Data from: The opportunity for balancing selection in experimental populations of Caenorhabditis elegans

The role of balancing selection in maintaining diversity during the evolution of sexual populations to novel environments is poorly understood. To address this issue, we studied the impact of two mating systems, androdioecy and dioecy, on genotype distributions during the experimental evolution of Caenorhabditis elegans. We analyzed the temporal trajectories of 334 single nucleotide polymorphisms (SNPs), covering 1/3 of the genome, and found extensive allele frequency changes and little loss of heterozygosities after 100 generations. As modeled with numerical simulations, SNP differentiation was consistent with genetic drift and average fitness effects of 2%, assuming that selection acted independently at each locus. Remarkably, inbreeding by self-fertilization was of little consequence to SNP differentiation. Modeling selection on deleterious recessive alleles suggests that the initial dynamics can be explained by associative overdominance, but not the later stages since much lower heterozygosities would be maintained. By contrast, models with selection on true overdominant loci can explain the heterozygote excess observed at all periods, particularly when negative epistasis or independent fitness effects were considered. Overall, these findings indicate that selection at single loci, including purging of recessive alleles, underlies most of the genetic differentiation accomplished during the experiment. Nonetheless they also imply that maintenance of genetic diversity may be due to balancing selection.

平衡选择（balancing selection）在有性种群适应新环境的演化过程中对维持遗传多样性的作用，目前仍不甚明确。为解决这一科学问题，我们以秀丽隐杆线虫（Caenorhabditis elegans）的实验进化为研究对象，分析了两种交配系统——雄雌同体型与雌雄异体型（androdioecy和dioecy）对基因型分布的影响。我们对覆盖基因组1/3区域的334个单核苷酸多态性（single nucleotide polymorphisms, SNPs）的时间动态轨迹进行了分析，结果显示，在历经100代演化后，群体出现了广泛的等位基因频率变化，但杂合度损失极小。通过数值模拟建模可知，假设每个位点上的选择作用独立发生，单核苷酸多态性的分化模式与遗传漂变及平均2%的适合度效应相符。值得注意的是，自交引发的近交对单核苷酸多态性分化几乎无影响。针对有害隐性等位基因的选择模型表明，群体的初始动态可通过关联超显性（associative overdominance）解释，但无法适配后期演化阶段——因为此时群体应维持更低的杂合度。与之形成对比的是，针对真正超显性位点的选择模型能够解释所有观测到的杂合子过剩现象，尤其是在考虑负上位性或独立适合度效应的情况下。综上，本研究结果显示，实验过程中产生的大部分遗传分化，源于单一位点上的选择作用（包括隐性等位基因的清除）。但同时也表明，遗传多样性的维持或许依赖于平衡选择。