The full-length mRNA of the high mobility group protein 1 coding gene(HMG1) was obtained by RACE-PCR from red crucian carp (Carassius auratus red var.),blunt snout bream(Megalobrama amblycephala),and their triploid and tetraploid progeny.The sequence contained an open reading frame of 579 nucleotides coding for 193 amino acids.The nucleotide identity of HMG1 was higher between the tetraploid hybrid and the maternal red crucian carp(99%) than between the tetraploid hybrid and the paternal blunt snout bream (97%).The nucleotide identity between the triploid hybrids and each parent(95%) was lower than that between the parents(98%).The protein identity between the tetraploid hybrid and each parent(100%) was higher than that between the triploid hybrid and each parent (97%).Our results suggest that interspecific hybridization generates a shock to the HMG1 gene in triploid hybrids,causing divergence of nucleotides.The HMG1 protein of the tetraploid hybrids was consistent with that of its parents,which reduced the barrier of cross incompatibility between alleles,providing the basis for the bisexual fertile tetraploid hybrids forming a new polyploid species in nature. The secondary and tertiary structures of the HMG1 protein contain eight helices,three switches,two DNA-binding domains in the N-terminus, and a long acidic tail in the C-terminus.Together,these data suggest that the HMG1 protein plays a role of protein-DNA interactions,facilitating various DNA-dependent activities in the nucleus.We also investigated the phylogeny of fish,amphibian,reptilian, bird,and mammalian HMG1 proteins.Our results suggest that HMG1 is an ancestral protein that has been highly conserved. These data provide clues as to how interspecific hybridization may form polyploid hybrids. The full-length mRNA of the high mobility group protein 1 coding gene (HMG1) was obtained by RACE-PCR from red crucian carp (Carassius auratus red var.), Blunt snout bream (Megalobrama amblycephala), and their triploid and tetraploid progeny. The sequence contained an open reading frame of 579 nucleotides coding for 193 amino acids. The nucleotide identity of HMG1 was higher between the tetraploid hybrid and the maternal red crucian carp (99%) than between the tetraploid hybrid and the paternal blunt snout bream (97 The nucleotide identity between the tetloidloid and each parent (95%) was lower than that between the parents (98%). The protein identity between the tetraploid hybrid and each parent (100%) was higher than that between the triploid hybrid and each parent (97%). Our results suggest that interspecific hybridization generates a shock to the HMG1 gene in triploid hybrids, causing causing divergence of nucleotides. The HMG1 protein of the tetraploid hybrids was consistent with that of its par ents, which reduced the barrier of cross incompatibility between alleles, providing the basis for the bisexual fertile tetraploid hybrids forming a new polyploid species in nature. The secondary and tertiary structures of the HMG1 protein contain eight helices, three switches, two DNA-binding domains in the N-terminus, and a long acidic tail in the C-terminus. these data suggest that the HMG1 protein plays a role of protein-DNA interactions, facilitating various DNA-dependent activities in the nucleus. We also investigated the phylogeny of fish, amphibian, reptilian, bird, and mammalian HMG1 proteins. These results suggest that HMG1 is an ancestral protein that has been highly conserved. These data provide clues as to how interspecific hybridization may form polyploid hybrids.