MODEL FOR THE EVOLUTION OF DISTYLY
MODEL FOR THE EVOLUTION OF DISTYLY
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DOI：
10.1086/283496
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发表时间：
1979-01-01
影响因子：
2.9
通讯作者：
CHARLESWORTH, B
中科院分区：
文献类型：
--
作者：
CHARLESWORTH, D;CHARLESWORTH, B
在不相容系统的理论得到支持之前，这种形态不太可能进化。研究的第一个问题是DISTHY中发现的类型的不亲和性的进化，有2个花粉和2个柱头类型。第一步，从自交亲和性开始，可能是突变成一种新的花粉类型，这样突变体就不能自交或使其他个体受精。给出了这种突变(假定为隐性突变)传播的条件。下一步，研究显性突变为一种新的柱头类型，它与原始的花粉类型不相容，但与新的类型相容。如果这种突变首先发生，几乎可以肯定它会被消除，但如果它发生在一个连锁的基因座上，就可以在对花粉突变具有多态的种群中传播。在紧密连锁的情况下，该模型产生了具有两种不亲和型的群体，其中一种为显性，另一种为显性，只有罕见的重组类型。计算机运行表明，导致更紧密联系的修饰基因将在这样的种群中传播。接下来研究花药和柱头位置差异的进化。这些可能会降低自交率。花药位置不太可能首先改变，因为这会降低雄性可育性，就像减少花粉转移到其他花的柱头上的机会一样；改变柱头位置对雌性可育性的影响可能会小于这一点，假设最初存在过剩的花粉，并且所有胚珠都可以受精，除非接收的相容花粉的数量低于某个阈值水平。如果不亲和系统是完美的，可以选择一个新的柱头位置，但与不亲和基因连锁不会影响结果。如果存在部分自交不亲和，而更亲和的形式是显性的，那么连锁对于影响柱头或花药位置的显性突变的传播是非常重要的；在连锁基因将建立多态的条件下，非连锁突变经常被消除或固定。计算机运行表明，在形态突变和不亲和基因座之间存在更紧密联系的选择，在两者都是多态的情况下。当进行最终形态改变的隐性突变引入时，它与其他基因座的连锁越紧密，传播就越快。在紧密连锁的平衡状态下，2个表型的频率大致相等，所有隐性等位基因均为1个纯合子，所有座位均为1个杂合子。自交亲和型是显性的，通常具有新柱头位置的体型也是如此。这与许多被区分的物种是一致的。自交亲和型为隐性的物种不能用这个模型来解释，但如果影响柱头反应的基因也控制了它的位置，就可以解释。本文还对该案例进行了研究。它具有与另一种情况类似的特性；尤其是，当被引入分离花粉反应基因的种群时，具有这种类型效应的突变有很强的连锁约束。该模型还可以解释柱头位置不同的物种，且有两种等频率的形式，每种形式对应一种不亲和型。用以前的模型很难解释这样的群体。
That morphological distyly is unlikely to evolve before the incompatibility system is a theory which is supported. The 1st problem studied is the evolution of incompatibility of the type found in distyly, with 2 pollen and 2 stigma types. The 1st step, starting from self-compatibility, may be a mutation to a new pollen type, such that the mutant cannot self-fertilize or fertilize other individuals. Conditions for the spread of such a mutation (assumed recessive) are given. Next, a dominant mutation to a new stigma type, incompatible with the original pollen type but compatible with the new type, is studied. Such a mutation is almost certain to be eliminated if it occurs first, but can spread in a population polymorphic for the pollen mutation provided that it occurs at a linked locus. With tight linkage, this model generates a population with 2 incompatibility types, one dominant to the other, with only rare recombinant types. Computer runs show that modifiers causing tighter linkage will spread in such populations. The evolution of differences in anther and stigma positions is studied next. These presumably reduce the selfing rate. Anther position may be unlikely to change first, as this will reduce male fertility by as much as the reduction in the chance of pollen transfer to the stigmas of other flowers; a changed stigma position will probably affect female fertility less than this, assuming that excess pollen is initially present and that all ovules can be fertilized unless the amount of compatible pollen received falls below some threshold level. If the incompatibility system is perfect, a new stigma position can be selected for, but linkage to the incompatibility locus does not affect the results. If there is partial self-compatibility and the more self-compatible form is dominant, linkage is very important for the spread of dominant mutations affecting stigma or anther position; an unlinked mutation is frequently either eliminated or fixed under conditions in which a linked gene will establish polymorphism. Computer runs show that there is selection for tighter linkage between morphology mutations and the incompatibility locus, in situations where both are polymorphic. When a recessive mutation making the final morphological change is introduced, it spreads faster the tighter its linkage to the other loci. There is again strong selection for tighter linkage, and at equilibrium with tight linkage there are approximately equal frequencies of 2 phenotypes, 1 homozygous for all the recessive alleles, and one heterozygous at all the loci. The more self-compatible form is dominant and so usually is the form with the new stigma position. This agrees with many distyled species. Species in which the more self-compatible form is recessive cannot be explained by this model, but can be accounted for if the gene affecting the stigma reaction also controls its position. This case is also studied. It has properties similar to the other case; in particular, there is a strong linkage constraint for mutations with this type effect when introduced into a population segregating for a pollen-reaction gene. This model can also explain species with differences in stigma position only, and with 2 equally frequent forms, each corresponding to 1 incompatibility type. Such populations are not easy to explain by the previous model.