Positive diversifying selection in avian Mx genes.

Mx proteins are interferon-induced GTPases that confer antiviral activities against RNA viruses. We analysed the molecular evolution of the Mx gene in birds using data on interspecific divergence in anseriform and galliform birds, and on intraspecific diversity in commercial chicken lines, local Chinese chicken breeds as well as in the mallard. The overall ratio of non-synonymous to synonymous substitution was unusually high, 0.80, indicating relaxed constraint or positive selection. Evidence for the latter was provided by that a total of 11-18 codons were found to have evolved under positive selection. The great majority of these codons are located in a region unique to birds at the N-terminal end of the Mx protein. We found an excess of non-synonymous polymorphisms relative to synonymous variants in all comparisons. This, together with positive Tajima's D values in the local Chinese chicken breeds and in the mallard suggests that balancing selection is acting in avian Mx genes. As such, Mx mimics the major histocompatibility complex system, indicating that heterozygous individuals are better off withstanding pathogen attack.