Weak population genetic structure in Eurasian spruce bark beetle over large regional scales in Sweden

The Eurasian spruce bark beetle, Ips typographus, is a major pest, capable
of killing spruce forests during large population outbreaks. Recorded
dispersal distances of individual beetles are typically within hundreds of
meters or a few kilometres. However, the connectivity between populations
at larger distances and longer time spans and how this is affected by the
habitat is less studied, despite its importance for understanding at which
distances local outbreaks may spread. Previous population genetic studies
in I. typographus typically used low resolution markers. Here, we use
genome-wide data to assess population structure and connectivity of I.
typographus in Sweden. We used 152 individuals from 19 population samples,
distributed over 830 km from Strömsund (63º 46' 8'' N) in
the north to Nyteboda (56º 8' 50'' N) in the south, to
capture processes at a large regional scale, and a transect sampling
design adjacent to a recent outbreak to capture processes at a smaller
scale (76 km). Using restriction site-associated DNA sequencing (RADseq)
markers capturing 1409-1997 SNPs throughout the genome, we document a weak
genetic structure over the large scale, potentially indicative of high
connectivity with extensive gene flow. No differentiation was detected at
the smaller scale. We find indications of isolation-by-distance both for
relative (FST) and absolute divergence (Dxy). The two northernmost
populations are most differentiated from the remaining populations, and
diverge in parallel to the southern populations for a set of outlier loci.
In conclusion, the population structure of I. typographus in Sweden is
weak, suggesting a high capacity to disperse and establish outbreak
populations in new territories.