Abstract:
Hybridization plays an important and underappreciated role in shaping
the evolutionary trajectories of species. Following the introduction of
a non-native organism to a novel habitat, hybridization with a native
congener may affect the probability of establishment of the introduced
species. In most documented cases of hybridization between a native and
a non-native species, a mosaic hybrid zone is formed, with hybridization
occurring heterogeneously across the landscape. In contrast, most
naturally occurring hybrid zones are clinal in structure. Here we report
on a long-term microsatellite dataset that monitored hybridization
between the invasive winter moth, Operophtera brumata(Lepidoptera: Geometridae), and the native Bruce spanworm, O.
bruceata, over a 12-year period. Our results document one of the first
examples of the real-time formation and geographic settling of a clinal
hybrid zone. In addition, by comparing one transect in Massachusetts
where extreme winter cold temperatures have been hypothesized to
restrict the distribution of winter moth, and one in coastal
Connecticut, where winter temperatures are moderated by Long Island
Sound, we find that the location of the hybrid zone appears to be
independent of environmental variables and maintained under a tension
model wherein the stability of the hybrid zone is constrained by
population density, reduced hybrid fitness, and low dispersal rates.
Documenting the formation of a contemporary clinal hybrid zone may
provide important insights into the factors that shaped other
well-established hybrid zones.
Keywords: Tension zone, hybrid fitness, introgression,
Lepidoptera, forest pest.