21Understanding the occurrence and spatial spread of infectious disease is a major challenge to 22 epidemiologists and evolutionary biologists. Current theory predicts the spread of highly 23 exploitative parasites at the front of spreading epidemics. However, many parasites rely on the 24 dispersal of their hosts to spread to new habitats. This may lead to a conflict between local 25 transmission and spatial spread, counteracting selection for highly virulent parasites. Yet, there 26 are no experimental tests of these hypotheses. Here we investigate parasite evolution in an 27 experiment creating conditions in cores and at fronts of spreading host-parasite populations, 28 using the freshwater host Paramecium caudatum and its bacterial parasite Holospora undulata. 29 We find that parasites from experimental range fronts induce higher rates of host dispersal than 30 parasites from the core. This divergence is accompanied by lower levels of virulence and 31 delayed development of infectious stages of front parasites. We validate these experimental 32 results by fitting an epidemiological model to time-series data independently obtained from the 33 experiment. This combined evidence suggests an evolutionary trade-off between host 34 exploitation (virulence) and host-mediated dispersal, resulting in a shift of the investment in 35 horizontal transmission. In conclusion, our results show that different segments of an epidemic 36 wave may be under divergent selection pressures, shaping the evolution of parasite life history. 37 These findings have important implications for our understanding of the interaction between 38 demography and rapid evolutionary change in spreading populations, which is crucial for the 39 management of emerging diseases, biological invasions and other non-equilibrium scenarios.40 41 42 43 2016; Penczykowski et al., 2016). In spatially homogeneous populations, theory often predicts 48 the spread of highly exploitative (i.e., virulent) parasites with a high transmission potential 49 (Andre and Hochberg, 2005). Outbreaks of disease in the wild, however, generally occur in 50 highly dynamic or patchy host populations, such as in metapopulations or in range expanding 51 and invasive species. Since many parasites spread to new habitats via host-mediated dispersal, 52 the evolution of such parasites will now depend not only on the strategies that maximise 53 transmission in response to changes in host demography (Nørgaard et al., 2019a; Penczykowski 54 et al., 2016), but also on a correlated response to the spatial sorting of hosts with different 55 dispersal capacities across a landscape (see (Calcagno et al., 2006; Perkins et al., 2013). 56 However, how the increased mobility of a host during range expansion processes (Phillips et 57 al., 2010) translates into the emergence and spread of infectious disease, remains unclear due 58 to the high complexity of ecological and evolutionary dynamics at play (see (Dunn and 59 Hatcher , 2015; Perkins et al., 2008; Torchin et al., 2003).
60By explori...