Evidence for Nonindependent Evolution of Adjacent Microsatellites in the Human Genome

The effects of transcription and recombination on mutational dynamics of short tandem repeats

Short tandem repeats (STR) are ubiquitous components of the genomic architecture of most living organisms. Recent work has highlighted the widespread functional significance of such repeats, particularly around gene regulation, but the mutational processes underlying the evolution of these highly abundant and highly variable sequences are not fully understood. Traditional models assume that strand misalignment during replication is the predominant mechanism, but empirical data suggest the involvement of other processes including recombination and transcription. Despite this evidence, the relative influences of these processes have not previously been tested experimentally on a genome-wide scale. Using deep sequencing, we identify mutations at >200 microsatellites, across 700 generations in replicated populations of two otherwise identical sexual and asexual Saccharomyces cerevisiae strains. Using generalized linear models, we investigate correlates of STR mutability including the nature of the mutation, STR composition and contextual factors including recombination, transcription and replication origins. Sexual capability was not a significant predictor of microsatellite mutability, but, intriguingly, we identify transcription as a significant positive predictor. We also find that STR density is substantially increased in regions neighboring, but not within, recombination hotspots.

Genetic variability, evidence of potential recombinational event and selection of LEI0258 in chicken

The chicken major histocompatibility complex (MHC) plays an important role in the immune response, disease resistance, productivity, and other important economic traits of the chicken. Therefore, a description of the polymorphisms of this region is crucial for understanding the genetic pattern of the MHC. The tandem repeat LEI0258 is located within the B region of the chicken MHC and is surprisingly strongly associated with serology. This marker has been used worldwide to provide a picture of the core area of the chicken MHC-B region and to categorize chicken MHC haplotypes. Thus, insight into the evolutionary pattern of LEI0258 may be useful for understanding MHC diversity. In the current study, 30 alleles of LEI0258 from 12 populations were screened and sequenced, and alleles that have previously been published in GenBank were also analyzed. The resulting 124 alleles were classified into four clusters according to the SNPs and indels found within the sequences flanking the repeats. Furthermore, a recombination region was identified between -30 and +43 that suggests that recombination may have played a role in the evolution of this MHC. Finally, strong evidence regarding the selection and evolutionary dynamics of the LEI0258 region is presented. Generally speaking, microsatellite is a classic anonymous marker which changes by genetic drift rather than by direct selection. Although, the genotypes of LEI0258 in MHC-B correlate with serology, its mechanism of inheritance and evolution was unclear. This study not only establishes a framework of further diversity or association studies in LEI0258, but also unraveling the reason what driving force and formulate the evolutionary dynamics of this region.

Measuring microsatellite conservation in mammalian evolution with a phylogenetic birth-death model

Microsatellites make up ∼3% of the human genome, and there is increasing evidence that some microsatellites can have important functions and can be conserved by selection. To investigate this conservation, we performed a genome-wide analysis of human microsatellites and measured their conservation using a binary character birth--death model on a mammalian phylogeny. Using a maximum likelihood method to estimate birth and death rates for different types of microsatellites, we show that the rates at which microsatellites are gained and lost in mammals depend on their sequence composition, length, and position in the genome. Additionally, we use a mixture model to account for unequal death rates among microsatellites across the human genome. We use this model to assign a probability-based conservation score to each microsatellite. We found that microsatellites near the transcription start sites of genes are often highly conserved, and that distance from a microsatellite to the nearest transcription start site is a good predictor of the microsatellite conservation score. An analysis of gene ontology terms for genes that contain microsatellites near their transcription start site reveals that regulatory genes involved in growth and development are highly enriched with conserved microsatellites.

Genome nucleotide composition shapes variation in simple sequence repeats

Simple sequence repeats (SSRs) or microsatellites are a common component of genomes but vary greatly across species in their abundance. We tested the hypothesis that this variation is due in part to AT/GC content of genomes, with genomes biased toward either high AT or high CG generating more short random repeats that are long enough to enhance expansion through slippage during replication. To test this hypothesis, we identified repeats with perfect tandem iterations of 1-6 bp from 25 protists with complete or near-complete genome sequences. As expected, the density and the frequency are highly related to genome AT content, with excellent fits to quadratic regressions with minima near a 50% AT content and rising toward both extremes. Within species, the same trends hold, except the limited variation in AT content within each species places each mainly on the descending (GC rich), middle, or ascending (AT rich) part of the curve. The base usages of repeat motifs are also significantly correlated with genome nucleotide compositions: Percentages of AT-rich motifs rise with the increase of genome AT content but vice versa for GC-rich subgroups. Amino acid homopolymer repeats also show the expected quadratic relationship, with higher abundance in species with AT content biased in either direction. Our results show that genome nucleotide composition explains up to half of the variance in the abundance and motif constitution of SSRs.

Mutation biases and mutation rate variation around very short human microsatellites revealed by human-chimpanzee-orangutan genomic sequence alignments

I have studied mutation patterns around very short microsatellites, focusing mainly on sequences carrying only two repeat units. By using human-chimpanzee-orangutan alignments, inferences can be made about both the relative rates of mutations and which bases have mutated. I find remarkable non-randomness, with mutation rate depending on a base's position relative to the microsatellite, the identity of the base itself and the motif in the microsatellite. Comparing the patterns around AC2 with those around other four-base combinations reveals that AC2 does not stand out as being special in the sense that non-repetitive tetramers also generate strong mutation biases. However, comparing AC2 and AC3 with AC4 reveals a step change in both the rate and nature of mutations occurring, suggesting a transition state, AC4 exhibiting an alternating high-low mutation rate pattern consistent with the sequence patterning seen around longer microsatellites. Surprisingly, most changes in repeat number occur through base substitutions rather than slippage, and the relative probability of gaining versus losing a repeat in this way varies greatly with repeat number. Slippage mutations reveal rather similar patterns of mutability compared with point mutations, being rare at two repeats where most cause the loss of a repeat, with both mutation rate and the proportion of expansion mutations increasing up to 6-8 repeats. Inferences about longer repeat tracts are hampered by uncertainties about the proportion of multi-species alignments that fail due to multi-repeat mutations and other rearrangements.

Heterogeneous distribution of SNPs in the human genome: microsatellites as predictors of nucleotide diversity and divergence

Understanding the forces that govern the distribution of single nucleotide polymorphisms is vital for many of their applications. Here we conducted a systematic search to quantify how both SNP density and human-chimpanzee divergence vary around different repetitive sequences. We uncovered a highly complicated picture in which these quantities often differ significantly from the genome-wide average in regions extending more than 20 kb, the direction of the deviation varying with repeat number and motif. AT microsatellites in particular are potent predictors of SNP density, long (AT)(n) repeat tracts tending to be found in regions of significantly reduced SNP density and low GC content. Although the causal relationships remain difficult to determine, our results indicate a strong relationship between microsatellites and the DNA that flanks them. Our results help to explain the mixed picture that emerges from other studies and have important implications for the way in which genetic diversity is distributed in our genomes.