Mutualism, parasitism and competition in the evolution of coviruses

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Abstract
Coviruses are viruses with the property that their genetic information is divided up among two or more different viral particles. I model the evolution of coviruses using information on both viral virulence and the interactions between viruses and molecules that parasitize them: satellite viruses, satellite RNAs and defective interfering viruses. The model ultimately, and inevitably, contains within it single–species dynamics as well as mutualistic, parasitic, cooperative and competitive relationships. The model shows that coexistence between coviruses and the self–sufficient viruses that spawned them is unlikely, in the sense that the quantitative conditions for coexistence are not easy to satisfy. I also describe an abrupt transition from mutualistic two–species to single–species dynamics, showing a new sense in which questions such as ‘Is a lichen one species or two?’ can be given a definite answer.