Genetic Characterization Of Hybridization Between Olive Flounder And Summer Flounder

Based on the hybridization and backcross experiments between olive flounder (Paralichthys olivaceus) and summer flounder (P. dentatus), chromosome counting, AFLP, mtDNA and nuclear gene partial sequences were employed to characterize genomic inheritance in these hybrid and backcross offspring. The genetic basis of asymmetric viabilibility in reciprocal hybrids and post-zygotic isolation between olive flounder and summer flounder were studied. The results are as following,1. Interspecific reciprocal crosses between Paralichthys olivaceus and P. dentatus yielded hybrids with different viabilities. Specifically, the hybrids of P. olivaceus female and P. dentatus male (H1) were found to be viable, while the reciprocal hybrids from P. dentatus female and P. olivaceus male (H2) were completely inviable. All the H2 individuals showed morphological deformities and died before first feeding. The chromosome analysis showed that H1 individuals had the same chromosome number as parents'. However, two chromosomes were missing in H2 offspring indicating that the latter were aneuploids. Genomic inheritance from the parents to F1 progeny was further examined by amplified fragment length polymorphism (AFLP) analyses, and the results indicated differences between reciprocal hybrids. Almost all AFLP bands (97.71%) present in parents were passed on to H1 individuals. In contrast, only 86.64% of the AFLP bands from parents were scored in H2 individuals. And frequency of lost parental bands was significantly higher in H2 than that in H1 and intraspecific crosses, which was probably associated with chromosomal elimination. In addition, higher segregation distortions were found in hybrids than in controls, although these differences were not significant.2. The H1 hybrids were bred to mature. We performed hybrid and maternal backcross experiments using olive flounder, summer flounder and their female H1 hybrids. Fertilization rates, hatching rates and combined fitness measure (CFM, product of fertilization rate and hatching rate) were calculated to examine the fitness between the two species. The CFMs of hybrid and backcross generations reduced significantly compared to pure olive flounder crosses. We also detected genomic changes which were possibly associated with the reduced fitness in backcross by amplified fragment length polymorphism (AFLP). The changes mainly involved loss of parental AFLP bands (backcross1, F1 hybrid female×olive flounder male, 3.96%; backcross2, F1 hybrid female×summer flounder male, 6.03%) and gaining of novel bands (backcross1, 5.63%; backcross2, 3.28%). Meanwhile, segregation distortions were observed in backcross generations. There were 27.40% and 31.18% of AFLP markers deviating from expected Mendelian ratio in two families, respectively.3. The transmission of mtDNA 16S rDNA, COⅠand nuclear rag1 partial sequences from parents to hybrid and backcross offspring were also analyzed. The hybrids possess identical sequences of 16S rDNA, COⅠto their maternal parents. The backcross progeny had high homology with their maternal parents (98%). These finding indicated that mitochondrial 16S rDNA and COⅠpartial sequences in artifical hybrids and backcrosses were inherited maternally. Nevertheless, mutiple haplotypes of 16S rDNA and COⅠgenes were found in backcross. The partial sequences of rag1 gene were identical in olive flounder and summer flounder, while multiple haplotypes of rag1 were found in H1, which was probably involved in recombination or genetic variation in hybrids.4. A phylogenetic tree was reconstructed based on the partial sequences of 16S rDNA and COⅠgenes in eight commercial flatfishes (Pleuronectiformes). The parameters of pairwise genetic distance among these flatfishes were calculated. Based on the crossing data among these flatfishes, the relationship between pairwise genetic distance and hybrid fitness was evaluated. The findings indicated that positive relationship might exist between genetic distance and hybrid fitness. The pairwise genetic divergence between species decreased as the interspecific hybrid fitness largened. These results implied that speciation clock might exsit in Pleuronectiformes.