The first steps toward a global pandemic: Reconstructing the demographic
history of parasite host switches in its native range
Abstract
Host switching allows parasites to expand their niches. However,
successful switching may require suites of adaptations and also may
decrease performance on the old host. As a result, reductions in gene
flow accompany many host switches, driving speciation. Because host
switches tend to be rapid, it is difficult to study their demographic
parameters in real-time. Fundamental factors that control subsequent
parasite evolution, such as the size of the switching population or the
extent of immigration from the original host, remain largely unknown. To
shed light on the host switching process, we explored the history of
independent switches by two ectoparasitic honey bee mites (Varroa
destructor and V. jacobsoni). Both switched to the western honey bee
(Apis mellifera) after it was brought into contact with their ancestral
host (Apis cerana), ~70 and ~12 years
ago, respectively. Varroa destructor subsequently caused worldwide
collapses of honey bee populations. Using whole-genome sequencing on 63
mites collected in their native ranges from both the ancestral and novel
hosts, we were able to reconstruct the known temporal dynamics of the
switch. We further found multiple previously undiscovered mitochondrial
lineages on the novel host, along with the genetic equivalent of tens of
individuals that were involved in the initial host switch. Despite being
greatly reduced, some gene flow remains between mites adapted to
different hosts. Our findings suggest that while reproductive isolation
may facilitate the fixation of traits beneficial for exploitation of the
new host, ongoing genetic exchange may allow genetic amelioration of
inbreeding effects.