Evidence of Different Ploidy Eggs Produced by Diploid F2 Hybrids of Carassius auratus (♀) × Cyprinus carpio (♂)

Based on the presence of three types of eggs with different diameters 0.13, 0.17 and 0.2 cm, we made two crosses: F₂ (♀) × diploid red crucian carp (♂), and F₂ (♀) × F₁₀ tetraploid (♂). The ploidy levels of the progeny of the two crosses were examined by chromosome counting and DNA content measurement by flow cytometer. In the offspring of the former cross, tetraploids, trip-loids, and diploid were obtained. In the progeny of the latter cross, tetraploids and triploids were observed. The production of the different ploidy level fish in the progeny of the two crosses provided a further evidence that F₂ might generate triploid, diploid and haploid eggs. The presence of the male tetraploid found in F₂ (♀) × diploid red crucian carp (♂) suggested that the genotype of XXXY probably existed in the tetraploid progeny. The gonadal structures of the tetraploids and triploids indicated that both female and male tetraploids were fertile and the triploids were sterile. We concluded that the formations of different ploidy level eggs from F₂ were contributed by endoreduplication and fusion of germ cells.