Wagner Günter P, Gabriel Wilfred
Institute for Zoology, University of Vienna, Althanstrasse 14, A-1090, Vienna, AUSTRIA.
Max Planck Institute for Limnology, Department of Physiological Ecology, P. O. Box 165, D-2320, Plön, FEDERAL REPUBLIC OF GERMANY.
Evolution. 1990 May;44(3):715-731. doi: 10.1111/j.1558-5646.1990.tb05950.x.
Finite parthenogenetic populations with high genomic mutation rates accumulate deleterious mutations if back mutations are rare. This mechanism, known as Muller's ratchet, can explain the rarity of parthenogenetic species among so called higher organisms. However, estimates of genomic mutation rates for deleterious alleles and their average effect in the diploid condition in Drosophila suggest that Muller's ratchet should eliminate parthenogenetic insect populations within several hundred generations, provided all mutations are unconditionally deleterious. This fact is inconsistent with the existence of obligatory parthenogenetic insect species. In this paper an analysis of the extent to which compensatory mutations can counter Muller's ratchet is presented. Compensatory mutations are defined as all mutations that compensate for the phenotypic effects of a deleterious mutation. In the case of quantitative traits under stabilizing selection, the rate of compensatory mutations is easily predicted. It is shown that there is a strong analogy between the Muller's ratchet model of Felsenstein (1974) and the quantitative genetic model considered here, except for the frequency of compensatory mutations. If the intensity of stabilizing selection is too small or the mutation rate too high, the optimal genotype becomes extinct and the population mean drifts from the optimum but still reaches a stationary distribution. This distance is essentially the same as predicted for sexually reproducing populations under the same circumstances. Hence, at least in the short run, compensatory mutations for quantitative characters are as effective as recombination in halting the decline of mean fitness otherwise caused by Muller's ratchet. However, it is questionable whether compensatory mutations can prevent Muller's ratchet in the long run because there might be a limit to the capacity of the genome to provide compensatory mutations without eliminating deleterious mutations at least during occasional episodes of sex.
如果回复突变很少，具有高基因组突变率的有限孤雌生殖种群会积累有害突变。这种机制，即所谓的穆勒棘轮效应，可以解释在所谓的高等生物中孤雌生殖物种为何稀少。然而，对果蝇中有害等位基因的基因组突变率及其在二倍体状态下的平均效应的估计表明，倘若所有突变都是无条件有害的，那么穆勒棘轮效应应该会在几百代内消除孤雌生殖昆虫种群。这一事实与专性孤雌生殖昆虫物种的存在相矛盾。本文分析了补偿性突变能够对抗穆勒棘轮效应的程度。补偿性突变被定义为所有能够补偿有害突变表型效应的突变。在稳定选择下的数量性状情形中，补偿性突变的速率很容易预测。结果表明，费尔森斯坦（1974年）的穆勒棘轮模型与本文所考虑的数量遗传模型之间存在很强的相似性，只是补偿性突变的频率有所不同。如果稳定选择的强度过小或突变率过高，最优基因型会灭绝，种群均值会偏离最优值，但仍会达到一个稳定分布。这个距离与在相同情况下有性生殖种群的预测基本相同。因此，至少在短期内，数量性状的补偿性突变在阻止因穆勒棘轮效应导致的平均适合度下降方面与重组一样有效。然而，从长远来看，补偿性突变是否能够阻止穆勒棘轮效应是值得怀疑的，因为基因组提供补偿性突变而不消除有害突变的能力可能存在限度，至少在偶尔的有性生殖时期是这样。
