, the ratio of nude and scattered fishes was 13:1 instead of 1:1 usually observed at crossing between nude and scattered carp. Such a shift in the ratio in experimental progeny was explained as being due to the high recombination frequency for gene N artificially suppressing the second meiotic division, resulting in triploids with genotype Nnn. All scattered fingerlings karyologically investigated were diploids. It was found that nude triploid Nnn and nude diploid Nn fish differed in the rate of reduction of scale cover and soft rays in the anal fin. This provides an opportunity to identify triploids simply by means of external examination.