EVOLUTION OF A PATHOGEN POPULATION IN HOST MIXTURES - SIMPLE RACE-COMPLEX RACE COMPETITION

The evolution of the genetic structure of a pathogen population was st udied in a varietal mixture with an epidemic simulator based on the mo del EPIMUL. The pathogen population was composed of simple races able io develop nn only one component of the mixture and a complex race whi ch developed on all mixture components, The effects on thf simple race -complex race competition of a cost of virulence, of density dependenc e and of differential adaptation were studied, The selection for simpl e or complex races in the pathogen population did not depend on initia l race frequencies. For a given multiplication rate, complex race freq uency increased faster when the spore dispersal gradient was shallow, when distribution of initial disease was generalized, when amount of i nitial disease was reduced and when the number of mixture components w as increased, This was attributed to a better efficacy of the mixture in controlling simple races. resulting in a higher relative fitness of the complex race. For measured values of density dependence: or diffe rential adaptation effects, the complex race was at a higher frequency after a mean number of pathogen cycles between 2.5 and 5. The effect of the cost of virulence tvas stronger asci, in certain situations, co uld result in selection for simple races, In the conditions of our sim ulations and with the effects tested, stabilization of the pathogen po pulation in host mixtures was unlikely io occur. However, more informa tion is needed concerning the rare at which complex races could evolve and how quickly mixture resistance could be eroded.