Résumé
Historical patterns of dispersal and population isolation are key
components shaping contemporary genetic diversity across landscapes and
require explicit consideration when examining the relative role of
different factors in driving the evolution of host specificity in
parasitic organisms. In this study, we investigate the worldwide
colonisation history of a common ectoparasite of seabirds, the tick Ixodes
uriae. This tick has a circumpolar distribution across both hemispheres,
but has repeatedly formed host-specific races within different regions. By
combining mitochondrial and nuclear data, we infer how this species spread
to its present-day distribution and how the colonisation process has
affected the geographic and host-associated structure of this tick within
regions. We demonstrate that I. uriae is highly structured and isolates
into four genetic groups that correspond to well-defined geographical
regions. Molecular dating suggests that the diversification of I. uriae
began in the early Miocene (22 Myr) and that this tick colonised most of
the Southern hemisphere before moving into northern latitudes via two
independent routes. However, no relationship between the degree of host
race divergence and colonisation history was evident, supporting previous
hypotheses that host specialization evolves relatively rapidly in this
parasite, but does not typically lead to speciation. We discuss the
possible historical and contemporary mechanisms of large scale dispersal
for this ectoparasite and how its biological characteristics may condition
current patterns of genetic diversity. More generally, our results
illustrate how combining broad-scale sampling and modern molecular tools
can help disentangle complex patterns of diversification in widespread
parasites.