| Since the first human whole genome was released, the whole genome technology has been applied in more and more biology researches. Especially, the rapid growth of the second-generation sequencing technology has been making population genetic widely used in researches on biodiversity, adaptation and demographic history. To date, how to efficiently dig out the resource behind the biology is still a challenge, however, the newest technology of sequencing is seems a optimal tool to deal this challenge. It makes great sense to study the basic mechanism behind the biodiversity, adaptation and its progression, by using the whole genomic and population genetic methodologies, as well as the conservation of endangered wild lives.Owing to its unique habitat in the arctic regions, the change of polar bear population size is always treating as a symbol of global climate change. Because of greenhouse effect and hunting, they were once in peril of extinction. Recently, population size of polar bear has recovered throug years of species conservation and international cooperation. However, the recover speed is still very slow and its habitat, i.e. the arctic floating ice is also still decreasing. The polar bear are facing unprecedently enormous pressure to survive. As the sign of global climate change, genetic polymorphism and diversity of polar bear have drawn our great attention nowdays in this research, we applied the whole genome sequencing and population genetic methodologies to investigate evolution and demorgraphic history of polar bear, and our main conclusions are as below:(1) We provided the first draft whole genome of polar bear. We de novo assembled the whole genome of polar bear by only using the next generation sequencing reads and assemble program designed and developed by BGI. The assembly contained 2.4 Gb, and its contig and scaffold N50 are 2.4 Kbp and 15.9 Mbp, respectively.(2) Polar bear had an independent origin relative to its closest species, brown bear. That means the theory that polar bear originated from a subgroup of brown bear lived in the ABC Island is rejected. We also estimated that the split time between polar bear and brown bear was more recent than previous results(the split time estimated by using the mitochondrial DNA data), which is only about 400 to 500 thousands years ago. Out results are consistent with the previous fossil results.(3) We also found frequent introgression between polar bear and brown bear. The introgression model indicated that frequent introgress happend during the earlier period of speciation of those two new species and these introgress were asymmetric since there was more gene flow from polar bear to brown bear than the reverse direction. Morever, it rejected the previous theory put forward by mitochondrial DNA analysis results that the polar bear originated from a subgroup of brown bear in ABC Island.(4) We found that genes realted to fat and cholesterol metabolism on the polar bear lineage have been under stronger positive selection than in brown bear. Especially, 9 of the top 16 genes under stronger positive selection are associated with cardiomyopathy and vascular diseases. This result demonstrated that the cardiovascular system might have been reorganization in polar bear in order to adapted the extremely cold environment in arctic climate.(5) We also found two candidate genes, LYST and AIM1, were associated with pigmentation are under positive selection in polar bear. LYST plays a critical role in pigment metablolism and its mutation can cause disorder traits correlate to pigment metablolism. AIMI is implicated in formation of human melanoma. Therefore, positive selection on these two genes may imply that they are likely related to formation of transparent fur with lack of pigment and reveal the molecular mechanism underlying the lack of pigment in polar bear. |
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