The dynamics of a senescence plasmid in fungal populations

Fungi normally do not senesce, but in some species mitochondrial plasmids a re known to occur that induce senescence. In this paper models for the dyna mics of a senescence plasmid in a fungal population are developed and analy sed. In the first model it is assumed that total fungal biomass density is constant, while in the second model the resource dynamics and its effect on fungal growth is modelled explicitly. An additional death rate describes t he effect of the plasmid on the senescent subpopulation. Plasmids can be tr ansferred to non-senescent fungus. Criteria for the coexistence of the non- senescent and senescent fungal strains are derived, all of which have a cle ar biological interpretation. It is shown that coexistence is not possible in the first model, but is possible in the second model for a large range o f parameter values. We show that the interplay between resource dynamics, f ungal growth and plasmid transmission is crucial for coexistence. We develo p a biological interpretation of how these mechanisms have to interact to p romote coexistence. A numerical study of the second model further clarifies the relations between the numerical value of several parameters and coexis tence of non-senescent and senescent fungal strains.