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The Recombinational Landscape in Daphnia pulex

Through the analysis of linkage disequilibrium from genome-wide sequencing data for multiple individuals from eight populations, the general features of the recombinational landscape are revealed in the microcrustacean Daphnia pulex. The data suggest an exceptionally uniform pattern of recombination across the genome, while also confirming general patterns that are inconsistent with existing population-genetic models for the relationship between linkage dis-equilibrium and physical distances between genomic sites. Patterns of linkage disequilibrium are highly consistent among populations, and average rates of recombination are quite similar for all chromosomes. There is no evidence of recombination hotspots, and although there does appear to be suppressed recombination in the vicinity of gene bodies, this effect is quite small. Although this species reproduces asexually in ∼ 80% of generations, the mean per-generation recombination rate per nucleotide site is ∼ 37× the per-nucleotide mutation rate. Contrary to expectations for models in which crossing-over is the primary mechanism of recombination, and consistent with data for other species, the gradient of linkage disequilibrium with increasing physical distance between sites is far too high at short distances and far too low at long distances, suggesting an important role for factors such as the nonindependent appearance of pairs of mutations on haplotypes and long-range gene-conversion-like processes. Combined with other observations on patterns of nucleotide variation, these results provide a strong case for the utility of D. pulex as a model system for the study of mechanisms of evolution in natural populations.