Vassilieva L L, Hook A M, Lynch M
Department of Biology, University of Oregon, Eugene 97403, USA.
Evolution. 2000 Aug;54(4):1234-46. doi: 10.1111/j.0014-3820.2000.tb00557.x.
Spontaneous mutation to mildly deleterious alleles has emerged as a potentially unifying component of a variety of observations in evolutionary genetics and molecular evolution. However, the biological significance of hypotheses based on mildly deleterious mutation depends critically on the rate at which new mutations arise and on their average effects. A long-term mutation-accumulation experiment with replicate lines of the nematode Caenorhabditis elegans maintained by single-progeny descent indicates that recurrent spontaneous mutation causes approximately 0.1% decline in fitness per generation, which is about an order of magnitude less than that suggested by previous studies with Drosophila. Two rather different approaches, Bateman-Mukai and maximum likelihood, suggest that this observation, along with the observed rate of increase in the variance of fitness among lines, is consistent with a genomic deleterious mutation rate for fitness of approximately 0.03 per generation and with an average homozygous effect of approximately 12%. The distribution of mutational effects for fitness appears to have a relatively low coefficient of variation, being no more extreme than expected for a negative exponential, and for one composite fitness measure (total progeny production) approaches constancy of effects. These results are derived from assays in a benign environment. At stressful temperatures, estimates of the genomic deleterious mutation rate (for genes expressed at such temperatures) is sixfold lower, whereas those for the average homozygous effect is approximately eightfold higher. Our results are reasonably compatible with existing estimates for flies, when one considers the differences between these species in the number of germ-line cell divisions per generation and the magnitude of transposable element activity.
自发突变为轻度有害等位基因已成为进化遗传学和分子进化中各种观察结果的一个潜在统一要素。然而，基于轻度有害突变的假说的生物学意义关键取决于新突变产生的速率及其平均效应。一项对秀丽隐杆线虫复制系进行的长期突变积累实验，通过单后代传代维持线虫，结果表明，反复自发突变导致每代适合度下降约0.1%，这比之前对果蝇的研究所表明的下降幅度小约一个数量级。两种相当不同的方法，即贝特曼-穆凯方法和最大似然法，表明这一观察结果，连同观察到的品系间适合度方差的增加速率，与适合度的基因组有害突变率约为每代0.03以及平均纯合效应约为12%是一致的。适合度的突变效应分布似乎具有相对较低的变异系数，并不比负指数分布预期的更极端，并且对于一种综合适合度测量（总后代产量），效应接近恒定。这些结果来自于在良性环境中的测定。在应激温度下，基因组有害突变率（针对在这种温度下表达的基因）的估计值低六倍，而平均纯合效应的估计值则高约八倍。当考虑到这些物种在每代生殖系细胞分裂数量和转座元件活性大小方面的差异时，我们的结果与对果蝇的现有估计相当吻合。
