Haj-Ahmad Y, Hickey D A
Nature. 1982 Sep 23;299(5881):350-2. doi: 10.1038/299350a0.
Frequency-dependent selection provides a means for maintaining genetic variability within populations, without incurring a large genetic load. There is a wealth of experimental evidence for the existence of frequency-dependent changes in genotypic fitness among a wide variety of organisms. Examples of traits which have been shown to be subject to frequency-dependent selection include the self-incompatibility alleles of plants, chromosomal rearrangements in Drosophila, visible mutations, enzyme variants and rare-male mating advantage in Drosophila. These experiments have been interpreted in a number of different ways. Principally, frequency dependence of genotype fitness may result from intergenotype facilitation due to the production of biotic residues, or from the differential use of resources by the competing genotypes. However, it has proved extremely difficult to isolate and identify any biotic residue of importance or, alternatively, to understand the manner in which genotypes partition the environment. Thus, the difficulty in the interpretation of experiments which show frequency-dependent selective effects stems largely from our lack of understanding of the exact physiological mechanisms which produce these frequency-dependent effects. The principal aim of this study was to investigate the mechanisms associated with frequency-dependent selection at the amylase locus in Drosophila melanogaster. The excretion of catalytically active amylase enzyme and its effect on food medium composition were correlated with the outcome of intraspecific competition between amylase-deficient and amylase-producing genotypes. Amylase-producing genotypes were shown to excrete enzymatically active amylase protein into the food medium. The excreted amylase causes the external digestion of dietary starch; this accounts for the frequency-dependent increase in the viability of the amylase-deficient mutants in mixed cultures, maintained on a starch-rich diet.
频率依赖型选择提供了一种在种群内维持遗传变异性的方式，而不会产生巨大的遗传负荷。有大量实验证据表明，在各种各样的生物体中存在基因型适合度的频率依赖性变化。已被证明受频率依赖型选择影响的性状例子包括植物的自交不亲和等位基因、果蝇的染色体重排、可见突变、酶变体以及果蝇中的稀有雄性交配优势。这些实验有多种不同的解释方式。主要地，基因型适合度的频率依赖性可能源于由于生物残基的产生而导致的基因型间促进作用，或者源于竞争基因型对资源的差异利用。然而，事实证明，要分离和识别任何重要的生物残基，或者理解基因型划分环境的方式极其困难。因此，解释显示频率依赖型选择效应的实验的困难很大程度上源于我们对产生这些频率依赖型效应的确切生理机制缺乏了解。本研究的主要目的是研究与黑腹果蝇淀粉酶基因座处的频率依赖型选择相关的机制。催化活性淀粉酶的排泄及其对食物培养基成分的影响与淀粉酶缺陷型和产生淀粉酶的基因型之间种内竞争的结果相关。已证明产生淀粉酶的基因型会将具有酶活性的淀粉酶蛋白排泄到食物培养基中。排泄出的淀粉酶会导致膳食淀粉的体外消化；这解释了在富含淀粉的培养基上混合培养时，淀粉酶缺陷型突变体存活率的频率依赖性增加。
