| Polyploid species are common in plants,but rare in animals,where polyploids are found primarily in insects,amphibians,reptiles and teleost fishes.Some bisexual fertile tetraploid lineages can be established by distant hybridization,which is of great significance in fish genetic breeding and biological evolution.In the subfamily distant hybridization of common carp（Cyprinus carpio L.,2n=100）（♀）×blunt snout bream（Megalobrama amblycephala,2n=48）（♂）,four different ploidies fertile offspring were first formed in F1.We further hybridized the offspring of two different ploidies in F1,that is,the female allotetraploid hybrids（4n=148,4nCB）was used as female parent,and the male crucian carp-like homodiploid fish（2n=100,2nNCRC）was used as the male parent for hybridization.An autotetraploid fish with chromosome number of 200（4n=200,4nNC）were obtained from this hybridization combination.At present,we have established an autotetraploid carp lineage（4n=200,4nNC-F1-F4）with excellent traits.This lineage can be used as the core parent to develop new triploid fish with the advantages of sterility,faster growth rate and good meat quality,and providing a new research model for the study of polyploid evolution.In this paper,we systematically studied the biological traits of the autotetraploid carp lineage,such as morphological traits,chromosome numbers,DNA contents,erythrocytes traits,gonadal structures,and genetic variation in5S rDNA and Hox genes.The results were as follows:（1）In terms of morphological traits,most of morphological traits of 4nNC-F1-F4 were significantly different from those of the original parents 2n COC and 2n BSB.However,there were no significantly difference from those of their parents 4nCB and 2nNCRC in most of morphological traits（except for the trait of having or no barbels）.Compared with 2nCOC and 2nBSB,most of countable traits of4nNC-F1-F4（such as lateral scale number and fin number）showed significantly differences,but no significantly differences with 4nCB and2nNCRC.Most of measurable traits of 4nNC-F1-F4（such as head width to body width,body width to body length,etc.）were significantly different from those of 2nCOC and 2nBSB,but not from those of 4n CB and 2n NCRC.（2）In terms of DNA contents and chromosome numbers,the results revealed that there was no significantly difference between the mean DNA content of 2nNCRC and 2n COC,no significantly difference between the mean DNA content of 4nCB and the sum DNA contents of that of 2nCOC and 2nBSB,and no significantly difference between the mean DNA content of 4nNC-F1-F4 and the double DNA contents of that of 2nCOC.By counting the number of chromosomes in metaphase phases,the results showed that there were 200 chromosomes in 4nNC-F1-F4,all derived from the original parent 2nCOC.（3）In terms of the appearance and nuclear volume of erythrocytes,the mean erythrocyte nuclear volume of 4n NC-F1 was twice than that of2nCOC,and a rare dumbbell-shaped erythrocyte with two nuclei was found in 4nNC-F1,which showed that 4n NC-F1 had tetraploid traits.（4）In terms of gonadal development,we observed gonadal structures in the male and female individuals of 10-month-old 4nNC-F3.The results showed that the ovary of 10-month-old 4nNC-F3 had developed well and contains oocytes in stages II,III,and IV,and the testis of 10-month-old 4nNC-F3 contained many mature spermatozoa.These results proved that the autotetraploid carp lineage had normal gonadal structures and bisexual fertility.（5）Through the comparative analysis of 5S rDNA and Hox gene sequences,the results showed that 4nNC-F1 had more genetic variation,including recombination type,maternal specific type and paternal specific type.These results provided a clear source for hybridization of 4nNC-F1,clearly reflecting instability in the newly established autotetraploid genomes.Furthermore,genetic variation analysis of the DNA sequences of these genes over successive generations（F1-F3）revealed that the genomic structure of 4nNC was undergoing rapid diploidization to maintain tetraploid stability. |
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