Post-glacial colonization of the Fennoscandian coast by a plant parasitic insect with an unusual life history

Species that exhibit very peculiar ecological traits combined with limited
dispersal ability pose a challenge to our understanding of ecological and
evolutionary mechanisms. This is especially true when they have managed to
spread over long distances, overcome physical barriers and colonise large
areas. Climate and landscape changes, trophic web relations as well as
life history all interact to shape migration routes and present-day
species distributions and their population genetic structures. Here we
analysed the post-glacial colonization of northern Europe by the gall
midge Contarinia vincetoxici, which is a monophagous parasite on the
perennial herb White swallowwort (Vincetoxicum hirundinaria). This insect
not only has a narrow feeding niche but also limited dispersal ability and
an exceptionally long dormancy. Gall midge larvae (n = 329) were collected
from 16 sites along its distribution range in Denmark, Sweden, and
Finland. Using microsatellite loci and knowledge of the species and the
regions’ history, we investigated the role of landscape change, host plant
distribution, insect population dynamics, and life history in shaping the
population genetic structure of the insect. We devoted particular interest
to the role of the insect’s presumed poor dispersal capacity in
combination with its exceptionally extended diapause. We found significant
levels of local inbreeding (95% highest posterior density interval =
0.42–0.47), low level within-population heterozygosity (mean HE = 0.45,
range 0.20–0.61) with private alleles in all populations except two. We
also found significant (p < 0.001) regional isolation-by-distance
patterns, suggesting regularly recurring mainly short-distance dispersal.
According to approximate Bayesian computations, C. vincetoxici appears to
have colonized the study area via wind-aided flights from remote areas
approximately 4600 to 700 years before present when the land has gradually
risen above the sea level. Extremely long dormancy periods have allowed
the species to “disperse in time”, thereby aiding population persistence
despite generally low census population sizes.