Study Of Bewick’s Swan Cygnus Columbianus Bewickii Migration Connectivity Based On Molecular Markers And Satellite Tracking

Migration and dispersion play an important role in shaping the genetic lineage and geographic pattern of bird populations,and is one of the determining factors affecting genetic diversity.The combination of molecular genetic markers and satellite tracking technology is an effective means of studying population migration connectivity.The Bewick’s Swan（Cygnus columbianus bewickii）is widely distributed in Eurasia,and previous studies have divided it into four migration routes based on telemetry data.Therefore,this study analyzed mitochondrial DNA sequences（1,026bp）and 14polymorphic microsatellite loci from 1,006 individuals based on feather samples obtained from 7 breeding and 15 wintering geographic regions,exploring the genetic diversity and structure,sex-specific dispersal,population dynamics and historical demography,and Gene flow among populations of the Bewick’s Swan.In addition,the genetic distribution pattern was linked to population distribution and satellite tracking results from previous studies on 171 Bewick’s Swan individuals,as well as wintering survey data from the past 50 years.The main results are as follows:1)Based on the analysis of mt DNA control region data,the breeding populations with the highest genetic diversity were found to be the NY（H_d:0.913;P_i:0.00368）,and the populations with the lowest genetic diversity were found to be the CD（H_d:0.358;P_i:0.00123）.Based on microsatellite data analysis,the genetic diversity of the seven breeding populations was similar.Based on the analysis of microsatellite and mt DNA markers,a pairwise genetic distance F_ST analysis was conducted on breeding populations.The microsatellite results showed significant differences between SY and CD and other areas,while the mt DNA results showed significant differences between the PB,LD and CD populations and other populations.The AMOVA analysis results showed that most of the genetic variance was within populations,with microsatellites accounting for 97.7%and mt DNA accounting for 88.72%.Furthermore,the STRUCTURE analysis based on microsatellite data showed that the most reasonable division was into Eurasian Clusters and East Asian Clusters when K=2.Finally,the BI and ML phylogenetic trees and haplotype network analysis based on mt DNA showed that the phylogenetic structure of all breeding populations was not clear.2)In GENECLASS,first-generation migrant individuals were tested in four breeding populations,among which only two male individuals were identified as first-generation migrants（P<0.01）.However,sex-biased dispersal analysis showed that the male-biased index m _AI was 0.138 for males and-0.136 for females（P=0.685）,F_ST was0.014 for males and 0.011 for females（P=0.685）,was 0.027 for males and 0.022 for females（P=0.447）.The three indices show that males are higher than females.And theν_AI of male was lower than the female,ν_AI was 11.711 for males and 13.606 for females（P=0.785）.The results indicate the presence of female-biased dispersal but not significantly so.Secondly,the highest gene flow between breeding populations was observed in YA-PB（25%）and YA-LD（22.6%）.The highest gene flow（population individual migration rate）between breeding and wintering populations was observed in CH-LD（18.4%）and CD-JA（20.1%）.Satellite tracking data provided new evidence of strong gene flow between the distant YA and LD populations in terms of spatial distance.Among the 32 individuals with complete migration routes captured and tracked in Yamal and Lena,one individual caught in Lena migrated to China for wintering before migrating back to Yamal for breeding the following spring,while the other 31 individuals returned to their original breeding grounds for continued reproduction.3)Based on nearly 50 years of wintering waterbird survey data,the number of Bewick’s Swan wintering in Japan was only 542 in 1970,while the survey count for2018/19 was 45,541,indicating a rapid increase in the overwintering population.Secondly,based on bottleneck analysis,although the IAM model showed evidence of heterozygosity excess in the PB,CY,LD,and CD populations,as well as in the inland and coastal clusters,and a shifted allelic frequency distribution in the CY population,no significant heterozygosity excess was detected in any population based on the SMM and TPM models.The analysis results were not significant in two or more models,therefore indicating that the breeding population of the Bewick’s Swan has not experienced recent genetic bottlenecks.Based on the calculated Garza-Williamson index results,the G-W index for all tested groups ranged between 0.42-0.47,which is lower than the critical value of 0.68.Therefore,it is inferred that the Bewick’s Swan population may have experienced a genetic bottleneck 100 generations ago.Finally,based on Tajima’s D value,Fu’s Fs neutrality test,mismatch distribution test,and expanded Bayesian skyline analysis,it is suggested that there has been no population expansion in the history of the breeding population.The following conclusions can be drawn from the above results:Bewick’s Swan distributed in Eurasia can be divided into Eurasian and East Asian clusters.The wintering habitat is likely to be a‘transfer station’to realize gene exchange among long-distance breeding populations.Bewick’s Swan has a variety of migration strategies and will migrate to different wintering habitats.Meanwhile,the following results were also discussed:Firstly,microsatellite data showed that the CD population had the highest genetic diversity,while mt DNA data analysis showed that the CD population had the lowest genetic diversity.This may be related to the different evolution rates between the two genetic markers,with microsatellites evolving faster than mt DNA.Therefore,the microsatellite results reflect recent genetic levels,while mt DNA reflects genetic levels in earlier generations.Secondly,microsatellite markers showed higher population genetic differentiation compared to mt DNA markers.This may be related to the possibility of female-biased dispersal in Bewick’s Swan,as familiarity with local resources is crucial for males in monogamous birds with a"resource defense"mating system.On the other hand,this is also related to the faster evolution rate of microsatellites compared to mt DNA.In addition,satellite tracking results support that recent the western Pacific migration routes of Bewick’s Swan do not intersect with other migration routes.Finally,the genetic bottleneck test showed that the CD population has not experienced a recent genetic bottleneck.According to satellite tracking data in recent years,the CD population overwinters in Japan.Based on the winter waterbird survey data in the past 50 years,the number of wintering Bewick’s Swan in Japan has rapidly increased from a very low value,while the surrounding wintering populations in Korea have shown a significant decline.Therefore,we infer that the rapid increase in the wintering population in Japan in recent years is not related to the dynamics of the breeding population of CD,but rather due to the degradation and disappearance of other wintering sites in East Asia,leading these individuals to change their migration routes and move to better wintering conditions in Japan.The hypothesis that the rapid increase in the number of Bewick’s Swan wintering in the Evros Delta on the Caspian migration route is due to individuals changing their migration routes from wintering on the North Sea coast to overwintering on the Mediterranean coast has also been confirmed by this study.