| The Oncomelania hupensis snail is the only intermediate host of Schistosoma japonicum, which determines the distribution of schistosomiasis. Marshland is the main snail habitats in Jiangsu Province. To study the distribution pattern of snails and its influence factors, which provides scientific reference for snail monitor and control measures. It’s also very important for achieving the goal of schistosomiasis transmission interrupted and elimination in Jiangsu Province. Based on database of marshland snails’ distribution, genetic and environmental factors, using spatio-temporal, genetic and gene-environment interaction analysis respectively, we conclude spatio-temporal distribution characteristics, population genetic structure and gene-environment interaction of snails, and clarify the spatio-temporal distribution rule and its influence factors.Firstly, based on spatial database of snails from 2001 to 2013, the global and local spatial autocorrelation analysis and spatial and temporal scanning method were carried out to identify the spatial-temporal distribution characteristics of snail and infected snail. The marshland was accounted for more than 90% of the total area of snail habitats, the area, marshland number and density of snails and infected snails showed a significantly downward trend(P < 0.05). The snails showed a global spatial cluster trend from 2001 to 2012(except for 2009) while the cluster of infected snails were in 2001 and from 2003 to 2007. The local hotspots of snails mainly concentrated in the upper reaches of the Yangtze River in Jiangsu Province(Nanjing City,Yangzhou City, etc.) from 2001 to 2007, while it moved to the lower reaches(Zhenjiang City) from 2008 to 2013. The local hotspots of infected snails mainlyconcentrated in Nanjing and Zhenjiang City from 2001 to 2003, the local hotspots of Nanjing City began to disappear after 2004 and the Zhenjiang City continued until2009. There were two high risk area of snails, and the relative risk(RR) of both were1.19, in the upper reaches of the Yangtze River with the radius for 14.46 km from2001 to 2005; the other was in the middle reaches of the Yangtze River, with the radius for 39.85 km from 2001 to 2006. The high risk area of infected snail, located in the upper reaches of the Yangtze River and the radius was 37.81 km, the RR was 3.00 from 2002 to 2007. There were spatial cluster distribution of snails and infected snail,the local cluster area moved from the upper reaches to the middle of the Yangtze River in Jiangsu Province.Secondly, microsatellite DNA marker technique was used to study the genetic diversity and genetic variation of snail population in three geographical location(North, Middle, South of the Yangtze River). The range of Na, Ho and He index were respectively 4.5 ~ 15, 0.10 ~ 0.35 and 0.77 ~ 0.92, These 3 indices of snail populations in the middle of the Yangtze River were greater than other population, but there was no significant difference(P > 0.05). The average number of pairwise differences between populations of snail populations in the South of the Yangtze River(Xinbei district of Changzhou City and Dantu district of Zhenjiang City) was relatively larger to other. Exact Test of Sample Differentiation Based on Genotype Frequencies showed no statistical significance between populations(P > 0.05);AMOVA showed that the main source of genetic variation came from an individual snail(69.32%); NJ and ME phylogenetic tree showed genetic relationship of snail population between the north and middle of the Yangtze River was close, and the south snail population was far to other populations. Mantel test showed there was no correlation between geographical distance and genetic distance. The marshland snail populations had a high genetic diversity along the Yangtze River in Jiangsu Province.Although the snail populations did not appear significant genetic differentiation, they had a certain extent of cluster trend based on geography.Finally, Normalized Difference Vegetation Index(NDVI), Wetness of snail habitats were gained from the Landsat satellite remote sensing images and data ofsnail control method was also collected. The generalized linear regression model and GMDR model were used to investigate the environmental factors and gene environment interaction effect on the distribution of snail. The natural environmental factors(NDVI, Wetness) and social factors(snail control) had no collinearity. The generalized linear regression equation of snails contained the snail control and wetness two independent variables while the infected snails contained wetness variable. GMDR model showed that the distribution of snails was affected by gene-environment interaction, the optimal model including C22 and T4-33 two microsatellite loci, Wetness, totally three factor interaction model, the OR was 90.86;The infected snail was affected by gene-gene interaction, optimal model including C22 and T4-33 two factors interaction model, the OR was 13.07. Environmental factors can impact the distribution of snails and the snail control was signficanlty effective. Gene-gene and gene-environment interaction may the factors to maintain the snail density.In conclusion, we studied the distribution pattern of marshland snails and its influencing factors at the middle-small scale in Jiangsu Provinve from 2001 to 2013,and found that there was a significant downward trend. The space cluster area moved from the upper reaches to the middle and lower reaches of the Yangtze River in Jiangsu Province, it suggested that we should pay more attention to the risk of transmission of schistosomiasis in the middle and lower reaches. Wetness was an important factor causing the snail density increasing, and keeping snail control was effective. Although marshland snail populations did not appear significant genetic differentiation, the impact of gene-gene and gene-environment interaction on the distribution of snail could not be to ignored. The combined effect need further study. |
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