On the evolutionary coexistence of parasite strains.
Classical models of parasite competition show that coexistence is impossible if different strains give complete cross-immunity. However, parasite coexistence is possible if some of the model assumptions are changed. For instance, coexistence is impossible if density-dependence operates only in hosts' fertility, but surprisingly becomes possible if hosts' mortality is density-dependent. Parasite strains can also coexist if a host already infected with one strain may become infected by another strain (superinfection). I examine here if these reasons for coexistence carry over to evolutionary timescales: in other words, suppose that potentially a continuum of parasite strains may arise by mutations; will evolution arrive at a halt? in that case, will only one or several strains persist? The paradigm and methods of adaptive dynamics are used in this study. It is found, under reasonably general assumptions, that a unique evolutionarily stable state for virulence, alpha(*), exist for both models. However, the pattern of the invasibility plots depends on the shape of the trade-off (between virulence and transmissibility, or superinfection rates) functions, and on the host demography. In many cases, the state alpha(*) is evolutionarily stable only with respect to small mutations, not to larger ones; hence, evolutionary dynamics will bring virulence to alpha(*) only if mutations are sufficiently small; for larger mutations, evolutionary dynamics are more complex and still mainly unresolved.
Pugliese
Andrea
A
Dipartimento di Matematica, Università degli Studi di Trento, Via Sommarive 14, 38050 Povo (TN), Italy. pugliese@science.unitn.it
D000818:Animals; D019143:Evolution, Molecular; D006790:Host-Parasite Interactions; D008954:Models, Biological; D009154:Mutation; D010271:Parasites / Q000235:genetics* / Q000276:immunology; D010272:Parasitic Diseases / Q000453:epidemiology / Q000276:immunology*; D015163:Superinfection / Q000453:epidemiology / Q000276:immunology