Competition among host-specific lineages of mites influences the extent of co-adaptation with their burying beetle hosts.

Reciprocal selection between symbiotic organisms and their hosts can generate variations in local adaptation between them. Symbionts often form species complexes with lineages partially adapted to various hosts. However, it is unclear how interactions among these lineages influences geographic variation in the extent of host-symbiont local adaptation. We addressed this shortcoming with experiments on burying beetles and their specialist phoretic mite in two adjacent woodlands. Burying beetles transport these mites to vertebrate carrion upon which they both reproduce. appears to be a species complex, with distinct lineages that specialise on breeding alongside different species. We found that in one wood (Gamlingay Woods), carries a mixture of mite lineages, with each lineage corresponding to one of the four species that inhabits this wood. However, two burying beetle species coexist in neighbouring Waresley Woods and here predominantly carries the mite lineage that favours . Mite lineage mixing alters the degree of local adaptation for both and the mites, affecting reproductive success variably across different woodlands. In Gamlingay, mite lineage mixing reduced reproductive success, while experimentally purifying mites lineage enhanced it. The near pure lineage of mites negligibly affected Waresley . Mite reproductive success varied with host specificity: Gamlingay mites had greatest reproductive success on Gamlingay beetles, and performed less well with Waresley beetles. By contrast, Waresley mites had consistent reproductive success, regardless of beetle's woodland of origin. We conclude that there is some evidence that and its specific mite lineage have coadapted. However, neither nor its mite lineage adapted to breed alongside other mite lineages. This, we suggest, causes variation between Waresley and Gaminglay Woods in the extent of local adaptation between beetles and their mites.