Study On The Biological Characteristics Of Two New Types Of Triploid Hybrid Fish

Distant hybridization is the crossbreeding of different species,genera or even more distantly related species,which fuses the genomes of different species and causes changes in genotypes and phenotypes of their offspring.It is widely used in the field of genetics and breeding.The triploid hybrid fish bred using distant hybrid breeding technology not only grow fast and have strong resistance to stress,but their sterile characteristics can ensure that they will not pollute wild fish germplasm resources even if they accidentally flow into the wild during the breeding process.At present,the homodiploid crucian carp-like fish lineage(2n NCRC,2n=100),improved diploid mirror carp lineage(2n IDMC,2n=100),autotetraploid carp lineage(4n NC,4n=200)have been established in the distant hybridization combination of common carp(Cyprinus carpio,2n COC,2n=100)(♀)and blunt snout bream(Megalobrama amblycephala,2n BSB,2n=48)(♂).In this study,two new types of triploid hybrid fish(3n NCRC and 3n MCC)were produced by distant crossing with the homodiploid crucian carp-like fish and the improved diploid mirror carp as the female parent,respectively,and the autotetraploid carp as the male parent.This study conducted a systematic study on their morphological characteristics,DNA content,chromosome number,red blood cell morphology,reproductive characteristics,and genetic variation of 5S r DNA and Hox genes.The results of the study are as follows:1.The morphological characteristics of 3n NCRC and 3n MCC and their parents 2n NCRC,2n IDMC,and 4n NC were studied,and the results showed that 3n NCRC and 3n MCC only showed hybridization characteristics in a few countable traits compared with their parents.For example,the number of lateral line scales of 3n NCRC was biased toward its male parent 4n NC,while the number of anal fin rays was biased toward its female parent 2n NCRC.However,there were more measurable traits that showed obvious hybridization characteristics.For example,the head length/body length of 3n NCRC was biased toward its male parent4 n NC,while the body width/body length was biased toward its female parent 2n NCRC.The body length/full length,head length/body length,and head width/head length of 3n MCC were biased toward its female parent 2n IDMC,while the tail stalk width/tail stalk length was biased toward its male parent 4n NC.In addition,there was no significant difference between 3n NCRC and 3n MCC in either countable traits or measurable traits.2.The ploidy level and chromosome number of 3n NCRC and3 n MCC and their parents 2n NCRC,2n IDMC,and 4n NC were identified.The results showed that the average DNA content of 3n NCRC and3 n MCC was equal to half of the sum of the DNA content of diploid maternal parent and tetraploid paternal parent.The chromosome count results showed that the number of chromosomes of 3n NCRC and 3n MCC was 150,thus confirming that they were both triploid hybrid fish.3.The blood cell morphology of 3n NCRC and 3n MCC and their parents 2n NCRC,2n IDMC,and 4n NC were compared,and the results showed that 3n NCRC,3n MCC and 4n NC have abnormal red blood cells with dumbbell-shaped nuclei,and the proportion of abnormal red blood cells in 4n NC was greater than that of the two types of triploid hybrid fish.4.The reproductive characteristics of 3n NCRC and 3n MCC were studied,and the results showed that the oocytes proliferated in large numbers in the ovary of 3n NCRC and 3n MCC,but only a few developed into primary oocytes phase II;and spermatogonia and abnormally developed sperm cells in the testis,with no mature sperm.It is speculated that their gonadal development is delayed and it is suspected that they are infertile.5.The genetic variation of the 5S r DNA genes of 3n NCRC and3 n MCC and their parents 2n NCRC,2n IDMC,and 4n NC were analyzed,and the results showed that: compared with their parents,3n NCRC had11 monomer types,nine recombination types and five new variation types,while 3n MCC had only two monomer types and two recombination types,indicating that it might be due to the complex genetic background of2 n NCRC has resulted in the complex composition and poor stability of the genome of 3n NCRC.6.The genetic variation of six Hox genes of 3n NCRC and 3n MCC and their parents 2n NCRC,2n IDMC,and 4n NC were analyzed,and the results showed that: compared with their parents,the Hox genes of both3 n NCRC and 3n MCC had great genetic variation,and they not only inherited part of the Hox genotypes(including some recombinant genotypes)of their parents,but also appeared some variant genotypes(including more abundant recombinant genotypes),while they also lost some genotypes at the same time.In addition,compared with 3n MCC,3n NCRC showed greater variation in the sequence structure of Hox genes.The results indicated that the hybrid origin of 3n NCRC and 3n MCC was clear.However,as a newly established triploid hybrid,the genome was extremely unstable,and 3n NCRC had a more complex genome composition.In this study,two new types of triploid hybrid fish were prepared based on the existing hybrid fish lineage using the technology of distant hybridization.It was found that both of them exhibited a certain degree of heredity and variability in both morphological characteristics and molecular level.The research results have important reference value for the selection and breeding of excellent new species of triploid hybrid fish.