Structure and Evolution of the MHC in a Model Marsupial

Project Summary

The Major Histocompatibility Complex (Mhc) contains a set of genes essential to the adaptive immune response in vertebrates. These genes encode molecules that present antigenic peptides to T cells and this presentation is a key step in initiating immune responses. The Mhc is the most polymorphic and the most gene dense region in the human genome, and the evolution of this highly dynamic region has been the focus of much investigation. Polymorphism at the Mhc has been linked to diverse traits from disease resistance to mate choice, making the Mhc the focus of much attention from conservation biologists. Information about Mhc structure and diversity in mammals has been limited to primarily one of the three mammalian subclasses, the eutherians (placental mammals). The human Mhc region has been completely sequenced and the homologous region is also or nearly completed for several other eutherians. Evidence that the Mhc, particularly the Mhc class I region, may have played an important role in the evolution of viviparous reproduction in mammals makes the analysis of this region in non-eutherian mammals critical. The characterization of the Mhc in an evolutionary lineage of mammals that parallels that of eutherians, the metatherians or marsupials, is the focus of this grant proposal.

The South American grey short-tailed opossum (Monodelphis domestica) has been developed as a model marsupial. This proposal has two specific research aims for the characterization of the opossum Mhc. They are designed to take advantage of the genomic tools being developed for the opossum including a genetic linkage map and Bacterial Artificial Chromosome libraries. The first aim of this proposal is to determine the complete content of the Mhc region in a specific partially inbred population of M. domestica. A BAC library will be screened to generate a contiguous set of overlapping clones covering the opossum Mhc region. The content of these BACs will be determined, ultimately by their complete sequencing. The second aim of this proposal is to focus specifically on the evolution, content, and expression of Mhc class I and class I related loci in the opossum. Currently there is evidence of at least four Mhc class I loci present in the opossum. The tissue specific pattern of expression will be determined for all four loci both in adult animals and at different stages of ontogeny.

This proposal also includes a training plan first established in a CAREER proposal awarded to the Principal Investigator. This project will involve both graduate and undergraduate students and provide training experience for junior investigators. In addition to direct research experience, students at all levels, including undergraduate, will participate in publications, presentations at conferences, and the opportunity to collaborate with domestic and international colleagues, the latter primarily in Australia. The collaborators participating in this proposal are also the leaders of the two current marsupial genome mapping projects. Both have expressed interest in hosting collaborative visits by the PI and his students. The work outlined in this proposal will be integrated to form a larger scale comparative genomic analysis with ongoing efforts on an Australasian marsupial, the tammar wallaby. In that way the project will contribute significantly to a larger comparative genomics effort and has significance for the Tree of Life initiative at NSF.