Abstract
C. elegans presents a low level of molecular diversity, which may be explained by its selfing mode of reproduction. Recent work on the genetic structure of natural populations of C. elegans indeed suggests a low level of outcrossing, and little geographic differentiation because of migration. The level and pattern of molecular diversity among wild isolates of C. elegans are compared with those found after accumulation of spontaneous mutations in the laboratory. The last part of the chapter reviews phenotypic differences among wild isolates of C. elegans.
Highlights
Natural variation and population genetics of Caenorhabditis elegans frequency of outcrossing, the geographic structure of populations and the fate of deleterious mutations
The nuclear mutation rate was estimated in Mutation accumulation (MA) lines to 2.1 × 10-8 per nucleotide site per generation (Denver et al, 2004). 17 out of 30 observed mutations were indels, among which insertions were dominant (13 out of 17). This contrasts with the pattern observed among wild isolates in putatively neutral regions such as transposon pseudogenes, where deletions are 2.8 times more frequent than insertions (Witherspoon and Robertson, 2003); data obtained on putative pseudogenes should be taken carefully, as Natural variation and population genetics of Caenorhabditis elegans selection may not be fully suppressed
At least in the locations sampled so far, C. elegans presents a low genetic diversity, comparable to that of humans and 20-fold lower than that found in Drosophila melanogaster
Summary
Work on molecular polymorphisms in C. elegans was based on a set of natural isolates that were collected worldwide over many years, each kept as a single selfing isogenic strain (available at CGC) These isolates come from North America (mostly California), Western Europe (including N2, the reference strain, isolated in Bristol, U.K.) and Adelaide (Australia); one isolate, CB4856, was sampled in Hawaii and is used for SNP mapping (see SNPs: introduction and 2-point mapping; Hodgkin and Doniach, 1997; Figure 1). Mutation accumulation (MA) lines are established by picking a single individual at each generation, limiting natural selection as much as possible (Figure 2) They represent the raw mutational process, whereas the diversity observed among wild isolates is the result of mutation as well as population structure and phenotypic selection. We first review for mitochondrial and nuclear DNA the observed molecular diversity in wild isolates, the mutation rate and pattern in MA lines, and compare them to reveal the action of selection in natural populations
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