scholarly journals The genomic landscape of recombination rate variation in Chlamydomonas reinhardtii reveals a pronounced effect of linked selection

2018 ◽  
Author(s):  
Ahmed R. Hasan ◽  
Rob W. Ness
Keyword(s):  
Chlamydomonas Reinhardtii ◽  
Recombination Rate ◽  
Natural Populations ◽  
Gc Content ◽  
Sexual Cycle ◽  
Rate Variation ◽  
Effective Population ◽  
Entire Genome ◽  
Recombination Rate Variation ◽  
Linked Selection

AbstractRecombination confers a major evolutionary advantage by breaking up linkage disequilibrium (LD) between harmful and beneficial mutations and facilitating selection. Here, we use genome-wide patterns of LD to infer fine-scale recombination rate variation in the genome of the model green alga Chlamydomonas reinhardtii and estimate rates of LD decay across the entire genome. We observe recombination rate variation of up to two orders of magnitude, finding evidence of recombination hotspots playing a role in the genome. Recombination rate is highest just upstream of genic regions, suggesting the preferential targeting of recombination breakpoints in promoter regions. Furthermore, we observe a positive correlation between GC content and recombination rate, suggesting a role for GC-biased gene conversion or selection on base composition within the GC-rich genome of C. reinhardtii. We also find a positive relationship between nucleotide diversity and recombination, consistent with widespread influence of linked selection in the genome. Finally, we use estimates of the effective rate of recombination to calculate the rate of sex that occurs in natural populations of this important model microbe, estimating a sexual cycle roughly every 770 generations. We argue that the relatively infrequent rate of sex and large effective population size creates an population genetic environment that increases the influence of linked selection on the genome.

scholarly journals Recombination Rate Variation and Infrequent Sex Influence Genetic Diversity in Chlamydomonas reinhardtii

2020 ◽  
Vol 12 (4) ◽  
pp. 370-380 ◽  
Author(s):  
Ahmed R Hasan ◽  
Rob W Ness
Keyword(s):  
Linkage Disequilibrium ◽  
Chlamydomonas Reinhardtii ◽  
Recombination Rate ◽  
Natural Populations ◽  
Effective Rate ◽  
Sexual Cycle ◽  
Rate Variation ◽  
Effective Population ◽  
Recombination Rates ◽  
Recombination Rate Variation

Abstract Recombination confers a major evolutionary advantage by breaking up linkage disequilibrium between harmful and beneficial mutations, thereby facilitating selection. However, in species that are only periodically sexual, such as many microbial eukaryotes, the realized rate of recombination is also affected by the frequency of sex, meaning that infrequent sex can increase the effects of selection at linked sites despite high recombination rates. Despite this, the rate of sex of most facultatively sexual species is unknown. Here, we use genomewide patterns of linkage disequilibrium to infer fine-scale recombination rate variation in the genome of the facultatively sexual green alga Chlamydomonas reinhardtii. We observe recombination rate variation of up to two orders of magnitude and find evidence of recombination hotspots across the genome. Recombination rate is highest flanking genes, consistent with trends observed in other nonmammalian organisms, though intergenic recombination rates vary by intergenic tract length. We also find a positive relationship between nucleotide diversity and physical recombination rate, suggesting a widespread influence of selection at linked sites in the genome. Finally, we use estimates of the effective rate of recombination to calculate the rate of sex that occurs in natural populations, estimating a sexual cycle roughly every 840 generations. We argue that the relatively infrequent rate of sex and large effective population size creates a population genetic environment that increases the influence of selection on linked sites across the genome.

scholarly journals DNA motifs are not general predictors of recombination in two Drosophila sister species

2018 ◽  
Author(s):  
James M. Howie ◽  
Rupert Mazzucco ◽  
Thomas Taus ◽  
Viola Nolte ◽  
Christian Schlötterer
Keyword(s):  
Recombination Rate ◽  
Large Scale ◽  
Strong Association ◽  
Natural Populations ◽  
Sister Species ◽  
Rate Variation ◽  
Sequence Motifs ◽  
Recombination Rates ◽  
The Difference ◽  
Recombination Rate Variation

ABSTRACTMeiotic recombination is crucial for chromosomal segregation, and facilitates the spread of beneficial and removal of deleterious mutations. Recombination rates frequently vary along chromosomes and Drosophila melanogaster exhibits a remarkable pattern. Recombination rates gradually decrease towards centromeres and telomeres, with dramatic impact on levels of variation in natural populations. Two close sister species, D. simulans and D. mauritiana do not only have higher recombination rates, but also exhibit a much more homogeneous recombination rate that only drops sharply close to centromeres and telomeres. Because certain sequence motifs are associated with recombination rate variation in D. melanogaster, we tested whether the difference in recombination landscape between D. melanogaster and D. simulans can be explained by the genomic distribution of recombination-rate associated sequence motifs. We constructed the first high resolution recombination map for D. simulans, and searched for motifs linked with high recombination in both sister species. We identified five consensus motifs, present in either species. While the association between motif density and recombination is strong and positive in D. melanogaster, the results are equivocal in D. simulans. Despite the strong association in D. melanogaster, we do not find a decreasing density of these repeat motifs towards centromeres and telomeres. We conclude that the density of recombination-associated repeat motifs cannot explain the large-scale recombination landscape in D. melanogaster, nor the differences to D. simulans. The strong association seen for the sequence motifs in D. melanogaster likely reflects their impact influencing local differences in recombination rates along the genome.

