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

It is often difficult to determine why parasites do not evolve broader niches, especially when there are closely related and ecologically similar hosts available. We used an experimental evolution approach to test whether source-sink demography or trade-offs drive specialization, and its underlying traits, in two microsporidian parasites infecting two brine shrimp species. In the field, both parasites regularly infect both hosts, but experiments have shown that they are partially specialized. We serially passaged the parasites on one, the other, or an alternation of the two hosts; after 10 passages, we assayed the infectivity, virulence, and spore production of the evolved lines. Our results indicated a weak between-host trade-off acting on infectivity, but a strong trade-off acting on spore production. Consequently, spore production maintained both parasites' overall pattern of specialization. This study highlights that when trade-off shapes differ among traits, one key trait can prevent the evolution of generalism.