Sex, horizontal transmission, and multiple hosts prevent local adaptation of Crithidia bombi, a parasite of bumblebees (Bombus spp.)

Local adaptation within host-parasite systems can evolve by several non-exclusive drivers (e.g., host species-genetic adaptation; ecological conditions-ecological adaptation, and time-temporal adaptation). Social insects, especially bumblebees, with an annual colony life history not only provide an ideal system to test parasite transmission within and between different host colonies, but also parasite adaptation to specific host species and environments. Here, we study local adaptation in a multiple-host parasite characterized by high levels of horizontal transmission. Crithidia bombi occurs as a gut parasite in several bumblebee species. Parasites were sampled from five different host species in two subsequent years. Population genetic tools were used to test for the several types of adaptation. Although we found no evidence for local adaptation of the parasite toward host species, there was a slight temporal differentiation of the parasite populations, which might have resulted from severe bottlenecks during queen hibernation. Parasite populations were in Hardy-Weinberg equilibrium and showed no signs of linkage disequilibrium suggesting that sexual reproduction is an alternative strategy in this otherwise clonal parasite. Moreover, high levels of multiple infections were found, which might facilitate sexual genetic exchange. The detection of identical clones in different host species suggested that horizontal transmission occurs between host species and underpins the lack of host-specific adaptation.     

A quantitative in vitro cultivation technique to determine cell number and growth rates in strains of Crithidia bombi (Trypanosomatidae), a parasite of bumblebees

The protozoan parasite Crithidia bombi and its host, the bumblebee Bombus terrestris, are used as a model system for the study of the evolutionary ecology of host-parasite interactions. In order to study these interactions we established a method for in vitro cultivation of single parasite strains. Additionally, a high-throughput method is developed for the determination of cell numbers in cultures by means of optical density (OD) measurements. The protocol for in vitro cultivation allowed for growing different strains on agar plates as well as in culture medium. A calibration curve for the relationship between cell number and OD has been developed. Subsequently, growth rates for different genotypes of C. bombi have been recorded. Significant differences in the growth rates and generation times between these genotypes were demonstrated. As this might be related to the virulence of the parasite, this relationship may be confirmed by in vivo growth rate determination. In comparison with conventional cell counting, the application of OD measurements allows for high-throughput experiments as the time taken to record each sample is reduced by a factor of 30. The in vitro cultivation method allows for controlled infection experiments in order to study host-parasite interactions.     

How a haemosporidian parasite of bats gets around: the genetic structure of a parasite,vector and host compared

Parasite population structure is often thought to be largely shaped by that of its host. In the case of a parasite with a complex life cycle, two host species, each with their own patterns of demography and migration, spread the parasite. However, the population structure of the parasite is predicted to resemble only that of the most vagile host species. In this study, we tested this prediction in the context of a vector‐transmitted parasite. We sampled the haemosporidian parasite Polychromophilus melanipherus across its European range, together with its bat fly vector Nycteribia schmidlii and its host, the bent‐winged bat Miniopterus schreibersii. Based on microsatellite analyses, the wingless vector, and not the bat host, was identified as the least structured population and should therefore be considered the most vagile host. Genetic distance matrices were compared for all three species based on a mitochondrial DNA fragment. Both host and vector populations followed an isolation‐by‐distance pattern across the Mediterranean, but not the parasite. Mantel tests found no correlation between the parasite and either the host or vector populations. We therefore found no support for our hypothesis; the parasite population structure matched neither vector nor host. Instead, we propose a model where the parasite's gene flow is represented by the added effects of host and vector dispersal patterns.