Bidirectional Introgression Between Polar Bear And ABC Brown Bear

Polar bears are marine mammals which live in floes near the Arctic Ocean.They major prey ring spot leopard,and they adapt to the cold weather whole year.Brown bears are an other taxon which has the closer genetic relationship with the polar bear.Polar bear and brown bear have the different distribution range,morphology,behavior and physiology.Introgression is the alleles from one species flow into the gene pool of the second divergent species by hybridization and back-crossing.Introgression exists in mammal extensively.It plays an important role in the evolution and adaptation for them.For example,the introgression between Canis produces new species,it prompts the evolution of Canis;the introgression between Bos has the important effect on their domestication and the adaptation of high altitude.Polar bear and ABC brown bear(the brown bears which live in Admiralty,Baranof and Chichagof islands of Alaska)have the introgression in the previous researches.However,the introgression direction of them and the adaptation result from the introgression are lack the enough researches.Polar bear and brown bear are two species which divergence in Pleistocene.There exist many alternate glacial and interglacial in Pleistocene.Thus,how they adapt the change of climate and what effect does introgression which after they divergence contribute to their adaptation.Based on this background,we use gphocs to analysis the demographic history and use PCAdmix,Dxy and rIBD identify total 8 introgression regions which length longer than 100 kb.From the analysis of demographic history and phylogenetic trees of introgression regions,we find the introgression of polar bear and ABC brown bear is bidirectional.This result is different from the many previous researches which the introgression is from polar bear to ABC brown bear only.4 regions suggest introgression is from polar bear to ABC brown bear.The rest 4 regions are from ABC brown bear to polar bear.Though annotation of the introgression regions,considering polar bear to ABC brown bear,it is major related to the olfactory receptors and X-linked genes.The intergenic regions are related to cartilage development and snRNA process.In addition,from ABC brown bear to polar bear,it is related to metabolism,immune,transcription,membrane trafficking and so on.Polar bear and brown bear have the sensitive olfaction,and polar bear lack the enough energy intake when climate change currently.Meanwhile,the cold weather makes the diffusion of odorous molecules slowly.Hence,sensitive olfaction can help them prey quickly.Although polar bear and brown bear are two divergent species,they have a series of admixture.They need overcome the barrier of reproduction for admixture.The X-linked genes major express in pituitary,testis and amniocyte,it suggests that these genes are maybe related to overcome the barrier of reproduction.Bidirectional introgression involved in different genes,it indicates that the two species can exchange the genetic information when necessary.Besides,we find the introgression ancestries are unequal in two haplotypes.One haplotype has the more ancestries than the other one.Comparing the recombination rate of polar bear and brown bear for these introgression regions,we find it is the coldspot in the polar bear and brown bear when the ancestries are equal in the two haplotypes,while it is the coldspot in the polar bear and hotspot in the brown bear when the ancestries are unequal in two haplotypes.Therefore,we suggest the unequal ancestries are result from the different recombination rate in the polar bear and brown bear.Moreover,in order to check the introgression regions whether result from the incomplete lineage sorting.We present the F4,which though the fastsimcoal2 to simulate the values of f4 are about zero with the model without the introgression.Thus,it proves the 8 introgression regions are not caused by incomplete lineage sorting.Finally,we use MK test find 15 genes under the positive selection of polar bear and brown bear.These genes are related to DNA damage repair,cell junction,neuro,immune and transcription.6 genes among 15 are predicted to influence the function of protein and they are major related to gene expression.In total,we find the bidirectional introgression between polar bear and ABC brown bear by demographic history and phylogenetic relationship of introgression regions.It is unsolved in previous researches.And we suggest the olfaction and reproduction are two important factors for polar bear and ABC brown bear survive in climate change during the Pleistocene.Elucidating the significant meaning of two species survive by introgression between themselves.Setting the fundament for know the polar bear and brown bear.Finding the interesting phenomenon of the unequal ancestries of two haplotypes and inspiring us to consider the causes of this phenomenon.With the climate change recently,the habitat of polar bear faces the threat,the knowledge of polar bear and brown bear can afford some guides for protection.