BAC Library Consturction And MHC Genomic Structure Of The Crested Ibis

The Crested ibis (Nipponia nippon) is a national first-class protected species in china, as well as one of the most endangered bird species in the world. It was almost extinct in the last century. Fortunately, after30-years protection, the number of individuals of ibis has increased significantly. Despite the expansion in population, this species is still vunerable to pathogens and other environment stresses and under the threat of being extinct, due to its low immune genetic variety caused by the severe bottleneck it had suffered. The major histocompatiility complex (MHC) is of great importance in vertebrate immune system, playing roles in the resistance to pathogens and adaption to environment. MHC also takes part in the recognition of relationships and mate choice, which is closely related to reproduction. Therefore, MHC is paid attention to and being studied by many scientists in different fields such as conservation biology and immunol genetics. It is effective and efficient to study MHC based on construction of bacterial artificial chromosome library and sequencing of BACs containing target MHC sequences. Since MHC contains lots of gene loci, it is necessary to acquire detailed and integrate sequence based on a physical map of MHC, for the purpose of learning MHC in an overall view.In this research, we constructed a crested ibis BAC library with4D-PCR screening system, providing about7.65-fold coverage of the crested ibis genome; we also generated an additional BAC library which provides even more coverage of the genome by using reverse-4D process. Based on the screening and sequencing of MHC positive BACs, we achieved, to our knowledge, the first physical map of MHC in Threskiornithidae birds, as well as sequences that totally span about475kb of crested ibis MHC. The ibis MHC is composed of three regions, the MHC core region, the BG region and the class Ⅰ region. It is still unknown whether the three parts of MHC were located on the same chromosome and closely linked to each other. A subsequent study to resolve this question will be done soon. Nevertheless,50gene loci have already been predicted and defined in the ibis MHC, including MHC Ⅰ、Ⅱα、Ⅱβ、 DM、TAP、BG and so on. While showing some similarity on the organization of MHC genes to that of chicken, the ibis MHC displays different and specific features of itself. The most notable difference between ibis and chicken MHC maybe the apparence of Ⅱα loci located next to Ⅱβ loci in the ibis core MHC region, which indicates a variety of evolution directions of MHC structure in birds. Compared to chicken and turkey MHC, the ibis MHC spans a much extended region with more MHC class Ⅰ and Ⅱ loci, suggesting that the "minimal essential MHC" may be limited in only a few bird orders, and more informations of avian MHC are waiting to be unveiled.