2.2 DNA extraction and sequencing
All individual samples were snap frozen in liquid nitrogen and stored at -80° before DNA extraction. Genomic DNA were extracted from the whole adult body using KingFisher™ Cell and Tissue DNA Kit using the sample preparation protocol “DNA Extraction from Single Insects”. After extraction, DNA concentrations were measured with a Qubit 3.0 Fluorometer using the dsDNA HS Assay Kit (Thermo Fisher Scientific) and Nanodrop 8000 to ensure an absorbance ratio at 260/280 between 1.7 and 2. We estimated DNA fragmentation using agarose gel electrophoresis stained with 2% GelRed and only retained samples with minimal degradation. Library preparation was performed with the Illumina TruSeq DNA PCR-free library preparation kit and then paired-end 2×150-bp sequenced on a NovaSeq6000 platform at SciLifeLab, Sweden. Library preparation failed for one G. calmariensis sample from Våtnäs and this sample was excluded from downstream analysis. The total number of samples with whole-genome resequencing data was thus 44 and we generated 1.6 Gbp of sequence data (>Q30) in total (out of 1.8 Gbp), corresponding to an average of 34.8 Mbp per sample.