The evolution of host specialization has been studied intensively, yet it is still often difficult to determine why parasites do not evolve broader niches - in particular when the available hosts are closely related and ecologically similar. Here, we used an experimental evolution approach to study the evolution of parasite specialization, and its underlying traits, in two sympatric parasites: Anostracospora rigaudi and Enterocytospora artemiae, microsporidians infecting the brine shrimp

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... franciscana and Artemia parthenogenetica. In the field, both parasites regularly infect both hosts, yet experimental work has revealed that they are each semi-specialized. We serially passaged the parasites on one, the other, or an alternation of the two hosts; after ten passages, we assayed the infectivity, virulence, and spore production rate of the evolved lines. In accordance with previous studies, A. rigaudi maintained a higher fitness on A. parthenogenetica, and E. artemiae on A. franciscana, in all treatments. Further, we showed that the origin of this specialization was not infectivity, which readily evolved and showed a weak trade-off among host species for both parasites. Instead, there was a strong trade-off in spore production, suggesting that the ability to exploit the host and produce spores is the barrier preventing the evolution of generalism in this system. Intriguingly, the trait-specific trade-offs appear to be very similar for both parasites. This study highlights that the shape of between-host trade-offs can be very heterogeneous across parasite traits, so that only some traits are pivotal to specialization.