Naturally occurring arsenic is toxic at extremely low concentrations, yet some species persist even in high arsenic environments. We wanted to test if these species show evidence of evolution associated with arsenic exposure. To do this, we compared allelic variation across 872 coding nucleotides of arsenic (+3) methyltransferase (as3mt) and whole fish as3mt gene expression from three field populations of Gambusia affinis, from water sources containing low (1.9 ppb), medium-low (3.3 ppb), and high (15.7 ppb) levels of arsenic. The high arsenic site exceeds the US EPA’s Maximum Contamination Level for drinking water. Medium-low and high populations exhibited homozygosity, and no sequence variation across all animals sampled. Eleven of 24 fish examined (45.8%) in the low arsenic population harbored synonymous single nucleotide polymorphisms (SNPs) in exons 4 and/or 10. SNP presence in the low arsenic population was not associated with differences in as3mt transcript levels compared to fish from the medium-low site, where SNPs were noted; however, as3mt expression in fish from the high arsenic concentration site was significantly lower than the other two sites. Low sequence variation in fish populations from sites with medium-low and high arsenic concentrations suggests greater selective pressure on this allele, while higher variation in the low population suggests a relaxed selection. Our results suggest gene regulation associated with arsenic detoxification may play a more crucial role in influencing responses to arsenic than polymorphic gene sequence. Understanding microevolutionary processes to various contaminants require the evaluation of multiple populations across a wide range of pollution exposures.

天然存在的砷在极低浓度下是有毒的，但有些物种甚至在高砷环境中仍然存在。我们想测试这些物种是否表现出与砷暴露相关的进化证据。为此，我们比较了砷 (+3) 甲基转移酶 ( as3mt ) 的 872 个编码核苷酸的等位基因变异，以及来自含有低 (1.9 ppb)、中低 (1.9 ppb) 水源的甘布西亚 (Gambusia affinis)三个田间种群的全鱼as3mt基因表达。 3.3 ppb）和高浓度（15.7 ppb）砷。该高砷地点超过了美国环保局规定的饮用水最高污染水平。中低和高群体表现出纯合性，所有采样动物都没有序列变异。在低砷种群中检查的 24 条鱼中，有 11 条 (45.8%) 在外显子 4 和/或 10 中具有同义单核苷酸多态性 (SNP)。与来自低砷种群的鱼相比，低砷种群中 SNP 的存在与as3mt转录水平的差异无关。中低位点，其中注意到 SNP；然而，砷高浓度位点的鱼中as3mt表达量显着低于其他两个位点。中低和高砷浓度地点的鱼类种群中的低序列变异表明对该等位基因的选择压力更大，而低种群中的更高变异表明选择宽松。我们的结果表明，与砷解毒相关的基因调控可能比多态性基因序列在影响砷反应方面发挥更重要的作用。了解各种污染物的微进化过程需要对各种污染暴露下的多个群体进行评估。