Autoselection

Abstract

Parasites such as helminths or rust fungi are organisms that make a living by exploiting the growth system of their host, diverting nutrients obtained or elaborated by the host to their own growth. All organisms have systems for reproduction that are based ultimately on the replication of nucleic acids; just as the growth system of an organism can be parasitized by other organisms, so the replication system of genes can be parasitized by other genes. We have already seen that a self-replicating system cannot be perfectly precise; nor can it be perfectly specific, replicating its own sequence and no other sequence whatsoever. This creates an opportunity for elements that are not autonomously self-replicating but that, rather, utilize the common replication system of the genome. I have already described a simple case of this sort in connection with Qß (Sec. 6). The virus itself is a conventional parasite that uses the host cell as a source of raw materials. These can be assembled into new viral genomes only through the viral replicase, a diffusible protein encoded by the viral genome. The replicase, however, will also direct the replication of incomplete viral genomes that do not themselves encode the replicase. Such incomplete viruses (often called “defective interfering particles”) are parasites of the viral replication system that often appear in the later stages of viral infection. The Qß experiments, indeed, for the most part document the evolution and diversification of such defective interfering particles.