Graphical analysis of evolutionary trade-off in sylvatic Trypanosoma cruzi transmission modes

The notion of evolutionary trade-off (one attribute increasing at the expense of another) is central to the evolution of traits, well-studied especially in life-history theory, where a framework first developed by Levins illustrates how internal (genetics) and external (fitness landscapes) forces interact to shape an organism׳s ongoing adaptation. This manuscript extends this framework to the context of vector-borne pathogens, with the example of Trypanosoma cruzi (the etiological agent of Chagas׳ disease) adapting via trade-off among three different infection routes to hosts-stercorarian, vertical, and oral-in response to an epidemiological landscape that involves both hosts and vectors (where, in particular, parasite evolution depends not on parasite density but on relative host and vector densities). Using a fitness measure derived from an invasion reproductive number, this study analyzes several different trade-off scenarios in cycles involving raccoons or woodrats, including a proper three-way trade-off (two independent parameters). Results indicate that selection favors oral transmission to raccoons but classical stercorarian transmission to woodrats even under the same predation rate, with vertical (congenital) transmission favored only when aligned with dominant oral transmission or (at trace levels) under a weak (convex) trade-off.