"Tandem repeat variation in partially selfing populations"Sudbrack, VitorTandem Repeat sequences (TRs) are a significant source of genetic structural variation within and between individuals. These sequences occur when a short DNA motif is repeated head-to-tail along a chromosome. Variation in TRs is thought to reflect a balance between purifying selection, amplification, unequal recombination, and genetic drift. The influence of these evolutionary processes on TRs is well understood in large haploid populations where individuals mate randomly. But many organisms are capable of selfing, which leads to inbreeding and an excess of homozygosity, with relevant consequences for the evolutionary processes that shape variation in TRs. Here, we use mathematical modeling to investigate the evolutionary dynamics of homologous TRs under the joint actions of purifying selection, amplification, unequal recombination, and genetic drift in a population of partially selfing diploids. We find that the equilibrium distribution of TRs in the population depends on the interaction between unequal recombination and selfing. When the selfing rate is low and homologous sequences within individuals tend to be different, unequal recombination reduces variation. However, when selfing is common and homologous sequences within individuals are more similar, unequal recombination increases the variation in TRs. In addition, because selfing increases variation between individuals relative to within individuals, purifying selection tends to be stronger in selfers compared to outcrossers. As a result, selfers have on average shorter TRs, in spite of experiencing increased genetic drift. Together, our findings suggest that selfing and inbreeding are important factors in shaping TRs and, more broadly, genetic structural variation. |
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