| Description |
Viruses are excellent tools to study the concepts and mechanisms of evolution. Intense selection, driven by conflicts with hosts, combined with large population size and fast replication allow viruses to adapt rapidly. While studies of RNA viruses have tended to focus heavily on evolution by point mutations, studies of DNA viruses have highlighted other mechanisms of adaptation, including copy number variation (CNV) and horizontal gene transfer (HGT). In this dissertation, I focus on viral evolution by HGT- the integration of genes from nonparent organisms, particularly hosts. In contrast to gradual evolution by point mutation, HGT provides the opportunity for leaps in adaptation by co-opting genes evolved in other genomes. Viruses use this strategy to good effect, and some encode multitudes of acquired genes. Many of these genes diverge to act as mimics or inhibitors to thwart host defenses. While HGT appears to be a widespread mechanism of viral adaptation, the mechanism of transfer has been a long-standing mystery. Because viral copies of host genes lack introns, it has been hypothesized that transfer occurs through an RNA intermediate; however, further evidence of this has been lacking. LINE-1 retrotransposons are known to reverse transcribe host mRNA, creating retrocopies in the human genome. We therefore hypothesized that this process could also deposit intron-less retrocopies in viral genomes. While LINE-1 retrotransposition leaves marks of its activity, these signatures may degrade quickly in a fast-evolving viral genome. To address this, we developed a strategy to select for HGT events in real time, using poxviruses as a model system. We discovered that LINE-mediated retrotransposition may be a primary means by which poxviruses gain host genes. In addition, we found that the location of newly acquired genes may have implications for their further evolution. These studies reveal new facets of poxvirus evolution and, likely, other DNA viruses, and add substantially to our understanding of the mechanisms of noncanonical gene flow. |
