PCA of genomic inversions in Atlantic cod. Ancient specimens (stars) were projected onto the first two principal components calculated by using individuals from modern populations (circles). The first principal component (PCA 1) separates genomic variation within each of the 4-Mbp-long regions (LG01, 9.1–26.2 Mbp; LG02, 18.5–24 Mbp; LG07, 13.6–23 Mbp; LG12, 1.3–13.6 Mbp) into distinct clusters (gray dotted ovals) that reflect the biallelic segregation of the three major inversion genotypes (AA, collinear; AB, heterozygote; BB, inverted). The number of SNPs (n) used per region is indicated. Mean heterozygosity values per genotype (presented in gray under each genotype; estimated by calculating the inbreeding coefficient F using a method of moments as implemented in VCFTOOLS v0.1.14) show the marked decrease in F values for the AB genotypes due to heterozygote excess. Ancient samples follow the trimodal cluster pattern of the modern individuals.