A GENERAL MODEL TO ACCOUNT FOR ENZYME VARIATION IN NATURAL POPULATIONS. III. MULTIPLE ALLELES
A GENERAL MODEL TO ACCOUNT FOR ENZYME VARIATION IN NATURAL POPULATIONS. III. MULTIPLE ALLELES
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解释自然群体中酶变异的通用模型。
DOI：
10.1111/j.1558-5646.1977.tb00985.x
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发表时间：
1977
期刊：
影响因子：
3.3
通讯作者：
J. Gillespie
中科院分区：
文献类型：
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作者：
J. Gillespie
最近对酶变体的电泳类别的生物化学探索揭示了大量先前怀疑但未检测到的变异（伯恩斯坦等人，1973; Singh等人，1975年）。这种变异的存在对自然选择通过杂种优势选择维持变异的假设提出了新的问题，因为随着等位基因数量的增加，杂种优势下的多态性条件变得非常有限（小岛健一博士，pers. comm.）。量化这一想法的一种方法是问：在具有n个等位基因的基因座上的n（n + 1）72个基因型的所有可能适合度的总集合中，什么分数导致所有等位基因分离的稳定多态性？通过在计算机上进行蒙特-卡罗模拟，通过为n（n + 1）72个基因型中的每一个分配均匀且独立地分布在单位区间上的适合度值，并应用具有n个等位基因的群体中内部平衡的存在性和稳定性的通常标准，可以容易地计算该分数（参见例如Mandel，1959）。当这个实验被重复了很多次，估计的比例的适应空间，产生稳定的内部平衡的结果。图1给出了这样一个模拟的结果，并很好地说明了小岛关于杂种优势假设的限制性本质的陈述的真实性。然而，这一论点有一个非常不现实的方面。在生物系统中，某些规律支配着相关基因型的适应度关系。这方面充分体现在健身组件的数量遗传学工作中（例如，Mukai等人，1972），该模型给出了杂合体适合度与两个亲本纯合体平均值的相关性估计。虽然相关基因型的适应性关系的规律尚不清楚，但很明显，它们可以从根本上改变图1给人的印象。例如，如果杂合子的适合度总是介于亲本纯合子之间，并且如果环境是恒定的，则不可能有稳定的多态性，并且稳定适合度的子集将是空的。另一方面，如果某种生物学原理决定了杂合子总是比它们的亲本纯合子更适合，那么产生稳定点的适合度空间的分数将比图1中给出的大得多。既然新的遗传变异正在自然种群中被发现，那么寻找生物学意义上的适合度分配的约束条件，并使大量等位基因得以维持，应该是一个主要的研究领域。在本文中，将检查本系列前几篇论文（吉莱斯皮和兰利，1974年;吉莱斯皮，1976年）中提出的限制，并证明通过在波动环境中平衡选择，可以轻易地实现大量等位基因的稳定存在。
Recent biochemical explorations into electrophoretic classes of enzyme variants have revealed a wealth of previously suspected but undetected variation (Bernstein et al., 1973; Singh et al., 1975). The existence of this variation poses new problems for the hypothesis that natural selection is maintaining the variation by heterotic selection since the conditions for polymorphism under heterosis become incredibly restrictive as the number of alleles increases (Dr. Ken-ichi Kojima, pers. comm.). One way to quantify this idea would be to ask: Of the total set of all possible fitnesses for the n(n + 1)72 genotypes at a locus with n alleles, what fraction leads to a stable polymorphism with all alleles segregating? This fraction can be easily calculated by a Monte-Carlo simulation on the computer by assigning each of the n(n + 1)72 genotypes a fitness value uniformly and independently distributed on the unit interval and applying the usual criterion for the existence and stability of an internal equilibrium in a population with n alleles (see, e.g. Mandel, 1959). When this experiment is repeated a large number of times, an estimate of the proportion of the fitness space which yields stable internal equilibria results. Figure 1 gives the results of such a simulation and illustrates well the truth of Kojima's statement about the restrictive nature of the heterosis assumption. There is, however, a very unrealistic aspect of this argument. In biological sysstems, certain laws govern the relationships of fitnesses in related genotypes. This aspect is amply born out in the work on the quantitative genetics of fitness components (e.g., Mukai et al., 1972) which gives estimates of the correlation of the heterozygote fitness to the mean of the two parental homozygotes. Although the laws governing the relationships of fitnesses of related genotypes are not known, it is clear they can radically change the impression given by Figure 1. For example, if heterozygotes were always intermediate in fitness between the parental homozygotes, and if the environment were constant, no stable polymorphism would be possible and the subset of stable fitnesses would be empty. On the other hand, if some biological principle dictated that heterozygotes are invariably more fit than their parental homozygotes, the fraction of the space of fitnesses yielding stable points would be much larger than given in Figure 1. Finding constraints on the assignment of fitnesses which are biologically meaningful and which will allow the maintenance of large numbers of alleles should be a major area of investigation now that new genetic variation is being uncovered in natural populations. In this paper, the constraints suggested in the previous papers in this series (Gillespie and Langley, 1974; Gillespie, 1976), will be examined and shown to allow readily the stable existence of large numbers of alleles by balancing selection in a fluctuating environment.