Effect of mutation mechanisms on variant composition and distribution in Caenorhabditis elegans

Effect of recombination on genetic diversity of Caenorhabditis elegans

Greater molecular divergence and genetic diversity are present in regions of high recombination in many species. Studies describing the correlation between variant abundance and recombination rate have long focused on recombination in the context of linked selection models, whereby interference between linked sites under positive or negative selection reduces genetic diversity in regions of low recombination. Here, we show that indels, especially those of intermediate sizes, are enriched relative to single nucleotide polymorphisms in regions of high recombination in C. elegans. To explain this phenomenon, we reintroduce an alternative model that emphasizes the mutagenic effect of recombination. To extend the analysis, we examine the variants with a phylogenetic context and discuss how different models could be examined together. The number of variants generated by recombination in natural populations could be substantial including possibly the majority of some indel subtypes. Our work highlights the potential importance of a mutagenic effect of recombination, which could have a significant role in the shaping of natural genetic diversity.

Fast genetic mapping using insertion-deletion polymorphisms in Caenorhabditis elegans

Genetic mapping is used in forward genetics to narrow the list of candidate mutations and genes corresponding to the mutant phenotype of interest. Even with modern advances in biology such as efficient identification of candidate mutations by whole-genome sequencing, mapping remains critical in pinpointing the responsible mutation. Here we describe a simple, fast, and affordable mapping toolkit that is particularly suitable for mapping in Caenorhabditis elegans. This mapping method uses insertion-deletion polymorphisms or indels that could be easily detected instead of single nucleotide polymorphisms in commonly used Hawaiian CB4856 mapping strain. The materials and methods were optimized so that mapping could be performed using tiny amount of genetic material without growing many large populations of mutants for DNA purification. We performed mapping of previously known and unknown mutations to show strengths and weaknesses of this method and to present examples of completed mapping. For situations where Hawaiian CB4856 is unsuitable, we provide an annotated list of indels as a basis for fast and easy mapping using other wild isolates. Finally, we provide rationale for using this mapping method over other alternatives as a part of a comprehensive strategy also involving whole-genome sequencing and other methods.

Evolution of the Mutational Process under Relaxed Selection in Caenorhabditis elegans

The mutational process varies at many levels, from within genomes to among taxa. Many mechanisms have been linked to variation in mutation, but understanding of the evolution of the mutational process is rudimentary. Physiological condition is often implicated as a source of variation in microbial mutation rate and may contribute to mutation rate variation in multicellular organisms.Deleterious mutations are an ubiquitous source of variation in condition. We test the hypothesis that the mutational process depends on the underlying mutation load in two groups of Caenorhabditis elegans mutation accumulation (MA) lines that differ in their starting mutation loads. "First-order MA" (O1MA) lines maintained under minimal selection for ∼250 generations were divided into high-fitness and low-fitness groups and sets of "second-order MA" (O2MA) lines derived from each O1MA line were maintained for ∼150 additional generations. Genomes of 48 O2MA lines and their progenitors were sequenced. There is significant variation among O2MA lines in base-substitution rate (µbs), but no effect of initial fitness; the indel rate is greater in high-fitness O2MA lines. Overall, µbs is positively correlated with recombination and proximity to short tandem repeats and negatively correlated with 10 bp and 1 kb GC content. However, probability of mutation is sufficiently predicted by the three-nucleotide motif alone. Approximately 90% of the variance in standing nucleotide variation is explained by mutability. Total mutation rate increased in the O2MA lines, as predicted by the "drift barrier" model of mutation rate evolution. These data, combined with experimental estimates of fitness, suggest that epistasis is synergistic.

Effect of mutation mechanisms on variant composition and distribution in Caenorhabditis elegans

Genetic diversity is maintained by continuing generation and removal of variants. While examining over 800,000 DNA variants in wild isolates of Caenorhabditis elegans, we made a discovery that the proportions of variant types are not constant across the C. elegans genome. The variant proportion is defined as the fraction of a specific variant type (e.g. single nucleotide polymorphism (SNP) or indel) within a broader set of variants (e.g. all variants or all non-SNPs). The proportions of most variant types show a correlation with the recombination rate. These correlations can be explained as a result of a concerted action of two mutation mechanisms, which we named Morgan and Sanger mechanisms. The two proposed mechanisms act according to the distinct components of the recombination rate, specifically the genetic and physical distance. Regression analysis was used to explore the characteristics and contributions of the two mutation mechanisms. According to our model, ~20-40% of all mutations in C. elegans wild populations are derived from programmed meiotic double strand breaks, which precede chromosomal crossovers and thus may be the point of origin for the Morgan mechanism. A substantial part of the known correlation between the recombination rate and variant distribution appears to be caused by the mutations generated by the Morgan mechanism. Mathematically integrating the mutation model with background selection model gives a more complete depiction of how the variant landscape is shaped in C. elegans. Similar analysis should be possible in other species by examining the correlation between the recombination rate and variant landscape within the context of our mutation model.