Spatial And Temporal Variation Of MHC Gene In Omei Tree Frog(Rhacophorus Omeimontis)

Discussing spatial and temporal variation of genetic diversity is beneficial for understanding the maintaining mechanism of genetic diversity in natural populations, which is one of the most essential subjects in evolutionary biology. Furthermore, the respective and relative roles of genetic drift and selection in the formation of genetic diversity in natural populations have always been the hotspot of this area. The major histocompatibility complex genes (MHC), which is known by its important role in recognizing self and foreign pathogens in immune system, is affected by evolutionary events such as selection and genetic drift. Therefore, it becomes the ideal molecular marker in clarifying the natural selection on adaptive evolution. On the other hand, neutrally evolving genetic markers, such as microsatellite and mtDNA which are solely affected by neutral events, can be used to compare with MHC gene to investigate the relative roles of natural selection and genetic drift in maintaining mechanism of genetic diversity in natural populations further.Omei tree frog (Rhacophorus omeimontis) belonging to Rhacophoridae, anura, amphibia breeds mainly around still water with diverse pathogens, which means it may possibly be influenced by selection. Furthermore, it may experience intense genetic drift and bottleneck effect whereby the fact that it presents severe male-biased sex ratio with polyandry breeding pattern correspondingly. In this study, we have collected 322 Omei tree frog samples in its distributions, analyzed the different variation patterns of MHC gene and microsatellite, evaluated the relative roles of genetic drift and selection in the formation of genetic diversity of MHC gene, and clarified the differentiation of local adaptation and evolutionary potential between populations. Our final results are as follows:On the temporal level, we have detected 53 MHC loci in 153 samples collected from HNBDGS during 3 years. Further analysis displayed that both MHC gene and microsatellite DNA presented high genetic diversity during 3 years, that the genetic variation among temporal populations differed during three years, and that patterns of MHC loci and microsatellite were different. Moreover, MHC gene was affected by balancing selection and the selecting sites were located on the presuming antigen banding sites which implied pathogen mediated balancing selection. However the MHC genetic variation at spatial levels needs further research because of the small temporal scales.While on the spatial level, we have detected 67 allele loci of MHC in 210 samples from 8 sites. Further analysis displayed as similar as temporal level that both MHC gene and microsatellite DNA present high genetic diversity between Omei tree frog populations, and that patterns of genetic diversity between populations have significant difference (P<0.001), and the variation patterns of MHC differed from those in microsatellite. Moreover, genetic structure analysis has shown that the 8 geographical populations can be divided into 3 genetic populations based on their significant genetic variation. The smaller effective population and stronger bottleneck effect suggested that MHC gene has been affected by intense genetic drift. Besides, MHC gene has been subject to selective pressures and the targeting sites were largely located on the presuming antigen banding sites implying pathogen mediated balancing selection. However, the effect of selection outweighed that of genetic drift, and the differential of genetic diversity between populations could be attributed to different selecting levels.