Introgressive Hybridization Between Siniperca Chuatsi And S.Kneri From Some Lakes In The Middle Reaches Of The Yangtze River

Both Siniperca chuatsi and S.kneri belong to Sinipercidae(Siniperca),which are unique freshwater fishes in East Asia with important economic and ecological values.S.chuatsi is mainly distributed in the north area from the Minjiang River,and S.kneri is distributed in the water system south of the Huaihe River,their distribution overlap in the Yangtze River system.S.chuatsi is highly similar to the S.kneri in morphology,and they are closely related sister species.With the expansion and deepening of wild sampling in the middle reaches of the Yangtze River,some samples were found possessing part of the morphological characteristics of both S.chuatsi and S.kneri(the posterior edge of the oral fissure extending below the posterior edge of the eye,while the eye size,the back hump is between S.chuatsi and S.kneri),which were temporarily called the "intermediate form".In this paper,the "intermediate form" collected from Poyang Lake and Dongting Lake in the middle reaches of the Yangtze River,weretaken as the research objects,and S.chuatsi from Heilongjiang River and S.kneri from Zhujiang River were used as references.The classification and attribution of "intermediate form" individuals and the hybridization type of hybrid progeny were analyzed at morphological and molecular genetics aspects,which provided the basic data for intra-species variation or inter-species hybridization in mandarinfish in the middle lake of the Yangtze River.The main results were as follows:1.Comparison of morphological differences between "intermediate form" and S.chuatsi,S.kneriThe countable characteristics(number of fins and gill rakes)and morphological framework characteristics of "intermediate form",S.chuatsi and S.kneri were compared and determined,and analysis of variance,principal component analysis,discriminant analysis,and STRUCTURE cluster analysis were performed.The results showed that:(1)the intermediate form was between S.chuatsi and S.kneri,in terms of eye size,relative position of the posterior edge of the maxilla to the posterior edge of the eye,and the bulge degree of the front back.(2)There were no significant differences in the countable traits(number of fin ray,number of gill raker)between the intermediate form and S.chuatsi and S.kneri.(3)Analysis of variance showed that in quantitative traits,the difference between the intermediate form with S.chuatsi and S.kneri mainly lied at the relative position of the maxillary bone to the eye,the size of the eye,the protruding degree of the head and back,and the length of the tail handle;principal component analysis showed some frame traits of the back of the head and the anterior part of the back of the head and body height were the main components of morphometric difference between the intermediate form and S.chuatsi and S.kneri.The discriminant accuracy of S.chuatsi,S.kneri and intermediate form was90%,81.8% and 85.4%,respectively,the average discriminatory accuracy was 85.6%.STRUCTURE cluster analysis showed that the morphometric characteristics of the intermediate form were between that of S.chuatsi and S.kneri,and inclined towards S.kneri.The quantitative analysis of morphology failed to resolve the classification of "intermediate form" individuals.2.Introgressive hybridization of lake S.chuatsi and S.kneri in the middle reaches of the Yangtze River based on microsatellite markersIn order to clarify their taxonomic relationship,the species-specific microsatellite loci were screened for identification.Results showed: Five speices-specific loci(T103,T063,T089,T135,W19517)were screened from 28 pairs of microsatellite markers.In genetic analysis and identification of intermediate types individuals using these five loci,16 individuals were inter-species hybrid progeny,and 9 of them were identified as backcross progeny.The results indicated that inter-species introgressive hybridization between S.chuatsi and S.kneri existed in themiddle Yangtze River.3.Single nucleotide polymorphisms based on target gene sequencing for assessing introgressive hybridization between S.chuatsi and S.kneri in the Middle Reaches of the Yangtze RiverBased on the enrichment and sequencing of target genes,463 gene sequences(Fst=1)were selected for the specific differences between the S.chuatsi and the S.kneri.A large sample of these differential genes was verified,and a total of 349 differential gene sequences were identified to discriminate S.chuatsi and S.kneri,including 458 diagnostic SNPs.224 diagnostic SNPs(only one SNP locus)were selected to identify 48 individuals of " intermediate form" from the Poyang Lake and Dongting Lake in the middle reaches of the Yangtze River,of which 8 individuals were hybrid progeny,8 were S.chuatsi and 32 were S.kneri.Newhybrids analysis showed that the hybrid offspring contained first-generation hybrid(2 individuals),first-generation backcross(1 individual),secondgeneration hybrid(1 individual)and fourth-generation backcross(4individuals),and some had backcrossed with S.chuatsi and S.kneri.In this study,458 diagnostic SNPs between the S.chuatsi and S.kneri was screened,a complex introgressive hybridization phenomenon between S.chuatsi and the S.kneri in the middle reaches of the Yangtze River was found.Therefore,the measures for genetic protection of mandarin fish resource in the Yangtze River should be enforced,strengtheningsupervision of cultured species to reduce the occurrence of escape,and monitoring the trend of introgressive hybridization in the future.