Recombination and the population structures of bacterial pathogens.
The population structures of bacterial species are complex and often controversial. To a large extent, this is due to uncertainty about the frequency and impact of recombination in bacteria. The existence of clones within bacterial populations, and of linkage disequilibrium between alleles at different loci, is often cited as evidence for low rates of recombination. However, clones and linkage disequilibrium are almost inevitable in species that divide by binary fission and can be present in populations where recombination is frequent. In recent years, it has become possible to directly compare rates of recombination in different species. These studies indicate that in many bacterial species, including Neisseria meningitidis, Streptococcus pneumoniae, and Staphylococcus aureus, evolutionary change at neutral (housekeeping) loci is more likely to occur by recombination than mutation and can result in the elimination of any deep-rooted phylogenetic signal. In such species, the long-term evolution of the population is dominated by recombination, but this does not occur at a sufficiently high frequency to prevent the emergence of adaptive clones, although these are relatively short-lived and rapidly diversify.
High-Resolution Mapping of Crossovers Reveals Extensive Variation in Fine-Scale Recombination Patterns Among Humans
Recombination plays a crucial role in meiosis, ensuring the proper segregation of chromosomes. Recent linkage disequilibrium (LD) and sperm-typing studies suggest that recombination rates vary tremendously across the human genome, with most events occurring in narrow “hotspots.” To examine variation in fine-scale recombination patterns among individuals, we used dense, genome-wide single-nucleotide polymorphism data collected in nuclear families to localize crossovers with high spatial resolution. This analysis revealed that overall recombination hotspot usage is similar in males and females, with individual hotspots often active in both sexes. Across the genome, roughly 60% of crossovers occurred in hotspots inferred from LD studies. Notably, however, we found extensive and heritable variation among both males and females in the proportion of crossovers occurring in these hotspots.
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