Trait-specific trade-offs prevent niche expansion in two parasites

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 host specialization, and its underlying traits, in two sympatric parasites: Anostracospora rigaudi and Enterocytospora artemiae , microsporidians infecting the brine shrimp Artemia franciscana and Artemia parthenogenetica . In the field, both parasites regularly infect both hosts, yet experimental work has revealed that they are each partially 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. The origin of this specialization was not infectivity, which readily evolved and traded off weakly between the host species for both parasites. Instead, the overall specialization was caused by spore production, which did not evolve in any treatment. This suggests the existence of a strong trade-off between spore production in A. franciscana and spore production in A. parthenogenetica , making this trait a barrier to the evolution of generalism in this system. 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.