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This dissertation, "Molecular Evolution of Infectious Bursal Disease Virus" by Chung-chau, Hon,, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Molecular Evolution of Infectious Bursal Disease Virus Submitted by Chung Chau HON for the degree of Doctor of Philosophy at The University of Hong Kong in November 2006 Infectious Bursal Disease Virus (IBDV) is a Birnavirus causing acute immunosuppression in young chickens. Evolutionary characteristics and molecular epidemiology of IBDV have not been investigated systematically. This thesis deals with evolutionary aspects of IBDV in six areas: 1) the evolutionary origin of IBDV, 2) the phylogenetic relationships among the strains, 3) the time scale of evolution of the strains, 4) demographic behavior of the strains, 5) recombination between the strains, 6) selection pressure acting on the strains. Phylogenetic methods, including various molecular clock models and coalescent concepts implemented in both likelihood and Bayesian framework, were employed to investigate these aspects. The genomes of Birnaviruses were found to be mosaic, suggesting that the IBDV genome may have a recombinant history from the ancient Birnaviruses of marine species. Multiple reassortment events among different strains of IBDV are suggested by the phylogeny of both of their genome segments. The phylogeny of genome segment B suggests an ancient relationship between the Australian and very virulent strains. Moreover, our results suggest the expansion of very virulent IBDV may be caused by the reassortment event of its genome segment B but not