Repeated out-of-Africa expansions of Helicobacter pylori driven by replacement of deleterious mutations

All genomes mutate but the consequences of the resulting deleterious mutational load are poorly understood. Helicobacter pylori lives in the human stomach, has a higher mutation rate than most bacteria and has accompanied anatomically modern humans in migrations including the out-of-Africa expansion more than 50,000 years ago. H. pylori from East Asia have accumulated at least 500 more non-synonymous mutations than African strains, which we propose is due to reduced efficacy of selection during the out-of-Africa bottleneck. H. pylori from Europe and the Middle East trace a substantially higher fraction of ancestry from modern African populations than the humans that carry them, which we find is due to at least three separate admixture events. African ancestry is elevated at positions in the genome where non-synonymous mutations are at high frequency in Asia. We propose that this is due to replacement of deleterious mutations that accumulated during the bottleneck, with the high overall African ancestry proportion due to clonal expansion of strains of African origin. We use simulations to show that a Muller9s ratchet like effect can lead to long-term segregation of deleterious mutations within bacterial populations after a bottleneck, despite high rates of homologous recombination, but that population fitness can be restored by migration of small numbers of bacteria from non-bottlenecked populations. Our results demonstrate that population bottlenecks can have long-term genomic and demographic consequences, even in species with enormous population sizes.