Abstract:In the catalog,the red crucian carp(Carassius auratus red var.) with 100 chromsomes belongs to the Cyprinidae,Cyprinidae subfamily,Carassius, with karyotype of 22m+34sm+22st+22t;and the common carp(Cyprinus carpio L.) with 100 chromosomes belongs to the Cyprinidae,Cyprinidae subfamily,Cyprinus,with karyotype of 22m+34sm+22st+22t.The previous studies had indicated that the first(F1)and second generation(F2)of hybrid fish of red crucian carp(♀)×common carp(♂)were diploid,and F2 hybrids could produce unreduced eggs and sperms,which mated each other to form fertile allotetraploid hybrid fish(F3).In order to explore the pathway of polyploidy occurrence in distant crossing of red crucian caip and common carp and the potential of producing unreduced gametes of F2 hybrids,we studied the chromosome in embryonic cell of F1 and F3 hybrid fish.The result showed that the F1 embryo was diploid,no haploid and polyploidy embryos were observed.While F3 embryos showed 100,150,200 and even 300 chromosomes,and it was inferred that the chromosomes number in germ cells of F2 had doubled one or more times,and the F2 produced diploid and polyploidy gametes,which mated each other to form F3 embryos with different ploidy.Taken together,it was concluded that F1 hybrids which come from crossing parents with similar genome size and karyotype,have not displayed polyploidization,while the diploid hybrid progenies could produce unreduced gametes,they fertilized to form polyploidy fish in F3.The pathway provided important guidance for the study of breeding polyploidy fish by distant crossing.