| Tilapia,originating from Africa,was introduced and widely cultured in China in the 1980 s.As tilapias’ high adaptability,once they escape from aquafarms into the natural water bodies,the local species diversity and ecosystem diversity will be damaged.Tilapia was added to China’s list of alien invasive species in 2014.Guangxi Province is one of the places in China where tilapias are raised in large quantities.Tilapias have been found in several rivers in south Guangxi,and the population has been stable and established.This paper aims to investigate the hybridization of these tilapia populations.Tilapia samples were investigated in six rivers(Red Water River,Youjiang River,Zuojiang River,Qianjiang River,Yujiang River,Xunjiang River)of south Guangxi,and then extracting the DNA of these samples,doing PCR amplifications on the coⅠ gene in mtDNA and enc1,ptr,sh3px3 genes in the cell nucleus,making homology alignment after sequencing,and setting up phylogenetic trees and performing genetic distance analysis,picking out heterozygous individuals and researching its distribution regularity of heterozygous individuals at last.The main results of this study are as follows:(1)Gene homology alignment results of coⅠ indicated that,except for the two samples(Heshan No.87 and Laibin No.11)with heterozygous female parents from Red Water River,the female parents of the other 495 tilapia samples from the six rivers were made up of four species: Oreochromis niloticus,Coptodon zillii,Oreochromis mossambicus and Oreochromis aureus.Among them,Coptodon zillii belongs to the genus Coptodon,while Oreochromis niloticus,Oreochromis mossambicus and Oreochromis aureus belong to the genus Oreochromis.Oreochromis aureus is most widely distributed in the six rivers of south Guangxi,while Oreochromis niloticus is least widely distributed in the six rivers of south Guangxi.(2)Phylogenetic trees and comparative analysis results of three nuclear genes indicated that,among 496 tilapia samples,91 were heterozygous samples,with a heterozygous rate of 18.3%.There are many hybridization cases of tilapia populations in Red Water River,Qianjiang River,Zuojiang River and Yujiang River,but just a few hybridization cases are found in Youjiang river.However,no hybridization was found in Xunjiang River.Most of the areas with high hybrid rate are midstream and upstream,while most of the areas with low hybrid rate are downstream,Xunjiang river is located at the downstream of the other 5 rivers,so the Xunjiang River has a hybrid rate of 0%.In general,the higher the proportion of Coptodon zillii,the lower the hybrid rate is in the same river.In addition,the occurrence of hybridization is also related to complex factors such as habitats,tilapia population distribution and hydrological conditions.The Oreochromis aureus occupies an important position in the process of tilapia hybridization,they are often cross-bred with Oreochromis niloticus,Oreochromis mossambicus also play a role in crossing with other tilapias.sh3px3 gene is highly sensitive in the heterozygous identification of tilapia,which is very suitable for the identification of tilapia species.More than 50% of the heterozygous samples were identified by this gene.(3)There are samples from Hengxian,Guigang in Yujiang River and Chongzuo in Zuojiang River,with large genetic distances from homozygous Oreochromis niloticus and Coptodon zillii,and numerical values of these genetic distances are all above 0.2000.The number of these special heterozygous samples was very large(36),accounting for 40.0% of the total of heterozygous samples.Tilapias in these sites have interbred with other fishes of different generas or even different families.It was found in the middle reach of Zuojiang River and whole areas of Yujiang River.(4)Except for Xunjiang River,hybridizations of wild tilapias are common and frequent in the other five rivers in south Guangxi,the hybridization of Youjiang river is in the preliminary stage,while the hybridization of Red Water River,Zuojiang River,Qianjiang River and Yujiang River is quite serious.Management department ought to strengthen the supervision of wild tilapias,and take corresponding measures to reduce the distribution and spread of tilapia populations,and to establish a perfect local fish protection mechanisms,in order to protect the fish resource diversity and integrity of rivers in south Guangxi. |
