Reference genome bias in light of species-specific chromosomal reorganization and translocations

Abstract

Summary Whole-genome sequencing efforts has during the past decade unveiled the central role of genomic rearrangements—such as chromosomal inversions—in evolutionary processes, including local adaptation in a wide range of taxa. However, employment of reference genomes from distantly or even closely related species for mapping and the subsequent variant calling, can lead to errors and/or biases in the datasets generated for downstream analyses. Here, we capitalize on the recently generated chromosome-anchored genome assemblies for Arctic cod ( Arctogadus glacialis ), polar cod ( Boreogadus saida ), and Atlantic cod ( Gadus morhua ) to evaluate the extent and consequences of reference bias on population sequencing datasets (approx. 15-20x coverage) for both Arctic cod and polar cod. Our findings demonstrate that the choice of reference genome impacts population genetic statistics, including individual mapping depth, heterozygosity levels, and cross-species comparisons of nucleotide diversity (π) and genetic divergence (D XY ). Further, it became evident that using a more distantly related reference genome can lead to inaccurate detection and characterization of chromosomal inversions, i.e., in terms of size (length) and location (position), due to inter-chromosomal reorganizations between species. Additionally, we observe that several of the detected species-specific inversions were split into multiple genomic regions when mapped towards a heterospecific reference. Inaccurate identification of chromosomal rearrangements as well as biased population genetic measures could potentially lead to erroneous interpretation of species-specific genomic diversity, impede the resolution of local adaptation, and thus, impact predictions of their genomic potential to respond to climatic and other environmental perturbations.