The overall goal of this project is to investigate how natural selection maintains variation in immune genes, using a wild rodent (the bank vole) and its pathogens (e.g. Borrelia) as study system. For this purpose, we have re-sequenced bank voles from two populations. We have also obtained rna-seq data to investigate functional effects of polymorphisms in immune genes and whether they show cis-regulatory variation (by testing for allele-specific expression). These data have been used to perform a genome-wide scan for signatures of balancing selection (i.e. natural selection that maintains variation within populations) and investigate the functional consequences of balanced polymorphisms. These analyses are now nearing completion. During 2026, we will focus on two new questions: (i) What is the relative importance of balancing selection within populations and divergent selection among populations as drivers of global genetic variation? For this purpose, we will obtain additional population samples of WGS data. (ii) How does expression of immune genes vary between seasons, sexes and populations? This will be based on existing data and a set of new samples sent for sequencing at NGI. We might also need to revisit previous analyses, prompted by revision of submitted manuscripts. Our study species, the bank vole (Myodes glareolus), is widely distributed in Europe and is a popular study species for various ecological and evolutionary questions. Råberg’s group has been using the bank vole as study species for research on the evolutionary ecology of host-pathogen interactions since 2006. Since 2017 we have generated several sets with NGS data, including Illumina WGS data (>30x) from N=56 bank voles, several sets with RNA-seq from various tissues from bank voles, and PacBio WGS of a closely related species (grey sided vole) for use as outgroup in population genomic analyses of the bank vole data. During 2026 we plan to resequence an additional 50 bank voles to complement the existing WGS data (samples collected, will be sent to NGI for sequencing within a couple of months).