Colonization of new habitats often reduces population sizes and may result in the accumulation of deleterious mutations by genetic drift. Compared to the genomic basis for adaptation to new environments, genome-wide analysis of deleterious mutations in isolated populations remains limited. In the present study, we investigated the accumulation of deleterious mutations in five endangered freshwater populations of threespine stickleback (Gasterosteus aculeatus) in the central part of the mainland of Japan. Using whole genome resequencing data,...