Variations in 5S rDNAs in diploid and tetraploid offspring of red crucian carp x common carp

Background: The allotetraploid hybrid fish (4nAT) that was created in a previous study through an intergeneric cross between red crucian carp (Carassius auratus red var., female) and common carp (Cyprinus carpio L., male) provided an excellent platform to investigate the effect of hybridization and polyploidization on the evolution of 5S rDNA. The 5S rDNAs of paternal common carp were made up of a coding sequence (CDS) and a non-transcribed spacer (NTS) unit, and while the 5S rDNAs of maternal red crucian carp contained a CDS and a NTS unit, they also contained a variable number of interposed regions (IPRs). The CDSs of the 5S rDNAs in both parental fishes were conserved, while their NTS units seemed to have been subjected to rapid evolution. Results: The diploid hybrid 2nF(1) inherited all the types of 5S rDNAs in both progenitors and there were no signs of homeologous recombination in the 5S rDNAs of 2nF(1) by sequencing of PCR products. We obtained two segments of 5S rDNA with a total length of 16,457 bp from allotetraploid offspring 4nAT through bacterial artificial chromosome (BAC) sequencing. Using this sequence together with the 5S rDNA sequences amplified from the genomic DNA of 4nAT, we deduced that the 5S rDNAs of 4nAT might be inherited from the maternal progenitor red crucian carp. Additionally, the IPRs in the 5S rDNAs of 4nAT contained A-repeats and TA-repeats, which was not the case for the IPRs in the 5S rDNAs of 2nF(1). We also detected two signals of a 200-bp fragment of 5S rDNA in the chromosomes of parental progenitors and hybrid progenies by fluorescence in situ hybridization (FISH). Conclusions: We deduced that during the evolution of 5S rDNAs in different ploidy hybrid fishes, interlocus gene conversion events and tandem repeat insertion events might occurred in the process of polyploidization. This study provided new insights into the relationship among the evolution of 5S rDNAs, hybridization and polyploidization, which were significant in clarifying the genome evolution of polyploid fish.