A pair of amino acid polymorphisms within the Drosophila melanogaster Malic enzyme (Men) locus presents an interesting case of genetic variation that appears to be under selection. The two alleles at each site are biochemically distinct, but their biological effects are unknown. One polymorphic site is near the active site and the other is buried within the protein. Strikingly, in twelve different populations, the first polymorphism is always found at approximately a 50:50 allelic frequency, whereas the second polymorphism is always found at approximately 90:10. The consistency of the frequencies between populations suggests that the polymorphisms are under selection and it is possible that balancing selection is at play. We used 16 lines of flies to create the nine genotypes needed to quantify both effects of the polymorphic sites and possible genetic background effects, which we found to be widespread. The alleles at each site differ, but in different biochemical characteristics. The first site significantly influences MEN K_m and V_max, whereas the second site affects the K_m and the V_max/K_m ratio (relative activity). Interestingly, the rarest allele is the most biochemically distinct. We also assayed three more distal phenotypes, triglyceride concentration, carbohydrate concentration, and longevity. In all cases, the phenotypes of the heterozygous genotypes are intermediate between those of the respective homozygotes suggesting that if balancing selection is maintaining the observed allele frequencies it is not through non-linear combinations of the biochemical phenotypes.

中文翻译：

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果蝇苹果酸酶基因座中单核苷酸多态性的生物效应。

果蝇苹果酸酶（Men）中的一对氨基酸多态性。）基因座提出了一个有趣的遗传变异案例，似乎正在选择中。每个位点的两个等位基因在生物化学上是不同的，但其生物学作用尚不清楚。一个多态性位点靠近活性位点，另一个多态性位点埋在蛋白质中。令人惊讶的是，在十二个不同的种群中，第一个多态性总是在大约50:50等位基因频率处发现，而第二个多态性总是在大约90:10等位基因发现。总体之间频率的一致性表明多态性处于选择状态，并且平衡选择可能在起作用。我们使用了16条果蝇来创建9个基因型，以量化多态性位点的影响和可能的遗传背景影响，这被我们发现是广泛的。每个位点的等位基因不同，但是具有不同的生化特征。第一个站点对MEN有重大影响K _m和V _max，而第二个位置会影响K _m和V _max / K _m的比率（相对活性）。有趣的是，最稀有的等位基因在生化上最独特。我们还分析了另外三种远端表型，甘油三酸酯浓度，碳水化合物浓度和寿命。在所有情况下，杂合基因型的表型都介于各个纯合子的表型之间，这表明如果平衡选择维持了观察到的等位基因频率，则不是通过生化表型的非线性组合。