Cytogenetic Analysis of Developmental Difference in Hybrid embryos Between Reciprocal Crosses in Distant Hybridization of Fishes

six reciprocal crosses were performed with common carp (Cyprinus carpio)♀×silver carp (Hypophthalmichthys molitrix)♂,crucian carp (Carassius auratus)♀×silver carp ♂ and white crucian carp (Carassius auratus cuvieri)♀×silver carp ♂ as well as silver carp ♀×common carp ♂,silver carp ♀×crucian carp ♂ and silver carp ♀×white crucian carp ♂.The direct hybrid embryos of common carp ♀×silver carp ♂,crucian carp ♀×silver carp ♂ and white crucian carp ♀×silver carp ♂ seemed to develop normally,and about 50% of the hybrided eggs produced larvae,although the viability of most of the hatched larvae was inferior.The development of the reciprocal hybrid embryos of silver carp ♀×common carp ♂,silver carp ♀×crucian carp ♂ and silver carp ♀×white crucian carp ♂ was abnormal,and the embryos successively died after gastrular stage.No normal hybrid embryos survived to hatching.Developmental cytogenetic analysis of the hybrid embryos between the reciprocal crosses showed that the direct embryos of common carp ♀×silver carp ♂,crucian carp ♀×silver carp ♂ and white crucian carp ♀×silver carp ♂ were almost euploids,containing 74 chromosomes of 50 originated from maternal species and 24 from paternal species,whereas the reciprocal hybrid embryos of silver carp ♀×common carp ♂,silver carp ♀×crucian carp ♂ and silver carp ♀×white crucian carp ♂ were basically aneuploids.The chromosome number observed from the reciprocal gastrular embryos varied considerally,usually ranging from 24 to 74,and a few of cells in which the chromosome number was less than 24 or more than 74 were also observed.The chromosome analysis of individual tail bud embryos revealed that most of the reciprocal hybrid embryos were hypodiploids,within which the loss of chromosomes had heavily occurred.The significant difference in embryonic development between the reciprocal crosses suggesed that the developmental fate of hybrid embryos might be related to the genome size between parental species.When the genome of maternal species was larger than that of paternal species,female nucleus and egg cytoplasm could not only coordinate male nucleus to engage in synchronus cell division and to preserve euploidy,but also bring about the potentiality of polyploid embryogenesis more easily.When the genome of maternal species was smaller than that of paternal species,female nucleus and egg cytoplasm had no the ability of harmonizing male nucleus to carry out synchronus development,and the aneuploid embryos were formed by chromosome elimination owing to asynchronus division and imbalancing segregation.The relationship between the difference of reciprocal crosses in distant hybridization of fishes and the natural polyploid species as well as the rate of embryonic development were analysed and discussed.Considerating the difference of reciprocal crosses and the potentiality of polyploidy,we suppose that the natural polyploid species,such as common carp,crucian carp and white crucdian carp,might have the potential capability of coordinating foreign allogenome,which is different from ordinary diploid species.