scholarly journals Variation in recombination rate affects detection of outliers in genome scans under neutrality

2020 ◽  
Author(s):  
Tom R. Booker ◽  
Sam Yeaman ◽  
Michael C. Whitlock
Keyword(s):  
Recombination Rate ◽  
Wide Distribution ◽  
Rate Variation ◽  
Summary Statistic ◽  
Genome Scans ◽  
Coalescent Process ◽  
Genome Wide ◽  
Genomic Landscape ◽  
Recombination Rate Variation ◽  
Linked Selection

AbstractGenome scans can potentially identify genetic loci involved in evolutionary processes such as local adaptation and gene flow. Here, we show that recombination rate variation across a neutrally evolving genome gives rise to mixed sampling distributions of mean FST, a common population genetic summary statistic. In particular, we show that in regions of low recombination the distribution of estimates have more variance and a longer tail than in more highly recombining regions. Determining outliers from the genome-wide distribution without taking local recombination rate into consideration may therefore increase the frequency of false positives in low recombination regions and be overly conservative in more highly recombining ones. We perform genome-scans on simulated and empirical Drosophila melanogaster datasets and, in both cases, find patterns consistent with this neutral model. Similar patterns are observed for other summary statistics used to capture variation in the coalescent process. Linked selection, particularly background selection, is often invoked to explain heterogeneity in across the genome, but here we point out that even under neutrality, statistical artefacts can arise due to variation in recombination rate. Our results highlight a flaw in the design of genome scan studies and suggest that without estimates of local recombination rate, interpreting the genomic landscape of any summary statistic that captures variation in the coalescent process will be very difficult.

scholarly journals Covariation in levels of nucleotide diversity in homologous regions of the avian genome long after completion of lineage sorting

2017 ◽  
Vol 284 (1849) ◽  
pp. 20162756 ◽  
Author(s):  
Ludovic Dutoit ◽  
Nagarjun Vijay ◽  
Carina F. Mugal ◽  
Christen M. Bossu ◽  
Reto Burri ◽  
...  
Keyword(s):  
Genetic Diversity ◽  
Related Species ◽  
Recombination Rate ◽  
Nucleotide Diversity ◽  
Large Population ◽  
Rate Variation ◽  
Lineage Sorting ◽  
Extant Species ◽  
Recombination Rate Variation ◽  
Linked Selection

Closely related species may show similar levels of genetic diversity in homologous regions of the genome owing to shared ancestral variation still segregating in the extant species. However, after completion of lineage sorting, such covariation is not necessarily expected. On the other hand, if the processes that govern genetic diversity are conserved, diversity may potentially covary even among distantly related species. We mapped regions of conserved synteny between the genomes of two divergent bird species—collared flycatcher and hooded crow—and identified more than 600 Mb of homologous regions (66% of the genome). From analyses of whole-genome resequencing data in large population samples of both species we found nucleotide diversity in 200 kb windows to be well correlated (Spearman's ρ = 0.407). The correlation remained highly similar after excluding coding sequences. To explain this covariation, we suggest that a stable avian karyotype and a conserved landscape of recombination rate variation render the diversity-reducing effects of linked selection similar in divergent bird lineages. Principal component regression analysis of several potential explanatory variables driving heterogeneity in flycatcher diversity levels revealed the strongest effects from recombination rate variation and density of coding sequence targets for selection, consistent with linked selection. It is also possible that a stable karyotype is associated with a conserved genomic mutation environment contributing to covariation in diversity levels between lineages. Our observations imply that genetic diversity is to some extent predictable.

scholarly journals Consequences of Recombination Rate Variation on Quantitative Trait Locus Mapping Studies: Simulations Based on the Drosophila melanogaster Genome

Genetics ◽  
2001 ◽  
Vol 159 (2) ◽  
pp. 581-588
Author(s):  
Mohamed A F Noor ◽  
Aimee L Cunningham ◽  
John C Larkin
Keyword(s):  
Quantitative Trait Locus ◽  
Drosophila Melanogaster ◽  
Qtl Mapping ◽  
Quantitative Trait ◽  
Recombination Rate ◽  
Genome Project ◽  
Gene Density ◽  
Rate Variation ◽  
Trait Locus ◽  
Recombination Rate Variation

Abstract We examine the effect of variation in gene density per centimorgan on quantitative trait locus (QTL) mapping studies using data from the Drosophila melanogaster genome project and documented regional rates of recombination. There is tremendous variation in gene density per centimorgan across this genome, and we observe that this variation can cause systematic biases in QTL mapping studies. Specifically, in our simulated mapping experiments of 50 equal-effect QTL distributed randomly across the physical genome, very strong QTL are consistently detected near the centromeres of the two major autosomes, and few or no QTL are often detected on the X chromosome. This pattern persisted with varying heritability, marker density, QTL effect sizes, and transgressive segregation. Our results are consistent with empirical data collected from QTL mapping studies of this species and its close relatives, and they explain the “small X-effect” that has been documented in genetic studies of sexual isolation in the D. melanogaster group. Because of the biases resulting from recombination rate variation, results of QTL mapping studies should be taken as hypotheses to be tested by additional genetic methods, particularly in species for which detailed genetic and physical genome maps are not available.

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