Mutualism, parasitism and competition in the evolution of coviruses

Coviruses are viruses with the property that their genetic information is d ivided up among two or more different viral particles. I model the evolutio n of coviruses using information on both viral virulence and the interactio ns between viruses and molecules that parasitize them: satellite viruses, s atellite RNAs and defective interfering viruses. The model ultimately and i nevitably, contains within it single-species dynamics as well as mutualisti c, parasitic, cooperative and competitive relationships. The model shows th at coexistence between coviruses and the self-sufficient viruses that spawn ed them is unlikely, in the sense that the quantitative conditions for coex istence are not easy to satisfy I also describe an abrupt transition from m utualistic two-species to single-species dynamics, showing a new sense in w hich questions such as 'Is a lichen one species or two? can be given a defi nite answer.