Horizontal gene transfer is when genes move between two organisms of different species. Not only do eukaryotes get some genes from viruses, but viruses also get some genes from eukaryotes. Genes from eukaryotes that viruses get allows the viruses to adapt and gain better control of the cells. Some genes that come from viruses can also aid in cellular communication and other functions. Through screening evalutionary trees they managed to find the frequency of eukaryot to virus transfers was around two times the frequency of eukaryot to virus transfers, and in total 2,841 protein families were effected, however this is just a conservative estimate. The study suggested that the transfer went in both directions at the same time, but more genes went in the direction of eukaryot to virus. Retrovirus(viruses that embed themselves in the host genome and replicate via transcription) transfer is underrepresented in the study. There is some cases where viruses transfer genes between different eukaryotes, mainly between unicellular organisms or between animals and plants and there closest unicellular relatives. Certain types of eukaryotes appear to have more genes transferred than most others indicating specific viruses that transfer a lot of genes may have affected them. There is also some evidence that the transfer of genes from archaea via viruses is responsible for many important genes in eukaryotes and that those genes were part of the process of eukaryotes evolving (before eukaryotes existed). This all provides insight into viral infection strategies, as proteases(enzymes that break down proteins) being some of the genes that appear to come from viruses indicates that proteases could be a common agent for infection.
