The role of alternative genetic codes in viral evolution and emergence.

Although the 'universal' genetic code is widespread among life-forms, a number of diverse lineages have evolved unique codon reassignments. The proteomes of these organisms and organelles must, by necessity, use the same codon assignments. Likewise, for an exogenous genetic element, such as an infecting viral genome, to be accurately and completely expressed with the host's translation system, it must employ the same genetic code. This raises a number of intriguing questions regarding the origin and evolution of viruses. In particular, it is extremely unlikely that viruses of hosts utilizing the universal genetic code would emerge, via cross-species transmission, in hosts utilizing alternative codes, and vice versa. Consequently, more parsimonious scenarios for the origins of such viruses include the prolonged co-evolution of viruses with cellular life, or the escape of genetic material from host genomes. Further, we raise the possibility that emerging viruses provide the selection pressure favoring the use of alternative codes in potential hosts, such that the evolution of a variant genetic code acts as a unique and powerful antiviral strategy. As such, in the face of new emerging viruses, hosts with codon reassignments would have a significant selective advantage compared to hosts utilizing the universal code.