Studies On Morphological Characteristics And Population Genetics In Xenophysogobio Boulengeri And X. Nudicorpa

The upper reaches of the Yangtze River is a hotspot of biodiversity research,which breeds a variety of endemic fish adapted to the local ecological environment.The upper reaches of the Yangtze River is also an area with abundant hydropower resources and it is the core region for hydropower development in China.Now,several large-scale hydropower projects have been built and some more are planned to be built.However,the construction of water conservancy projects has a serious impact on fish,especially endemic fish.Xenophysogobio boulengeri and X.nudicorpa,which belongs to family Cyprinidae,order Cypriniformes,are the only two species of Xenophysogobio and endemic fish in the upper reaches of the Yangtze River.The two species both exhibit many similar features,both of which are small benthic fish,preferring flowing water environment,producing drifting eggs and sharing basically overlapping distribution areas,etc.But they also have some obvious difference,such as resources abundance,the presence or absence of scales,etc.They provide a useful model system for studying biological adaptability.The construction of hydropower stations such as the Three Gorges Dam,Xiangjiaba and Xiluodu had resulted in the compression and fragmentation of the habitats of X.boulengeri and X.nudicorpa,and a significant decreased in resources abundance.In order to clarify the difference of environmental adaptability between the two species and the potential impact of hydraulic engineering construction on them,the history and present situation of distribution pattern,morphological characteristics and population genetics of the two species were systematically studied by using methods of historical data collection,field sampling and molecular markers,so as to provide scientific basis for management and protection.The main results of this study are as follows:1.The investigation of catchs and fish resources of early life showed that the distribution of X.boulengeri and X.nudicorpa overlapped in some area,but differed in the main distribution areas.X.boulengeri was more abundance than X.nudicorpa in the upper reaches of the Yangtze River from Jiangjin to Yibin section,but it was contrary in the lower reaches of Jinsha River,Panzhihua to Qiaojia section.The average temperature of the section from Panzhihua to Qiaojia of the Jinsha River was significantly lower than that of the upper reaches of the Yangtze River,which showed that the X.nudicorpa was more suitable for low temperature environment than X.boulengeri.Based on the Maxent ecological model,the suitable habitats for X.boulengeri and X.nudicorpa in the Last Glacial Maximum(22,000 years ago),Mid Holocene(6,000 years ago),current period(1950-2000 average)and future period(2050)were simulated.The results showed that the upper reaches of the Yangtze River,where the two species overlapped,were shelters.2.Twenty-five morphological characteristics were selected to analyze the morphological divergence of X.boulengeri and X.nudicorpa.The results were as follows:(1)A pair of barbels and three pairs of chin bristles were found in the X.nudicorpa based on 62 specimens.The length of the second pair of chin bristles was different in different specimens.In most specimens,the second pair of chin bristles was approximately equal to the length of the first pair's.Historic records showed that there were two pairs of chin bristles in the X.nudicorpa.The second pair of whiskers degenerated,and only could be seen in a few specimens.The results of this study were inconsistent with the literature.(2)PCA analysis showed that there was obvious morphological divergence between the two species of X.boulengeri and X.nudicorpa,but some individuals overlapped.There was no obvious morphological divergence among the geographic populations of X.boulengeri,but existed between the Jinsha River and Yangtze River populations in X.nudicorpa.(3)Independent sample t-test results showed that there were significant differences in morphological indices between the two species,including eye diameter,head length behind eyes,pectoral fin length,pelvic fin length,hip fin height,dorsal fin height,caudal stalk length,head length,pectoral fin anterior distance,pectoral ventral distance,dorsal snout distance,dorsal tail distance,barbels length,the first chin bristles length,the second chin bristles length and the third chin bristles length.The morphological differences between the Jinsha River and Yangtze River populations were mainly manifested in pelvic fin length,dorsal fin base length,tail handle length,head length,pectoral fin anterior distance,pelvic fin anterior distance,eye distance and barbels length.3.The genetic diversity,genetic structure and population dynamics of the two species were compared and analyzed based on mitochondrial DNA(Cyt b and D-loop)and SSR markers.The results were as follows:(1)The level of genetic diversity was high among the Yangtze River populations in X.boulengeri and X.nudicorpa,but was lower in the Jinsha River populations of X.nudicorpa.(2)For X.boulengeri,there was no significant genetic divergence among the geographic populations,but the analysis based on Cyt b sequence showed that there was sympatric divergence.In contrast,there was significant geographic genetic divergence among populations of X.nudicorpa.The correlation analysis between geographic distance and genetic distance showed that the population divergence of X.nudicorpa belonged to the Isolation by Distance(IBD)model.(3)Population dynamics analysis showed that there were population expansion events in the Yangtze River populations of X.boulengeri and X.nudicorpa during the late Pleistocene,but not in the the Jinsha River population of X.nudicorpa.4.Based on RAD-seq technique,126,799 SNP loci were obtained to analysis population structure and population history of X.boulengeri and X.nudicorpa.The results were as follows:(1)The level of genetic diversity in the populations of X.boulengeri was slightly higher than that of in X.nudicorpa,and there was significant divergence between the two species.For X.nudicorpa,there was a significant geographic divergence between the Jinsha River and Yangtze River populations.These results were consistent with those based on mitochondrial DNA and SSR dataset.(2)Structure analysis showed that there was no gene introgression between X.boulengeri and X.nudicorpa.(3)Population dynamics analysis based on PSMC showed that the population expansion events began at 0.012 Ma in the both two species,but the expansion rate of X.nudicorpa was lower than that of X.boulengeri,which may be one of the reasons for the smaller population size of X.nudicorpa at present.(4)GO annotation results showed that these selected sites between the two species were mainly associated to membrane,signal transduction,energy metabolism,response to heat,and response to cold,which play an important role in response to temperature changes.It indicated that there were differences of the two species in the response to temperature changes.The selective sites between the Jinsha River and Yangtze River populations were mainly associated to membrane,integral component of membrane,and developmental process,which indicated that there were differences in response to temperature change between the two populations.In conclusion,although the external morphology and habits were same or similar between X.boulengeri and X.nudicorpa,their main distribution areas,measurable and countable traits,and genetic showed significant differences,reflecting their differences in local adaptability.