At some point, perhaps after retroviruses had sprinkled host genomes with multiple copies of the f gene, a baculovirus picked it up through recombination. Unlike retroviruses, baculoviruses don't insert their genes into their hosts' genomes. They do steal genes, however, by mixing host DNA with their own. "The way they generate diversity in their genome is by recombination," Blissard says.
No matter how the original f gene moved from the fruit fly into viruses, it had to change a lot to win its current starring role in infectivity, Blissard says. He and Lung found that while viral F proteins naturally migrate to the surface of a host cell, F proteins native to the cell appear to stay on the inside. "Membrane fusion goes on all over the place inside the cell," Blissard says. "But fusion in cells is regulated by fairly large complexes of proteins. Viruses can accomplish membrane fusion with a one-protein machine." The F in fruit flies may not even be a fusion protein.
If the F protein is so important, how did baculoviruses get along without it? These viruses actually generate two types of infectious particles, Blissard explains: one type specialized to infect gut cells, and another that spreads infection in other parts of the insect. The latter type uses F to enter cells. An ancestor of today's baculoviruses "might have just existed as a diarrheal-type virus, reproducing itself at a low level in the gut, letting the host survive," Blissard hypothesizes. "Picking up the f gene may have caused a radical change in the pathology of this virus, because a virus that co-exists nicely with its host will function a lot differently than one
Source:Cornell University News Service