Multiple host shifts by the emerging honeybee parasite, Varroa jacobsoni

Hostshifts are a key mechanism of parasite evolution and responsible for the emergence of many economically important pathogens. Varroa destructor has been a major factor in global honeybee (Apis mellifera) declines since shifting hosts from the Asian honeybee (Apis cerana) >50years ago. Until recently, only two haplotypes of V.destructor (Korea and Japan) had successfully hostshifted to A.mellifera. In 2008, the sister species V.jacobsoni was found for the first time parasitizing A.mellifera in Papua New Guinea (PNG). This recent hostshift presents a serious threat to world apiculture but also provides the opportunity to examine hostshifting in this system. We used 12 microsatellites to compare genetic variation of V.jacobsoni on A.mellifera in PNG with mites on A.cerana in both PNG and surrounding regions. We identified two distinct lineages of V.jacobsoni reproducing on A.mellifera in PNG. Our analysis indicated independent hostshift events have occurred through small numbers of mites shifting from local A.cerana populations. Additional lineages were found in the neighbouring Papua and Solomon Islands that had partially hostshifted to A.mellifera, that is producing immature offspring on drone brood only. These mites were likely in transition to full colonization of A.mellifera. Significant population structure between mites on the different hosts suggested hostshifted V.jacobsoni populations may not still reproduce on A.cerana, although limited gene flow may exist. Our studies provide further insight into parasite hostshift evolution and help characterize this new Varroa mite threat to A.mellifera worldwide.