PIM-SM(Protocol Independent Multicast-Sparse Mode) is a main multicast routing pro-tocol in the IPv6(Internet Protocol version 6).It can use either a shared tree or a shortest path tree to deliver data packets,consequ...PIM-SM(Protocol Independent Multicast-Sparse Mode) is a main multicast routing pro-tocol in the IPv6(Internet Protocol version 6).It can use either a shared tree or a shortest path tree to deliver data packets,consequently the multicast IP lookup engine requires,in some cases,two searches to get a correct lookup result according to its multicast forwarding rule,and it may result in a new requirement of doubling the lookup speed of the lookup engine.The ordinary method to satisfy this requirement in TCAM(Ternary Content Addressable Memory) based lookup engines is to exploit parallelism among multiple TCAMs.However,traditional parallel methods always induce more re-sources and higher design difficulty.We propose in this paper a novel approach to solve this problem.By arranging multicast forwarding table in class sequence in TCAM and making full use of the intrinsic characteristic of the TCAM,our approach can get the right lookup result with just one search and a single TCAM,while keeping the hardware of lookup engine unchanged.Experimental results have shown that the approach make it possible to satisfy forwarding IPv6 multicast packets at the full link rate of 20 Gb/s with just one TCAM with the current TCAM chip.展开更多
Major histocompatibility complex （MHC） is a family of highly polymorphic genes activating adaptive immunity in vertebrates. However, the underlying mecha- nism of MHC evolution is still not fully understood. In this...Major histocompatibility complex （MHC） is a family of highly polymorphic genes activating adaptive immunity in vertebrates. However, the underlying mecha- nism of MHC evolution is still not fully understood. In this study, we investigated genetic variation of three classical MHC class I genes in the giant panda （Ailuropoda mela- noleuca） and tested for selection effect and recombination event across exonic and intronic sequences to understand maintenance mechanism of polymorphism at Aime-MHC class I genes. In total, we isolated 21 MHC class I haplotypes （exon 2-intron 2-exon 3） from 46 captive giant pandas, of which eight were for Aime-C, seven for Aime-I and six for Aime-L; however, we only identified six unique sequences from these haplotypes. The subsequent maximum-likeli- hood and Chi-square analyses both detected evidence of recombination acting on the 21 haplotypes. These results indicate that the giant panda still retains a relatively high adaptive variation at Aime-MHC-I genes, and that the intronic segments have been homogenized along evolu- tionary time by recombination and subsequent genetic drift.We calculated nucleotide substitution rates of the antigen- binding regions （exons 2 and 3） and the noncoding intron 2, and found two pieces of evidence supporting the presence of balancing selection in the giant panda： an excess of nonsynonymous over synonymous substitutions at the antigen-binding sites, and an obviously higher synonymous substitutions in the exons than nucleotide substitutions in the intron. Thus, this study reveals that balancing selection and recombination together shape the diversity pattern at Aime- MHC-I loci of the giant panda.展开更多
基金Supported by the National High-Tech Research and De-velopment Plan of China (No. 2007AA01Z2a1)the Na-tional Grand Fundamental Research 973 Program of China (No. 2007CB307102)
文摘PIM-SM(Protocol Independent Multicast-Sparse Mode) is a main multicast routing pro-tocol in the IPv6(Internet Protocol version 6).It can use either a shared tree or a shortest path tree to deliver data packets,consequently the multicast IP lookup engine requires,in some cases,two searches to get a correct lookup result according to its multicast forwarding rule,and it may result in a new requirement of doubling the lookup speed of the lookup engine.The ordinary method to satisfy this requirement in TCAM(Ternary Content Addressable Memory) based lookup engines is to exploit parallelism among multiple TCAMs.However,traditional parallel methods always induce more re-sources and higher design difficulty.We propose in this paper a novel approach to solve this problem.By arranging multicast forwarding table in class sequence in TCAM and making full use of the intrinsic characteristic of the TCAM,our approach can get the right lookup result with just one search and a single TCAM,while keeping the hardware of lookup engine unchanged.Experimental results have shown that the approach make it possible to satisfy forwarding IPv6 multicast packets at the full link rate of 20 Gb/s with just one TCAM with the current TCAM chip.
基金supported by a special grant(SG1411)for the giant panda from the State Forestry Administration of China
文摘Major histocompatibility complex （MHC） is a family of highly polymorphic genes activating adaptive immunity in vertebrates. However, the underlying mecha- nism of MHC evolution is still not fully understood. In this study, we investigated genetic variation of three classical MHC class I genes in the giant panda （Ailuropoda mela- noleuca） and tested for selection effect and recombination event across exonic and intronic sequences to understand maintenance mechanism of polymorphism at Aime-MHC class I genes. In total, we isolated 21 MHC class I haplotypes （exon 2-intron 2-exon 3） from 46 captive giant pandas, of which eight were for Aime-C, seven for Aime-I and six for Aime-L; however, we only identified six unique sequences from these haplotypes. The subsequent maximum-likeli- hood and Chi-square analyses both detected evidence of recombination acting on the 21 haplotypes. These results indicate that the giant panda still retains a relatively high adaptive variation at Aime-MHC-I genes, and that the intronic segments have been homogenized along evolu- tionary time by recombination and subsequent genetic drift.We calculated nucleotide substitution rates of the antigen- binding regions （exons 2 and 3） and the noncoding intron 2, and found two pieces of evidence supporting the presence of balancing selection in the giant panda： an excess of nonsynonymous over synonymous substitutions at the antigen-binding sites, and an obviously higher synonymous substitutions in the exons than nucleotide substitutions in the intron. Thus, this study reveals that balancing selection and recombination together shape the diversity pattern at Aime- MHC-I loci of the giant panda.
