Accumulation of deleterious mutations in landlocked threespine stickleback populations

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, we first conducted phylogenomic analysis and confirmed at least two independent freshwater colonization events in the central mainland from ancestral marine ecotypes. Next, analyses of single nucleotide polymorphisms (SNPs) showed a substantial reduction of heterozygosity in freshwater populations compared to marine populations. Reduction in heterozygosity was more apparent at the center of each chromosome than the peripheries and on X-chromosomes c...