Recent duplication and interlocus gene conversion

Authors: Thorsten B H Reusch Helmut Schaschl K Mathias Wegner

Publish Date: 2004/08/21

Volume: 56, Issue: 6, Pages: 427-437

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Abstract

Using a bacterial artificial chromosome BAC library we analysed a 995 kb genomic segment containing the major histocompatibility class II genes of a teleost the threespined stickleback Gasterosteus aculeatus Experiments with G aculeatus have provided direct evidence for balancing selection by pathogens and mate choice driving MH class II beta polymorphism Two sets of paralogous class II alpha genes and beta genes in a tandem arrangement were identified designated GaacDAA/DAB and GaacDBA/DBB Expression analysis of the beta genes using singlestrand conformation polymorphism revealed that both gene copies are expressed Based on an analysis of pairwise nucleotide polymorphisms we estimate that the duplication into two paralogous class II loci occurred only 12–24 million years ago 1–2 orders of magnitude more recently than in other fish bird or mammalian species At the 3′direction of the classical MH loci we identified another seven genes or gene fragments two of which small inducible cytokine complement regulatory factor are related to immune function in other vertebrates None of these genes were associated with MH class II genes in zebrafish suggesting a markedly different organisation of the MH class II region in sticklebacks and presumably across bony fishes When the nucleotide substitution pattern of the novel class II beta genes was analysed together with a representative sequence sample isolated from fish in northern Germany n=27 we found that the peptide binding region of the GaacDAB and GaacDBB loci had undergone an interlocus gene conversion P=0007 In accordance we found a 10 to 20fold higher frequency of CpGislands on the MH class II segment compared to other species a feature that may be conducive for interlocus recombinationThis project was jointly funded by the Deutsche Forschungsgemeinschaft Re 1108/4 and 1108/5 HeisenbergProgram and the Max–Planck Society We thank Robert Bogden Amplicon Express and K Mohr and W Zimmermann Agowa Berlin for technical support and excellent cooperation S Liedtke for assistance and M Milinski for continual support and encouragement

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