Experimental Evolution of a Trypanosome Parasite of Bumblebees and its Implications for Infection Success and Host Immune Response

Selection on basic growth properties of parasites may have many consequences for parasite traits, infection outcome, or host responses to infection. It is known that genotypes (strains) of the trypanosome parasite of bumblebees Crithidia bombi vary widely in their growth rates in their natural host, Bombus terrestris, as well as when cultured in medium. To test for changes in growth rates and their consequences, we here experimentally evolved six strains of C. bombi for fast and slow growth under controlled conditions in culture medium. Subsequently, we infected the evolved lines in live host and found that lines selected for slow growth attained higher infection intensity in the live bumblebee than those evolved for fast growth, whilst the immune response of the host was the same to both kinds of lines. These results fit the expectation that attenuation through rapid adaptation to a different environment, the culture medium, makes the parasite less successful in its next host. Selection for fast growth therefore does not necessarily lead to higher parasite success or more transmission. Hence, insect trypanosome pathogens can be attenuated by experimental evolution in the culture; this could inform important aspects of host-parasite evolution and perhaps vaccine development.