Gene flow and adaptive potential in a generalist ectoparasite

Abstract

Background: In host-parasite systems, relative dispersal rates condition genetic novelty within populations and thus their adaptive potential. Knowledge of host and parasite dispersal rates can therefore help us to understand current interaction patterns in wild populations and why these patterns shift over time and space. For generalist parasites however, estimates of dispersal rates depend on both host range and the considered spatial scale. Here, we assess the relative contribution of these factors by studying the population genetic structure of a common avian ectoparasite, the hen flea Ceratophyllus gallinae, exploiting two hosts that are sympatric in our study population, the great tit Parus major and the collared flycatcher Ficedula albicollis. Previous experimental studies have indicated that the hen flea is both locally maladapted to great tit populations and composed of subpopulations specialized on the two host species, suggesting limited parasite dispersal in space and among hosts, and a potential interaction between these two structuring factors.

Results: C. gallinae fleas were sampled from old nests of the two passerine species in three replicate wood patches and were genotyped at microsatellite markers to assess population genetic structure at different scales (among individuals within a nest, among nests and between host species within a patch and among patches). As expected, significant structure was found at all spatial scales and between host species, supporting the hypothesis of limited dispersal in this parasite. Clustering analyses and estimates of relatedness further suggested that inbreeding regularly occurs within nests. Patterns of isolation by distance within wood patches indicated that flea dispersal likely occurs in a stepwise manner among neighboring nests. From these data, we estimated that gene flow in the hen flea is approximately half that previously described for its great tit hosts.

Conclusion: Our results fall in line with predictions based on observed patterns of adaptation in this host-parasite system, suggesting that parasite dispersal is limited and impacts its adaptive potential with respect to its hosts. More generally, this study sheds light on the complex interaction between parasite gene flow, local adaptation and host specialization within a single host-parasite system.

Keywords: Ceratophyllus gallinae; Dispersal; Ecological specialization; Ficedula albicollis; Habitat fragmentation; Local adaptation; Multi-host system; Parus major; Population genetics; Spatial scale.

Fig. 1

Location of the sampling sites on Gotland

Fig. 2

Hierarchical levels of the study design

Fig. 3

a F_is (f ± SE) and (B) F_st (θ ± SE) for each locus, over all infrapopulations. A star (*) next to the locus label indicates cases where p < 0.05, all markers gave a significant estimate in (b)

Fig. 4

Discriminant analysis of principal components representing among-nest genetic structure at the among patch scale. Proportion of among group variation is 47.0% on the horizontal and 29.7% on the vertical axis

Fig. 5

Discriminant analysis of principal components representing among-nest genetic structure within patches. a Fleringe: Proportion of between group variation is 41.2% on the horizontal and 26.1% on the vertical axis. b Hall: Proportion of between group variation is 35.5% on the horizontal and 20.4% on the vertical axis. c Hammarsänget: Proportion of between group variation is 38.6% on the horizontal and 34.8% on the vertical axis

Fig. 6

Per infrapopulation average relatedness (blue lines) ± SE (black lines). Red lines show the upper and lower bounds of expected values under a random hypothesis. A star (*) next to the nest label indicates cases where p < 0.05