Shifting patterns of natural variation in the nuclear genome of caenorhabditis elegans

Background: Genome wide analysis of variation within a species can reveal the evolution of fundamental biological processes such as mutation, recombination, and natural selection. We compare genome wide sequence differences between two independent isolates of the nematode Caenorhabditis elegans (CB4856 and CB4858) and the reference genome (N2). Results: The base substitution pattern when comparing N2 against CB4858 reveals a transition over transversion bias (1.32:1) that is not present in CB4856. In CB4856, there is a significant bias in the direction of base substitution. The frequency of A or T bases in N2 that are G or C bases in CB4856 outnumber the opposite frequencies for transitions as well as transversions. These differences were not observed in the N2/CB4858 comparison. Similarly, we observed a strong bias for deletions over insertions in CB4856 (1.44:1) that is not present in CB4858. In both CB4856 and CB4858, there is a significant correlation between SNP rate and recombination rate on the autosomes but not on the X chromosome. Furthermore, we identified numerous significant hotspots of variation in the CB4856-N2 comparison. In both CB4856 and CB4858, based on a measure of the strength of selection (k(a)/k(s)), all the chromosomes are under negative selection and in CB4856, there is no difference in the strength of natural selection in either the autosomes versus X or between any of the chromosomes. By contrast, in CB4858, k(a)/k(s) values are smaller in the autosomes than in the X chromosome. In addition, in CB4858, k(a)/k(s) values differ between chromosomes. Conclusions: The clear bias of deletions over insertions in CB4856 suggests that either the CB4856 genome is becoming smaller or the N2 genome is getting larger. We hypothesize the hotspots found represent alleles that are shared between CB4856 and CB4858 but not N2. Because the k(a)/k(s) ratio in the X chromosome is higher than the autosomes on average in CB4858, purifying selection is reduced on the X chromosome.